Optimization of Wet Grinding Conditions of Sheets Made of Stainless Steel

Journal of Manufacturing and Materials Processing

Article Optimization of Wet Grinding Conditions of Sheets Made of Stainless Steel

Akira Mizobuchi 1,* and Atsuyoshi Tashima 2

1 Graduate School of Technology, Industrial and Social Sciences, Tokushima University, Tokushima 7708506, Japan 2 Ishihara Kinzoku Co., Ltd., Tokushima 7700873, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-88-656-9741

Received: 9 November 2020; Accepted: 4 December 2020; Published: 7 December 2020

Abstract: This study addresses the wet grinding of large stainless steel sheets, because it is difficult to subject them to dry grinding. Because stainless steel has a low thermal conductivity and a high coefficient of thermal expansion, it easily causes grinding burn and thermal deformation while dry grinding on the wheel without applying a cooling effect. Therefore, wet grinding is a better alternative. In this study, we made several types of grinding wheels, performed the wet grinding of stainless steel sheets, and identified the wheels most suitable for the process. As such, this study developed a special accessory that could be attached to a wet grinding workpiece. The attachment can maintain constant pressure, rotational speed, and supply grinding fluid during work. A set of experiments was conducted to see how some grinding wheels subjected to some grinding conditions affected the surface roughness of a workpiece made of a stainless steel sheet (SUS 304, according to Japanese Industrial Standards: JIS). It was found that the roughness of the sheet could be minimized when a polyvinyl alcohol (PVA) grinding wheel was used as the grinding wheel and tap water was used as the grinding fluid at an attachment pressure of 0.2 MPa and a rotational speed of 150 rpm. It was shown that a surface roughness of up to 0.3 µm in terms of the arithmetic average height could be achieved if the above conditions were satisfied during wet grinding. The final surface roughness was 0.03 µm after finish polishing by buffing. Since the wet grinding of steel has yet to be studied in detail, this article will serve as a valuable reference.

Keywords: wet grinding; stainless steel; ground surface; grinding machine; grinding wheel

1. Introduction Typically, grinding [1] is the final process of machining and is an important technique that adopts a specific polishing process for the manufacturing of advanced products and surfaces. Grinding is widely used in the manufacturing of several parts that require smooth finishes and fine tolerances. This process accounts for approximately 20–25% of the total investment in machining operations [1]. A number of studies have been done on the grinding types. For example, research has been carried out on external cylindrical grinding [2–5], surface grinding [6,7], and belt grinding [8]. The surface roughness of ground parts [9] is an essential factor in the evaluation of a grinding process and is an important criterion for selecting machine tools, dressings, and grinding wheels. The surface condition has a great effect on these factors. Stainless steel is a high value-added material with outstanding designability, corrosion resistance, and functionality. The metal surface of a steel sheet is used as is, with no need for plating or painting. For this reason, stainless steel is processed in various ways depending on the application domain, which can vary widely from familiar Western-style tableware to nuclear power plants [10]. Additionally, grinding and polishing is a particularly important machining process that increases the added value of a stainless steel

J. Manuf. Mater. Process. 2020, 4, 114; doi:10.3390/jmmp4040114 www.mdpi.com/journal/jmmp J. Manuf. Mater. Process. 2020, 4, 114 2 of 13 sheet. Polishing for stainless steel sheets includes mechanical polishing by micro-machining using abrasive grains [11,12] and chemical polishing by etching in solution or electrolytes [13–20]. Chemical polishing has several limitations in terms of processing. Mechanical polishing requires no special equipment compared to that needed for chemical polishing, and it has almost no limits on the size of the workpieces that can be processed. Mechanical polishing renders it possible to polish large stainless steel sheets with a length of 1 m or more on one side. However, dry grinding is the mainstream for large stainless steel-sheets, and wet grinding is rarely performed. There are hardly reports on wet grinding used with a grinding wheel. The grinding of large stainless steel sheets at a processing site is carried out using the dry method. In this method, grinding is first performed by a flap wheel to remove surface irregularities, and then finish polishing is performed by a buffing wheel. A flap wheel is a rotary grinding tool that performs grinding by pressing an abrasive cloth arranged in a radial pattern against the workpiece while rotating it. Since stainless steel is a material with a low thermal conductivity and a high coefficient of thermal expansion, burning and thermal deformation are easily caused by the grinding wheel during dry processing in the absence of cooling. Using a flap wheel in dry processing ensures a good workability, but a large amount of chips and abrasive grains fly as dust during the process, causing health hazards to on-site workers and neighboring residents [21], as well as potentially causing a dust explosion [22]. In addition, flap wheel grinding is a horizontal axis grinding, where the feed direction of the shaft and the rotational direction of the shaft are always in the same direction, resulting in longitudinal streaks on the stainless steel surface along the grinding direction. Vertical stripes on the stainless steel surface can be clearly visually confirmed by the reflection of light. These stripes cause a distortion of the reflected image. This strain of these stripes causes a decrease in the smoothness and design of a stainless steel sheet. It is possible to make the vertical streaks inconspicuous by lowering the pressing pressure, but if more smoothness and design quality are required, a greater number of steps are required and the productivity consequently deteriorates. The quality of a stainless steel surface obtained a flap wheel is limited. In this research, in order to solve the problems that occur during dry grinding, instead of horizontal axis dry grinding by a flap wheel, vertical planetary rotation wet grinding by a grinding wheel is proposed. In the case of the vertical planetary rotation type, the cutting direction of the abrasive grains can be dispersed to ensure the smoothness of the surface. In addition, wet processing can suppress the burning and thermal deformation of dust and work material during processing. No flap wheel is used, but a grinding wheel is. When applying wet grinding with a grinding wheel to the grinding of a large stainless steel sheet, the selection of the wheel for grinding the sheet is an important consideration. This is because the practical application of this technology would not be possible without selecting a grinding wheel that is capable of grinding stainless steel sheets in a wet process and that can process them to the surface finish levels obtained by the current grinding process. There are five aspects of a grinding wheel that must be considered: abrasive grain type, grain size, bond type, grade, and structure; the performance of a grinding wheel is determined by the combination of these factors. The success or failure of wet grinding is determined by the appropriateness of these choices. In this study, we made several types of grinding wheels, performed the wet grinding of stainless steel sheets, and examined the wheels that could be suitable for the wet grinding of stainless steel sheets.

2. Materials and Methods

2.1. Wet Grinding Apparatus As no wet grinding machine for large stainless steel sheets currently exists, we developed a prototype based on an existing dry grinding machine. To determine the prototype specifications, we studied specifications such as spindle speed, grinding wheel feed rate, and working fluid supply. Figure1 shows an external view of the developed wet grinding machine and the attachment points for the grinding wheels. The machine was built following the gantry style with a raised frame. The machine dimensions are 2800 mm (maximum height) 4750 mm (maximum width) 9260 mm × × J.J. Manuf.Manuf. Mater.Mater. Process.Process. 20202020,, 44,, 114x FOR PEER REVIEW 33 of 13 14

