Analysis of the Influence of Blanking Clearance on the Wear of The

materials

Article Analysis of the Inﬂuence of Blanking Clearance on the Wear of the Punch, the Change of the Burr Size and the Geometry of the Hook Blanked in the Hardened Steel Sheet

Jacek Mucha 1,* and Jacek Tutak 2

1 The Faculty of Mechanical Engineering and Aeronautics, Department of Mechanical Engineering, Rzeszów University of Technology, Al. Powsta´nców Warszawy 8, 35-959 Rzeszów, Poland 2 The Faculty of Mechanical Engineering and Aeronautics, Department of Applied Mechanics and Robotics, Rzeszów University of Technology, Al. Powsta´nców Warszawy 8, 35-959 Rzeszów, Poland; [email protected] * Correspondence: [email protected]

Received: 18 March 2019; Accepted: 12 April 2019; Published: 17 April 2019

Abstract: In this thesis, the results of an experimental analysis of blanking angled hooks with a punch of a sloping face in a thin steel sheet with a hardness of 55 HRC are presented. The blanking punch was made of K340 cold-work tool steel. Tests were carried out for three values of clearance, 5%, 10%, and 15% of sheet thickness. The results of the analysis of the influence of the number of cuts made by the punch on the growth of the burr at the sheet edge were presented. Moreover, the influences of the clearance on the initial values of burr (bh), deflection (Hb), and the bending radius (Rb) of the hook have been shown. The influence of the friction path on the intensity of degradation of working surfaces and the blanking edges of the punch was also demonstrated. The obtained results allow the selection of the proper clearance and new tool materials for blanking blades working in particularly difficult tribological conditions.

Keywords: blanking; burr height; sloping face of the punch; damage of the punch; sheet C45

1. Introduction During blanking, due to the pressure on the blanking edge of the punch and the die, a complex stress distribution is achieved. After plasticizing the material, depending on the method of the process, diﬀerent geometry of the intersection surface and burr size can be obtained. In the case of deformation of the sheet material, the punching force causes the tools to be loaded. The greatest pressures are close to the tool’s cutting edges. The pressure of the punched material on the punch surface and the friction causes intensive tool wear [1]. The vector of reaction forces for the tools is inversely directed to the working movement of the punch. This causes a bending of the sheet, which results in the lateral action of forces. The clearance between the tool edges determines the value and orientation of the contact force. The model of reaction forces and the bending moment is shown in Figure1[2].

Materials 2019, 12, 1261; doi:10.3390/ma12081261 www.mdpi.com/journal/materials Materials 2019, 12, 1261 2 of 16 Materials 2019, 12, x FOR PEER REVIEW 2 of 16

𝐹 cutting force acting on the upper die ` 𝐹 cutting force acting on the lower die

𝐹 lateral force acting on the upper die ` 𝐹 lateral force acting on the lower die coefficient of friction

𝐹 horizontal force acting on the upper die ` 𝐹 horizontal force acting on the lower die 𝐹 vertical force acting on the upper die ` 𝐹 vertical force acting on the lower die 𝑀 moment l lever arm C clearance

FigureFigure 1.1. The system of forces andand pressurespressures duringduring punching.punching.

TheThe blankingblanking processprocess isis widelywidely used to manufacturemanufacture sheet metal parts from different different materials. TheThe cutcut elementselements areare usuallyusually subjectedsubjected toto furtherfurther plasticplastic working,working, butbut therethere areare alsoalso thosethose thatthat areare directlydirectly usedused inin thethe assemblyassembly processprocess ofof complexcomplex products.products. TheThe blankingblanking ofof thethe elementselements withwith aa punch ofof flatflat surfacesurface hashas beenbeen veryvery wellwell presentedpresented inin aa numbernumber ofof worksworks [[3–17].3–17]. WhenWhen punchingpunching withwith aa flatflat face,face, thethe edgeedge pressurepressure onon thethe sheetsheet isis uniform.uniform. For punch with a slopingsloping face,face, therethere isis aa gradualgradual blankingblanking ofof thethe sheetsheet materialmaterial [[18]18].. The location of the punching forceforce on the sheet material changes. AsAs thethe punchpunch penetratespenetrates intointo thethe sheet,sheet, thethe materialmaterial separationseparation moves along the blanking edges. BlankingBlanking withwith thethe inclinedinclined blanking edge allows the lowering of the maximum pressure force [[18].18]. TheThe angleangle ofof inclinationinclination ofof thethe faceface ofof thethe punchpunch alsoalso influencesinfluences thethe amountamount ofof deformationdeformation ofof thethe stampedstamped elementelement [[19].19]. Gürün Gürün and and others others [19], [19], showed showed that that blanking blanking axisymmetric axisymmetric elements elements with with a apunch punch with with an an inclined inclined surface surface causes causes deformation deformation of the of shape the shape in two in perpendicular two perpendicular surfaces. surfaces. They Theypresented presented the influence the influence of the of angle the angleof inclination of inclination of the of punch the punch face on face the on maximum the maximum punching punching force. force.They Theyshowed, showed, in the in case the caseof punching of punching DC01 DC01 sheet sheet material material with with a athickness thickness of of 1 1 mm, mm, that that for thethe greatergreater angleangle ofof punchpunch faceface inclinationinclination thethe maximummaximum forceforce andand totaltotal workwork is lower. However, aa larger strokestroke ofof thethe punchpunch isis necessarynecessary forfor blanking.blanking. In addition,addition, it has beenbeen shownshown thatthat aa largerlarger angleangle ofof inclinationinclination ofof thethe stamp’sstamp's faceface causescauses greatergreater deformationdeformation ofof thethe shapeshape ofof thethe finalfinal product.product. TheThe useuse ofof punchespunches withwith aa slopingsloping faceface resultsresults inin aa reductionreduction ofof thethe punchingpunching force.force. However, tootoo largelarge an an angle angle can can cause cause large large burr burr [3]. [3]. Lin Lin [20 [2],0], presents presents the the results results of blanking of blanking with with a punch a punch with twowith faces two inclinedfaces inclined at the at angle the ofangle 12◦ .of The 12°. tests The were tests carriedwere carried out using out ausing 0.77 mma 0.77 thick mm sheet thick metal. sheet Theymetal. analyzed They analyzed the influence the influence of clearance of clearance on the punchon the wear.punch wear. TheThe intensityintensity of of the the burr burr height height increase increase depends depends on theon the speed speed of wear of wear of blanking of blanking tool edges tool edges [4–6]. In[4–6]. the In case the of case blanking of blanking thin sheets thin sheets with specialwith sp magneticecial magnetic properties, properties, it is importantit is important to choose to choose the appropriatethe appropriate clearance, clearance, so theso the area area of theof the hardened hardened material material will will be be the the smallest smallest [ 4[4].]. ToolTool wearwear isis particularlyparticularly important important when when blanking blanking thin thin sheets. sheets. The qualityThe quality of the intersectionof the intersection surface insurface the blanking in the processblanking is influencedprocess is influenced by the type by of punchthe type material of punc [5h, 6material]. Works [5,6]. [4–6] Works present [4 the–6] influence present the of the influence punch materialof the punch on its material wear (tungsten on its wear carbide, (tungsten M2, carbide, M3: 2). M2, Tool M3: wear 2). depends Tool wear on depends the size on of thethe clearance,size of the punchclearance, material, punch and material, sheet material and sheet [7– 10material,21,22]. [7–10,21,22]. When blanking When hard blanking materials, hard the materials, problem is the to obtainproblem the is right to obtain quality the of right cut surface.quality of Fazily cut surface. and other Fazily authors and [23other], presented authors [23], an analysis presented of thean blankinganalysis of anthe element blanking from of AZ31Ban element magnesium from AZ31B alloy. As magnesium a result of heating alloy. As the a punched result of material, heating they the increasedpunched material, the smooth they blanking increased zone the and smooth significantly blanking reduced zone and the significantly punching force. reduced In the the case punching of steel sheetsforce. withIn the high case hardness of steel obtained sheets with after heathigh treatment,hardness theobtained punching after clearance heat treatment, should be the considerably punching higherclearance than should for soft be materials considerably [24]. Whenhigher blanking than fo veryr soft hard materials materials, [24]. very When high blanking pressures very occur hard on thematerials, face (at very the blankinghigh pressures edge). occur Such on a cyclically the face (at loaded the blanking punch is edge). more Such worn a out cyclically on the loaded face side punch [24]. Increaseis more worn of the out tool on wear the face resistance side [24]. can Increase be achieved of the tool by applying wear resistance hard coatings. can be achieved In [21], by the applying authors havehard outlinedcoatings. theIn [21], effect the of authors the anti-wear have outlined coating onthe the effect friction of the and anti-wear wear conditions coating on of the tool friction materials. and Inwear [7], conditions the authors of presented tool materials. the issues In of[7], the the influence authors ofpresented heat treatment the issues and theof the hardness influence of punches of heat treatment and the hardness of punches on the course of change in the quality of the cut surface. The higher the hardness of the America Iron and Steel Institute (AISI) D2 steel punches was, the lower

