A survey of flexible forming processes in Japan

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International Journal of Machine Tools & Manufacture 46 (2006) 1939–1960 www.elsevier.com/locate/ijmactool

A survey of flexible forming processes in Japan J.M. Allwooda,, H. Utsunomiyab,c a

Institute for Manufacturing, University of Cambridge, Mill Lane, Cambridge CB2 1RX, UK b Division of Materials and Manufacturing Science, Osaka University, Suita, Japan c National Institute for Materials Science, Tsukuba, Japan Received 19 December 2005; accepted 18 January 2006 Available online 11 April 2006

Abstract In the past two decades there has been a significant growth of interest in flexible forming processes in Japan, but many new developments are little known elsewhere, particularly when the processes have been reported only within the Japanese literature. This paper presents a thorough survey of recent and current work on flexible forming processes in Japan. The survey was achieved through a series of visits to Japanese laboratories, followed by a literature search, and circulation of a draft of the paper to the key groups active in this area for review and feedback. The processes are described and illustrated, and where possible, brief descriptions of the process capability and limits are given. A classification scheme is developed to organise the flexible processes into a single table and illustrate the possibilities for future innovations in this area. r 2006 Elsevier Ltd. All rights reserved. Keywords: Flexible; Forming; Japan

1. Introduction The past 20 years has seen extensive development of novel flexible forming processes in Japan, yet many of these developments are largely unknown elsewhere. As interest in flexible forming is now growing world wide, this paper attempts to provide a broad survey and introduction to Japanese developments in the area. For clarity, a flexible forming process will be defined to be a process in which the geometry of the formed product can be changed by the control of an actuator without requiring a different tool set. Thus conventional strip rolling is flexible (the thickness of the strip may be controlled), but conventional extrusion and closed-die forging are inflexible (the same product is produced regardless of ram speed or forging force.) Many of the flexible processes described in this paper are described as ‘incremental’ implying that a small region of the workpiece is deformed at any instant, but the location of the Corresponding author. Tel.: +44 1223 338 181; fax: +44 1223 338 076.

E-mail addresses: [email protected] (J.M. Allwood), [email protected] (H. Utsunomiya). 0890-6955/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2006.01.034

deformation is moved around the workpiece under computer control. ‘Incremental’ processes are thus a subset of ‘flexible’ processes. After the scale economies of mass production have allowed widespread access to what were originally luxury products, manufacturers now attempt to grow by competing on product differentiation as much as on price. This leads to short product life cycles, with lower batches of a given product model, and profitability is as much dependent on the speed at which new models and products are introduced as on the control of direct cost. In turn this has led to development of flexible manufacturing technologies, and particularly in the world of machining, there is a rich market of highly flexible machinery able to shift between different products without delays for manual setup operations. In contrast to these developments in machining, however development of flexible forming technologies has been delayed, and particularly in Europe and the US, such interest is more recent. However, in Japan, work on the development of flexible forming processes is more widespread, mainly driven by the realisation by domestic manufacturers that the only way to compete with low labour cost neighbours would be to

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develop more flexible technologies able to produce diverse products without incurring high labour costs. But, despite the existence of a rich seam of activity developing novel processes, many of the innovations in Japan are not well known, if at all, in the West. Largely this is because Japanese academics have a strong internal literature, particularly through the Japan Society for the Technology of Plasticity, in which to publish work, and as a consequence many exciting developments remain unknown in the West. Two early reviews on incremental forming have been found in the Japanese literature: Asao and Nakamura [1] reviewed work on dieless forming, and introduced two categories of flexible forming—‘flexible die forming’ and ‘dieless forming.’ All the flexible processes they identify are actuated by thermal means; Nakamura et al. [2] give a review of technologies, mainly developed at the Hitachi Corporation, for which they worked, for flexible forging of steam turbine blades. Only one recent review aiming to give an introduction to novel technologies in English has been identified: Shima [3] provided a survey of developments in incremental forming. The present paper aims to give a comprehensive survey of developments in flexible forming. The intention of the paper is to improve international appreciation of the rich and diverse developments in this area in Japan, to draw attention to the achievements of key groups, and to motivate continuing expansion of interest in this area outside Japan. The methodology behind the paper has been a tour of several laboratories by both authors followed by a literature survey. A draft-working version of the paper was then circulated to all the groups known to be working in the area, to verify the descriptions of their own innovations and to request information about any missing developments. Sections 2 and 3 of the paper give brief descriptions of the novel process designs, and an overview of their flexible capability. The sections are divided broadly between bulk and sheet processes, the division being drawn according to whether a realistic finite element model of the process would require brick elements or might be approximated using shell elements. Section 4 attempts to provide some categorisation of the processes, to assist in identification of opportunities for further development, prior to a brief discussion in Section 5. 2. Flexible bulk-forming processes In bulk-forming processes, deformation generally arises due to a compressive state of stress achieved by opposed tools acting across the product: the rolls in rolling; the dies in forging; the ram and die in extrusion. Options for creating flexible forms of these processes include the addition of new tool motion, having tools with a smaller region of influence, which can be moved relative to the workpiece under computer control, or segmenting the tools

