Chapter - 2 FUNDAMENTALS OF FLOW FORMING
"A wise system of education will at least teach us how little man yet knows, and how much he has still to learn "
.... Sir John Luhbok Chapter - 2 FUNDAMENTALS OF FLOW FORMING
Flow Forming is comparatively a recent trend in metal working. It can be defined as a volumetric, high energy rate, rotary forming process of plastically deforming the metal to the shape of a rotating mandrel by applying considerable force through one or more power assisted rollers. The process is employed in accurate manufacture of rotationally symmetric hollow parts of any desired contour and carried out on a specialised form of lathe fitted with hydraulic drives and copying (or contour tracing) device. More recently, NC and CNC systems have been introduced to transfer the skill from the machine operator to the machine itself with a view to increase the speed, accuracy and consistency of flow formed products.
2.1 HISTORY OF EVOLUTION
Flow forming is an improved derivative of an old established sheet metal forming process known as spinning. The principle of conventional spinning might have been originated many centuries ago when the first potter's wheel was used in Egypt [1]. It is said that at the beginning of the tenth century, the art of metal spinning used to be applied in China as means of making hollow vessels from sheet metals [2]. From these distant beginnings, this art of spinning progressed gradually through the Dark ages, the Renaissance and the Industrial Revolution, - and today it provides the basis for a new technique of forming metal in cold state.
The process was introduced into Europe sometime in fourteenth century and after a period of nearly five hundred years, the craft was taken to USA in or about 1840 [3]
In its beginning, the art was carried out on relatively simple equipment using stick type wooden hand tools for manufacturing almost exclusively the useful hollow wares
5 and similar items, mostly domestic utensils and general household articles such as water mugs, bowls, vessels etc. [4]. This hand spinning method was practised only on thin gauge materials of softer grades such as copper base alloys, and a very high degree of skill and considerable experience were required of the operator [4].
Subsequently for spinning thicker gauge materials requiring greater power to drive the machine and application of greater force to the tool by the operator, provision was made for some form of power assisted tools (rollers) to supplement the strength of the operator, - and such was the beginning of Power Spinning. With the development of machinery during the industrial revolution of 19th century, means were available to spin metal with power assisted rollers, the first known development being a German Patent in the late 1860's[5].
Use of adequate power to assist the forming rollers provided the means for plastic deformation of metal to aid in obtaining desired shape and extensive development work took place in the technique of heavy duty power spinning almost simultaneously in several countries, namely, Sweden (since 1930), Germany (1940), England (1946) and USA (1947) in order to meet highly stringent demands of accuracy, finish and improved mechanical properties of many vital rotationally symmetric hollow parts required in various fields of industry, particularly in the field of gas turbine engine, aircraft, atomic & space research and armament and defence besides many general industrial applications. This resulted in appearance of a new metal working technique that referred to here as Flow Forming [5,6].
The machine tool industry varied design features of lathes and vertical boring mills with hydraulic and electronic controls to produce the flow forming machine in use today [5]. Several machine designs were marketed concurrently and a variety of names were given by various machine-makers or by the countries practising the process, such as power shear spinning, hydrospinning, flow spinning, roll forming, spin forging, shear forming, flow turning, flow forming, etc [4, 7], Of these names, the popularly adopted ones are flow turning and flow forming.
6 Eventhough a derivative of old established conventional spinning, the technique of flow forming today is capable of producing parts to such a close limits of accuracy, precision and surface finish that can never be dreamed of in the parent process of orthodox spinning. This is a typical example of the offspring outgrowing the parent.
2.2 PHYSICAL CONCEPTS & DESCRIPTION OF THE PROCESS [1, 2, 7, 8]
As already mentioned, several names are given to the process of flow forming by various machine manufacturers or by the countries practising the process, such as power shear spinning, hydrospinning. flowspinning, roll forming, shear forming, spin forging, flow turning, flowforming etc. Out of these names, the author feels that the name "Flow Forming" is best suited since it neither connotes a particular manufacturer, nor it indicates any cutting operation and is adequate to describe the process in which the workpiece metal is plastically deformed to flow into a new form. However, to remove the confusion, it would be better to clear the concepts of two associated processes, namely :-
(a) Conventional Spinning (b) Flow Forming
2.2.1 Conventional Spinning
In conventional metal spinning, a flat circular sheet metal blank of diameter greater than that of the finished part is held by pressure between a small disc mounted on the tail-stock and a mandrel mounted on the head-stock of a spinning lathe. The mandrel is rotated alongwith the metal blank and the pressure is applied on the unsupported portion of the rotating blank either manually by a stick type hand tool supported by hinged bracket in the case of Hand Spinning (Fig. 2.1), or by hydraulic power applied through the roller in the case of Power Spinning (Fig. 2.2) in order to deflect the unsupported portion of the blank metal and gradually force it over the rotating mandrel by a series of strokes. The number of strokes or intermediate passes of the roller depends on the thickness and ductility of work material and also on the ratio of the supported to unsupported diameter of the blank. In both hand and
7 [HEAD STOCK
\ POINT
Fig 2.1 :Schematic sketch showing conventional hand—spinning of a hollow metallic conical part
3 FLAT BLANK
Fig.2.2 : Schematic sketch showing conventional power spinning of a hollow metallic cone
9 power spinning, there is no intentional reduction in the thickness of the blank metal and during the spinning process the metal-flow follows nearly the laws of equal surface area, i.e. the superficial area of the finished spun part is same as the area of the blank from which it is made. The limiting factor is the ratio of the blank diameter to the finished part diameter which should not exceed the value of 1.5, otherwise the circular blank inclines to form folds during spinning.