Materials 2019, 12, 1261 3 of 16 onMaterials the course2019, 12, ofx FOR change PEER REVIEW in the quality of the cut surface. The higher the hardness of the America3 of 16 Iron and Steel Institute (AISI) D2 steel punches was, the lower the punch wear and the change in the punch wear and the change in the quality of the cut surface were. Blanking asymmetrical outline the quality of the cut surface were. Blanking asymmetrical outline elements is more complex than elements is more complex than blanking circular elements. Subramonian [8] presented the influence blanking circular elements. Subramonian [8] presented the influence of the size of the radius of the of the size of the radius of the cut profile on the wear of the blanking edge. Blanking tests were carried cut profile on the wear of the blanking edge. Blanking tests were carried out for 0.25 mm thin sheet out for 0.25 mm thin sheet metal and several radius values: R = 0.15, 0.25, 0.5, and 1 mm. He metal and several radius values: R = 0.15, 0.25, 0.5, and 1 mm. He characterized the influence of the characterized the influence of the radius/sheet thickness ratio on the normalized stress value. The radius/sheet thickness ratio on the normalized stress value. The work is important from the point work is important from the point of view of thin-element blanking. Guo [9] conducted a circular of view of thin-element blanking. Guo [9] conducted a circular punch blanking test with a 0.15 mm punch blanking test with a 0.15 mm diameter and 0.2 mm thick sheet metal. In the case of blanking diameter and 0.2 mm thick sheet metal. In the case of blanking round elements, their size, thus the round elements, their size, thus the punch diameter, influenced the punch wear. The influence of the punch diameter, influenced the punch wear. The influence of the SKD-11 steel punch diameter on the SKD-11 steel punch diameter on the quality of die-cut sheet metal parts was described by quality of die-cut sheet metal parts was described by Lawanwong [10]. He also showed how using Lawanwong [10]. He also showed how using regenerated punches affects the quality of elements regenerated punches affects the quality of elements during blanking. during blanking. Often, in the production processes of sheet metal parts, flange trimming is applied after the Often, in the production processes of sheet metal parts, flange trimming is applied after the pressing process. Appropriate direction of the sheet surface inclination to the punch results in an pressing process. Appropriate direction of the sheet surface inclination to the punch results in an improvement of the quality of the intersection surface [25]. Golovashchenko [26], presented the improvement of the quality of the intersection surface [25]. Golovashchenko [26], presented the influence of the method of supporting the cut sheet from the aluminum alloy AA6111-T4 and the influence of the method of supporting the cut sheet from the aluminum alloy AA6111-T4 and the clearance on the geometric quality of the intersection surface and on the formation of burr. The shearing clearance on the geometric quality of the intersection surface and on the formation of burr. The was made for an aluminum alloy sheet and a flat-faced cutter. In the next paper [27] he presented shearing was made for an aluminum alloy sheet and a flat-faced cutter. In the next paper [27] he a modified shearing process in the case of using an inclined sheet surface to the face of the cutter. presented a modified shearing process in the case of using an inclined sheet surface to the face of the He showed that the inclination angle of the sheet can be chosen with a relatively large clearance so cutter. He showed that the inclination angle of the sheet can be chosen with a relatively large that a flat surface of the intersection without burrs will be obtained. The mechanism of separation clearance so that a flat surface of the intersection without burrs will be obtained. The mechanism of of materials during shearing can be changed by modifying tool arrangement [28]. In paper [29], separation of materials during shearing can be changed by modifying tool arrangement [28]. In paper the authors presented the results for the DP500 high-strength sheet shearing tests using an additional [29], the authors presented the results for the DP500 high-strength sheet shearing tests using an element supporting the sheet material. Separation of material with an open cut line is related with an additional element supporting the sheet material. Separation of material with an open cut line is intense influence of lateral forces on the tool flank surface [30]. As in the case of blanking, similarly, related with an intense influence of lateral forces on the tool flank surface [30]. As in the case of during the shearing process: Clearance affects the wear of tools [31]. Cho in his paper [31] described blanking, similarly, during the shearing process: Clearance affects the wear of tools [31]. Cho in his the influence of the direction of inclination of the DP980 sheet in relation to the punch, on the quality paper [31] described the influence of the direction of inclination of the DP980 sheet in relation to the of the intersection surface. He characterized the failure mechanism of blanking punches in the case punch, on the quality of the intersection surface. He characterized the failure mechanism of blanking of using different directions of the sheet surface in relation to the punch surface. In order to reduce punches in the case of using different directions of the sheet surface in relation to the punch surface. the defects of the intersection surface some modifications of the shearing process can be done [32,33]. In order to reduce the defects of the intersection surface some modifications of the shearing process Supporting the material in shearing process, with 20% of tool clearance, reduces the burr on the sheet can be done [32,33]. Supporting the material in shearing process, with 20% of tool clearance, reduces separation edge [32]. Mackensen and others [33], showed that the proper arrangement of the trimmed the burr on the sheet separation edge [32]. Mackensen and others [33], showed that the proper sheet in relation to the surface of the punch causes a reduction in the blanking total work. Tests for arrangement of the trimmed sheet in relation to the surface of the punch causes a reduction in the two different sheet-punch arrangements were made for materials used in the automotive industry blanking total work. Tests for two different sheet-punch arrangements were made for materials used (including high-strength steels). in the automotive industry (including high-strength steels). In order to obtain cut out and bent fragments of sheets, punches with a sloping face are used [18]. In order to obtain cut out and bent fragments of sheets, punches with a sloping face are used In the case of blanking a triangular hook (Figure2), there is a gradual deviation of the blanked material, [18]. In the case of blanking a triangular hook (Figure 2), there is a gradual deviation of the blanked in the direction of the punch movement. The burr on the edge of the sheet and the size of the deviation material, in the direction of the punch movement. The burr on the edge of the sheet and the size of of the hook from the sheet surface, make it difficult to insert the hook into the other element during the the deviation of the hook from the sheet surface, make it difficult to insert the hook into the other assembly process. element during the assembly process.

Figure 2. View of triangular hooks obtained by partial blanking.

In the abovementioned manuscripts, the changes of the cut-part shape were not studied. The material area at the intersection surface, the quality of the intersection surface, and the burr were

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In the abovementioned manuscripts, the changes of the cut-part shape were not studied. MaterialsThe material 2019, 12 area, x FOR at PEER the intersection REVIEW surface, the quality of the intersection surface, and the burr4 were of 16 described as important issues. The partial blanking of parts of thin and very hard sheet material is not describedwell researched as important and described. issues. The partial blanking of parts of thin and very hard sheet material is not wellThe researched paper presents and described. the results of research on the influence of different clearance, during partial blankingThe paper with apresents sloping the face, results on the of geometrical research on qualitythe influence of the of hook. different The influenceclearance, of during clearance partial on blankingthe change with of burra sloping height face, and on the the partially geometrical bended quality material, of the made hook. of The 55 HRC influence hardness of clearance steel sheet, on thewas change discussed. of burr height and the partially bended material, made of 55 HRC hardness steel sheet, was discussed. 2. Materials and Methods 2. Materials and Methods 2.1. The Blanking Experiment 2.1. TheThe Blanking hooks were Experiment cut in a sheet with a thickness of t = 0.5 mm. The sheet was made of C45 (1.0503) non-alloyThe hooks carbon were steel cut for in thermala sheet with improvement a thickness [34 of]. t = Table 0.5 mm.1 shows The thesheet chemical was made composition of C45 (1.0503) and non-alloyTable2 shows carbon the mechanicalsteel for thermal properties improvement after heat [3 treatment.4]. Table 1 shows the chemical composition and Table 2 shows the mechanical properties after heat treatment. Table 1. Chemical composition (average), [%].