to allow some re-configuration. Generally, this will lead to a similar pattern of deformation as in the non-flexible conventional process, but when the contact area between tool and workpiece becomes small compared to the workpiece, it is possible that the region of deformation will no longer extend across the workpiece, so a quite different process is created. For instance, open die forging, when one tool is greatly reduced in size becomes similar to a hardness test. The sub-sections below describe work on flexibility in four major bulk-forming processes—extrusion, drawing, rolling and forging—and concludes with a description of recent work, which aims to leave the part geometry thicker as a result of forming. 2.1. Flexible extrusion processes In conventional forward extrusion the cross-section of a billet is reduced and shaped, but the axis of the billet remains linear. In tube extrusion, the billet is hollow and an internal mandrel defines the cross-section of the bore of the product. Thus flexibility can be introduced only by allowing control of the cross-section of the die and mandrel, which defines the final external and internal cross-section of the product. Both possibilities, and their combination, are illustrated in Fig. 1: Makiyama and Murata [4] show a moveable external die to allow production of rectangular section solid bars of variable height; Makiyama et al. [5] show control of the cylindrical bore of an extruded tube—where a needle-shaped internal mandrel is inserted further into a die, or withdrawn, in order to control the bore which can be varied continuously during the extrusion; Hara et al. [6] show both approaches combined in extruding hollow square section tubes. In all three processes, the mandrel or die can be moved during extrusion to give a variable cross-section product, but to date the processes have been tested only on cold lead or warm pure aluminium in order to minimise extrusion forces. 2.2. Flexible drawing processes The same means to allow control of external profile in extrusion could be applied to a drawing process, but no work in this area has been identified. However, a further option exists in drawing, arising from it being a fundamentally tensile rather than compressive process. Based on original work by Prof. J. Alexander in the UK, Kobatake et al. [7] developed a continuous dieless drawing process, illustrated in Fig. 1D. In this process, the bar or wire is locally heated by high-frequency induction, and immediately cooled to create a short hot zone of local deformation. Control of the tensile force in the workpiece, or the drawing speed, allows controllable profile reduction. While no further developments in this area have been identified, the process is significant in demonstrating the value of a locally heated zone to allow controllable deformation.

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Fig. 1. Flexible extrusion processes with controllable: (A) external profile, (B) internal bore, (C) external and internal profiles, and (D) Flexible drawing.

The control of external cross-section demonstrated in Fig. 1 could be applied to any convex polygonal profile, but cannot allow controllable circular cross-sections. However, the helical tube rolling process illustrated in Fig. 2A and described by Takizawa and Murata [8,9] and Takizawa and Kimura [10] (but attributed by them to developments in the USSR with the earliest cited reference

being that of Blazynski [11]) allows drawing of a circular product with controllable external diameter, by varying the location of the three external forming rolls. Takizawa and Murata demonstrate this approach for drawing and rolling a hot steel tube, with an inner mandrel, but it has also been applied to solid rods. Their analysis shows that the tube formed by this process will not be circular due to bulging of

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Forming direction Ft  y 120°

y Rolling surface 

Reeling surface  x

z v1 v0 Mandrel  (A) Mother pipe Roll 



Reduced pipe  : Elevation angle  : Offset angle



(B) Fig. 2. Tube diameter reduction: (A) with three external rolls and an internal mandrel and (B) with eight external rolls and no internal mandrel.

the workpiece between the three roll bites, and that the specific energy consumption of the process decreases markedly with feed speed, as the ‘redundant deformation’ in repeated rolling of each part of the tube greatly increases the work required. An interesting alternative approach to tube compression named a ‘planetary roller reducer’ is presented by Kiuchi and Shintani [12,13] and illustrated in Fig. 2B. In contrast to the helical tube rolling process, this approach uses six or eight rolls around the circumference of the tube, but requires no internal mandrel so is applied principally to reduce the bore of the tube rather than to reduce tube wall thickness. Kiuchi and Shintani demonstrate that the roundedness achieved by the planetary process appears to be better than that of the helical process.

This process uses neither front tension nor back compression but controls the workpiece feed rate by varying the angle b, so is strictly neither drawing nor extrusion, but is clearly related to that of Takizawa and Murata. Following the example of tube compression spinning described below [14] an extension of the helical rolling approach would be to allow cyclic radial motion of the forming rolls so that the process can be used for noncircular cross-sections, but this has apparently not been explored yet. Similarly, it would appear to be possible to combine these processes whose flexibility influences the product cross-section with bending processes such as those described in Section 3.3 to allow flexible control of both cross-section and axis.