(maximum length). The maximum size of the workpiece to be ground is 16 mm thick × 1524 mm wide (maximum length). The maximum size of the workpiece to be ground is 16 mm thick 1524 mm wide × 6100 mm long. The machine is equipped with two main shafts, and each× shaft has four 6100 mm long. The machine is equipped with two main shafts, and each shaft has four countershafts. ×countershafts. The countershafts are passively rotated by the active rotation that is transmitted by the Thegears countershafts of the main are shafts, passively which rotated are directly by the activeconnected rotation to the that motor. is transmitted The spindle by the speed gears can of thebe mainincreased shafts, from which 39.5 are to directly 395 rpm connected with a gear to the ratio motor. of 1:1. The The spindle movement speed can of bethe increased grinding from wheel 39.5 is toimplemented 395 rpm with as a gearepicyclic ratio rotation, of 1:1. The which movement reduce ofs thethe grinding unevenness wheel of is the implemented finished surface as epicyclic and rotation,improves which grinding reduces efficiency. the unevenness The number of of the main finished shafts surface to be used and improvesis selectable, grinding and each effi ciency.can be Thecontrolled number independently of main shafts from to be a used control is selectable, box. The and grinding each can wheel be controlled is fixed with independently an adhesive from to aa controlstainless box. steel The disc grinding (SUS430, wheel JIS) iswith fixed a diameter with an adhesive of 150 mm to a and stainless a thickness steel disc of 2 (SUS430, mm. The JIS) grinding with a diameterwheel head of at 150 the mm tip andof the a thicknessspindle is ofequipped 2 mm. Thewith grinding four flanges wheel with head a 150 at mm the tipdiameter. of the spindleAs shown is equippedin Figure with2, a magnet four flanges is embedded with a 150 in mmthe flange, diameter. maki Asng shown it easy in to Figure detach2, athe magnet stainless is embedded steel disc injig theto which flange, the making grinding it easy wheel to detach is attached, the stainless thus reducing steel disc jigthe to labor which required the grinding to change wheel the is attached,grinding thuswheel. reducing The spindle the labor speed, required the feed to change rate of the the grinding gantry unit, wheel. and The the spindle feed rate speed, of the the grinding feed rate wheel of the gantryhead are unit, controlled and the feedthrough rate ofan the inverter. grinding Moreover, wheel head the are control controlled box allows through for an the inverter. configuration Moreover, of thevarious control settings. box allows The gantry for the unit configuration runs on a rack of various gear drive, settings. with The the gantry gear shifting unit runs performed on a rack by gear an drive,inverter. with A themotor-driven gear shifting screw performed jack system by an drives inverter. the A grinding motor-driven wheel screw head. jack The system feed rate drives of thethe grindinggantry unit wheel can head.be set Thefrom feed 0.01 rateto 0.5 of m/s, the gantry and th unite feed can rate be of set the from grinding 0.01 to wheel 0.5 m /heads, and can the be feed set ratefrom of 0.01 the to grinding 0.2 m/s. wheel The grinding head can solution be set from uses 0.01 tap to water 0.2 m without/s. The grinding mixing additives. solution uses The tap grinding water withoutfluid drains mixing at a additives.rate of approximately The grinding 3 fluidL per drainsminute at from a rate the of center approximately of the main 3 L shaft, per minute as shown from in theFigure center 2. The of thegrinding main fluid shaft, is asfiltered shown and in recirculated. Figure2. The As grinding only tap fluidwater is is filteredused, the and waste recirculated. treatment Asis less only burdensome tap water is used,and the the envi wasteronmental treatment impact is less burdensomeis smaller than and that the environmentalof using chemicals. impact The is smallergrinding than fluid that also of prevents using chemicals. thermal deformation The grinding and fluid dust also dispersal, prevents removes thermal the deformation swarf, and and reduces dust dispersal,the processing removes heat. the The swarf, pressing and force reduces of the the grindi processingng wheel heat. is controlled The pressing by air force pressure of the to grinding provide wheelan appropriate is controlled pressure by air pressureand allo tow providefor grinding an appropriate along the pressurecurvature and of allowthe material. for grinding The alonggrinding the curvaturewheel head of pressure the material. operates The grindingbetween 0.1 wheel and head 0.4 MPa. pressure operates between 0.1 and 0.4 MPa.

Wheel head

Table

Wheel head Flange

(a) External view (b) Attachment points

Main sh aft

cou ntershafts

Grinding wheel

Stainless steel plate

FigureFigure 1.1. Developed wet grinding machine andand attachmentattachment pointspoints forfor thethe grindinggrinding wheels.wheels.

J. Manuf. Mater. Process. 2020, 4, 114 4 of 13 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 4 of 14

Flange

Magnet Fluid drain

Magnet

FigureFigure 2. 2. MagnetMagnet attachment attachment and and grinding grinding fluid fluid drain. drain. 2.2. Selecting a Grinding Wheel Experimentally 2.2. Selecting a Grinding Wheel Experimentally Figure3 shows the machining part of the wet tabletop grinding equipment designed to select the Figure 3 shows the machining part of the wet tabletop grinding equipment designed to select optimum grinding wheel for the wet grinding of a stainless steel sheet. It serves as a model of the the optimum grinding wheel for the wet grinding of a stainless steel sheet. It serves as a model of the grinding wheel mounted on the large grinding machine, as shown in Figure1. The grinding wheels grinding wheel mounted on the large grinding machine, as shown in Figure 1. The grinding wheels used in the experiment were commercially available grinding wheels that were cut into a cylindrical used in the experiment were commercially available grinding wheels that were cut into a cylindrical shape with a diameter of approximately 25 mm and a thickness of approximately 5 mm. The grinding shape with a diameter of approximately 25 mm and a thickness of approximately 5 mm. The grinding wheel was glued to the flange shaft, and the shaft was held using a collet chuck inside the spindle. wheel was glued to the flange shaft, and the shaft was held using a collet chuck inside the spindle. The spindle was fixed to the stand of the grinding machine. The rotation speed of the spindle was The spindle was fixed to the stand of the grinding machine. The rotation speed of the spindle was set set through the controller. A constant in-feed pressure was applied only in the z-axis direction of the through the controller. A constant in-feed pressure was applied only in the z-axis direction of the table; thus, no motions along x- and y-axes were provided. This surface pressure was calculated by table; thus, no motions along x- and y-axes were provided. This surface pressure was calculated by measuring the pressure with a three-component dynamometer installed on the table. The workpiece measuring the pressure with a three-component dynamometer installed on the table. The workpiece was stainless steel (SUS304, JIS). This steel sheet was fixed with a jig inside a water tank mounted was stainless steel (SUS304, JIS). This steel sheet was fixed with a jig inside a water tank mounted on on the table, while the tank was filled with sufficient tap water to immerse the surface of the steel the table, while the tank was filled with sufficient tap water to immerse the surface of the steel sheet. sheet. The grinding time was set to 1 min. Each grinding wheel performance was evaluated in terms The grinding time was set to 1 min. Each grinding wheel performance was evaluated in terms of the of the abrasion loss on the grinding wheel and the surface roughness of the steel sheet. The grinding abrasion loss on the grinding wheel and the surface roughness of the steel sheet. The grinding conditions were set at a spindle speed of 12,000 rpm and an average surface pressure of 14.7 kPa along conditions were set at a spindle speed of 12,000 rpm and an average surface pressure of 14.7 kPa the z-axis. The dimensions of the workpiece were 80 80 8 mm. The surface roughness of the steel along the z-axis. The dimensions of the workpiece were× 80× × 80 × 8 mm. The surface roughness of the sheet was measured using a surface roughness measuring instrument (Mitutoyo SJ-410); the arithmetic steel sheet was measured using a surface roughness measuring instrument (Mitutoyo SJ-410); the average height of the assessed profile before grinding was Ra = 3.0 µm. arithmetic average height of the assessed profile before grinding was Ra = 3.0 μm.

