A new method of burr-free slitting

Journal of Mechanical Working Technology, 3 (1979) 1 0 1 - - 1 1 8

101

© Elsevier Scientific Publishing C o m p a n y , A m s t e r d a m -- Printed in The Netherlands

A NEW METHOD

OF BURR-FREE

SLITTING

MASAO MURAKAWA

Department of Mechanical Engineering, Nippon Institute of Technology (Japan) and TEIZO MAEDA

Department of Precision Machinery Engineering, University of Tokyo (Japan)

(Received O c t o b e r 2, 1978; a c c e p t e d March 9, 1979)

Industrial Summary Conventional r o t a r y slitting is n o w widely used for dividing a wide strip into a plurality o f narrow strips or for t r i m m i n g the unnecessary a n d / o r unacceptable edge p o r t i o n of relatively wide sheet metal stock. However, with conventional slitting, burr f o r m a t i o n on the slit edge is inevitable. To eliminate this drawback associated with c o n v e n t i o n a l slitting, a new slitting m e t h o d has been developed by the authors which causes no burr at all to occur on the slit edge. This new slitting m e t h o d consists of t w o stages, namely, (i) that of first subjecting the flat-rolled material to a shearing action along the longitudinal axis of the material to cause it to be partially sheared, by use of a pair of circular slitting cutters set at a distance apart of less than the thickness of the material, and (ii) then transferring the partially-sheared material to the nip of a pair o f cylindrical rolls which are preset to a gap which is substantially equal to the thickness of the material, in order to push back and separate the material into a n u m b e r o f strips without any burr on their edges. The process is very simple, in that the horizontal c u t t e r clearance can always be set at zero value, regardless of the material and the thickness. Apart f r o m the simple o p e r a t i o n of setting the second-stage flattening rolls to a gap which is substantially equal to the material thickness, o n l y the a d j u s t m e n t o f cutter spacing is needed, to suit the t y p e and thickness of the material. This feature will result in a r e d u c t i o n of operational cost. Moreover, the wear o f c u t t e r discs according to this m e t h o d is found to be very small w h e n c o m p a r e d with that for the c o n v e n t i o n a l slitting m e t h o d . In addition, the second stage rolls s h o w hardly any wear at all. Further, industrial significance of the m e t h o d resides in that it is applicable to various kinds of ductile material which w o u l d be liable to cause relatively high levels of burr as the c u t t e r is w o r n away.

1. I n t r o d u c t i o n W h e n a s l i t t i n g o r t r i m m i n g o p e r a t i o n is c a r r i e d o u t o n a c o i l e d s t r i p , i t is essential -- from the viewpoint of quality control -- to maintain so-called "burrs" at a minimum level, since such burrs are of sharp knife-like edge shape,

102

with enhanced hardness due to work hardening, and could possibly damage the adjacent coil stock surface in contact with them during transportation or in storage. Moreover, while processing, these burrs would come into contact with the various machine components -- such as tension pads, feeding rollers etc. -- of the slitting machine, resulting in the damage of such components. Further, when applying a soft covering such as a layer of plastic resin on and around the formed strips to obtain a covered or sheathed product, the material of such covering would be damaged by the h~arr. In view of such defects caused b y the burrs, it has long been one of the greatest concerns in the manufacture of coil stock to prevent such burrs from occurring in the first instance, since various methods to remove burrs mechanically have proved to be inadequate from the viewpoint of either quality or economics. 2. Earlier method of burr-free slitting (Counter-Cutting method) In the light of the above, there has been proposed recently a method for preventing, in principle, burr formation during the slitting of coiled stock. In this method, so-called "Counter-Cutting" [ 1], there is provided a second pair of cutter discs in addition to and adjacent to a first pair of cutter discs (Fig. 1), the relative arrangement of the first pair to the second pair being such that the bodily displacement required for partially shearing the mate-

- - / / i / / / /

-

(a) THE FIRST STAGE

(b)THE SECOND STAGE Fig. 1. Diagrammatic illustration of the principle of Counter-Cutting.