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2.3. Flexible rolling processes All rolling processes have one degree of flexibility as the roll gap may be varied, and this opportunity is exploited by Ayada et al. [15] to manufacture tapered leaf springs. A further degree of flexibility can be achieved by reducing the extent of contact between roll and strip/ring/section, and achieving reduction over multiple passes, with the location of the contact area adjusted between passes. Beyond this, the rolling equipment must be re-designed, either with increased actuation, or a different configuration or rolls

Fig. 3. Flexible rolling with moveable rolls: (A–C) axial ring rolling process for wheel profiling and (D) wedge rolling.

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relative to the workpiece to allow more variety in the stresses imposed in the deformation region. In the first category, Omori et al. [16] describes an axial ring rolling process for railway wheels, shown in Fig. 3A–C, with contact between roll and disk over only a short section of the radius, but with the axial roll moving radially during processing in order to allow production of a range of wheel profiles without new tooling. Yamada et al. [17] also working at Hitachi Corporation used a similar controlled movement of the rolls to allow flexible in-plane bending by wedge rolling, shown in Fig. 3D. In the second category, two novel approaches have been found. Kaneko et al. [18] have explored a strip spreading configuration shown in Fig. 4, with the axis of the roll parallel to the feed direction of the strip-allowing a much greater spreading of strip width than in conventional rolling. This work was based on a similar process by Yosida [19] who produced flat wires from a round wire by consecutive transverse rolling. The process by Kaneko et al. differs in having an idle transverse roll able to travel across the width of the strip, where that of Yosida was driven. The strip is fed a short distance, then the roll completes a traverse and the strip is fed again, and the process can reliably achieve spreading of 20%. The other approaches relate to section rolling. Two approaches to shaping of I-beams are shown in Fig. 5 driven by the commercial attraction of being able to control the height of the section without tool changes. In Fig. 5A, the approach of Hiroguchi et al. [20] based on original work by Aoyagi et al. [21], leads to tensile stretching of the section web, as the upper and lower flanges are forced apart by two pairs of rollers whose angle to the perpendicular may be controlled. In Fig. 5B, in the web height reduction process of Kusaba et al. [22] the beam is compressed between two flange rollers, and buckling of the flanges and web is inhibited by two pairs of narrow rolls whose separation can be controlled. Hiroguchi et al. give no details of the degree of stretching that can be achieved but Kusaba et al. demonstrate significant

Fig. 4. Strip spreading.

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Fig. 5. Controllable shape rolling of sections for (A) web stretching of I-beams, (B) web compression of I-beams and (C) combined rolling.

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reduction in the number of rolling passes required to achieve a given geometry due to the increased reduction possible with their flexible process. Fig. 5C illustrates a combined rolling approach by Yamada et al. [17] for bending shaped beams, related to the method for in-plane bending by forging illustrated in Fig. 13. 2.4. Flexible forging processes There are three broad means by which forging can be made flexible: the workpiece can be moved relative to the press, so that deformation is built up with multiple strikes; a press could be designed with multiple actuators; a reconfigurable tool could be designed to fit between the press and the workpiece with sufficient stiffness and strength to transmit the forging force. Apparently, very little activity has occurred in Japan in this area to date. The use of controlled motion of a workpiece relative to the press has been developed in the UK and Germany, but apparently has not yet been explored in Japan, although the work on incremental hammering of sheets reported in Section 3.2 below is related to this approach. The second category of work has been examined at the Hitachi Corporation for flexible manufacture of turbine blades [23–25] and is reviewed in Section 3.2 below (and illustrated in Fig. 13) as it is closely related to other work on strip hammering. Osakada [26] has developed work with a double action press to allow controlled motion of the external container die. However, to date this has mainly been used to improve die filling and hence product precision particularly in forging helical gears, rather than to increase flexibility. Although reconfigurable tools of the type used by [6] for control of external profile in extrusion could be used in forging, no evidence of this has yet been found. 2.5. Flexible thickening processes The final category of bulk forming processes is one, which has generally received little attention—forming processes, which make a product thicker rather than thinner. This approach has possible applications to avoid joining processes or un-necessary material removal, but potentially is also important for recycling—reversing the normal manufacturing processes which make components smaller. Three broad approaches have been identified. Saito et al. [27] describe a novel rolling apparatus shown in Fig. 6A comprising a five-stand tandem rolling mill with inter-stand ‘guide shoes’ to prevent strip buckling when inter-stand compression (rather than the normal tension) is applied. This allows a rolling regime in which the elongation and spread of the final product can be controlled, and results show overall elongation in the range 95–135%. In later work, Utsunomiya et al. [28] report on trials in which stands two and four of the five stand mill were rotated 901 to allow rolling of square wires from initially round wires. The effect of compressive and tensile inter-stand stress on final cross-section is explored,