Spindle Jig Workpiece

Jig Jig Grinding wheel

Figure 3. Wet tabletop grinding equipment. Figure 3. Wet tabletop grinding equipment. 2.3. Hardness of Grinding Wheels 2.3. Hardness of Grinding Wheels Table1 lists the wheels used in the grinding and provides a short description of each wheel. WhiteTable alundum 1 lists wasthe usedwheels as used the abrasive in the grinding grain. As an ad bonding provides agent, a short we descri usedption either of vitriﬁed, each wheel. resin, Whiteor sponge alundum bonds. was For used the spongeas the abrasive bond grinding grain. As wheels, a bonding we used agent, acetanilide we used from either polyvinyl vitrified, alcohol resin, or(PVA) sponge as thebonds. bonding For the agent. sponge For bond the hardnessgrinding andwheels, structure, we used we acetanilide used the valuesfrom polyvinyl provided alcohol by the (PVA)grinding as wheelthe bonding manufacturer. agent. For The the hardness hardness ranged and from structure, G to O we (JIS), used and the the values structure provided was 9. Tobyrefer the grindingto the grinding wheel wheelsmanufacturer. in a shorthand The hardness notation, ranged we indicate from G the to vitriﬁed O (JIS), grindingand the structure wheels by was grain 9. sizeTo refer to the grinding wheels in a shorthand notation, we indicate the vitrified grinding wheels by

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 5 of 14 grain size and the grade of hardness, and we classify the resin bond and sponge bond grinding wheels by grain size and the type of bonding agent (B or SP).

J. Manuf. Mater. Process. 2020, 4, 114 Table 1. Wheels used in the grinding. 5 of 13 Label Grain Size Bond Grade and the grade of hardness, and180G we classify #180 the resin bondVitrified and spongeG bond grinding wheels by grain size and the type of bonding agent180H (B or SP).#180 Vitrified H 180J #180 Vitrified J 180KTable 1. Wheels#180 usedVitrified in the grinding. K 180O #180 Vitrified O Label240K Grain#240 Size Vitrified Bond GradeK 240O180G #240 #180 Vitrified Vitrified O G 400G180H #400 #180 Vitrified Vitrified G H 180J #180 Vitrified J 400H #400 Vitrified H 180K #180 Vitrified K 400K180O #400 #180 Vitrified Vitrified K O 400N240K #400 #240 Vitrified Vitrified N K 400SP240O #400 #240 Sponge Vitrified - O 400B400G #400 #400 Resinoid Vitrified D G 400H #400 Vitrified H 400K #400 Vitrified K We performed the Rockwell400N hardness #400test on each Vitrified grinding wheel N because the sponge grinding wheels had no hardness value 400SPavailable in #400a specification Sponge sheet. The - hardness measuring instrument was a handmade machine that 400Bwas an improved #400 version Resinoid of the universal D testing machine. Figure 4 shows the Rockwell hardness measurements of the grinding wheels. A steel ball with a sphereWe diameter performed of the6.35 Rockwell mm was hardnessused as testthe indenter, on each grinding and the wheeltest load because was theset spongeat 588.4 grinding N. The hardnesswheels had of the no hardnesssponge grinding value available wheel was in a approx specificationimately sheet. 100 HRL. The hardnessThis value measuring corresponded instrument to the hardnesswas a handmade of K for a machine vitrified that bond. was The an hardness improved of version the resin-bond of the universal grinding testing wheel machine. (approximately 40 HRL)Figure was slightly4 shows higher the Rockwell than the hardness degree of measurements hardness (400H) of theof the grinding vitrified wheels. bond. A steel ball with a sphereWe diameter set two ofconditions 6.35 mm wasfor selecting used as the a grinding indenter, wheel: and the it test had load to be was capable set at 588.4of wet N. grinding The hardness and haveof the an sponge adequate grinding hardness wheel to wasprevent approximately self-sharpen 100ing HRL. loading. This valueWe used corresponded the grinding to thewheels hardness shown of inK forTable a vitrified 1 on a stainless bond. The steel hardness sheet to of find the the resin-bond grinding grinding wheels with wheel the (approximately least abrasion 40loss HRL) and wasthe smallestslightly highersurface than roughness. the degree of hardness (400H) of the vitrified bond.

Figure 4. Rockwell hardness measurements of grinding wheels. Figure 4. Rockwell hardness measurements of grinding wheels. We set two conditions for selecting a grinding wheel: it had to be capable of wet grinding and 3.have Results an adequate and Discussion hardness to prevent self-sharpening loading. We used the grinding wheels shown in Table1 on a stainless steel sheet to ﬁnd the grinding wheels with the least abrasion loss and the 3.1. Abrasion Losses and Abrasion Conditions of Grinding Wheel smallest surface roughness. Figure 5 shows the average abrasion loss of the grinding wheel thickness after grinding. We conducted3. Results andthree Discussion trials for each type of grinding wheel and then calculated the average abrasion loss from the abrasion losses of the three grinding wheels. Before grinding, the thickness of the grinding 3.1. Abrasion Losses and Abrasion Conditions of Grinding Wheel wheel was 5 mm. For the vitrified grinding wheels, the abrasion loss was the highest with hardness Figure5 shows the average abrasion loss of the grinding wheel thickness after grinding.

We conducted three trials for each type of grinding wheel and then calculated the average abrasion loss from the abrasion losses of the three grinding wheels. Before grinding, the thickness of the grinding wheel was 5 mm. For the vitriﬁed grinding wheels, the abrasion loss was the highest with hardness J. Manuf. Mater. Process. 2020, 4, 114 6 of 13 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 6 of 14 valuesvalues of G, H, and J and thethe smallestsmallest withwith aa hardnesshardness value value of of K. K. Regarding Regarding the the abrasion abrasion condition condition of ofthe the grinding grinding wheels wheels during during grinding, grinding, those withthose hardness with hardness values ofvalues G, H, andof G, J wore H, and out immediatelyJ wore out immediatelyafter the start after of machining. the start of The machining. resin bond The and resi spongen bond grindingand sponge wheels grinding exhibited wheels less exhibited abrasion less loss abrasionthan the loss vitriﬁed than grinding the vitrified wheels. grinding wheels.

Figure 5. Average abrasion loss of the grinding wheel thickness after grinding. Figure 5. Average abrasion loss of the grinding wheel thickness after grinding. Figure6 shows the conditions of each wheel—180G, 400G, and 400SP—after machining. After grinding, the surfaceFigure of6 theshows grinding the conditions wheel was of almost each wheel—180G, level. The grinding 400G, wheels and 400SP—after were worn down machining. almost evenlyAfter grinding,because of the the surface uniform of application the grinding of thewheel indentation was almost load. level. Their The original grinding thickness wheels of thewere 180G worn and down 400G almostgrinding evenly wheels because changed of substantially.the uniform application In contrast, of the the thickness indentation of the load. 400SP Their wheel original was approximately thickness of thethe 180G same and as before 400G machining.grinding wheels Moreover, changed the entiresubstant surfaceially. ofIn thecontrast, grinding the wheelthickness was of stained the 400SP gray. wheelThe staining was approximately of the surface suggestedthe same as the before adhesion machinin of theg. swarf Moreover, to the pores.the entire surface of the grinding wheel was stained gray. The staining of the surface suggested the adhesion of the swarf to the pores. Based on the abrasion condition of the vitrified grinding wheels with lower hardness values, we hypothesize that releasing occurred because the hardness was substantially low for the machining conditions. Furthermore, the vitrified, sponge, and resin bond grinding wheels with higher hardness values experienced loading due to10mm the accumulation of10mm swarf adhering to 10mmthe pores. The abrasion loss on the grinding wheels used for grinding was governed by the hardness and did not depend on the type of bonding agent or the size of the abrasive grains.