2.2.2 Flow Forming
Flow forming is a further development of conventional power spinning. In flow forming, a metal blank or preform of the same diameter as the maximum diameter of the finished part is formed over a rotating mandrel. While the blank metal and the mandrel (which are locked together) rotate, the forming roller follows the mandrel contour maintaining a preset gap. This gap between the mandrel and the roller acts as an orifice through which the metal is flowed (extruded). In this manner, the blank or preform metal is plastically deformed to mandrel contour and proper (reduced) wall thickness following nearly the principle of equal volume. Fig. 2.3 shows schematically the flow forming of a simple conical part from a flat round blank metal.
Compared with drawing operation in which the part is formed as a whole, the part in flow forming is shaped by increment forming. The process must not be confused with swaging or upsetting operations, since the metal is not reduced in diameter as in swaging or gathered to increase thickness as in upsetting. In a sense, it utilises a 3-dimensional variation of the basic rolling process and combines rolling, shearing and bending in one operation [3] and essentially it is a point deformation metal forming process.
2.2.3 Comparison with Conventional Spinning
Upon first consideration it may appear that the process of flow forming is quite similar to conventional power spinning process because both the processes have certain commonalities in that they are normally cold forming methods, they are chipless machining or forming
10 :HEAD STOCK —, —- THICKER FLAT _ | -TBLANK
Fig.2.3(a) : Flow forming of a hollow conical part schematically shown
11 STARTING MACHINE HEAD STOCK POSITION
The upper heilf indioatea starting position with metal blank before formliig, -while the lover half •hoira the end position after flo-w forming the coni Fig.2.3(b) : Scheme for flow forming the hollow pointed cone.
12 operations carried out on specialised form of a lathe and the product formed by both the processes are generally conical, tubular and curvilinear shaped or their combinations having rotational symmetry. But similarities end here only and it must be understood that there are some basic differences between the two processes. Some distinguishing characteristics of flow forming as compared to conventional power spinning are outlined as under: -
(1) While in conventional spinning, each element of the blank metal undergoes radial displacement from its original position, there is no appreciable change in the radial position of each element of the finished part from its initial position in the blank in the case of flow forming. (2) Unlike the conventional spinning in which the blank diameter is considerably greater than the finished part diameter, the blank diameter in flow forming is exactly same as that of the finished part overall diameter. (3) Whereas there is practically no intentional thinning of metal in conventional spinning, a large reduction in metal thickness is accomplished in flow forming. This means that the metal for the finished part in flow forming is obtained not from the diameter of the blank as in conventional spinning, but from the thickness of the blank. (4) The metal flow during conventional spinning follows nearly the law of equal surface area, the superficial area of the blank metal and the finished part being equal, but the metal flow in flow forming follows mainly the principle of equal volume, the surface area of the finished part being more than that of the blank metal. (5) In contrast to the conventional spinning which is essentially a bending and flaring operation, the flow forming is a point deformation squeezing operation in which the metal undergoes plastic deformation in a very small region of work material in contact with the forming rollers. (a) CONICAL (l») TUBULAR
n
(e) CURVILINEAR
Fig.2.4 : Three basic catagories of parts produced by flow forming process
14 2.3 PROCESS CHARACTERISTICS [9, 10, 11, 12, 13]
The parts produced by flow forming process may be broadly divided into three basic categories, namely, conical shapes, cylindrical shapes and curvilinear shapes, or combination of these as shown in Fig. 2.4.
2.3.1 Conical Shapes
The basic shape on which the flow forming process is founded is the cone. Based on this conical shape, simple formulae have been evolved to determine the thickness of the flat circular blank required to produce a desired wall thickness of the cone or alternately, to determine the wall thickness of the cone which will result from a given thickness of the blank. The formula known as the "Sine Law", is expressed as
t = T sin a or, T = (see Fig 2.5) sin a
Where T = thickness of the blank t = wall thickness of the conical part a = angle subtended by the wall of the cone and the centre line, i.e. one half the included cone angle.
Fig. 2.5 also provides an explanation as to the axial displacement of the material in the process and illustrates that the original thickness of the flat blank is transposed parallel to the centre line. The limiting value of the angle a is generally accepted as 15° i.e. a cone with an included angle upto 30° can be flowformed. For producing cones of smaller included angles than 30* a second flowing operation will be required or, alternately, a pre-machined blank may be used The necessary formula may be written as
15 Fig.2.5 : 'Sine latv' as applied in flow forming of cone
16 f\
n
SECOND STAGE FIRST STAGS BLANK
TJ—Tjptxijcx/mixijS
Fig.2.6 : Method for second stage flow forming
17 T2 sin a Tl= . ~ (see Fig 2.6) sin p where Ti = thickness of the wall at the first flow form or preform stage.
T2 = wall thickness of the final produce (cone shaped) a = one half of the included cone angle of the first flow formed cup blank or preform P = one half of the included cone angle of the final product
From the forgoing formula, the following mteranees may be drawn :-
(i) The depth of any conical part produced by flow forming is not only dependent on the blank diameter (as in deep drawing or conventional spinning), but also on the blank thickness. The blank diameter is, in turns, dictated by the major (overall) diameter at the open end of the finished cone (including any flange) to be flowformed.