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showing sharper corners, and larger cross-sectional area when compressive stress is used. Prior work by the same group [29] on rolling U and H section wires on a related ‘satellite-mill’ also showed better filling of roll grooves in shape rolling with compressive inter-stand stresses. A second approach to thickening is demonstrated by Iura et al. [30] who describe a mechanism for forming a thick collar on a solid bar, by a combination of barbending under compression and rotation, as illustrated in Fig. 6B. Tests were conducted on steel bars at room temperature, showing fractional increases of diameter up to 1.6 and requiring up to 30 revolutions. The third approach to thickening that has been identified is that of Matsumoto et al. [31,32] who have explored flexible tube thickening with a process shown in Fig. 6C whereby a tube is rotated with or without axial compression, and locally heated by a movable laser. This allows local thickening of the tube wall, but the results show a thickening of only around 4% relative to the original, which is increased to 5% if axial force is included. Matsuki et al. [33] explored a combination of the processes of [31] and [30] showing that the addition of bending allowed reduction of the axial compressive force to around one half the previous level, but that the degree of bending possible before buckling occurs is limited. 3. Flexible sheet forming processes In contrast to bulk forming which is generally compressive, deformation in sheet forming may arise from various stress configurations, and in particular can take advantage of bending and shear deformations which are not possible for thicker bulk products, but which allow permanent deformation of a product with reduced tool forces. The three sub-sections below essentially describe processes arising from three such states of stress:

  

deformation in spinning is largely due to shearing of the sheet, normal to its plane, and similar deformation is seen in incremental sheet forming or tube compression; hammering or impact processes lead to local compression across the sheet, which in turn causes lateral expansion within the plane of the sheet; bending processes combine compressive and tensile states within the plane of the sheet.

The broad category of sheet processes includes processes working on geometrically similar products—particularly, tubes and disks. 3.1. Flexible spinning and incremental sheet forming Conventional spinning is not a flexible process, as it requires a specific mandrel for each product. Three approaches to making this process flexible have been identified: Kitazawa et al. [34] have examined spinning of pre-formed shells as illustrated in Fig. 7A—initially

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Fig. 6. Forming processes leading to thicker products by (A) rolling, (B) bending, compression and rotation and (C) rotation and compression with local heating.

forming a dome shape, then reversing the product so that the tool works against the curvature of the dome, allowing formation of sharp corners, even without a mandrel; Shima

et al. [35] developed a flexible spinning process shown in Fig. 7B with two opposed rollers to either side of the workpiece, moving in tandem to create a localized, but

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Fig. 7. Flexible spinning processes (A) of pre-formed shells, (B) with two opposed rotors and (C) with a moving blank holder.

controllable deformation; Matsubara [36], Kawai et al. [37] and Aihara and Matsubara [38,39] have explored mandrel free spinning processes with that of Aihara and Matsubara shown in Fig. 7C with only one tool, but with controllable movement of a blank-holder, allowing creation of close to spherical components from a flat disk. Comparison of these three approaches suggests that there are two main choices in the development of flexible spinning processes: should the outside edge of the workpiece be held in a blank holder? (Sometimes a distinction is made between the

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names ‘‘spin forming’’ and ‘‘shear spinning’’ to indicate this choice.) Should one moving roller or two be used to create the formed shape? In addition, the approach of Kitazawa et al. is a reminder that the final formed shape can be achieved via an intermediate form, whose stiffness will influence the response of the workpiece to further motion of the tools. Of the various products illustrated in the above papers, the most dramatic is that of Aihara and Matsubara, who are able to form a shape close to a sphere (with circular or polygonal cross-section) from an initial disk, using an external blank holder and one moving tool only. The spinning process, in which a tool with small contact area moves across the workpiece, has inspired a number of processes with related geometry. The most well developed of these, generally known as incremental sheet forming, has been commercialised by the Amino Corporation, and is now well known outside Japan .The original developments of this process in Japan were by Iseki et al. [40] in which a small tool moves in Cartesian co-ordinates (as opposed to polar co-ordinates in spinning) over a sheet supported only by a blank-holder frame as illustrated in Fig. 8A. Two common variants of this configuration are when the centre of the workpiece is held over a post (Fig. 8B, [41]) with the blank-holder frame pulled parallel to the post to stretch the sheet and similarly when instead of a post, the frame is pulled over a template product, Fig. 8C, [42]. Strictly this second variant is not flexible, as it requires a template die, but the die can be made from wood or polymer for instance by rapid prototyping, so is at least cheaper than a full rigid tool for pressing. Matsubara [43,44] and Iseki [45] have demonstrated forming a sheet over a template with the workpiece held centrally and without a blank-holder constraining the sheet edges, in order to achieve the deepdrawing of a square cup as shown in Fig. 8D. Iseki has also examined use of a water jet instead of a stiff tool [46], Fig. 8E. Tanaka et al. [47,48] and Yoneyama and Naganawa [49], have explored micro-incremental sheet forming as illustrated in Fig. 8F to form products of size around 8 mm [47,48] or 200 mm [49]. Yoshikawa et al. [50] appear to be the only group to have tried incremental sheet forming with an upper and lower tool, as shown in Fig. 8G. After more than a decade of intensive effort, the dream of a dieless forming process based on incremental sheet forming remains elusive, due to the difficulty of achieving a specified geometric accuracy when the deformation created by a step in actuator position depends on the current deformed shape of the workpiece and the location of the tool relative to the blankholder. However, incremental sheet forming with a one-sided die is well established, a commercial machine has been developed for the purpose by the Amino corporation under license to Matsubara [51]. The use of the process by Toyota to produce a prototype car body for motor shows is described by Matsui and Matsuda [52] and to produce personalised dental prosthetics is described by Tanaka et al. [53] and illustrated in Fig. 8H.