10mm 10mm 10mm 10mm 10mm 10mm (a) 180G (b) 400G (c) 400SP Figure 6. Conditions of the 180G, 400G, and 400SP wheels after machining. 10mm 10mm 10mm Based on the abrasion condition of the vitrified grinding wheels with lower hardness values, we hypothesize that releasing occurred because the hardness was substantially lowfor the machining (a) 180G (b) 400G (c) 400SP conditions. Furthermore, the vitrified, sponge, and resin bond grinding wheels with higher hardness values experiencedFigure loading 6. Conditions due to theof the accumulation 180G, 400G, an ofd swarf 400SP adhering wheels after to themachining. pores. The abrasion loss on the grinding wheels used for grinding was governed by the hardness and did not depend on the 3.2.type Comparison of bonding of agent Loss Ratios or the and size Surface of the abrasiveProperties grains. of Steel Sheet The grinding ratio is commonly used as an abrasion resistance indicator of a grinding wheel and 3.2. Comparison of Loss Ratios and Surface Properties of Steel Sheet is the value obtained by dividing the workpiece removal volume by the abrasion volume of the grindingThe wheel grinding consumed ratio is during commonly the grinding. used as anIn these abrasion experiments, resistance the indicator workpiece of a removal grinding volume wheel wasand minimal is the value and obtained difficult to by measure. dividing In the this workpiece paper, the removal grinding volume ratio is by the the amount abrasion of improvement volume of the ingrinding surface wheel roughness consumed divided during by the the abrasion grinding. losses In these of the experiments, grinding wheel, the workpiece which is removal called the volume loss rate.was minimalThe amount and of di ffiimprovementcult to measure. in surface In this roughn paper,ess the was grinding calculated ratio isby the subtracting amount of the improvement arithmetic averagein surface height roughness of the divided assessed by profile the abrasion after grinding losses of from the grinding the arithmetic wheel, whichaverage is calledheight theof lossthe

J. Manuf. Mater. Process. 2020, 4, 114 7 of 13

rate.J. The Manuf. amount Mater. Process. of improvement 2020, 4, x FOR PEER in REVIEW surface roughness was calculated by subtracting the 7 arithmetic of 14 average height of the assessed profile after grinding from the arithmetic average height of the assessed assessed profile before grinding. The abrasion loss on the grinding wheel used the values shown in profile before grinding. The abrasion loss on the grinding wheel used the values shown in Figure4. Figure 4. Figure7 shows the loss ratio of each wheel. For the vitrified grinding wheels, the loss ratio was Figure 7 shows the loss ratio of each wheel. For the vitrified grinding wheels, the loss ratio was the smallestthe smallest for for the the wheels wheels with with the the lowest lowest hardnesshardness values. values. Moreover, Moreover, the theratio ratio was wasthe highest the highest for for grindinggrinding wheels wheels with with the the highest highest hardness hardness values.values. When When the the grain grain size size was was different different but the but hardness the hardness waswas similar, similar, the the loss loss ratio ratio did did not not show show muchmuch difference. difference. To To the the extent extent that that the theloss lossratio ratio could could be be interpretedinterpreted as theas the grinding grinding performance, performance, thethe wheels with with the the highest highest hardness hardness values values had the had best the best grindinggrinding performance. performance. The The loss loss ratioratio of the 400SP 400SP wheel wheel was was almost almost the thesame same as the as 400K the 400Kwheel. wheel. Moreover,Moreover, the the 400B 400B wheel wheel presented presented the the highesthighest loss ratio. ratio. The The amount amount of improvement of improvement in surface in surface roughness was slightly less than that for the sponge wheel; however, it was affected by the difference roughnessroughness was was slightly slightly less less than than that that for for thethe sponge wheel; wheel; however, however, it was it was affected affected by the by difference the difference in the abrasion loss on the grinding wheels. in the abrasion loss on the grinding wheels.

Figure 7. Loss ratio of each wheel. Figure 7. Loss ratio of each wheel. Figure8 shows the surface properties of the stainless steel sheet. A digital microscope (Keyence, Figure 8 shows the surface properties of the stainless steel sheet. A digital microscope (Keyence, VHX-500)VHX-500) was was used used to observeto observe the the surface surface ofof thethe st steeleel sheet. sheet. Figure Figure 8a8 showsa shows thethe surface surface condition condition of of the stainlessthe stainless steel steel sheet sheet before before grinding. grinding. ThisThis stee steell sheet sheet surface surface had had a No. a No. 1 finish 1 ﬁnish and andwas wascleaned cleaned withwith acid acid after after heat heat treatment. treatment. In In other other words,words, th thee surface surface was was etched etched with with acid. acid. Figure Figure 8b–f8 showsb–f shows the surfacethe surface conditions conditions after after grinding grinding for for thethe 180G 180G,, 180O, 180O, 240K, 240K, 400G, 400G, and and 400SP 400SP wheels, wheels, respectively. respectively. The observationThe observation point point was was set set near near the the outerouter circumferencecircumference of of the the wheel. wheel. Scratch Scratch marks marks from fromthe the abrasiveabrasive grains grains were were clearly clearly visible visible on on thethe surfacessurfaces machined machined with with the the 180G, 180G, 180O, 180O, 240K, 240K, and 400SP and 400SP wheels. Clear boundaries were present between the areas subjected to grinding and those that were wheels.wheels. Clear Clear boundaries boundaries were were present present betweenbetween the the areas areas subjected subjected to togrinding grinding and andthose those that were that were not. Moreover, few scratch marks were present on the surfaces machined with the 400G wheels. not. Moreover, few scratch marks were present on the surfaces machined with the 400G wheels.

Ground surface Ground surface Ground surface Ground surface Basis material Basis material Basis material Basis material 0.1mm 0.1mm 0.1mm 0.1mm 0.1mm 0.1mm (a) Before grinding (b) 180G (c) 180O

(a) GroundBefore surface grinding (b) 180G (c) 180O Ground surface Ground surface

GroundBasis surfacematerial Basis material Basis material Ground surface Ground surface 0.1mm 0.1mm 0.1mm Basis material Basis material Basis material (d) 240K (e) 400G (f) 400SP 0.1mm 0.1mm 0.1mm Figure 8. Surface properties of the stainless steel sheet (a) Before grinding (b) 180G (c) 180O ( d) 240K (e) 400G (f) 400SP. (d) 240K (e) 400G (f) 400SP

Figure 8. Surface properties of the stainless steel sheet (a) Before grinding (b) 180G (c) 180O (d) 240K (e) 400G (f) 400SP. J. Manuf. Mater. Process. 2020, 4, 114 8 of 13 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 8 of 14

3.3.3.3. Surface Surface Properties Properties of of Steel Steel SheetSheet afterafter Rough Grinding and and Semi-Finish Semi-Finish Grinding Grinding TheThe results results outlined outlined above above suggestsuggest thatthat it is possib possiblele to to grind grind a asteel steel sheet sheet with with only only one one grinding grinding wheel.wheel. However, However, it it was was challengingchallenging to select a a single single grinding grinding wheel wheel with with a low a low abrasion abrasion loss loss that that couldcould satisfy satisfy the the requirementrequirement of of obtaining obtaining the the arithmet arithmeticic average average height height of the of assessed the assessed profile profile of 0.3 of 0.3μµmm or or less less before before buffing. buffing. Therefore, Therefore, we weselected selected multiple multiple candidates candidates from fromthe grinding the grinding wheels, wheels, as asshown shown in in Table Table 1,1, and and performed performed grinding grinding using using th theseese wheels. wheels. To Tomaintain maintain low low costs, costs, we imposed we imposed a a constraintconstraint that that only only two two grinding grinding wheels wheels could could be used: be used: one for one rough for roughgrinding grinding and another and anotherfor semi- for semi-finishfinish grinding. grinding. First,First, for for rough rough grinding, grinding, thethe 180K,180K, 180O, 240K, 240K, and and 240O 240O grinding grinding wheels, wheels, which which had had large large amountsamounts of of shavings shavings on on the the etchedetched surface,surface, were selected as as candidates. candidates. After After rough rough grinding grinding with with thisthis grinding grinding wheel, wheel, semi-finished semi-finished grinding grinding was was performed performed using using the the 400SP 400SP wheel. wheel. Figure Figure9 9shows shows the arithmeticthe arithmetic average average height height (Ra) (Ra) and and the largestthe largest peak peak to valleyto valley height height (Ry) (Ry) of of the the assessed assessed profile profile of of the steelthe sheetsteel sheet after grindingafter grinding it with it thewith four the candidatefour candidate wheels wheels for rough for rough grinding grinding and and the 400SPthe 400SP wheel wheel for semi-finish grinding. When rough grinding was performed, the grinding wheels with for semi-finish grinding. When rough grinding was performed, the grinding wheels with smaller smaller grain sizes minimized the surface roughness. Subsequently, when semi-finish grinding was grain sizes minimized the surface roughness. Subsequently, when semi-finish grinding was performed, performed, the grinding wheel with a larger grain size minimized the surface roughness. When the grinding wheel with a larger grain size minimized the surface roughness. When machining to machining to a #180 finish, the in-feed was very deep, creating deep scratches in the steel sheet. a #180 finish, the in-feed was very deep, creating deep scratches in the steel sheet. Additionally, Additionally, the 400SP wheel could not be machined to the depth of the scratches. The grinding thewheel 400SP combinations wheel could that not becould machined reach a to surface the depth roughness of the scratches. of approximately The grinding Ra = wheel0.3 μm combinations were the that240K–400SP could reach and a240O–400SP surface roughness combinations. of approximately Ra = 0.3 µm were the 240K–400SP and 240O–400SP combinations. 2 Grinding conditions 1st grinding m