C MnTable P 1. Chemical Cr composition Si S(average), Ni [%]. Mo Fe C0.48 Mn 0.73 0.011P Cr0.09 0.35Si 0.01 S 0.02 Ni 0.002Mo otherFe 0.48 0.73 0.011 0.09 0.35 0.01 0.02 0.002 other Table 2. Mechanical properties of hardened steel C45. Table 2. Mechanical properties of hardened steel C45. Yield Strength Tensile Strength Elongation Hardness YieldRe Strength [MPa] TensileRm [MPa]Strength ElongationA5 [%] HardnessHRC Re [MPa]335 Rm 2285[MPa] A5 30 [%] 55HRC 335 2285 30 55

The blanking process was carried out with a punch with a 12◦ sloped face. The most important dimensionsThe blanking of the blankingprocess was punch carried are presentedout with a inpunc Figureh with3. During a 12° sloped the tests, face. the The punch most movement important dimensionswas 50 strokes of the/min. blanking The tests punch were are carried presented out in for Figure three 3. values During of the punching tests, the clearance punch movement (clearance was(C)/sheet 50 strokes/min. thickness ( t)):The 5%, tests 10%, were 15% carried (Figure out4a). for The three stroke values of the of punch punching was clearance set so as to (clearance obtain a (penetrationC)/sheet thickness depth H (t=)):1.2 5%, mm 10%, (Figure 15% 4(Figureb). 4a). The stroke of the punch was set so as to obtain a penetration depth H = 1.2 mm (Figure 4b).

Figure 3. TheThe basic basic geometry geometry of the punch blanking tool. Sheet metal blanking with high strength properties requires, for punches, the selection of a material with good strength and relatively high resistance to abrasive and adhesive wear. In the case of short series of blanked products, the economics of the material used for the tools is important. The punch was made of K340 Isodur tool steel from Böhler Edelstahl Company (Düsseldorf, Germany). Steel with a content of ~8% chromium is produced in the technology of electroslag remelting process (ESR) [35]. This steel is characterized by, among other things, high adhesive resistance to wear and compressive strength. The basic alloying additions are presented in Table3. Thanks to the micro-addition of aluminum, the oxide passivation system is improved, where passivation of the surface takes place. . (a) (b)

Figure 4. Characteristic elements of the tool and material arrangement: (a) Outline of the die hole and punch; (b) clearance (C) and blanking depth (H) of the punch in the sheet.

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After passivation, this layer reduces the tendency of particles of punched material adhesion to the cutting surface of a punch. The hardness of theArticle punch after heat treatment was 62 HRC. Materials 2019, 12, x FOR PEER REVIEW 5 of 16

Sheet metal blanking with high strength properties requires, for punches, the selection of a material with good strength and relatively high resistance to abrasive and adhesive wear. In the case of short series of blanked products, the economics of the material used for the tools is important. The punch was made of K340 Isodur tool steel from Böhler Edelstahl Company (Düsseldorf, Germany). Steel with a content of ~8% chromium is produced in the technology of electroslag remelting process (ESR) [35]. This steel is characterized by, among other things, high adhesive resistance to wear and compressive strength. The basic alloying additions are presented in Table 3. Thanks to the micro- addition of aluminum, the oxide passivation system is improved, where passivation of the surface (a) (b) takes place. After passivation, this layer reduces the tendency of particles of punched material adhesionFigureFigure to 4. 4. theCharacteristicCharacteristic cutting surface elements elements of ofa of thepunch. the tool tool andThe and material materialhardness arrangement arrangement: of the punch: (a (a) )Outline Outlineafter heat of of the the treatment die die hole hole and andwas 62 HRC.punch;punch; (b (b) )clearance clearance (C (C) )and and blanking blanking depth depth ( (HH) )of of the the punch punch in in the the sheet. sheet. Table 3. Chemical composition of the cold-work tool steel K340 (in weight percent). Table 3. Chemical composition of the cold-work tool steel K340 (in weight percent). C Si Mn Cr Mo V Other Micro-Alloying C Si Mn Cr Mo V Other micro-alloying 1.10 0.90 0.40 8.30 2.10 0.50 Al, Nb 1.10 0.90 0.40 8.30 2.10 0.50 Al, Nb

2.2.2.2. Measurement Measurement of of the the Hook Hook Profile Profile and and the the Size Size of of the the Burr Burr TheThe measurement measurement of of the the hook hook profile profile radius radius wa wass determined determined using using the the non-contact non-contact method method by by usingusing the the GOM GOM Atos optical scanning andand measuringmeasuring system. system. The The Atos Atos Core Core 3D 3D scanner scanner is is a systema system of ofstereoscopic stereoscopic cameras, cameras, which which work work by by triangulation triangulation (i.e.,(i.e., calculationcalculation of the intersection of of a a specific specific planeplane with with a a ray ray in in three-dimensional three-dimensional space). space). The The scanner scanner projects projects a a system system of of bands bands on on the the inspected inspected surfacesurface of of the the piece. piece. The The projected projected bands bands are are reco recordedrded by by the the two two stereoscopic stereoscopic cameras cameras (Figure (Figure 5),5), providingproviding a a phase-shift phase-shift image image from from the the sine sine distribution distribution of of intensity intensity on on the camera detectors.

Measuring area 135 × 100 mm

Working distance 170 mm

Point spacing 0.05 mm

Sensor dimensions 206 × 205 × 64 mm

FigureFigure 5. 5. IdeaIdea of of the the AtosCore AtosCore 135 135 measurement measurement system system and and main main camera camera parameters. parameters.

InIn each each case case of of the the measurement, measurement, first first a scan a scan of the of thesheet sheet with with the punch the punch was performed. was performed. The samplesThe samples were were placed placed in the in thesame same position position relative relativelyly to tothe the scanner. scanner. The The obtained obtained data data allowed allowed the the remodelingremodeling of of the the sheet sheet surface surface with with a a punched punched hook hook.. After After establishing establishing the the appropriate appropriate longitudinal longitudinal andand transverse transverse surfaces surfaces (Figure (Figure 6a),6a), the the measuring measuring line line with with points points was was determined determined (Figure (Figure 7a). 7Thea). characteristicThe characteristic points points were weredefined defined based based on the on length the length of the of linear the linear hole holesection section in the in hook the hook axis. axis.On theirOntheir basis, basis, the radius the radius of the ofhook the ( hookRb) was (R determined.b) was determined. An example An exampleof defining of points defining in the points program in the andprogram the circle and thedescribed circle described on them is on shown them isin shown Figure in 6b Figure (the 6remainingb (the remaining points are points hidden are hidden for better for readabilitybetter readability of the measurement of the measurement principle). principle). The size Theof the size actual of the deviation actual deviation (Hb) of the (H hookb) of from the hook the main sheet was also measured (Figure 7a). The measurements of Hb and Rb were made for five samples. Materials 2019, 12, x; doi: FOR PEER REVIEW www.mdpi.com/journal/materials

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from the main sheet was also measured (Figure7a). The measurements of Hb and Rb were made for Materialsﬁve samples. 2019,, 12,, xx FORFOR PEERPEER REVIEWREVIEW 6 of 16

((a)) ((b))

FigureFigure 6. 6. ((a())a )ExampleExample Example ofof of positioningpositioning positioning basebase base surfacessurfaces surfaces inin in thethe the measuringmeasuring measuring systemsystem system andand and ((b (b)) )thethe the principleprinciple principle ofof of bendingbending radius radius measurement. measurement.

((a)) ((b))

FigureFigure 7. 7. BurrBurrBurr characteristiccharacteristic characteristic measurementmeasurement measurement pointspoints points ((b (bhh)h) )andand and measurementsmeasurements measurements ofof of RRbb b andandand HHbb::b (:((a))a Location)Location Location ofof of basebase points points to to determine determine the the be bendingnding radius radius of of the the hook hook and and (b (b)) )readread read pointspoints points ofof of thethe the burrburr burr values.values. values.