(ii) The included angle of the finished conical part and the thickness of flat starting blank (for any specified normal thickness of the wall of the finished part) are dependent on each other. However, the bottom thickness as well as the axial thickness of the finished cone remain same as the original thickness of the starting blank.
(iii) It is not possible to reduce the metal thickness on conical parts other than in a single pass of the forming roller at the same angle. To carry out more than one flowing operation, if necessary, additional mandrels will be required with tapers to the newly calculated included angles and the machines must be reset for these angles.
The conical parts with varying wall thickness can be produced by using proformed blanks as shown in Fig 2.7.
18 PREFORMED BLANK
lis VARYINg WAT.T, THICIQ>rE3S Tfe j& 11 — Tj •in« t-2— T2 «iia.a
Fig.2.7: flow forming of a conical part with varying wall thickness
J 9 2.3.2 Cylindrical Shapes (From Flat Circular Blanks)
Based on the two preceeding formulae, it would be a logical assumption that flow forming of parallel walled cylindrical parts from the flat starting blank is an impossible task, as the sine of zero degree is zero which means that a blank of infinitely large thickness would be required to produce even an infmitesimally thin wall thickness of the finished part. However, the flow forming of parallel walled cylindrical parts from the flat circular blanks can be accomplished by first forming a shallow cup and then flowing operation can be carried out at a single pass by using a specially designed dual purpose forming roller provided that the following conditions are complied with.
(a) d/D> 0.60, and (b) T/D>0.01
Where d = inside diameter of the finished cylindrical part D = diameter of the initial flat blank T = thickness of the flat blank
The relationship for combined cupping (by spinning) and cylindrical flow forming is shown in Fig. 2.8.
2.3.3 Cylindrical Shapes (From Cylindrical Blanks)
Cylindrical shapes or tubular parts with thin wall can be flowformed by starting with a cylindrical blank (preform) machined, extruded, fabricated or forged which are shorter in length and greater in thickness than the finished parts, but with the same basic inside diameter This particular process, also called sometimes as "Flow Turning" or "Tube Spinning", does not follow the sine law as applied in the flow forming of conical parts but depends primarily on the volume displacement relationship. The formula in common use for determining the size of the cylindrical blank may be written as L.T.d.t. = \X.AX
20 ^
d T % 0.6 * 56 O.Ol D D -Ik Fig.2.8 : Relationship for combined cupping (by spinning) & cylindrical flow forming
2] SINE LAW NOT APPLICABLE DEPENDS ON VOL. DISPLACEMENT RELATION L.T.( ^ t T Fig.2.9 : Volumetric relationship in flow forming of cylindrical shapes 22 It (d . t) L = x (see Fie. 2.9) T (d.T) v & ' Where L = required length of the cylindrical blank 1 = length of the finished cylinder d = inside diameter of both cylindrical blank and finished cylinder T = wall thickness of cylindrical blank t = wall thickness of finished cylinder The ratio (d.t) / (d.T) is a correction factor which can safely be ignored when flow forming large diameter cylinder with very thin walls. Flow forming or flowturning of cylindrical or tubular parts may be accomplished by one or both of the two methods, namely, "forward flowing" or backward (reverse) flowing". Fig. 2.10 illustrates the technique of forward flowing (drawing type) where the metal is flown ahead to the forming roller and in the same direction as the roller feed. During forming the deformed metal is in tension and the undeformed metal in front of the roller is essentially stress free. This technique provides excellent control of length especially since the metal is moved only once during the flowing pass. Fig 2.11 illustrates the technique for backward or reverse flowing (extruding type) where the metal is extruded beneath the forming roller and in opposite direction of the roller feed. The underformed portion ahead of the roller is in compression and the deformed metal is essentially stress free. This method saves considerable time as shorter roller feed is required to form the same finished length. Other benefits include shorter mandrel, longer parts than the machine travel and simpler tooling The flow forming of cylindrical parts is generally a multipass process in which the part becomes thinner and longer with each pass of the forming rollers. There is no mathematical formula for optimum deformation However, as a general rule, a machine with 23 * Metal tlovrm ah«ad of roller- In. mam i direction ae roUer- feed. Sxoellant control of langth, metal being ra.O'voA only oxioe ixx f ollo-frlrxa paaa f=^^^ E3- I I Znl /"A FEED Fig.2.10 : Technique of forward flow forming of tube 24 Metal la extruded beneath roller in opposite direction of roller feed. Shorter mandrel, pert longer than the slide travel, time savins method Ln i i B • ' W 71 FEED Fig.2.11 : Reverse flow forming technique for tubular parts * Gradual tiilnninfj of TVO.11 * Amount of deformation defined fay aiaxne Sine 1 * Contoui—-tracing device -with template required (to synchronise axial and radial feed) VARYING TTAUL THICKNESS UNIFORMLY .THICK BLANK ti —T eina t2 -T -in,. t- -T sa< ^7 Fig.2.12 : Flow forming of curvilinear (spherical) shape of varying -wall thickness from flat blank of uniform thickness 26 1*! — t/elncc! T3 -t/ilnog