μ Spindle revolution: 12000rpm 2nd grinding A verage surface pressure: 14.7kPa

1 Surface roughness Ra 0 180K→400SP 180O→400SP 240K→400SP 240O→400SP (a) Arithmetic average height 10 Grinding c o n d itio n s 1st grinding m μ 8 Spindle rev o lu tion : 12000rpm 2nd grinding A verage su rface p ressu re: 14.7kPa 6

2 Surface roughnessRy 0 180K→400SP 180O→400SP 240K→400SP 240O→400SP (b) Largest peak to valley height

FigureFigure 9. 9.Arithmetic Arithmetic average average height height and and largest largest peak peak to to valley valley height height of of the the assessed assessed proﬁle profile of of the the steel sheetsteel after sheet grinding. after grinding.

FigureFigure 10 10 shows shows the the lossloss ratiosratios forfor rough and semi-finish semi-finish grinding. grinding. To To calculate calculate the the loss loss ratio, ratio, wewe first first obtained obtained the the arithmetic arithmetic average average height height of theof assessedthe assessed profile profile when when grinding grinding with thewith 400PVA the rough400PVA grinding rough wheel. grinding Subsequently, wheel. Subsequently, we divided we thisdivided number this bynumber the added by the abrasion added abrasion loss, which loss, we obtainedwhich we by obtained summing by thesumming abrasion the lossabrasion on the loss rough on the grinding rough grinding wheel wheel and the and abrasion the abrasion loss loss on the 400PVAon the wheel.400PVA Even wheel. though Even thethough 240K–400SP the 240K–400SP combination combination improved improved the surface the surface roughness roughness the most, it hadthe most, the lowest it had loss the ratiolowest at loss 5.19. ratio This at was 5.19. because This was the because added abrasionthe added loss abrasion was high. loss was The 240O–400SPhigh. The

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 9 of 14

240O–400SP combination had the highest loss ratio at 8.70, and its surface roughness was Ra = 0.319 J. Manuf.μm, similar Mater.J. Manuf. Process. Mater.to the Process.2020 240K–400SP, 42020, 114, 4, x FOR PEERcombination. REVIEW Here, the added abrasion loss was 0.09 9 mm. of 14 By9 of 13 comparing240O–400SP the loss combination ratio, it was had thepossible highest to loss esti ratiomate at 8.70,the combination and its surface in roughness which this was valueRa = 0.319 became smaller. combinationμm, similar had the to highestthe 240K–400SP loss ratio combination. at 8.70, and Here, its surface the added roughness abrasion was loss Ra was= 0.3190.09 mm.µm, By similar to comparing the loss ratio, it was possible to estimate the combination in which this value became the 240K–400SP combination. Here, the added abrasion loss was 0.09 mm. By comparing the loss ratio, smaller. it was possible to estimate the combination in which this value became smaller.

Figure 10. Loss ratios for rough and semi-finish grinding. FigureFigure 10. Loss10. Loss ratios ratios for for rough rough an andd semi-finish semi-ﬁnish grinding. grinding.

FigureFigure 11Figure 11 shows shows 11 shows the the condition thecondition condition of theof ofthe steel the steel steel sheet sheet sheet after afterafter grinding grinding with with with the the the 180K–400SP 180K–400SP 180K–400SP and and and240K– 240K–400SP 240K– combinations.400SP 400SPcombinations. combinations. The 180K–400SP The The 180K–400SP 180K–400SP combination combination combination left scratch left left scratchscratch marks marks marks on on theon the the steel steel steel sheet sheet surface, surface, surface, while while while the the 240K–400SPthe 240K–400SP combination combination left left the the surface surface virtuall virtuallyy scratch-free.scratch-free. Scratch Scratch marks marks should should be removed be removed 240K–400SP combination left the surface virtually scratch-free. Scratch marks should be removed by by re-buffing,by re-buffing, which which means means that that another another round round of of grinding grinding isis required.required. If Ifthe the scratch scratch marks marks are veryare very re-buffing, which means that another round of grinding is required. If the scratch marks are very deep, deep, deep,the product the product is considered is considered to tobe be defective. defective. The The lossloss ratioratio ofof the the 180K–400SP 180K–400SP combination combination was was the product is considered to be defective. The loss ratio of the 180K–400SP combination was higher higherhigher than 8.than We 8. decidedWe decided to exclude to exclude it itfrom from the the selection selection based on on the the final final finish finish condition, condition, which which than 8. Weshowed decided scratch to marks. exclude it from the selection based on the final finish condition, which showed showed scratch marks. scratch marks.

Ground surface Ground surface Ground surface Ground surface Basis material Basis material Basis material Basis material 0.1mm 0.1mm

0.1mm 0.1mm (a) 180K–400SP (b) 240K–400SP

Figure 11. Condition( aof) 180K–400SPthe steel sheet after grinding ( bwith) 240K–400SP the 180K–400SP and 240K–400SP combinations. FigureFigure 11. 11.Condition Condition of theof steelthe sheetsteel aftersheet grinding after grinding with the with 180K–400SP the 180K–400SP and 240K–400SP and 240K–400SP combinations. combinations.Subsequently, to select a grinding wheel for semi-finish grinding, the steel sheet was ground Subsequently,using the 400K, to select400N, 400B, a grinding and 400PVA wheel wheels for semi-finish after rough grinding,grinding with the the steel 240O sheet wheel. was ground using the 400K,Subsequently, 400N,Figure 400B, 12 showsto and select the 400PVA aarithmetic grinding wheels average wheel after height for rough semi-finish of the grinding assessed grinding, with profile the ofthe 240O the steel steel wheel. sheet sheet wasafter groundthe usingFigure semi-finishthe 400K,12 shows 400N,grinding. the 400B, arithmetic The and surface 400PVA roughness average wheels height of the after 400K of rough the and assessed 400N grinding wheels profile with was the higher of the240O than steel wheel. that sheet of the after the semi-finishFigure400B grinding. and 12 400PVAshows The the wheels. surfacearithmetic The roughness smallest average value ofheight thewas 400Kobtained of the and assessed for 400N the 240O–400SP wheelsprofile of was thecombination. higher steel sheet than after that the of the 400Bsemi-finish andJ. Manuf. 400PVA grinding. Mater. wheels.Process. The 2020 Thesurface, 4, x FOR smallest PEERroughness REVIEW value of was the 400K obtained and for400N the wheels 240O–400SP was higher combination. than 10 that of 14 of the 400B and 400PVA wheels. The smallest value was obtained for the 240O–400SP combination. 2 Grinding con d ition s 1st grinding m

μ S p in d le re v o lu tio n : 12000 rp m 2nd grinding Average su rface pressure: 1 4 .7 kPa

Surface roughness Ra 0 240O→400K 240O→400N 240O→400SP 240O→400B Figure 12. Arithmetic average height of the assessed profile of the steel sheet after the semi-finish grinding. Figure 12. Arithmetic average height of the assessed profile of the steel sheet after the semi-finish grinding.