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Fig. 8. Incremental sheet forming: (A) single sided, (B) with a support post, (C) with a one-sided die, (D) without a blank-holder, (E) with a water jet, (F) at micro-scale and (G) applied to dental prosthetics.

Two other configurations related to spinning are for incremental forming of tubes—at mid section, or at an open end. Matsubara [54] examined a tube nosing process

illustrated in Fig. 9A, in which a CNC controlled forming tool was used to compress the open end of a tube into a variety of axi-symmetric or polygonal forms. This a

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Fig. 9. Incremental tube forming: (A) tube nosing (B) tube compression spinning and (C) tube beading.

pproach was also used to examine expansion of an open tube end [55,56]. Relatedly, Matsubara [57,58], Murakami [59] and Murata [14] have explored tube compression

spinning with one hemispherical tool (Matsubara) one narrow rotor (Murakami) or two rotors (Murata) mounted co-axially to the tube moving to cause compression of the

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Fig. 10. Incremental deep drawing: (A) with a single tool and (B) with a multiple headed tool.

tube surface. Murata’s approach is illustrated in Fig. 9B and has been used both at the ends of an open tube, and at mid-section. Murakami’s approach illustrated in Fig. 9C uses an internal mandrel with groove and a narrow roller tool pressing material into the groove. Matsubara’s work on shaping the open end of tubes was intended to give a particular pyramidal form, and examines the design of tool paths to achieve a desired geometry without defects. In contrast, the intention of Murata’s work on open tube ends [60] was to create a short section of reduced diameter at the open end of the tube. In both cases, a distribution of thickness in the formed part is shown, with increased thickness partway between the onset of deformation and the open end. Murata has also examined compression (or parallel) spinning of the mid-section of aluminium [14] or magnesium [61] tubes. In the latter case he used heaters

within the narrow rotors to heat the tube to a range of temperatures above and below the recrystallization temperature, showing that at temperatures around 300–400 1C diameter reduction from 38 to 5 mm is possible. A natural extension of Murata’s approach is to apply a cyclic radial motion to the rotors, to allow production of some of the polygonal forms created by Matsubara [54], and while this approach was evident in Prof. Murata’s laboratory, it has not apparently been reported in the literature to date. A third class of variant from conventional spinning is in incremental deep drawing. This is explored by Shima et al. [62] and illustrated in Fig. 10A. A punch smaller than the diameter of the drawn cup acts adjacent to a partial die and blank-holder, and after each stroke the blank-holder force is released and the workpiece rotated such that without change of tools, a variety of cup diameters and heights may

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Fig. 11. Sheet forming by stretching over a controllable array of pins.

Fig. 12. Sheet hammering processes: (A) over an anvil, (B) with an X–Y table, (C–D) with a robot and (E) at micro scale.