FLOTTFORMED PART UNIFORM "WALL THICKNESS Fig.2.13 : Flow forming of curvilinear (parabolic) shape of constant wall thickness from a contoured blank 27 a single roller is capable of taking 20% to 30% reduction in thickness per pass while the machines with two or more rollers may take 40% to 60% reduction per pass. Total reduction in thickness which is possible upon a given part is dependent primarily upon the percentage reduction and the work hardening characteristics of the material of the part. 2.3.4 Curvilinear Shapes Curvilinear shapes (parabolic, half elliptical, hemispherical etc) can be flow formed either from flat blanks resulting in a gradual thinning of wall (Fig. 2.12) or from the taper/contoured blanks giving a constant wall thickness of the finished part (Fig. 2.13). The optimum amount of deformation in a given part is defined by the same sine law. In flowforming of curvilinear shapes, a contour-tracing (hyro-copying) device or attachment with suitable template to synchronise the axial and the radial feeds of the forming rollers is required to be used. Today it is easily done on CNC flow forming machines. 2.4 PROCESS PARAMETERS The objective parameters or end results characterising the process such as flow formability, improvement in mechanical properties, dimensional accuracy and surface finish as well as power requirement of the machine are influenced by various controlling input parameters as outlined under : 2.4.1 Process Variables (a) Surface speed of forming (normally ranges from 300 to 600 m/min). (b) Longitudinal feed or forming roller axial travel rate (normally in the range from 0.5 to 1.25 mm per revolution). (c) Deformation rate or percentage reduction in metal thickness per pass (should not exceed 75%). (d) Coolant and lubricant used at the interface of the workpiece and the rollers 28 2.4.2 Preform or Blank Quality (a) Preform symmetry (should be within 0.06 mm). (b) Thickness and thickness variation (variation should not exceed 0.05 mm). 2.4.3 Tool Geometry (a) Mandrel shape, accuracy and finish (b) Edge radius (r) and contact angle (5) of the forming rollers (normally r = 2-2.5 times thickness reduction, 8=15° to 40 ). 2.4.4 Work Material (a) Hardness, strength and ductility (b) Purity and cleanliness (c) Microstructure 2.5 ADVANTAGES OF FLOW FORMING 2.5.1 Saving in Material Flow forming, a volumetric forming process, "moves" rather than "removes" material. Since the scrap losses in the form of chips are practically eliminated, tremendous savings in material are realised. Simple blanks can be axially flown into complex forms eliminating, in many cases, the need for expensive machining, welding or deep drawing operations. 2.5.2 Improved Part Quality Cold working during flow forming refines, elongates and reorients the grain structure of the finished parts in the direction of flow (parallel to the part contour), resulting in substantial increase in the tensile strength, yield strength and hardness In some cases, part 29 strength is increased as much as 100%, thus offering the opportunity of replacing expensive high strength material with lower cost material. Moreover, flow formed parts have high resistance to fatigue failure. Despite high forming pressures employed in the process, the surface have no ruptures, tears or fractures 2.5.3 Good surface finish & dimensional accuracy Surface finish on the inside diameter of a flowformed part is a direct reflection of the mandrel finish but the surface finish on outside diameter, depending upon various controlling parameters or variables, generally range from 0.8 to 1.6 microns which is comparable to commercial grinding finish. The process is capable of producing ID. and wall thickness tolerances of ± 0.05 mm. However, the stable repeatative accuracy of the work is depended to a large extent on the tolerances of the starting blank or preform, the accuracy of the mandrel, the rigidity of the machine and how closely the forming roller follows the mandrel 2.5.4 Low tooling cost The basic tooling for flow forming consists of a mandrel, forming roller and tracing template. This simplified tooling generally cost a fraction of what is required for conventional metal forming process (e.g. about one-tenth of the tooling cost for deep drawing operation), and has a long life under normal operating conditions. 2.5.5 Less finished scrap Serious flows or foreign inclusions in the workpiece material are often revealed during flow forming operation. This early discovery in material imperfection reduces further processing which is a decided saving since the flaw or even part failure, in case of other processes, would normally not be detected until final inspection. 30 2.5.6 Design Freedom Flow forming offers a greater opportunity to the creative designer and development engineer for alternate practical solution to his design. The combination of advantages such as material saving and low tooling cost promotes experimental and prototype work by flowforming. 