Figures 13 and 14 show the loss ratios and steel sheet surface conditions, respectively. To calculate the loss ratio of each semi-finish grinding wheel, we first obtained the arithmetic average height of the assessed profile when grinding with the wheel after rough grinding with the 240O wheel. After that, we divided this number by the added abrasion loss, which we obtained by summing the abrasion loss on the 240O wheel and the abrasion loss on the tested grinding wheel. The 400B wheel exhibited a small abrasion loss and presented the highest loss ratio. However, comparing the steel sheet surfaces after grinding, the 400B wheel left several scratch marks. Moreover, the vitrified grinding wheels left almost no scratch marks on the machined surface, and the loss ratios were smaller than those of the other wheels.

Figure 13. Loss ratios of the steel sheet after the semi-finish grinding.

0.1mm 0.1mm

(a) 240O–400K (b) 240O–400N

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 10 of 14 J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 10 of 14

2 2 Grinding con d ition s 1st grinding m Grinding con d ition s

μ 1st grinding m S p in d le re v o lu tio n : 12000 rp m

μ S p in d le re v o lu tio n : 12000 rp m 2nd grinding Average su rface pressure: 1 4 .7 kPa 2nd grinding Average su rface pressure: 1 4 .7 kPa

1 1 Surface roughness Ra Surface roughness Ra 0 0 240O→400K 240O→400N 240O→400SP 240O→400B 240O→400K 240O→400N 240O→400SP 240O→400B

Figure 12. Arithmetic average height of the assessed profile of the steel sheet after the semi-finish Figure 12. Arithmetic average height of the assessed profile of the steel sheet after the semi-finish J. Manuf.grinding. Mater. Process. 2020, 4, 114 10 of 13 grinding. Figures 13 and 14 show the loss ratios and steel sheet surface conditions, respectively. To Figures 13 and 14 show the loss ratios and steel sheet surface conditions, respectively. To calculateFigures the 13loss and ratio 14 showof each the semi-finish loss ratios andgrinding steel sheet wheel, surface we first conditions, obtained respectively. the arithmetic To calculateaverage calculate the loss ratio of each semi-finish grinding wheel, we first obtained the arithmetic average heightthe loss of ratio the ofassessed each semi-finish profile when grinding grinding wheel, wi weth firstthe obtainedwheel after the rough arithmetic grinding average with height the of240O the height of the assessed profile when grinding with the wheel after rough grinding with the 240O wheel.assessed After profile that, when we grindingdivided withthis thenumber wheel by after the rough added grinding abrasion with loss, the which 240O wheel.we obtained After that, by wheel. After that, we divided this number by the added abrasion loss, which we obtained by summingwe divided the this abrasion number loss by theon addedthe 240O abrasion wheel loss, and whichthe abrasion we obtained loss on by the summing tested thegrinding abrasion wheel. loss summing the abrasion loss on the 240O wheel and the abrasion loss on the tested grinding wheel. Theon the 400B 240O wheel wheel exhibited and the a abrasion small abrasion loss on theloss tested and grindingpresented wheel. the highest The 400B loss wheel ratio. exhibitedHowever, a The 400B wheel exhibited a small abrasion loss and presented the highest loss ratio. However, comparingsmall abrasion the losssteel and sheet presented surfaces the highestafter grindi lossng, ratio. the However, 400B wheel comparing left several the steel scratch sheet surfacesmarks. comparing the steel sheet surfaces after grinding, the 400B wheel left several scratch marks. Moreover,after grinding, the vitrified the 400B grinding wheel left wheels several left scratch almost marks. no scratch Moreover, marks the on vitrified the machined grinding surface, wheels and left Moreover, the vitrified grinding wheels left almost no scratch marks on the machined surface, and thealmost loss noratios scratch were marks smaller on than the machinedthose of the surface, other wheels. and the loss ratios were smaller than those of the otherthe loss wheels. ratios were smaller than those of the other wheels.

Figure 13. Loss ratios of the steel sheet after the semi-finish grinding. FigureFigure 13.13. LossLoss ratiosratios ofof thethe steelsteel sheetsheet afterafter thethe semi-ﬁnishsemi-finish grinding.grinding.

0.1mm 0.1mm 0.1mm 0.1mm

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 11 of 14 (a) 240O–400K (b) 240O–400N (a) 240O–400K (b) 240O–400N

0.1mm 0.1mm

Figure 14. SteelSteel sheet sheet surface surface conditions. conditions.

ForFor coarse finishing,finishing, itit appearsappears that that grinding grinding wheels wheels with with higher higher hardness hardness values values are are suitable suitable for formachining machining an an etched etched surface. surface. In In contrast, contrast, for for semi-finishing, semi-finishing, grinding grinding wheelswheels withwith lower hardness values areare suitablesuitable for for machining machining a nascenta nascent surface. surfac Meanwhile,e. Meanwhile, unlike unlike the resin the bondresin grindingbond grinding wheels, wheels,the sponge the grindingsponge grinding wheels left wheels no visible left no scratch visible marks scratch on marks the grinding on the grinding surface, and surface, the loss and ratio the loss was ratioapproximately was approximately half the value half the of the value 400B of wheel.the 400B wheel. TheThe improvement improvement in surface surface roughness roughness was consider considereded to be due due to to the the multi-pore multi-pore structure structure and and elasticityelasticity of the the PVA PVA grinding wheel. In In the the case case of of a a multi-pore multi-pore structure, structure, it it is is presumed presumed that that loading loading duedue to to chips of stainless steel sheet is is unlikely to to occur, prevents an increase in frictional resistance duringduring grinding, grinding, and and has has the the effect effect of moderate we wear.ar. Since Since the the PVA PVA grinding wheel is an organic binderbinder and and has has a a multi-pore multi-pore structure, structure, when it is is pressed pressed against against a a work work material, material, the the abrasive abrasive grains grains are pushed into the binder and behave so that the abrasive grain cutting edges are aligned. Therefore, deep scratches are unlikely to occur on a ground surface.

3.4. Grinding Using a Large Wet-Grinding Machine Guided by the results obtained above, we used the large wet-grinding machine to grind large steel sheets. The size of the workpiece was 1219 mm × 2438 mm × 8 mm. The grinding conditions were changed to a spindle speed of 150 rpm, a surface pressure of 0.2 MPa, and a lateral speed of 15.4 mm/s. The grinding wheel had no dressing Figure 15 shows the surface condition and surface roughness when grinding with the 180K– 400SP and 240K–400SP combinations. The number of grinding sessions was two for rough grinding and 10 for semi-finish grinding. The figure shows the characteristics reflected on both steel sheet surfaces. However, there were various fine scratches on the steel sheet surface, and these scratches could not be removed even after ten grinding sessions with the 400SP wheel. Such scratches are prone to remain even after buffing.

(a) 180K–400SP (b) 240K–400SP

Figure 15. Surface condition and surface roughness when grinding with the 180K–400SP and 240K– 400SP combinations.