103 rial by the first pair o f cutter discs is repeated, inversely, by the second pair of cutter discs, to secure complete severing of the original half-sheared material, with no burr being formed on the slit edge. The Counter-Cutting method indeed seems to be useful in respect of its burr-free feature. However, in practice the method reveals a number of drawbacks. In the first place, compared with conventional slitting (where the material is cut through completely by the first -- and only -- pair of cutters), the number of cutter discs in Counter-Cutting is naturally doubled, and consequently there would be more work involved in maintenance, such as exchanging, regrinding and realignment of the cutters. Moreover, strictly speaking, for the success of the method it is essential to maintain the alignment of the t w o pairs of cutter discs extremely precisely, and it is also required that the guide devices for the uncoiled strip should operate very precisely. In consideration of such a specific arrangement, which leads to a very high precision in the design and manufacture of the slitting stand and its associated devices, it is inevitable that the production cost of the slitting machine should become two or three times as much as that of a conventional arrangement having a similar production capacity. Thus, in consideration of these limitations, it will be practically impossible to a d o p t the Counter-Cutting method successfully to suit thin gauge strip having, for example, a thickness of 0.5 mm or less. 3. Improved burr-free slitting (Roll Slit method} In order to eliminate the drawbacks inherent in Counter-Cutting, as stated above, an improved burr-free slitting method (called "the Roll Slit" method [2,3] ) has been developed by the authors, and its principles will be described hereinafter. As shown schematically in Fig.2, this m e t h o d comprises two stages, namely: (i) the first stage of partial shearing or penetration of a raw flat material, and (ii) the second stage of pushing back this partially-sheared material to its original thickness. Referring to the details, Fig.2 (a) illustrates the first stage of partially shearing a flat material b y the engaging function of the opposing cutters 1, 2 and 1', 2', fixed respectively to a pair of rotatable shafts and having such working parameters as "overlap L" and "clearance C". As shown in Fig.2 (b) and Fig.2 (c), the second stage of the Roll Slitting is obtained from the insertion and feeding of this partially-sheared material into an opposing pair of cylindrical rolls spaced a distance apart substantially equal to the material thickness, so that there occurs a flattening or pushing-back action by the rolls over both surfaces of the material, with the result that the material is finally separated or severed into three products I, II and III. Referring n o w to Fig.3, which shows the metal flow in the Roll Slitting process, the blank is shown in Fig.3(a) to be displaced bodily b y the upper cutter discs 1 and 2 into the space of the lower cutter discs 1' and 2' of Fig.2, the deformed part of the blank being still in an unseparated state. The blank

104 1

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(a)THE FIRST STAGE

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Fig.2, Diagrammatic illustration of the principle of Roll Slitting. Fig.3. Metal flow in Roll Slitting.

is then shown in Fig.3 (b) in the state of partial engagement between the pressing-down rolls 3 and 4 of the flattening stand. The sheared-off faces a are in the slitting line of the blank, while burnished faces b are formed in the areas adjacent to the face a, in opposed relationship with each other. As the half-sheared blank is driven into the spacing between the rolls, the shear droops formed in the outer corners of the sheared edges of the blank are caused to be diminished to s0, due to the progress of the pressing-down effect of the rolls, and new droops ~ are increasingly formed on the opposite outer corners of the sheared edges to the droop ~, as the degree of engagement with the rolls progressively increases. In Fig.3(c) is shown the blank engaged further with the pressing-down rolls

105

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(b) Fig.4. Example of cut-off surface from Roll Slitting (mild steel sheet of 2.3-mm thickness). 3 and 4, in which the plastic deformability of the blank reaches the point that cracking occurs, for example, from the leading ends A and A' of the deformed areas of the blank, the cracks then growing further until they finally join. Thus the initial partially-sheared blank is now parted into the final slit products I, II and III as seen in Fig.2 (c). In Fig.4 (a), in which further details of the cut-off face of the slit product II are illustrated, there is shown an aspect of the product having a small survival droop ~ ' and a fractured surface f generated between the sheared face and the burnished face b, yet having no burrs at all. It should be noted that the droop ~ formed at the first stage is now relatively small in the final slit product, while there is obtained in contrast a relatively large droop ~' from the second pressing-down or flattening stage. In Fig.4 (b) is presented an example of the longitudinal cut-off surface of a burr-free product of Roll Slitting, as viewed from the arrow direction shown in Fig.4(a). Finally, consideration of the two burr-free slitting methods of CounterCutting and Roll Slitting might lead to an impression t h a t both have some correlation with _previously developed burr-free blanking methods [4--6] in respect of their pure burr-free principles. However, it will be noted that, although all of the techniques have a burr-free feature in c o m m o n , the former group of techniques, (i.e. burr-free slitting) belong to a quite different field of art from the latter group (burr-free blanking) and consequently that both of the groups have quite different problems to be solved for quite different industrial significances or advantages to be obtained. 4. Successful conditions for Roll Slitting

4.1 Selection o f overlap value L It will be appreciated on one hand that the raw material will be severed during the first stage if the overlap L (presented with a minus value when the upper and lower cutters are apart from each other in the vertical direction as in the case of Fig.2) is preset too much toward the positive direction and will result in a conventional slit product with burr, and on the other hand