2.6 LIMITATIONS OF FLOW FORMING Some of the major limitations of the flow forming process are as follows : (a) The process is restricted to the manufacture of hollow axisymmetrical parts. (b) The thickness of the metal that can be flowformed today is limited to 25 - 30 mm with about 60% reduction in wall thickness per pass. (c) Spring back of material after flow forming may be a problem which can be minimised by increasing the roller feed rate, thereby compromising with desired surface finish of the part. 2.7 APPLICATIONS OF FLOW FORMING Prior to its wide spread adoption in about the year 1953, flow forming was used principally in the manufacture of utensils, kitchen ware, precision television tubing etc., which were of thin sections. Now much heavier sections are flow formed easily. Rocket motor tubes, warhead casing, cartridge cases, which were hitherto manufactured by conventional spinning and other production processes, are now being produced by flow forming technique. In aeronautical industries, certain critical items of jet engines are made by this process. For space research, sections and large size parts are being flow formed, and flow forming process has been playing an important role in the manufacture of critical components for defence, aerospace as well as industrial applications. For space research applications where strength to weight ratio is of utmost importance, the application of flow forming is inescapable 31 It is not possible to list out in detail the applications of flow forming because the types of components that can be effectively manufactured by this process are enormous. It is sufficient to mention a few as in Fig. 2.14 just to give an idea. 2.8 FLOW FORMESG MACHINES [3, 4] A flow forming machine, in its simplest form, may be considered to be a specialised form of lathe fitted with hydraulic drives, forming rollers and contour tracing devices. More recently CNC systems have been introduced to transfer the skill from the machine- operator to the machine itself with a view to increase the efficiency of operation as well as the accuracy and precision of the flow formed products. All machines for flow forming of metals work on the same basic principle but may differ in design from one machine-manufacturer to another. Fig. 2.15 shows a schematic line diagram of a horizontal flow forming machine which consists of the following main units: 1) A sturdy & rigid base on which are mounted all the important units like headstock, tailstock, carriage, main bed, etc. 2) Headstock complete with drive unit 3) Tailstock & hydraulic unit 4) Carriage main Bed/slide way 5) Carriage (with separate hydraulic power pack) which carries the roller heads fitted with forming rollers. The most commonly used machines are of horizontal type. However, one major drawbacks of the horizontal models is the very large floor space requirement. Therefore, some manufacturers have gone for vertically designed machines to conserve valuable floor space. Ofcourse there are certain components, which for various reasons would not be practical to be flow formed in vertical orientation, and for these components, vary large honzontal type machines are suitable 32 Fig. 2.14 : Some typical components produced by flow forming process 33 Flow forming machines are available with single forming roller, or two forming rollers, or even three rollers, the selection being dependent on the types of components to be produced. As a guideline, for simple axi-symmetric components requiring not very high accuracy & consistancy, the single roller machines can serve the purpose. However, for high degree of accuracy and surface finish as well as for maintaining tight geometrical tolerances, the two-roller or three-roller machines are more preferable respectively for flow forming of conical parts and cylindrical parts. Fig. 2.16 to Fig. 2.18 show the pictorial views of a number of flow forming machines of different types and models made by various manufacturers of the world, whereas Tables 2.1 to 2.3 list out their important technical specifications. Table 2.1 : Technical Specifications of Flow Forming Machine Make : M/s. Kieserling & Albrecht Gmbh & Co., Solingen, Germany Technical Data Model Model Model AS 23.40 AS 23.60 AS 24.50 Machine Type Horizontal, Two Rollers, CNC Max. Parts Dia (mm) 500 900 1000 Drive Motor : (i) Main Spindle (kW) 45 110 74 (ii) Hydraulic (kW) 30 74 45 Spindle Speed (rpm) 120 90 90 (Variable thro' DC drive motor) to 1200 to 900 to 900 Feed Force : (i) Longitudinal slide (fcN) 200 250 240 (ii) Cross slide (kN) 120 500 180 Table 2.2 : Technical Specifications of Flow Forming Machine Make : L&F Industries, USA Machine Type Horizontal, Single Roller Machine Size (inches) 26x20 45x20 60x30 Work swing (inches) 26 45 60 Work length, maxe (inches) 20 20 30 Travel main-slide (inches) 20 20 30 Travel cross-slide (inches) 14 22 30 Spindle drive motor (h.p) 10 20 30 Force per slide (lbs) 4000 8000 8000 Net weight (lbs) 6500 17000 14000 34 P-RTV-TC MOTOR WORK MKTAL CARRIAQE WITH ROT.T.FiR HEAD Fig.2.15 : Schematic sketch of a two—roller flow forming machine 35 Fig. 2.16 : A Single Roller Flow Forming Machine (Leifeld & Co, Germany) Fig. 2.17 : A Three Roller Flow Forming Machine (Leifeld & Co., Germany) 36 Fig. 2.18 : A horizontal two rollers CNC flow forming machine & its control panel (M/s. Kieserling Albrech Gmbh & Co, Germany Model 23.40) 37 Table 2.3 : Technical Specifications for Leico Range of Flow Forming Machines Make Leifeld Und Co., Germany Type Horizontal, Auto/CNC, One/Two/Three Rollers Parts Diameter (mm) 30/400,60/560, 100/650 Longitudinal slide travel, max (mm) 400,800, 1300,2400 Feed Force (longitudinal & cross) (kN) 70,90, 150,300,400 Motor Rating (i) Main (kW) 22, 30, 75, 90, 132 (ii) Hydraulic (kW) 18.5,30,55,120 Speed Range : (I) 9-12 steps (rpm) 36-365, 195-975,200-1200 (ii) Infinitely variable (rpm) 220/820, 165/500 Overall Weight (kg) 9000, 11000, 15000,32000 2.9 MECHANICS OF FLOW FORMING OF CONES Metal working processes may broadly be classified into two basic categories. These are : (1) Chip forming; i.e. conventional machining (2) Chipless forming, i.e. metal forming In the first category of processes, the shape of the workpiece consisting of a metal or alloys is altered by removing unwanted metal from the workpiece, whereas in the second category of processes, the workpiece shape is altered by plastic deformation of the workpiece metal. Thus, in the chip forming or conventional machining processes, it is mainly the waste metals or chips that are plastically deformed, whereas in the chipless forming or simply metal forming processes, it is the workpiece that is subjected to plastic deformation. 