WithWith the the increaseincrease ofof the the punch punch wear wear surfaces, surfaces, the burrthe burr height height changes changes [13–17 [13]. By−17]. indirect By indirect method, method,through through measuring measuring the height the of height burrs onof theburrs elements, on thethe elements, itelements, is possible itit isis to possiblepossible determine toto whichdeterminedetermine exploitation whichwhich exploitationphase the punches phase the are punches in [12]. are The in measurement [12]. The meas ofurement the burr of on the the burr edge on the of the edge hook of the was hook made was on madea stand, on witha stand, a table with adjusted a table byadjusted a micrometer by a micrometer screw, and screw, using aand TAYLOR using a HOBSON TAYLOR Surtronic HOBSON 3+ Surtronicprofilographometer. 3+ profilographometer The measurement.. TheThe measurementmeasurement accuracy was accuracyaccuracy 0.5 m. Measurements waswas 0.5  m. on Measurements the selected section on the at selectedthe measuring section pointsat the (Figuremeasuring7b)were points made (Figure for the 7b) same were places made onfor five the samples.same places The on graphs five samples. show the Theaverage graphs value show from the the average measurements value from on five the samples. measurements The principle on five of samples. determining The theprinciple surfaces of is determiningshown in Figure the surfaces8a, and theis shown scheme in ofFigure burr measurement8a, and the scheme in Figure of burr8b. measurement in Figure 8b. After performing a hook scan with the ATOS system, the samples were cut using wire electrical discharge machining. The cut hooks were placed on the stand with a digital camera, where macro photographs were taken. To measure the burr, the samples were prepared in such a way that the bent hook was cut off. In this way, an easy access to the intersection edge on which the burr height was measured was obtained.

((a)) ((b))

Figure 8. TheThe ideaidea ofof burrburr measurement:measurement: ((a)) SystemSystem ofof basebase surfacessurfaces andand ((b)) measurementmeasurement scheme.scheme.

After performing a hook scan with the ATOS system, the samples were cut using wire electrical discharge machining. The cut hooks were placed on the stand with a digital camera, where macro

Materials 2019, 12, x FOR PEER REVIEW 6 of 16

(a) (b)

Figure 6. (a) Example of positioning base surfaces in the measuring system and (b) the principle of bending radius measurement.

(a) (b)

Figure 7. Burr characteristic measurement points (bh) and measurements of Rb and Hb: (a) Location of base points to determine the bending radius of the hook and (b) read points of the burr values.

With the increase of the punch wear surfaces, the burr height changes [13−17]. By indirect method, through measuring the height of burrs on the elements, it is possible to determine which exploitation phase the punches are in [12]. The measurement of the burr on the edge of the hook was made on a stand, with a table adjusted by a micrometer screw, and using a TAYLOR HOBSON Surtronic 3+ profilographometer. The measurement accuracy was 0.5  m. Measurements on the selected section at the measuring points (Figure 7b) were made for the same places on five samples. Materials 2019 12 The graphs, , 1261 show the average value from the measurements on five samples. The principle7 of 16 of determining the surfaces is shown in Figure 8a, and the scheme of burr measurement in Figure 8b.

Materials 2019, 12, x FOR PEER REVIEW 7 of 16 photographs were taken. To measure(a) the burr, the samples were prepared in such( ba )way that the bent hook was cut off. In this way, an easy access to the intersection edge on which the burr height was measuredFigureFigure was 8. The obtained.8. The idea idea of burr of burr measurement: measurement: (a) System(a) System of base of base surfaces surfaces and and (b) ( measurementb) measurement scheme. scheme. 3. Results and Discussion 3. ResultsAfter and performing Discussion a hook scan with the ATOS system, the samples were cut using wire electrical discharge machining. The cut hooks were placed on the stand with a digital camera, where macro 3.1. Bend of the Hook 3.1. Bend of the Hook In the initial phase of blanking, the punch deforms the material at the surface contact. When the punchIn pierces the initial the phase sheet, theof blanking, pressure phasethe punch ends deform for a parts the of material the punch at surface.the surface Further contact. movement When the of thepunch punch pierces results the insheet, a material the pressure cut and phase bending ends of fo ther a hook.part of For the a punch 5% blanking surface. clearance, Further movement the initial valueof the punch of the results deflection in a material radius is cut smaller and bending than when of the blanking hook. For with a 5% ablanking clearance clearance, of 10% the and initial 15% value of the deflection radius is smaller than when blanking with a clearance of 10% and 15% (Figure (Figure9). Increasing the clearance from 5% to 10% caused the decrease of the deflection ( Hb) to approximately9). Increasing 21.4%.the clearance A further from increase 5% into clearance 10% caused by 5%–15% the decrease resulted inof athe reduction deflection in deflection (Hb) to value.approximately The deflection 21.4%. (atA furtherC = 15%) increase decreased in clearance compared by 5%–15% to that obtainedresulted in at a 5% reduction clearance in deflection by 29.4%. value. The deflection (at C = 15%) decreased compared to that obtained at 5% clearance by 29.4%. The The described dependences and changes in Hb and Rb values were observed for blanking process with adescribed sharp punch. dependences and changes in Hb and Rb values were observed for blanking process with a sharp punch.

Figure 9. C R H Figure 9. The inﬂuenceinfluence ofof thethe blankingblanking clearanceclearance ((C)) onon thethe radiusradius Rbb andand thethe hookhook Hbb deviation after thethe ﬁrstfirst punchingpunching strokes.strokes.

During the tests, the cutting edge and the surfaces of the punch were worn out. During the During the tests, the cutting edge and the surfaces of the punch were worn out. During the blanking and measurement tests, the biggest differences in changes in Hb and Rb values were observed blanking and measurement tests, the biggest differences in changes in Hb and Rb values were observed for the blanking clearance of 5% and 15%. Therefore, the further analysis presents the results of for the blanking clearance of 5% and 15%. Therefore, the further analysis presents the results of changing the geometry of the hook in the range of up to 60,000 cuts for 5% and 15% clearance. changing the geometry of the hook in the range of up to 60,000 cuts for 5% and 15% clearance. During punching with a clearance of 5%, the bend radius (Rb) increased as the number of cuts During punching with a clearance of 5%, the bend radius (Rb) increased as the number of cuts made by the punch increased (Figure 10a). A significant increase in the radius value was observed made by the punch increased (Figure 10a). A significant increase in the radius value was observed since 30,000 cuts had been achieved. A further increase in the number of hook cuts caused a more since 30,000 cuts had been achieved. A further increase in the number of hook cuts caused a more intensive increase in the radius of the hook’s deflection. After about 30,000 cuts, the Hb value began intensive increase in the radius of the hook's deflection. After about 30,000 cuts, the Hb value began to grow to a lesser extent, up to around 40,000 cuts. Further, as the number of punch cuts increased, to grow to a lesser extent, up to around 40,000 cuts. Further, as the number of punch cuts increased, the radius of the hook deflection did not increase. On the other hand, the Rb has increased quite the radius of the hook deflection did not increase. On the other hand, the Rb has increased quite significantly from the first cuts to 60,000 cuts. On the curve of the change in the size of Rb, there can be significantly from the first cuts to 60,000 cuts. On the curve of the change in the size of Rb, there can be observed an increase after reaching about 30,000 cuts. Blanking with a clearance of 5% did not cause a significant change in Hb for the examined range of cuts.

Materials 2019, 12, 1261 8 of 16 observed an increase after reaching about 30,000 cuts. Blanking with a clearance of 5% did not cause a signiﬁcant change in H for the examined range of cuts. Materials 2019, 12, x FOR PEERb REVIEW 8 of 16

(a) (b)

FigureFigure 10. 10. TheThe influence inﬂuence of of changes changes in in HHbb andand RRb bvaluesvalues depending depending on on the the number number of of cuts; cuts; for for blanking blanking clearance:clearance: (a (a) )5% 5% and and ( (bb)) 15%. 15%.