106

that if the overlap is preset t o o much toward its mi:nus direction the material will be only pushed back with no separation during the second stage. Accordingly, the first key point for successful Roll Slitting is to select the appropriate overlap L in the first stage. In Fig.5 there is shown the successful overlap range for Roll Slitting obtained from experiment using a specially made test slitter [2,3] where the metal strip is pulled through both the cutters and the rolls by means of a horizontal force applied along the length axis o f the strip. As seen in the graph, the results show that if an overlap in the first stage is selected such that the corresponding shear force or pulling force exists in the proximity of the respective maximum points of load, then that overlap will be successful. The results also show that the successful overlap lies in a range where the corresponding roll force or pulling force at the second stage is decreasing from the peak value. SHEARING FORCE AT THE FIRST STAGE ~ x -J --1

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107

4.2 Selection o f clearance value C The second key point for successful Roll Slitting is the selection of the appropriate cutter clearance value for the first stage. As shown in Fig.6, it is anticipated that if a large positive clearance (see Fig.2) is selected, a burr-free feature will not be obtained. This matter was further studied and the result is shown in Fig.7. From the graph it is found that the marginal clearance for

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Fig.6. I n f l u e n c e o f c u t t e r c l e a r a n c e o n t h e first stage of Roll Slitting, for l - r a m t h i c k n e s s mild steel strip. R e m a r k s : ® g o o d slit surface; o fair slit surface; ~ p o o r slit s u r f a c e ; × b a d slit surface (slit with burr); the arrows indicate the unsuccessful range (no separation).

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108

successful burr-free slitting is n o t absolute, but relative to the thickness of the material. As seen f r o m the trend of Fig.7, a clearance value of +5% is marginal, with either a thickness of 1 m m or 3.2 m m for t he test material. Also, it is generally seen th at as the clearance grows in t he negative direction, the success range increases. Although this may suggest t hat the clearance should be preset at a negative value, it.is desirable in general to preset the clearance at zero, since as the clearance increases in the negative direction not only is the profile of the cut-off face worsened (Fig.8) with poor breadth accuracy AB of the slit p r o d u c t (Fig.6), but also more elaborate work in setting of the cutters will be needed. (CUTTER PENETRATION = 6 9 %t)

2_ -9.4

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+3.1 (%t)

Fig.8. Influence of clearance over the profile of the cut-off face (work material: 1.6-mm thickness mild steel strip).

5. Influence o f radius o f cutting edge One of the problems anticipated for the industrial application of Roll Slitting is the manner in which the success range, the breadth accuracy and the cross-sectional profiles, vary as the tip profile of the cutter disc is rounded, for example, by its wear. Figure 9 shows the relationship between the success range and the radius R applied t o the cutting edge of t he cutter discs, with the clearance as parameter. It is shown in the figure t h a t as R grows, b o t h the upper and lower successful cut t er penetration limits (shown in the figure as A and B, respectively) in the first stage to allow separation in the second stage, rise, regardless of the clearances, and thus the successful range as a whole moves in such direction that the ratio of c u t t e r penetration (=(t+L)/t) increases. The test result also shows t hat for 3% clearance the success range narrows as the radius R increases to m ore than 0.1 mm and finally disappears at a radius of 0.25 mm. Figure 10 shows the relationship bet w e e n the clearance C and t he success range with R being preset to zero and 0.2 mm, respectively. F r o m the graph

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it is obvious t h a t at a large R value a relatively wide success range can be obtained if a small negative clearance is selected. Figure 11 shows t h e relationship b e t w e e n t h e change o f AB and t h e c u t t e r p e n e t r a t i o n r a t i o w h e n t h e c u t t e r tip radius is large (R = 0.2 m m ) , t h e p a r a m e t e r being C. As s h o w n in this figure, a small AB is o b t a i n e d at a C value ranging f r o m zero t o positive, as in t h e case o f Fig.6 in w h i c h R =0. The c h a r t also shows t h a t o n c e t h e C value is d e t e r m i n e d AB varies little b y the p e n e t r a t i o n ratio in t h e first stage, this being also t r u e in the case w h e r e R = O . In a d d i t i o n , as seen in Fig.12 which shows AB versus C for various R

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values, AB is little affected b y the R value and is determined exclusively by the C value, except when the latter has a positive value. In Fig.13 is shown the variation of cut-off face profile obtained by the Roll Slit method as the R value is changed, the parameter being the clearance. As seen in the figure, the profile is little influenced b y the change of R value.

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Fig.12. Relationship b e t w e e n breadth deviation and clearance for various c u t t e r tip radii (work material: 1.6-ram thickness mild steel strip).

111 R=Omm

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Fig.13. Variation of sectional profile of roll slit material for various cutter-tip radii (work material: 1.6-mmthickness mild steel strip).