2.9.1 Classification of metal forming processes Numerous metal forming processes are now employed in metal working industry and there is no general agreement in their classification. One of the best ways to classify them is based on the states of stress according to which the processes may be grouped into three categories [16] : 38 Table 2.4 : Classification of Major Metal Forming Processes (A) SQUEEZING MECHANISM APPROX, INSTANTANEOUS FORMING PR0CES5 5R. STATE OF ZONE OF 5TRTE OF'"/.- No STRESS RT DEFORMATION DEFORMATION NAME DIAGRAM 50URRE (CR055 HATCHED) 1 CLOSED DIE V/HOLE N0N5TEPDY FORGING *§& fsf- PRRT (DROP OR PRESS: rT WHOLE COINING NON5TERDY PART WHOLE UPSETTING N0N5TERDY PRRT (OPEN DIES) Wc UPSETTING WHOLE All t NONSTERDY (CLOSED DIES! ULi ~&- PRRT EXTRUDING ZONE FLATS I ROUNDS NEPR STEADY roS J7.Aw&C -^ (DIRECT OR FORVflflQ) ORIFIC EXTRUDING ZONE FLATS & ROUNDS' '7//. '/, S NERR STEADY (1H01RF.CT OR nnCKVPRO) liir ORIFIC EXTRUDING 1 ZONE HOLLOW SHAPES J=4 UNDER NONSTERDY (REVERSE CANNING) '/V V/r PUNCH ROLLING FLATS, ZONE ROUNDS I OTHER UNDER STEADY SHAPES (V1TH0UT fe er FRONT I FJflCK PULL) •*&• ROLLS FLOW FORMING I ZONE OF TUBES r •Cf UNDER STEADY (EXTRU01MG TYPE) ROLLER FLOW FORMING tfi. ZONE OF TUBES > UNDER STEADY (EXTRUDING TYPE) £? ROLLER ZONE FLOW FORMING UNDER 5TFADY OF CONES W '"I POLLER ZONE 0 SWAGING AND UNDER NONSTERDY KNEED ING • \3" / TOOLS Table 2.4 : Classification of Major Metal Forming Processes (B) DRAWING MECHANISM FORMING PROCESS RPPROX. IN5THNTANE0U5 SR. 5TRTE OF ZONE OF STATE OF No. STRESS RT DEFORMATION •'• DEFORMATION NAME DIAGRAM 5QURRE ICR055 HUTCHED) WIRE & BAR ZONE IN -E| STEADY DRRWING •&- DIE 1_ ZONE IN TUBE DRRWING STEADY -^ DIE "^ DEEP DRAWING WHOLE OF CUP5,B0XE5 NON5TERDY wk & FLANGE I OTHER SHAPES m EMBOSSING L* ZONE IN (HL50 BENDING DIE NONSTEADY Ex DIMPLING) ~£p- CAVITY < r MUUUEIl ZONE IN 1 jf BULGING /A •• J\ /// DIE NONSTEADY CAVITY ZONE ROLL /7 A. BETWEEN STEADY FORMING V te "V V ^6T R0LLE5 Table 2.4 : Classification of Major Metal Forming Processes (C) BENDING MECHANISM APPROX. IN5TRNTRNE0U5 FORMING PR0CE55 5R. STATE OF ZONE OF STRTE OF No. 5TRE55 RT OEFORMflTION DEFORMATION NAME DIAGRAM SOURRE (CR055 HnrCIIED) IN BEND ' 1 STRAIGHT LiLL.^ AND IN N0N5TEADY FLANGING ^~ FLANGE 2 STRAIGHT IN BEND FLANGING AND IN NON5TERDY (CONCAVE ^ -r FLANGE FLANGES ) 41 (a) Squeezing operations in which materials are principally subjected to compressive loading. (b) Drawing operations in which materials are principally in tension. (c) Bending operations in which moments must be applied, thereby inducing tension on one side of the part while compression on the other side. Table 2.4 illustrates the major metal forming processes under the above classifications. 2.9.2 Category of Flow Forming Flow forming is one of the recent fttocesses- falling under the category of squeezing operations. Instead of forming a part as a whole (as in deep drawing operations), the part is shaped in flow forming by incremental forming. Thus flow forming is essentially a 'point deformation' metal forming process. The deformation takes place in a small region where the roller contacts the workpiece, i.e. the instantaneous zone of plastic deformation is the zone under the roller with a steady state of deformation as shown at serial No. 8 under Squeezing Mechanism in Table 2.4. 2.10 REVIEW OF THEORETICAL ANALYSIS & MECHANICS OF FLOW FORMING Because of the difficulty of applying exact mathematical methods to the problems of interest in flow forming technology, only approximate methods of solution are employed in analysing the mechanics of plastic deformation in flow forming. There are a number of approximate methods for treating the flow forming problems, but none of the solutions is perfect because of the fact that by necessity assumptions are required that may only very generally describe the physical behaviour and the mechanism of the system. Furthermore, some solutions will give average stresses and strains whereas others give local distribution. In fact, the precise state of stress and mechanics of plastic deformation in flow forming is still ill-defined. Thus in the analysis of a flow forming problem, no categorical guidance can be given, although some useful hints and tips are often possible to be obtained from several research papers appeared on the subject. It appears that flow forming of hollow axi- 42 symmetric sheetmetal parts has no doubt aroused considerable interest among the research workers since more than three decades Paulton & Colding [18] considered flow forming of hollow cone as a combination of bending and rolling process, whereas Kalpakcioglu [19] assumed a simple shear mechanism for the theoretical analysis. The complex straining effect was introduced into the solution by Avitzur and Yang [20] and also by Kobayashi et al [21], More recently, Slater [22] has given approximate upper bound estimates for the dimensionless tangential force component. 