InIn the the case case of of blanking blanking hooks hooks with with 15% 15% clearance clearance (Figure (Figure 10b),10b), both the Rb radiusradius and and the the Hbb bendbend of of the the hook hook increased increased as as the the number number of cuts of cuts increased increased to a to similar a similar one oneas at as clearance at clearance of 5%. of The 5%. sizeThe of size the of deflection the deflection (Hb) of (H theb) ofhook the until hook the until excision the excision of abou oft 30,000 about 30,000cuts increased cuts increased and then and began then tobegan decrease. to decrease. The change The change of punching of punching clearance clearance from from5% to 5% 15% to 15%indicated indicated that thatafter after the first the first cuts cuts Rb valuesRb values were were higher higher by about by about 15.4% 15.4% and andHb deflectionsHb deflections were were higher higher by 16.6%. by 16.6%. With With 60,000 60,000 cuts, cuts,the differencesthe differences in the in change the change in value in value were weredifferent: different: Rb wasR blesswas by less about by about1% and 1% H andb wasH largerb was by larger about by 50%.about The 50%. 1% Thevalue 1% change value changecan be considered can be considered that the thatvalue the was value in the was margin in the of margin error. ofTo error. a greater To a extent,greater the extent, clearance the clearance had an hadinfluence an influence on the onchange the change of the of R theb bendRb bendhook. hook. In spite In spite of this, ofthis, it can it canbe concludedbe concluded that that the thechange change in clearance, in clearance, and and therefore therefore wear, wear, had had significantly significantly changed changed the the geometry geometry ofof the the hook hook during during the the blanking blanking process process with with a a use use of of a a sloping-face sloping-face punch. punch. FigureFigure 1111 showsshows macro photographs ofof exemplaryexemplary sections sections of of a a cut-out cut-out hook hook made made with with the the use use of ofa sloping-facea sloping-face punch. punch. In In the the case case of of a 5%a 5% clearance, clearance, as as the the number number of of cuts cuts increased, increased, the the deviation deviation of ofthe the hook hook increased increased and and the the hook hook radius radius decreased decrease (ford (for 60,000 60,000 cuts) cuts) (Figure (Figure 11 11a).a). The The intensive intensive wear wear of ofthe the punch punch caused caused an increase an increase in the in clearance the clearance between between the blanking the blanking edges of theedges tools. of Thethe progressivetools. The progressivewear of the wear punch of influencedthe punch influenced the change the in thechange geometry in the ofgeometry the hook of and the thehook size and of the the size clearance. of the clearance.The size of The the size burr of onthe the burr edge on the of the edge sheet of the also sh waseet also changing was changing during during the blanking the blanking process. process. As the Asnumber the number of cuts of made cuts by made the punchby the increasedpunch increased (C = 5%), (C the = 5%), burr the on theburr edge on the of the edge sheet of the also sheet increased. also increased.Increasing Increasing the clearance the by clearance 10%, i.e., by to 10%, the value i.e., to of the 15%, value resulted of 15%, in the resulted hook springing. in the hook After springing. the first Aftercut with the afirst 15% cut clearance, with a 15% the bendclearance, radius the was bend greater radius than was for greater the hook than cut for with the ahook clearance cut with of 5%. a clearanceThe increase of 5%. in clearance The increase also causedin clearance a significant also caus increaseed a significant in the size ofincrease the burr. in Thethe valuesize of of the the burr. hook Thedeflection value of was the the hook highest deflection after the was first the cut highest for 10% after blanking the first clearance cut for 10% (Figure blanking 11b). clearance As the number (Figure of 11b).cuts madeAs the by number the punch of cuts increased, made by its the wear punch increased, increased, which its wear had anincreased, influence which on the had decrease an influence of the onhook the deflection decrease value.of the hook The largest deflection deflection value. of The the largest hook ( Cdeflection= 10%) could of the be hook due to(C the = 10%) increase could in thebe dueinfluence to the ofincrease the moment in the of influence reaction forcesof the ofmoment the cut of material. reaction However, forces of insuthe fficutcient material. clearance However, caused insufficientincomplete clearance springing caused of the material,incomplete as inspringing the case of of the punching material, with as ain clearance the case of of punching 15% (Figure with 11 c).a clearanceThe corresponding of 15% (Figure values 11c). of The parameters correspondingHb and valuesRb for of the parameters element, H firstb and blanking Rb for the 30,000 element, cuts first and blanking60,000 cuts, 30,000 are presentedcuts and 60,000 in Table cuts,4. The are hardnesspresented of in tools Table and 4. clearanceThe hardness between of tools blanking and clearance edges [ 9] betweenhas an influence blanking on edges the quality[9] has ofan the influence punched on elements. the quality Sheet of the metal punched blanking elements. with low Sheet clearance metal blankingcauses more with intensive low clearance wear ofcauses the blanking more intens edgeive of wear punches of the [6 –blanking8]. edge of punches [6−8].

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(a)

(b)

(c)

FigureFigure 11. 11. Cross-sectionCross-section of of the the hook hook for for a a pa particularrticular number number of of punch punch cuts: cuts: (a (a) )CC == 5%;5%; (b (b) )CC == 10%;10%; andand (c (c) )CC == 15%.15%.

Table 4. Size Rb and Hb depending on the clearance and the number of cuts made by the punch. Table 4. Size Rb and Hb depending on the clearance and the number of cuts made by the punch. Clearance, C Clearance, C Clearance, C Clearance, C Number of Cuts 5% 10% 15% 5% 10% 15% Number of cuts 5% 10% 15% 5% 10% 15% Rb [mm] Hb [mm] Rb [mm] Hb [mm] First cuts 13.15 13.52 14.05 1.96 1.42 1.21 30,000First cuts 13.92 13.15 14.0113.52 14.05 14.61 1.96 2.11 1.42 1.52 1.21 1.36 60,00030,000 15.75 13.92 15.7714.01 14.61 15.71 2.11 2.12 1.52 1.67 1.36 1.28 60,000 15.75 15.77 15.71 2.12 1.67 1.28 3.2. Burr Height 3.2. Burr Height When blanking carbon sheets with high hardness, as the number of cuts increased, a significant increaseWhen in blanking burr height carbon was observedsheets with [11 high]. Blanking hardness, with as too the low number a clearance of cuts caused increased, faster a significant wear of the increaseblanking in edge. burr Theheight roundness was observed of the blanking[11]. Blanking edge with changed too thelowstress a clearance distribution caused in faster the tool wear edges of thecontact blanking area. edge. Increasing The roundness the clearance of the from blanking 5% to 10% edge resulted changed in a the burr stress increase distribution of the average in the valuetool edgesof 63% contact (Figure area. 12—solid Increasing line). the Increasing clearance the from clearance 5% to 10% byanother resulted 5% in resulteda burr increase in another of the increase average of valuethe burr of 63% by 12% (Figure of the 12 value—solid obtained line). asIncreasing for a 10% the clearance. clearance The by presented another values5% resulted and their in another changes increaserefer to theof the average burr by value 12% of of the the burr value from obtained five points as for (as a in 10% Figure clearance.7b). The presented values and their changesDuring therefer tests, to the it wasaverage observed value of that the on burr the from rounded five points edges (as (point in Figure 6 in Figure 7b). 7b) the burr is slightly larger. The largest difference between the average value of the burr on the edge (for points 1 to 5) and the one measured at the rounding of the edge (point 6) was obtained for the 15% clearance (Figure 12—dashed line). The smallest difference between the values of the burr was observed in the case of punching with a clearance of 10%. The higher burr values at the edge of the semicircular metal sheet were caused by the non-linear movement of the blanking edge at this location. In point 6, the rounding of the edges was affected by the deformations caused by blanking with linear edges. It was a relatively small area, located near the top of the punch with a radius of 0.4 mm (Figures3 and4). The hooks were characterized by high elasticity, so that they did not plastically deform in the subsequent assembly process. The hook blanking tests were carried out for carbon steel sheet hardened to 55 HRC. In the case of a 5% blanking clearance the largest increase in burr, at the tested point on the edge of the hole, occurred up to 20,000 cuts (Figure 13). Up to 30,000 40,000 cuts, the burr − Figure 12. Effect of clearance on the size of the burr (after the first cuts with a punch).

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(a)

(b)

(c)

Figure 11. Cross-section of the hook for a particular number of punch cuts: (a) C = 5%; (b) C = 10%; and (c) C = 15%.