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be formed. Shima et al. demonstrate a limiting draw ratio (initial blank diameter/inner diameter of formed cup) of around 1.55, but this is dependent on the diameter of the cup relative to the punch. One other extension of spinning related to deep drawing is the rotary drawing process of Kawai et al. [63] illustrated in Fig. 10B. In this process the forming tool of spinning is replaced by a multi-roller head, which rotates at a rate around 20–50 times less than the mandrel while forming the workpiece by moving along a computer controlled path. In the case illustrated, with a cylindrical mandrel, the process can be used for deep drawing, and a drawing ratio of 3.33 is possible. The rotary head can also be moved along a free path away from the mandrel to allow flexible spinning. Finally in this section, Iseki [64] based on an original suggestion by Nakajima [65], has explored the possibility of a re-configurable one-sided die being used for incremental stretching of the sheet. This process is shown in Fig. 11, in which the die comprises an array of pins mounted on screw threads, which are adjusted in turn by a robotic screwdriver. A less ambitious, but more immediately useable

variant of this approach is to have a segmented blankholder within a conventional stamping process, as explored by Yagami et al. [66]. 3.2. Flexible sheet forming using hammering The distinction between sheet and bulk-forming processes is less certain in hammering processes, where the through-thickness compressive stress created by hammering action creates the in-plane expansion which leads to sheet deformation. The mechanics of the process are thus rather similar to forging, but the term ‘hammering’ is generally used when the workpiece is a sheet. Hammering is of course an ancient process, and has been used by swordsmiths (producing Japanese Samurai swords for instance), blacksmiths and panel beaters as a means to form metal throughout time. The interest in hammering recorded here arises when the tool is moved by a computercontrolled process. The earliest work on flexible sheet forming identified in this survey is that of Nakajima [67] illustrated in Fig. 12A

Fig. 13. Incremental strip bending: (A) intermittent partial forging, (B) bending out of plane, (C) in-plane bending due to wedge shaped die and (D) with twisting.

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Fig. 14. Peen forming.

whose flexible computer controlled hammering process anticipated the development of incremental sheet forming by more than 10 years, yet was able to form a diverse range of shapes. Nakajima’s process used a small hammer driven by a hydraulic cylinder acting on an anvil which could be rotated with two degrees of freedom, so the actuation choice included both the location of the impact, and the alignment of the hammer with the anvil. Similar studies were conducted by Hasebe and Shima [68] and Matsubara et al. [69]. Mori et al. [70] worked specifically on the computer control of a process in which the workpiece is held over a die, Fig. 12B, and the actuation is the location of the hammer. Tanaka et al. [71] explored a hammering process illustrated in Figs. 12C,D, as a pre-cursor to later work on incremental sheet forming. The deformation is created by opposed elastic upper and lower tools of various shapes and the workpiece is held in a frame moved by robot arm. The robot presents the sheet to the hammer pair allowing control of the location and alignment of the strike. Kotani et al. [72] describe a similar configuration, with the workpiece held by a bolt through a hole in the sheet. Saotome and Okamoto [73] describe a micro version of hammering illustrated in Fig. 12E to make products in the range 10–500 mm. The hammer is mounted on a cantilever, which is deflected by a piezoelectric actuator and the machine is sufficiently small to be mounted entirely within the vacuum chamber of an SEM to allow precise feedback of the deformed shape. The above processes are all applied to sheets, with the area of contact between hammer and sheet being small compared to the dimensions of the sheet. When the area of contact is relatively large compared to the workpiece dimensions, the hammering process becomes similar to that of forging. Fig. 13A illustrates the intermittent partial forging process developed at Hitachi Corporation by Nakamura et al. [23] with a variant in Fig. 13B [24] in which the hammer makes contact across the product, but only for a narrow length, and can then be used to create out-of-plane bending. In contrast, the process developed by Jin and Murata [74,75] is designed to achieve bending of the strip within its own plane. The process is illustrated in

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Fig. 13C showing a wedge shaped hammer and anvil whose contact spans most of the width of the strip and by creating a linear variation in thickness reduction across the strip, causes the strip to bend. The flexibility of this process is limited, as it depends primarily on the angle of the hammer wedge, but some flexibility can be imposed by controlling the distance moved by the strip between strokes referred to by the authors as the pitch. Their results demonstrate that for a 25 mm wide strip of nearly pure Aluminium, a bend with outer radius between 50 and 120 mm can be achieved, depending on strip thickness, and that varying the pitch allows a range of about 25 mm in outer bend radius to be controlled. Finally, in Fig. 13D, a variant of the intermittent partial forging process is shown [25] where it is combined with workpiece twisting to allow production of turbine blades. While not usually considered a hammering process, the mechanics of peen forming are similar to those of hammering, and the peen forming work of Kondo [76] is included here and illustrated in Fig. 14. Peen forming can produce only shallow curvatures, but it has an attractive feature that by varying the impact speed, both convex and concave curvatures can be achieved. In addition, the peen forming process acts to suppress the occurrence of fracture and can be used for difficult to work alloys. 3.3. Flexible bending processes Typically bending processes form a bend across an entire cross-section of a workpiece, so unlike the processes related to spinning and hammering, are not localised. Such processes can be made flexible in two ways: by allowing control of the bend angle at a particular location; by controlling the distribution of the bend over some length of the product. The first approach is explored for bending sheets along a line by Yang and Shima [77] who show four options for the design of punch and die as illustrated in Fig. 15A–D. The flexibility of the process arises from control of the depth of the punch stroke, which allows bending to a range of angles, and allows compensation for springback—by over-bending. Experimental results showed that the tool force was lower using the curved and oval dies. Yang et al. [78] apply a related method to ‘L-bending’ with an intelligent punch tool which includes a transverse actuator to allow appropriate over-bending to compensate for spring-back. Both of these methods are used for creating single sharp bends, but in the second approach, continuous sheet bending is achieved by threeroll bending, and Mori et al. [79] report on the optimal control of this process illustrated in Fig. 15E in which the top roll may be moved both horizontally and vertically. The control system included a ‘fuzzy learning process’ to allow for control of plates and sheets with unknown properties. Examples of both of these forms of flexibility have been applied to tubes. Nakamura et al. [80] and Murata et al. [81] have developed processes for CNC bending of tubes to