Figure 16 shows the surface condition and roughness when grinding with the 400K–400SP, 400N–400SP, and 400SP combinations; the condition after buffing is also shown. The number of grinding sessions was set to 3 for the 400K, 400N, and 400SP wheels. Buffing was performed until the steel sheet was glossy, and no scratches were found on the steel sheet surface after buffing. The surface roughness was also very low. When only the 400SP wheel was used, 15 grinding sessions were required to achieve a similar grinding surface. Though it required more and longer grinding sessions, the No. 1 substrate was machined, and the surface roughness was low, with almost no fine

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 11 of 14

0.1mm 0.1mm

Figure 14. Steel sheet surface conditions.

For coarse finishing, it appears that grinding wheels with higher hardness values are suitable for machining an etched surface. In contrast, for semi-finishing, grinding wheels with lower hardness values are suitable for machining a nascent surface. Meanwhile, unlike the resin bond grinding wheels, the sponge grinding wheels left no visible scratch marks on the grinding surface, and the loss ratio was approximately half the value of the 400B wheel. The improvement in surface roughness was considered to be due to the multi-pore structure and elasticity of the PVA grinding wheel. In the case of a multi-pore structure, it is presumed that loading

J.due Manuf. to chips Mater. Process.of stainless2020, 4 ,steel 114 sheet is unlikely to occur, prevents an increase in frictional resistance11 of 13 during grinding, and has the effect of moderate wear. Since the PVA grinding wheel is an organic binder and has a multi-pore structure, when it is pressed against a work material, the abrasive grains are pushed into the binder and behave so that the ab abrasiverasive grain cutting edges are are aligned. aligned. Therefore, Therefore, deep scratches are unlikely to occur on aa groundground surface.surface.

3.4. Grinding Using a Large Wet-Grinding Machine Guided by the results obtained above, we used the large wet-grinding machine to grind large steel sheets. The size of the workpiece was 1219 mm 2438 mm 8 mm. The grinding conditions steel sheets. The size of the workpiece was 1219 mm ×× 2438 mm ×× 8 mm. The grinding conditions were changed to to a a spindle spindle speed speed of of 150 150 rpm, rpm, a surface a surface pressure pressure of 0.2 of 0.2MPa, MPa, and and a lateral a lateral speed speed of 15.4 of 15.4mm/s. mm The/s. grinding The grinding wheel wheel had hadno dressing no dressing. Figure 1515 shows shows the the surface surface condition condition and and surface surface roughness roughness when when grinding grinding with with the 180K–400SP the 180K– and400SP 240K–400SP and 240K–400SP combinations. combinations. The number The number of grinding of grinding sessions sessions was two was for two rough for grindingrough grinding and 10 forand semi-finish 10 for semi-finish grinding. grinding. The figure The shows figure the shows characteristics the characteristics reflected reflected on both steelon both sheet steel surfaces. sheet However,surfaces. However, there were there various were fine various scratches fine on scratche the steels on sheet thesurface, steel sheet and surface, these scratches and these could scratches not be removedcould not evenbe removed after ten even grinding after ten sessions grinding with sessio thens 400SP with wheel. the 400SP Such wheel. scratches Such are scratches prone toare remain prone evento remain after even buffi ng.after buffing.

(a) 180K–400SP (b) 240K–400SP

Figure 15. 15. SurfaceSurface condition condition and and surface surface roughness roughness when when grinding grinding with withthe 180K–400SP the 180K–400SP and 240K– and 240K–400SP400SP combinations. combinations.

Figure 16 shows the surface condition and roug roughnesshness when grinding grinding with with the the 400K–400SP, 400K–400SP, 400N–400SP, andand 400SP400SP combinations;combinations; the conditioncondition afterafter bubuffingffing isis alsoalso shown.shown. The number of grinding sessions was set to 3 for the 400K, 400N, and 400SP wheels. Buffin Buffingg was performed until the steel sheetsheet waswas glossy, glossy, and and no no scratches scratches were were found foun ond the on steel the sheetsteel surfacesheet surface after bu afterffing. buffing. The surface The roughnesssurface roughness was also was very also low. very When low. only When the 400SP only wheel the 400SP was used, wheel 15 was grinding used, sessions 15 grinding were requiredsessions to achieve a similar grinding surface. Though it required more and longer grinding sessions, the No. 1 J.were Manuf. required Mater. Process. to achieve 2020, 4, x aFOR similar PEER REVIEWgrinding surface. Though it required more and longer grinding 12 of 14 substratesessions, the was No. machined, 1 substrate and was the machined, surface roughness and the wassurface low, roughness with almost was no low, fine with scratches almost observed. no fine scratchesConsidering observed. the machining Considering quality, the e ffimachiningciency, and quality, cost, grinding efficiency, with and various cost, #400grinding wheels with was various found #400to be wheels superior was when found using to be a singlesuperior 400SP when wheel. using a single 400SP wheel.

(i) Buffing (ii) Grinding 0.030µ m R a 0.130µ m R a

50mm

(a) 400K–400SP (b) 400N–400SP (c) 400SP

Figure 16. Surface condition and roughness when grinding with the 400K–400SP, 400N–400SP, Figure 16. Surface condition and roughness when grinding with the 400K–400SP, 400N–400SP, and and 400SP combinations. 400SP combinations. FigureFigure 17 shows thethe machinedmachined surfaces surfaces under under the the conventional conventional method method (dry (dry machining) machining) with with a ﬂap a flapwheel wheel and theand proposed the proposed method method (wet machining)(wet machining) with thewith 400SP the 400SP wheel. wheel. In both In cases, both ginghamcases, gingham checks checkswere reﬂected were reflected on the steel on the sheet steel surface. sheet surface. Note that Note the checksthat the in checks Figure in17 aFigure appear 17a distorted. appear distorted. Moreover, Moreover, the checks in Figure 17b are clearly visible. The proposed method appears to have caused less distortion on the steel sheet surface than the conventional method.

20mm 20mm

(a) Conventional method (b) Proposed method

Figure 17. Machined surfaces under the conventional method with a flap wheel (a) and the proposed method with the 400SP wheel (b).

4. Conclusions The study aimed to develop a practical application of wet grinding and polishing technology for large stainless steel sheets. Specifically, we evaluated wet grinding technology for machining the substrate of No. 1 finished stainless steel sheet to a ground surface. The following results were obtained: 1. A wet grinding machine for large stainless steel sheets was developed. 2. The PVA grinding wheel was suitable for grinding stainless steel sheets. 3. The PVA grinding wheel provided a good surface condition with an arithmetic average height of the assessed profile of Ra = 0.3 μm. After this grinding, the Ra parameter was 0.03 μm when buffing was performed. 4. No scratch marks or distortions were present on the steel sheet surface, and the machining quality was excellent. 5. The use of wet processing suppressed the generation of dust and prevented atmospheric dispersal, thereby improving the working environment. 6. In this paper, we described the effectiveness of the PVA grinding wheel in the grinding of stainless steel sheets. We are continuing our experiments to explain this effectiveness more logically.

J. Manuf. Mater. Process. 2020, 4, x FOR PEER REVIEW 12 of 14 scratches observed. Considering the machining quality, efficiency, and cost, grinding with various #400 wheels was found to be superior when using a single 400SP wheel.

(i) Buffing (ii) Grinding 0.030µ m R a 0.130µ m R a

50mm

(a) 400K–400SP (b) 400N–400SP (c) 400SP

Figure 16. Surface condition and roughness when grinding with the 400K–400SP, 400N–400SP, and 400SP combinations. Figure 17 shows the machined surfaces under the conventional method (dry machining) with a J. Manuf. Mater. Process. 2020, 4, 114 12 of 13 flap wheel and the proposed method (wet machining) with the 400SP wheel. In both cases, gingham checks were reflected on the steel sheet surface. Note that the checks in Figure 17a appear distorted. Moreover,the checks inthe Figure checks 17 inb areFigure clearly 17b visible. are clearly The proposedvisible. The method proposed appears method to have appears caused to less have distortion caused lesson the distortion steel sheet on the surface steel than sheet the su conventionalrface than the method. conventional method.