Table 4. Size Rb and Hb depending on the clearance and the number of cuts made by the punch.

Clearance, C Clearance, C Number of cuts 5% 10% 15% 5% 10% 15% Materials 2019, 12, 1261 10 of 16 Rb [mm] Hb [mm] Materials 2019, 12, x FOR FirstPEER cutsREVIEW 13.15 13.52 14.05 1.96 1.42 1.21 10 of 16 increase stabilized. Blanking30,000 with a 13.92punch in14.01 the range14.61 from 40- 2.11 to 60 1.52 thousand 1.36 strokes had already causedDuring a significant the tests, increase60,000 it was inobserved the burr. 15.75 that In on the15.77 the last rounded cut15.71 range, edges 2.12 the process(point 1.67 6 of in punch 1.28Figure wear 7b) the was burr more is intense,slightly hence,larger. theThe burr largest on thedifference edges of between the sheet the increased. average value Blanking of the the burr hook on with the aedge clearance (for points of 10% 1 3.2.resultedto 5) Burr and inHeight the aless one intense measured burr at increase, the rounding but with of the a higher edge value(point compared6) was obtained to blanking for the with 15% a clearance of(Figure 5%. The12— curvedashed in line). the range The smallest of up to difference 30,000 cuts between was gentler the values rising, of the and burr the was burr observed had increased in the When blanking carbon sheets with high hardness, as the number of cuts increased, a significant morecase of intensively. punching with In the a clearance case of blanking of 10%. withThe higher a clearance burr values of 15%, at a the more edge intense of the increase semicircular of the metal burr increase in burr height was observed [11]. Blanking with too low a clearance caused faster wear of wassheet observed were caused up to by a the value non-linear of about movement 30,000 cuts. of Forthe blanking all clearance edge values, at this thelocation. cut range In point from 6, 0 the to the blanking edge. The roundness of the blanking edge changed the stress distribution in the tool 30,000rounding40,000 of the is edges the range was ofaffected intense by burr the increasedeformations at the caused edges of by the blanking sheet. Thewith punch linear had edges. a sloping It was edges contact− area. Increasing the clearance from 5% to 10% resulted in a burr increase of the average face,a relatively the contact small (path area, of located friction) near of the the separated top of the material punch with with a the radius side surfaceof 0.4 mm of the (Figures punch 3 was and not 4). value of 63% (Figure 12—solid line). Increasing the clearance by another 5% resulted in another theThe same hooks (along were the characterized blanking edge). by high Therefore, elasticity, we decidedso that tothey analyze did not the plastically value of the deform burr growth, in the increase of the burr by 12% of the value obtained as for a 10% clearance. The presented values and forsubsequent the two extremeassembly clearance process. values The ofhook 5% andblanking 15%, dependingtests were oncarried the number out for of carbon cuts. steel sheet their changes refer to the average value of the burr from five points (as in Figure 7b). hardened to 55 HRC. In the case of a 5% blanking clearance the largest increase in burr, at the tested point on the edge of the hole, occurred up to 20,000 cuts (Figure 13). Up to 30,000−40,000 cuts, the burr increase stabilized. Blanking with a punch in the range from 40- to 60 thousand strokes had already caused a significant increase in the burr. In the last cut range, the process of punch wear was more intense, hence, the burr on the edges of the sheet increased. Blanking the hook with a clearance of 10% resulted in a less intense burr increase, but with a higher value compared to blanking with a clearance of 5%. The curve in the range of up to 30,000 cuts was gentler rising, and the burr had increased more intensively. In the case of blanking with a clearance of 15%, a more intense increase of the burr was observed up to a value of about 30,000 cuts. For all clearance values, the cut range from 0 to 30,000−40,000 is the range of intense burr increase at the edges of the sheet. The punch had a sloping face, the contact (path of friction) of the separated material with the side surface of the punch was not the same (along the blanking edge). Therefore, we decided to analyze the value of the burr growth,FigureFigure for the 12. 12. twoEffectEffect extreme of of clearance clearance clearance on on the the values size of the of 5% burr and (after 15%, the dependingfirst first cuts with on a thepunch). number of cuts.

FigureFigure 13.13. InfluenceInfluence of the number of cuts on the sizesize of thethe burrburr atat thethe fifthfifth measurementmeasurement pointpoint (Figure(Figure7 7b).b). Furthermore, during the tests, it was also observed that the burr along the straight edge was Furthermore, during the tests, it was also observed that the burr along the straight edge was not not the same (Figure 14). After the first punch cuts, the burr along the straight edge was the same. the same (Figure 14). After the first punch cuts, the burr along the straight edge was the same. After After further cuts, the irregularity of the burr increased. The higher the punch wear, the larger the further cuts, the irregularity of the burr increased. The higher the punch wear, the larger the variation variation of the burr on the straight edge. At the pattern roundness, the height of the burr changed in a of the burr on the straight edge. At the pattern roundness, the height of the burr changed in a slightly slightly different way. When blanking material, the face and cutting edge were more loaded (material different way. When blanking material, the face and cutting edge were more loaded (material impact). Cyclical pressure loading close to the cutting edge caused the punch fatigue wear. Thus, impact). Cyclical pressure loading close to the cutting edge caused the punch fatigue wear. Thus, the the burr was not proportional to the burr on the straight line. The examples of the measurement results, burr was not proportional to the burr on the straight line. The examples of the measurement results, in the characteristic points on the sheet edge, for the three values of the number of punch cuts (C = 5%) in the characteristic points on the sheet edge, for the three values of the number of punch cuts (C = are presented in Figure 15. 5%) are presented in Figure 15.

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Figure 14. The burr on the edge of the sheet for exemplary hooks cut out with a clearance of 5%. FigureFigure 14. TheThe burrburr onon thethe edgeedge ofof thethe sheetsheet forfor exemexem exemplaryplary hooks hooks cut out with a clearance of 5%.

Figure 15. Burr height values measured along the blanking edge for five measuring points (C = 5%). Figure 15. BurrBurr heightheight valuesvalues measuredmeasured along along the the blanking blanking edge edge for for five ﬁvefive measuring pointspoints ((CC == 5%).5%). In the case of blanking with 15% clearance (Figure 16), the burr edge on the edge was much InIn thethethe casecasecase of ofof blanking blankingblanking with withwith 15% 15%15% clearance clearanceclearance (Figure (F(Figureigure 16), 16),16), the burrthethe burrburr edge edgeedge on the onon edge thethe wasedgeedge much waswas highermuchmuch higher than for the 5% clearance (Figure 15). The greater the friction path with a single stroke, the higherthan for than the 5%for clearancethe 5% clearance (Figure 15(Figure). The 15). greater The thegreater friction the pathfriction with path a single with stroke,a single the stroke, more the more the burr on the edges of the sheet increased. Within the value of around 30,000 cuts, the burr moreburr on the the burr edges on ofthe the edges sheet of increased. the sheet Within increased. the value WithinWithin of aroundthethe valuevalue 30,000 ofof aroundaround cuts, the 30,00030,000 burr proportionallycuts,cuts, thethe burrburr proportionally expanded along the linear edge of the sheet. From about 30,000 cuts with a clearance proportionallyexpanded along expanded the linear along edge of the the linear sheet. edge From of about the sheet. 30,000 From cuts about with a 30,000 clearance cuts of with 15% a (Figures clearance 13 of 15% (Figures 13 and 16), the burr size change was not proportional. After 60,000 punch cuts, the ofand 15% 16 ),(Figures the burr 13 size and change 16), the was burr not size proportional. change waswas After notnot 60,000proportional.proportional. punch cuts,AfterAfter the 60,00060,000 variation punchpunch of cuts,cuts, wear thethe of variation of wear of the punch cutting surfaces was significant. Photographs of hook samples blanked variationthe punch of cutting wear of surfaces the punch was cutting signiﬁcant. surfaces Photographs was significant. of hook Photographs samples blanked of hook with samples a clearance blanked of with a clearance of 15% are shown in Figure 17. The variation of wear was observed to such an extent with15% area clearance shown in of Figure 15% are 17. shown The variation in Figure of wear17. The was variation observed of to wear such was an extentobserved that to the such gap an between extent that the gap between the blanking edges had changed, and the material separation surface thatthethat blanking thethe gapgap edgesbetweenbetween had thethe changed, blankingblanking and edgesedges the material hadhad chch separationanged, and surface the material displacement separation was observed surface displacement was observed (Figure 17—detail, Figure 18). The burr on the part edge practically was displacement(Figure 17—detail, was observed Figure 18 (Figure). The burr 17—detail, on the part Figure edge 18).18). practically TheThe burrburr was onon thethe not partpart obtained edgeedge practicallypractically (Figure 18). waswas not obtained (Figure 18). not obtained (Figure 18).