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Fig. 15. Flexible bending of sheets with: (A) conventional V-bending, (B) air bending, (C) a curved die, (D) an oval die and (E) three roll bending from.

allow flexible shaping of an initially straight tube into freeform shape. Nakamura et al. use an internal spherical plug to avoid tube collapse, and create bending through a moveable-bending roll as shown in Fig. 16A. Murata and Aoki extrude the tube through a die mounted on a spherical bearing that can be re-directed in two directions as shown in Fig. 16B. Murata and Aoki’s process is the simpler, not requiring an internal plug, and has been fully commercialised by Nissin Co. Ltd. Experimental results demonstrate bending of curves with radius from 2.5 to 20 times the diameter of the tube. Subsequent development of this process [82] shows that it can be applied also to square

tubes, although the limiting ratio of curve radius to tube side is approximately 8. An extreme version of the second form of flexibility in bending is illustrated by the shear bending process of Goodarzi et al. [83] illustrated in Fig. 16C. In this process a Z-bend is created in a tube by shearing. The tube is supported internally by two mandrels that tessellate prior to deformation. It is enclosed externally by two full dies, which slide relative to each other to create two right angle bends. The extent of shearing, the speed of deformation, and the axial force applied to one end of the tube can all be controlled. Here the angle of bending is fixed at 901

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Fig. 16. Controllable tube bending: (A) with an internal spherical plug, (B) with a die on a spherical bearing and (C) shear bending.

(although it could also be set to any other fixed angle by appropriate die design), but the extent of shearing/bending is controllable. Analysis of the process shows that the

precise selection and control of axial force applied to the tube during the shearing process is critical to successful deformation.

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Fig. 17. Laser bending of sheets.

All the above processes achieve bending by mechanical actuation. It can also be achieved by local application of heat—through so called ‘laser bending.’ This process is flexible as the heat source, often a laser, can be moved freely over the surface of the sheet, but has relatively limited flexibility due to the limited curvatures that can be achieved. Otsu et al. [84] describe exploration of this process illustrated in Fig. 17 in which experiments are carried out on a 50 mm square 1.2 mm thick sample of stainless steel, with various scanning paths and speeds. Although the curvatures achieved were modest—around 0.006 mm 1—the process was sufficiently unpredictable to suggest that future work would be needed on the development of spatial feedback to allow closed loop control. An interesting variant of this process described by Osakada et al. [32] involves scanning the laser co-axially along a section of pipe, to allow controllable change in pipe-cross section, while retaining a linear pipe axis. The curvatures achieved in the pipe cross-section appear to be markedly greater than those of the earlier work on flat sheet. 4. Categorisation of flexible forming processes A survey of this type inevitably presents an interesting question: given the range of novel flexible forming processes described here, what other processes might be explored in future? The idea of classifying forming processes dates back to the work by Thomsen et al. [85] who organised processes according to the stress state during deformation. Related work in Germany led to the DIN classification, and the work of translating Thomsen et al.’s book appears to have stimulated work by Kudo [86] who proposed a classification of forming processes (not limited to flexible processes) with six main parameters: initial workpiece geometry; mode of deformation; state of stress; deformation sequence; type of tool; type of power transmission to workpiece. Based on the survey of flexible