2.10.1 Deformation Energy Method Deformation mechanism in flow forming of cones is shown schematically in Fig. 2.19. The initial blank thickness (T) and the final wall thickness of the cone (t) for the half-cone angle ( a ) are related by the equation t= Tsina (2.1) In view of the complexity of the actual deformation process, several analytical solutions for basic quantities such as forces or power requirement in flow forming based on varying approximation and simplification has been proposed. All these solutions utilise the deformation energy method. Ideal Process : Kalpakcioglu [19], assuming the flow forming of sheet metal cone as a simple shear mechanism, proposed an idealised process as shown in Fig. 2.20. The analysis is like the model of a deck of cards sliding over each other which has been widely used in metal cutting with a single point cutting tool. The difference here is that the elements in Fig. 2.20 are concentric hollow cylinders sliding over each other axially and forming a hollow cone while at the same time fulfilling the requirements of eqn (2.1). It can be easily seen from this figure that the shear strain is the distance an element moves axially divided by the thickness of the element. Therefore, the expression for the shear strain is 43 R I) t 0 II 0 • 0 L c ? u a *- 0 ' 0 0 0 c » C I II I I II I g 0 L IO DO 0 U GO 2- O rd CO o O ^ ^ ° o 2 Bo* to 5 SB «t-i w So O O cy o WT3 cvi 44 y = cot a (2.2) This is for the case where the flange is straight, i.e. 8 = 90°. If the oiriginal blank is preformed such that the preform half angle 8 has a value between 90° and 0°, the expression for the shear strain becomes y = cot a - cot 6 (2.3) Kalpakcioglu [24] has further shown that strain rates occuring during average cone forming conditions vary from 102 to 6.5 x 102 per second depending on the roller corner radius or 'lead-in'. These rates approach that experienced in impact loading. As a result, actual allowable deviation from the 'sine law' is quite limited. From eqn. (2.3), it can be easily seen that for the same mandrel half angle a, the greater the preform angle the less will be the value of shear strain. For this ideal case, the roller cones radius r is taken as zero. For a strain hardening material, shear stress x will be a function of shear strain y. Furthermore, since T Vs. y curves of materials are more scare than a Vs..8 curves, it would be better to use the relationship between y and s from the deformation energy theory which may be written as y = V3 E y l or e = ~ = - (cot a - cot 6) (2..4) V3 3 where s is the principal true strain in a tensile test. If closely examined, it will be noted that the axial thickness of the elements in the region under roller changes. However, the measurement of actual parts flow formed with different values of r (roller corner radius) have shown that the variation in axial thickness from the original blank thickness is within ±10% At this point, it would be interesting to see 45 how well the idealised process agrees with the actual flow pattern of a metal formed by the process. Kalpakcioglu [26] who used the methods of plugged holes and grid line techniques, observed that the original blank is twisted about the axis of the mandrel during flow forming process. This distortion is not uniform in the thickness direction, hence the result is a twisting of all radial cross-sectional planes. Force Analysis : One of the most important parameters in flow forming process is the power required to carryout a particular forming operation. It has been experimentally observed that while the axial and radial forces in flow forming may be much larger than the tangential force component, i.e. torque, it is the tangential force that is responsible for most of the power consumption. The external work input during flow forming of a hollow axi-symmetric cone can be closely approximated by F, . dl, where F, is an instantaneous tangential force component acting on the forming roller and dl is the distance of contact between the roller and the cone during an infinitesimal time interval dt Equating the external work input to the work of deformation permits the following energy balance : f (fj)dl = dv jad^ (2.5) where a and z are effective stress and infinitesimal strain respectively, and dv is the infinitesimal volume of the cone being flow formed. Therefore the eqn (2.4) can now be written as = T sin a f J a ds = or (Fy T . f. sin a .om . e (2 6) 46 where T is the initial blank thickness, f is the roller feed rate and a is one-half the cone angle. The mean effective stress am in eqn. (2.5) should be evaluated at the total effective strain s achieved. Kalpakcioglu [ 19] determined the effective strain by assuming simple shear deformation under roller and has given the expression for tangential force Ft as under c T e - COt a F, = T»i» sinaon V^ 1 i.e. F, = —7= T • f • o" • cos a (2.7) V3 where effective strain does not include redundant straining. Avitzur and Yang [20] used a more sophisticated analysis in determining the strain rate under the roller. They set up the strain rate tensor components from the instantaneous velocities under the circular tool. They neglected, however, any redundant straining caused by repeated passage of the roller over the same metal when the feed is smaller than the extent of