Figure 16. Burr height values measured along the blanking edge for ﬁvefive measuring points ((CC == 15%). Figure 16. BurrBurr heightheight valuesvalues measuredmeasured alongalong thethe blankingblanking edgeedge forfor fivefive measuringmeasuring pointspoints ((C == 15%).15%). 3.3. The Mechanism of Wear and Damage to the Punch

As previously presented, the burr was not the same on the edges of the sheets, hence it was decided to make a brief analysis of the mechanism of wear and damage to the punches. In the blanking process with 5% clearance, after exceeding 60,000 cuts, an incremental increase in the value of the burr

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size was observed. For 63,000 cuts, the punch had microcracks and bigger wear of the blanking edges (Figure 19). The tests were discontinued and the punch was replaced. The next punch was damaged after about 65,000 cuts (Figure 20). It was observed that up to the limit of 60,000 cuts, the burr on the edge did not increase incrementally. Hence, for comparative purposes, the deﬂection (H ) and bending Figure 17. Burr (bh) on the edge of the sheet for hook cuts out with a clearance of 15%.b radius (Rb) analysis of the hook and the size of the burr (bh) of their values have been prepared for Materials60,000Materials 2019 cuts. ,2019 12, x, 12FOR, x FORPEER PEER REVIEW REVIEW 12 of 1216 of 16

Figure 18. Zoom area (in Figure 17) near the separation of the material of the hook deflection.

FigureFigureFigure 17. Burr 17.17. Burr (bh) on (bbhh )the on edge the edge of the of sheet the sheet for hook for hook cuts cutsout with out with a clearance a clearance of 15%. of 15%. 3.3. The Mechanism of Wear and Damage to the Punch As previously presented, the burr was not the same on the edges of the sheets, hence it was decided to make a brief analysis of the mechanism of wear and damage to the punches. In the blanking process with 5% clearance, after exceeding 60,000 cuts, an incremental increase in the value of the burr size was observed. For 63,000 cuts, the punch had microcracks and bigger wear of the blanking edges (Figure 19). The tests were discontinued and the punch was replaced. The next punch was damaged after about 65,000 cuts (Figure 20). It was observed that up to the limit of 60,000 cuts, the burr on the edge did not increase incrementally. Hence, for comparative purposes, the deflection (Hb) and bending radius (Rb) analysis of the hook and the size of the burr (bh) of their values have been FigureFigure 18. Zoom 18. Zoom area area(in Figure (in Figure 17) near 1717)) near nearthe separation thethe separationseparation of the ofof material thethe materialmaterial of the ofof hook thethe hookhook deflection. deﬂection.deflection. prepared for 60,000 cuts. 3.3. The3.3. MechanismThe Mechanism of Wear of Wear and Damageand Damage to the to Punch the Punch As previouslyAs previously presented, presented, the burrthe burr was wasnot notthe samethe same on theon edgesthe edges of the of sheets,the sheets, hence hence it was it was decideddecided to make to make a brief a brief analysis analysis of the of themechanism mechanism of wear of wear and anddamage damage to the to thepunches. punches. In the In the blankingblanking process process with with 5% clearance,5% clearance, after after exceeding exceeding 60,000 60,000 cuts, cuts, an incremental an incremental increase increase in the in valuethe value of theof burrthe burr size sizewas wasobserved. observed. For For63,000 63,000 cuts, cuts, the punchthe punch had hadmicrocracks microcracks and andbigger bigger wear wear of the of the blankingblanking edges edges (Figure (Figure 19). 19).The Thetests tests were were discon discontinuedtinued and andthe punchthe punch was wasreplaced. replaced. The Thenext next punch punch was wasdamaged damaged after after about about 65,000 65,000 cuts cuts (Figure (Figure 20). 20).It was It wasobserved observed that thatup to up the to limitthe limit of 60,000 of 60,000 cuts, cuts, the burrthe burr on the on edgethe edge did notdid increasenot increase incrementall incrementally. Hence,y. Hence, for comparative for comparative purposes, purposes, the deflectionthe deflection

(Hb) (andHb) andbending bending radius radius (Rb) (analysisRb) analysis of the of hookthe hook and andthe sizethe sizeof the of burrthe burr (bh) of(b htheir) of their values values have have been been (a) (b) (c) preparedprepared for 60,000 for 60,000 cuts. cuts. FigureFigure 19. Tool afterafterblanking blanking about about 63,000 63,000 elements elements with with 5% clearance:5% clearance: (a) the (a) principal the principal blanking-punch blanking- punchelements; elements; (b) cutting (b) cutting edge wear; edge (wear;c) fatigue (c) fatigue wear. wear.

The blanking of hooks in the hardened steel sheets (55 HRC) caused the surfaces of flank and tool face of the punch (Figure 19a) to be abrasively worn (Figure 19b—lower) and fatigue worn (Figure 19b—upper and Figure 19c). Near the top of the punch (Figure 19b—upper), fatigue fractures and material damages were visible. The abrasive wear along the blanking edge occurred on the leading (face) surface. The abrasive wear on the face of the punch near the top had a slightly larger size. The contact area of the punch with the material was initially larger. Only after material cutting out, the hook was gradually bent. During the gradual blanking, there was a micro-slip between the surface (a ) (a) (b) (b) (c) (c) of the bending material of the hook and the face of the punch. FigureOnFigure 19. the Tool 19. flank Tool after surface after blanking blanking ofthe about punch about 63,000 with63,000 elements the elements sloping with with 5% face clearance:5% (Figure clearance: ( 21a) a),the (a) the principalthewear principal wasblanking- blanking- not the same Figure 20. A punch with a sloping frontal surface after damage and punch material separation (C = (Figurepunchpunch elements;21b). elements; This (b) is cutting ( dueb) cutting to edge the edge wear; fact wear; that (c) fatigue the (c) fatigue length wear. wear. of the friction path was different: The largest was 5%, 65,000 cuts). at the top of the punch. The surface near the cutting edge must pierce the material, the material is cut out by the punch’s cutting edges. The deflection of the hook was obtained by gradual material

FigureFigure 20. A 20. punch A punch with with a sloping a sloping frontal frontal surface surface after afte damager damage and andpunch punch material material separation separation (C = (C = 5%, 65,0005%, 65,000 cuts). cuts).

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The blanking of hooks in the hardened steel sheets (55 HRC) caused the surfaces of flank and tool face of the punch (Figure 19a) to be abrasively worn (Figure 19b—lower) and fatigue worn (Figure 19b—upper and 19c). Near the top of the punch (Figure 19b—upper), fatigue fractures and material damages were visible. The abrasive wear along the blanking edge occurred on the leading (face) surface. The abrasive wear on the face of the punch near the top had a slightly larger size. The contact area of the punch with the material was initially larger. Only after material cutting out, the hook was gradually bent. During the gradual blanking, there was a micro-slip between the surface of the bending material of the hook and the face of the punch.