processes presented here, five parameters are derived to allow process classification, and assist in the predictive search for as yet unidentified flexible processes. A first approach to developing a classification is to consider the migration of the means used to create flexibility between different types of workpiece. The most well developed example of this in the above survey arises in Section 3.1 where work towards a flexible spinning process has led to developments of incremental sheet forming and flexible tube forming. Similarly, the actuation required to control diameter in helical tube rolling is similar to that to control gauge in conventional strip rolling. A rich source of inspiration in searching for novel flexible processes is to pursue this approach—given a process design applied to one type of workpiece, how could it be applied to other types of workpiece? Secondly, the comments at the beginning of Section 2 indicated three broad sources of flexibility: controlled movement of a set of tools (as with control of gauge in strip rolling); reduction of tool size combined with multiple passes of the workpiece past the tool (as with incremental sheet forming); use of multiple actuators or reconfigurable tools (as with flexible extrusion.) These three categories usefully divide many of the processes described here. Thirdly, the conventional use of the terms ‘bulk’ and ‘sheet’ to separate forming processes has proved to be problematic, as processes such as sheet hammering are ‘bulk’ processes in the same sense as conventional strip rolling, even though they are applied to sheet workpieces. Instead, it seems to be useful to separate processes according to whether they lead to a high state of hydrostatic stress (or pressure) in the workpiece or not. Thus, rolling and extrusion give high pressures, while V-bending and incremental sheet forming do not. Finally, in some processes such as rolling or extrusion, deformation occurs continuously while in others such as V-bending or hammering the deformation is intermittent. In addition, the interaction between tool and workpiece may be primarily sliding (as in extrusion), rolling, impact, non-contact (as in laser bending) or stationary contact (as in stretching). Although many processes with intermittent deformation are impact processes, strip spreading is an interesting example of a process where the deformation is intermittent—deformation must cease in order to allow feeding of the strip—but with a rolling form of contact. This discussion provides five parameters by which flexible forming processes may be classified, and has been used to create Table 1. The columns of the table indicate the workpiece type, and the rows are grouped according to intermittent or continuous deformation, high or low hydrostratic stress, and the three main sources of flexibility. The processes which depend on heating are grouped separately although it could be argued that the use of widespread or localised heating is a sixth parameter within the previous categorisation. Within each cell of the table, processes are grouped according to the five types of contact. Although some combinations of these five

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Table 1 Categorisation of flexible forming processes

parameters are impossible (processes having continuous deformation cannot have impacting contact between tools and workpiece), the table suggest that there may be around 300 classes of flexible process, of which only 25 have been identified to date. Of the areas that have not yet been explored, the broad category of flexible forging (processes with intermittent deformation applied to bulk type products) is a surprising gap although work is developing in this area outside Japan. Similarly, work on processes with multiple actuators or reconfigurable tools has had little attention. Local softening of material by heating as part of another mechanically actuated process also appears to be worthy of exploration, as actuation to move a heat source is relatively cheap. 5. Discussion This paper has attempted to provide a comprehensive survey of recent and current work on flexible forming process development in Japan. Clearly, it cannot be entirely comprehensive, as many developments within companies will be confidential, but based on information in the public

domain it is hoped that the survey is close to complete. The survey has concentrated on description of the process configuration, and has attempted to give basic information on the ‘process window’ of each design—the range of flexibility that could be achieved without encountering forming limits. The survey has examined forming of ductile solid materials, so has excluded processes based on powder materials, or processes that combine forming and joining— both of which areas have great potential for flexibility and are in active development. Most of the processes described in this survey are at a pre-industrial stage of development, with research teams showing what is possible, although some notable exceptions exist for example with the development of commercial equipment for CNC tube bending and incremental sheet forming. It is clear that full industrial adoption of such novel processes depends both on having a sufficiently broad process window to provide real economic benefit in allowing manufacture of a wide range of products without dedicated tooling, and also on the process achieving product quality comparable to that of the conventional process route with fixed tooling. This problem is the major limit on the adoption of incremental sheet forming, where

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geometric accuracy remains a problem unless a one-sided die is used, and suggests that further integration of flexible processes with sensors allowing feedback control is a necessary development. The categorisation of processes in Table 1 provides some interesting suggestions about where new developments may be sought. A characteristic of the organisation of this table is that the high hydrostatic stress processes could equally lead to product thinning or thickening—and the development of novel thickening processes has been one of the interesting discoveries of the survey. For any thinning process based on tensile hydrostatic stress, there is likely to be a related process, allowing thickening. However, any process with high hydrostatic stress requires redundant work, and the absence of work on low hydrostatic stress process for bulk products suggests an interesting area of exploration in seeking low energy forming processes. Table 1 also shows that some conventional forming processes— for instance hydro-forming—have apparently not yet led to flexible developments. Japan is not the only country where developments in flexible forming have occurred, and the work of this paper has not attempted to compare work in Japan with that elsewhere. That task will be pursued in future, but in concluding this paper it is clear that the range of inventiveness brought to the design of flexible forming processes in Japan in the past 20 years is impressive and should be a significant inspiration to groups elsewhere attempting to support industrial requirements for low-cost low-batch production.

[8] [9]

[10]

[11]

[12] [13]

[14]

[15]

[16]

[17]

[18]

Acknowledgements [19]

The authors are grateful to the developers of the processes listed here for their co-operation on the work of this paper, and to the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan for funding the travel of the first author.

[20]

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