the plastic zone. They derived the following equation for the tangential force F, as Vl + 5 F, = ( 2 n Ro f cos a + A ) (2.8) 2 « V5 •7t»R0 Ro = instantaneous cone radius at contact with roller f = roller feed a = half the cone angle 1 = square strain rate ratio R 47 A = JA ^ d9 \cnJ a = mean flow stress m n = fraction of revolution swept out during time interval A t T = initial blank thickness The quantity 5 involves strain rate ratios which are assumed to be independent of R and 9 and, together with oZ/cn, can be obtained from the approximate equations of a torus if the roller is assumed to have this shape. If the strain rates — (duz I 59) are R neglected, then 6 is zero, and similarly, if A is assumed to be small compared with 2 n R<, f cos a then the force eqn (2.8) due to strain rates — ( 5uz / 59) alone reduces to R cos a Ft = ami f —T^- (2.9) V3 which is same as the simple eqn. (2.6) derived from the shear work of deformation for an average effective stress. Kobayashi, Hall and Thomsen [21], assumed in their analysis that the bending takes place simultaneously with shearing at point P of Fig. 2.21 and is followed by unbending as the roller passes during each revolution of the cone. They have calculated theoretically the total effective strain with reference to Fig. 2.21; and evaluated the tangential force Ft from the simple shear theory. Kobayashi, Hall, Thomson [21] have also shown that approximate values of the radial force component Fr and the axial force component Fa can be evaluated from the value of tangential force component F, assuming that the normal stress p is uniformly distributed over the contact area between the roller and the cone. A graphical and approximate numerical method was used by them to get the contact area in the three mutually perpendicular direction so that for the assumed stress condition AT- A Q Fr = F.. — , and Fa = F, — (2 10) At At 48 CONTACT AREA £,- DEFORMATION COUNTER OF FLANGE BEFORE TO COMES INTO THE DEFORMATION ZONE <«> Fig. 2.21 .Straining of an element in cone spin forging (a)Tangential section (b)axial section (c)radial section 49 Where Ai\ Aa and At are the three projections of the contact area between the roller and cone in the three coordinate directions r, z and 0 respectively. 2.10.2 Approximate Upper Bound Estimates for Dimensionless Tangential Force Flow forming of sheet metal cones is essentially an unsteady state process since the positions of the elements in the plastically deforming region change as the cone is formed and the diameter increases. At the same time, for a constant rotational speed of the mandrel, the forming speed (velocity) of the elements increases from the nose to the base of the cone. The effects of strain hardening and temperature are also not easy to assess. Nevertheless Slater & Joorabchian [27, 28] presented an upper-bound estimate for dimensionless tangential force component assuming (a) plain strain deformation and (b) axisymmetric deformation by considering single triangular velocity field on a Median plane. Assuming Plain Strain Deformation, the expression for the dimensionless tangential component force (Ft / a . T f) has been given as ^-^ + sin2p - sin2«j).sin(p + (|)) + (1-R) R — sin P . sin 50 77 UPPER—BOUND ESTIMATE AXISYMMETRIC DEFORMATION' / UPPER—BOUND ESTIMATE PLAIN STRAIN DEFORMATION EXPERIMENTAL .2 VALUES FOR 70/30 BRASS (ar~4,SiO MPa) 1 1 O 0.2 0.4, O.O FRACTIONAL REDUCTION IN THICKNESS (R) Comparsion of the approximate upper—bound estimates -with experimental values of the dimensionless tangential component force 70/30 brass 51 Assuming Axi-symmetric Deformation, with straight lines of tangential velocity discontinuity on a Median plane, Slater [28] obtained the expression for the dimensionless tangential component force as under : sin (2.12) Approximate upper bound estimates for dimensionless tangential component force as derived by Slater [28] appear to provide acceptable correlation with experimental values during flow forming of 70/30 Brass sheetmetal cones carried out by him. The values of dimensionless tangential component force during flow forming of cones as predicted by approximate upper bound estimates are produced in Fig. 2.22. For the purpose of comparison, the experimentally observed values obtained by him from flow forming of 70/30 brass sheetmetal cones also included in the same figure. 2.11 LOCALISED DEFORMATION: MAIN ASSET OF FLOW FORMING PROCESS One of the applications of large scale deformation using point loads (compressive) is in the process of flow forming. In other metal forming process such as forging, bending, rolling, drawing, extrusion, etc., the area of contact between the tool and the work is large, and therefore, the force and the power requirements is of considerable importance in designing such metal forming machines. By adopting incremental/point or localised deformation process, the forces and power requirement can be largely reduced and this technique is accomplished in flow forming process. In flow forming process, the deformation takes place in a small region where the forming roller contacts the work [17]. This trend of localising the deformation zone to a small region of the work has the following main advantages: 52 (a) large specific pressures are brought into action. (b) there is a considerable reduction in the forming forces required. (c) the force characteristic is almost flat and consequently the size of the machine required to perform the process is reduced. 53