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blanking, and bending the punch stroke to the depth H (Figure4b) separates the material along the The blankingThe blanking of hooks of hooks in the in hardened the hardened steel steelsheets sh (55eets HRC) (55 HRC) caused caused the surfaces the surfaces of flank of flankand and blankingtool face edge of theof the (puncha) punch. (Figure The face19a) of( bto) the be punch abrasively wasinclined worn (Figure at an angle 19b—lower)( cα)). The and area fatigue of pressure worn toolbetween face of thethe sheetpunch and (Figure the punch 19a) movesto be alongabrasively the edge. worn The (Figure bending 19b—lower) of the cut part,and fatigue in the transverse worn (FigureFigure 19b—upper 19. Tool after and blanking 19c). Nearabout the63,000 top elements of the punch with 5% (Figure clearance 19b—upper),: (a) the principal fatigue blanking fractures- and (Figureplane, 19b—upper caused the and pressure 19c). toNear beconcentrated the top of th neare punch the blanking(Figure 19b—upper), edge at a certain fatigue width. fractures As a result and of materialpunch elements;damages (wereb) cutting visible. edge The wear; abrasive (c) fatigue wear wear. along the blanking edge occurred on the leading materialblanking damages a certain were number visible. of The hooks abrasive on the wear surface along of the the punch, blanking damage edge occurredoccurred (Figureon the leading19b). (face)(face) surface. surface. The abrasive The abrasive wear wearon the on face the of face the of punch the punch near thenear top the had top a had slightly a slightly larger larger size. Thesize. The contactcontact area ofarea the of punch the punch with withthe material the material was initwasially init larger.ially larger. Only Onlyafter aftermaterial ma terialcutting cutting out, theout, the hook hookwas graduallywas gradually bent. bent.During During the gradual the gradual blanking, blanking, there therewas awas micro-slip a micro-slip between between the surface the surface of theof bending the bending material material of the of hook the hookand the and face the of face the of punch. the punch. On theOn flank the flanksurface surface of the of punch the punch with withthe slopin the sloping faceg (Figure face (Figure 21a), 21a),the wear the wearwas not was the not same the same (Figure(Figure 21b). 21b).This isThis due is to due the to fact the that fact the that length the length of the of friction the friction path waspath different:was different: The largest The largest was was at theat top the of top the of punch. the punch. The surface The surface near thenear cutting the cutting edge edgemust mustpierce pierce the material, the material, the material the material is cut is cut out byout the by punch’s the punch’s cutting cutting edges. edges. The deflectionThe deflection of the of hook the hookwas obtainedwas obtained by gradual by gradual material material blanking,blanking, and bending and bending the punch the punch stroke stroke to the to depth the depth H (Figure H (Figure 4b) separates 4b) separates the material the material along along the the blanking edge of the punch. The face of the punch was inclined at an angle (α). The area of pressure blanking edge of the punch. The face of the punch was inclined at an angle (α). The area of pressure betweenbetween the sheet the sheet and the and punch the punch moves moves along along the edge. the edge. The bending The bending of the of cut the part, cut part,in the in transverse the transverse Figure 20. A punch with a sloping frontal surface after damage and punch material separation (C = plane,plane, causedFigure caused the 20. pressureA the punch pressure to with be to aconcentrated slopingbe concentrated frontal near surface thenear blanking afterthe blanking damage edge and edgeat apunch certain at a certain material width. width. separationAs a resultAs a result 5%, 65,000 cuts). of blankingof blanking(C = a5%, certain 65,000 a certain number cuts). number of hooks of hooks on the on su therface su rfaceof the of punch, the punch, damage damage occurred occurred (Figure (Figure 19b). 19b).

(b) (a) (a) (b) (b) Figure 21. Tool after(a) blanking 60,000 elements with 15% clearance: (a) the blanking part of the punch; FigureFigure 21. Tool 21. Tool after after blanking blanking 60,000 60,000 elem elementsents with with 15% 15% clearance: clearance: (a) the (a) blanking the blanking part part of the of punch; the punch; (b) flank wear. (b) (Figureflankb) ﬂank wear. 21 wear.. Tool after blanking 60,000 elements with 15% clearance: (a) the blanking part of the punch; (b) flank wear. PunchingPunchingPunching with with anwith intermediate an an intermediate intermediate clearance clearance clearance between between between 5% 5%and 5% and 15%, and 15%, 15%,i.e., i.e., C i.e., =C 10%, =C 10%,= 10%,caused caused caused the thewear the wear wearof of of thethe blankingthe blankingOn blanking the edge flank edge edgeof surfacethe of of thepunch the punchof punch the to apunch to greater to a greatera withgreater extent the extent extent slopingthan than thanin facethe in in the case(Figure the case ofcase the of21 of thea), two the the two remainingtwo wear remaining remaining was clearances not clearances the clearances same (Figure(Figure(Figure 22a). 2221 a).22a).b).Blanking BlankingThis Blanking is thedue the hooks tothe hooks the hooks withfact with thatwiththe the clearancethe clearancethe length clearance C Cof = the 10% C friction= in10% aa C45 C45in path a steel steelC45 was sheet, steelsheet, different: withsheet, with hardness Thewithhardness largest hardness 55 55 HRC, was 55 HRC,atcaused HRC,the caused top chipping caused of chipping the chippingpunch. of the of the blankingThe of blanking thesurface blanking edge nearedge and theedgeand abrasive cutting abra andsive wearabra edge wearsive of must the wearof face thepierce of offace the thethe of face K340 material,the of K340 steelthe K340thesteel punch. material punch.steel A bandpunch. isA cut of A bandoutabrasive bandof byabrasive ofthe material abrasive punch material wear’s material cutting wear near wear near the edges. chipped thenear Thechipped the edgedeflection chipped edge of the edgeof of punchthe the of punch the hook is punch visible is wasvisible is on obtainedvisible theon the punch on punch bythe face gradual punch face (Figure (Figure face material 22 (Figureb). 22b).blanking 22b). , and bending the punch stroke to the depth H (Figure 4b) separates the material along the

(b) (a) (a) (b) FigureFigure 22. 22.The The punch punch after after blanking blanking out out about about 60,000 60,000 hooks hooks with with a clearance a clearance of 10%: of 10%: (a) A (a fragment) A fragment of Figure 22. The punch after blanking out about 60,000 hooks with a clearance of 10%: (a) A fragment theof blanking the blanking edge edge of the of punch the pu withnch visiblewith visible chippings chippings and ( band) abrasive (b) abrasive wear onwear the on face the close face toclose the to of the blanking edge of the punch with visible chippings and (b) abrasive wear on the face close to blankingthe blanking edge. edge. the blanking edge.

Materials 2019, 12, 1261 14 of 16

4. Conclusions This experimental research has shown how the determined value of clearance, in the process of partial blanking of a C45 sheet with a hardness of 55 HRC, affects the size of burrs on the edges of the sheet. In addition, the obtained results of the Hb deviation of the hook and the size of its radius, Rb, allowed to assess how the wear of the punch influences the course of changes in the geometric values of the cut element. Based on the results achieved from the research, the following conclusions can be formulated. 1. In the case of K340 material of the punch, it was possible to cut the hooks to a range of approximately 60,000 cuts. Longer exploitation caused high fatigue of cold-work tool steel material. The size of the blanking clearance very clearly affected the intensity of wear of the blanking edge of the punch and the amount of deflection of the hooks. 2. During the tests after the first cuts with a punch, only for the clearance of 15% the value of the hook deviation Hb = 1.2 mm was obtained. In the case of the other two partial blanking experiments with 5% and 10% clearance, the Hb values were higher than assumed. For a 5% clearance, the Hb value obtained after the first cut was 62.5% higher, while for C = 10% the deflection was 18.3% higher. As the number of cuts increased, the wear of the punch caused significant changes in the deflection value. 3. The use of a punch with a sloping face for partial blanking of the hooks in the C45 sheet, resulted in irregular wear of the punch. Blanking and bending an element with an open separation line results in irregular wear of the blanking edges. As the number of cuts made by the punch increased, the size of the burr was diversified. 4. The value of the burr for the three blanking clearance values was different. The lowest initial burr values were obtained for a 5% clearance, and the biggest one for 15% clearance. However, the largest increase in burr values was observed during blanking with a clearance of 5%. After 60,000 cuts, with 5% clearance, on the linear edge the burr value increased by 133%.

Author Contributions: Conceptualization, J.M. and J.T.; investigation, J.M. and J.T.; data curation, J.M.; visualization, J.T.; writing—original draft preparation, J.M.; writing—review and editing, J.M. and J.T.; project administration, J.M.; funding acquisition, J.T. Funding: This Research was financed by the Ministry of Science and Higher Education of the Republic of Poland—statutory activity No. DS.MA.17.001. Acknowledgments: We acknowledge financial support by Rzeszów University of Technology within the funding program Open Access Publishing. Conflicts of Interest: The authors declare no conflict of interest.

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