20mm 20mm

(a) Conventional method (b) Proposed method

Figure 17. MachinedMachined surfaces surfaces under under the the conventi conventionalonal method method with with a a flap ﬂap wheel ( (aa)) and and the the proposed proposed method with the 400SP wheel ( b).

4. Conclusions Conclusions The study study aimed aimed to to develop develop a apractical practical application application of ofwet wet grinding grinding and and polishing polishing technology technology for largefor large stainless stainless steel steel sheets. sheets. Specifically, Specifically, we we evaluated evaluated wet wet grinding grinding technology technology for for machining machining the the substrate ofof No.No. 1 1 finished finished stainless stainless steel steel sheet sheet to a groundto a ground surface. surf Theace. following The following results were results obtained: were obtained:1. A wet grinding machine for large stainless steel sheets was developed. 1.2. AThe wet PVA grinding grinding machine wheel for was large suitable stainless for grinding steel sheets stainless was developed. steel sheets. 2.3. TheThe PVA PVA grinding wheel providedwas suitable a good for grinding surface condition stainless withsteel ansheets. arithmetic average height of 3. Thethe assessedPVA grinding profile wheel of Ra provided= 0.3 µm. a good After surface this grinding, condition the with Ra parameter an arithmetic was average 0.03 µm height when ofbu theffing assessed was performed. profile of Ra = 0.3 μm. After this grinding, the Ra parameter was 0.03 μm when 4. buffingNo scratch was marks performed. or distortions were present on the steel sheet surface, and the machining quality 4. Nowas scratch excellent. marks or distortions were present on the steel sheet surface, and the machining 5. qualityThe use was of wet excellent. processing suppressed the generation of dust and prevented atmospheric dispersal, 5. Thethereby use improvingof wet processing the working suppressed environment. the generation of dust and prevented atmospheric 6. dispersal,In this paper, thereby we described improving the the eff ectivenessworking environment. of the PVA grinding wheel in the grinding of stainless 6. Insteel this sheets. paper, We we are described continuing the our effectiveness experiments of to the explain PVA thisgrinding effectiveness wheel morein the logically. grinding of stainless steel sheets. We are continuing our experiments to explain this effectiveness more Authorlogically. Contributions: Data curation, A.M.; Methodology, A.M. and A.T.; Investigation, A.M. and A.T.; Writing—original draft, A.M.; Writing—review & editing, A.M. and A.T. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Malkin, S.; Guo, C. Grinding Technology; Theory and Application of Machining with Abrasives; Industrial Press Inc.: New York, NY, USA, 2008. 2. Li, G.F.; Wang, L.S.; Yang, L.B. Multi-parameter optimization and control of the cylindrical grinding process. J. Mater. Process. Technol. 2002, 129, 232–236. [CrossRef] 3. Chatterjee, S.; Rudrapati, R.; Kumar pal, P.; Nandi, G. Experiments, analysis and parametric optimization of cylindrical traverse cut grinding of aluminium bronze. Mater. Today Proc. 2018, 5, 5272–5280. [CrossRef] 4. Gupta, R.; Shishodia, K.S.; Sekhon, G.S. Optimization of grinding process parameters using enumeration method. J. Mater. Process. Technol. 2001, 112, 63–67. [CrossRef] 5. Pi, V.N.; The, P.Q.; Khiem, V.H.; Huong, N.N. Cost optimization of external cylindrical grinding. Appl. Mech. Mater. 2013, 312, 982–989. [CrossRef] J. Manuf. Mater. Process. 2020, 4, 114 13 of 13

6. Pi, V.N.; Tung, L.A.; Hung, L.X.; Ngoc, N.C. Experimental Determination of Optimum Replaced Diameter in Surface Grinding Process. J. Environ. Sci. Eng. 2017, 5, 85–89. 7. Rana, P.; Lalwani, D.I. Parameters optimization of surface grinding process using Modified ε constrained Differential Evolution. Mater. Today Proc. 2017, 4, 10104–10108. [CrossRef] 8. Pandiyan, V.; Caesarendra, W. Tegoeh Tjahjowidodo, and Gunasekaran Praveen, Predictive Modelling and Analysis of Process Parameters on Material Removal Characteristics in Abrasive Belt Grinding Process. Appl. Sci. 2017, 7, 363. [CrossRef] 9. Hecker, R.L.; Liang, S.Y. Predictive modeling of surface roughness in grinding. Int. J. Mach. Tools Manuf. 2003, 43, 755–761. [CrossRef] 10. Omura, K.; Kunioka, S.; Furukawa, M. Product Development on Market Trends of Stainless Steel and Its Future Prospects. Nippon Steel Tech. Rep. 2010, 99, 9–19. 11. Syreyshchikova, N.V.; Guzeev, V.I.; Ardashev, D.V.; Pimenov, D.Y.; Patra, K.; Kapłonek, W.; Nadolny, K. A Study on the Machinability of Steels and Alloys to Develop Recommendations for Setting Tool Performance Characteristics and Belt Grinding Modes. Materials 2020, 13, 3978. [CrossRef] 12. Sankaranarayanan, G.; Shireesha, K.; Kennedy, A.X. Taguchi analysis in optimization of belt grinding of stainless steel 304. Int. J. Mach. Mach. Mater. 2009, 5, 41–59. 13. Rokosz, K.; Solecki, G.; Mori, G.; Fluch, R.; Kapp, M.; Lahtinen, J. Effect of Polishing on Electrochemical Behavior and Passive Layer Composition of Different Stainless Steels. Materials 2020, 13, 3402. [CrossRef] [PubMed] 14. Kim, T.; Lee, H. Preliminary Study on Fluidized Bed Chemical Mechanical Polishing (FB-CMP) Process for Stainless Steel 304 (SS304). Micromachines 2020, 11, 705. [CrossRef][PubMed] 15. Edyta, L.; Pawel, L.; Ginter, N. Electrochemical Polishing of Austenitic Stainless Steels. Materials 2020, 13, 2557. 16. Muhannad, A.O.; Eanna, M.; Barry, O.; Inam, U.A.; Dermot, B. Laser Polishing of Additive Manufactured 316L Stainless Steel Synthesized by Selective Laser Melting. Materials 2019, 12, 991. 17. Fang-Jung, S.; Chih-Cheng, H. Surface finishing of hardened and tempered stainless tool steel using sequential ball grindings, ball burnishing and ball polishing process on a machining centre. J. Mater. Proc. Technol. 2008, 205, 249–258. 18. Ohmori, H.; Katahira, K.; Komotori, J.; Mizutani, M. Functionalization of stainless steel surface through mirror-quality finish grinding. CIRP Ann. Manuf. Technol. 2008, 57, 545–549. [CrossRef] 19. Xiaokai, H.; Zhitang, S.; Weili, L.; Fei, Q.; Zefang, Z.; Haibo, W. Chemical mechanical polishing of stainless steel foil as flexible substrate. Appl. Surf. Sci. 2012, 258, 5798–5802. 20. Kao, P.; Hocheng, H. Optimization of electrochemical polishing of stainless steel by grey relational analysis. J. Mater. Proc. Technol. 2003, 140, 255–259. [CrossRef] 21. Jakobsson, K.; Mikoczt, Z.; Skerfving, S. Deaths and tumours among workers grinding stainless steel: A follow up. Occup. Environ. Med. 1997, 54, 825–829. [CrossRef] 22. Cashdollar, K.L. Overview of dust explosibility characteristics. J. Loss Prev. Process. Ind. 2000, 13, 183–199. [CrossRef]

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