6. Work hardening of Roll Slit product It is possible that the slit edge may cause a problem, due to its work hardening in particular, when the strip material cut b y this method is directly bent or welded along the cut-off edge surfaces. Figure 14 shows the work-hardening curves of conventionally-slit products and roll split products, respectively. It may be deduced, that in the case of Roll Slitting a strongly work-hardened strip is obtained, since the strip is pressed d o w n in the second-stage rolls. However, the strip edge according to Roll Slitting is in fact equally, or only a little more, work hardened when compared with the product of conventional slitting, as shown in Fig.14 (the Vicker's Hardness Number of the material is a b o u t 120). On the other hand, from the flat bending test in which the strip was bent through 180 ° in its longitudinal direction, with the outer peripheral portion of the bent material being the burr side in the case of conventional slitting and the burnished side in the case of Roll Slitting, it was found that conventionally-slit strip was liable to cause large visible cracks adjacent to the burr portion, b u t that the roll-slit strip showed no such defects.

112

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Fig. 14. Work hardening curves for (A) Roll Slitting, (B) conventional slitting (work material: 2.3-mm thickness mild steel strip).

7. Roll slittability and the material F r o m the results o f field tests using a p r o t o t y p e practical Roll Slitter (Fig. 15) it was f o u n d t h a t o r i e n t e d silicon steel, as well as p u r e and alloyed alumin i u m coil strips, all o f which were o f 0 . 3 - m m thickness, c o u l d be successfully slit with n o b u r r and with a n e a t c u t - o f f face. It is n o t believed t h a t strips o f

113

Fig.15. Proto-type practical slitter employing the Roll Slit method.

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UPPER LiMiT PENETRATION X0(°/o) Fig.16. Relationship between the upper limit penetration and the lower limit penetration f o r R o l l S l i t t i n g . ( T h e d o t t e d l i n e is f o r X 1 = X 0 ).

114

such low thickness have been successfully handled by the prior Counter-Cut method. Generally speaking, if the width of the success range per unit thickness of material is taken as the roll slittability index, aluminium and its alloys show the best roll slittability and pure copper, steel and alloyed steels show the second best, while 60/40 brass shows a poor roll slittability. According to the experimental results shown in Fig.16 in which various commercial metal sheets were roll slit by the test slitter, it was found that X1 (the lower limit penetration below which the material is only pushed back without separation in the second stage) was roughly proportional to X0 (the upper limit penetration above which the material is completely severed with burr in the first stage). From this result the success range per unit thickness (Xo--XI) is anticipated also to be approximately proportional to X0, since X1 is proportional to X0. Since the value of X0 in turn represents the so-called shear ductility of the material to be slit, the Roll Slit method, it is deduced, can be suitably applied with a wide successful overlap range to such materials as have good shear ductility. In respect of another roll slittability index, i.e. the a m o u n t of shear droop 13 of the second stage, it was found that this can be correlated to the uniform elongation 6 of the work material in simple tension, as shown in Fig.17. Since

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115

the a m o u n t of ~ is known to be correlated also to the elongation,/~ is consequently anticipated to be proportional to a. From further experimental data (not presented herein) this anticipation is f o u n d to be valid, with the ratio of /3/~ having a value of approximately 1.5. 8. Wear of the cutter disc Oriented silicon steel coil of 0.3-mm thickness with a thin insulation film on both surfaces was selected as the test material for tests comparing the wear of cutters in conventional slitting with the wear of cutters in roll slitting. The experiments were carried o u t by means of a specially designed mini slitter line [3,7]. Although this material is required to be slit with virtually no burr formation -- since the existence of such burr on the product edge will cause, for example, the severe disadvantage of short circuit in use -- it is nevertheless a fact that this material causes the severest cutter wear with premature growth of burr if it is slit by the conventional method, as shown in Fig.18. The only counter-measure has been to grind off the burr by means of an expensive grinding line.

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Fig.18. R e l a t i o n s h i p b e t w e e n t h e b u r r h e i g h t a n d t h e c u t t e r r e v o l u t i o n s ( w o r k m a t e r i a l : 0 . 3 - m m t h i c k n e s s o r i e n t e d silicon steel coil).

116 c

END FACE

1 Fig.19. (a) and (b), SEM p h o t o m i c r o g r a p h s of the w o r n tip-portion of the cutter-disc which is presented as (c) (work material: 0.3-mm thickness oriented silicon steel coil).

In the case of Roll Slitting, however, the material was successfully slit without burr for up to 10000 integrated cutter revolutions for the initial cutter overlap value, plus a further 5000 revolutions for the second deeper overlap reset, affording a total cutter life of 15000 integrated revolutions (the cutter material being JIS S K D l l ) . Figures 19(a) and 19(b) show SEM (Scanning Electron Microscope) photomicrographs of the cutter tip-portion (Fig.19(c)) after 10000 cutter r e v o l u t i o n s when employed in the Roll Slit method. From these photomicrographs it is found that the cutter was worn away abrasively, leaving the carbides in relief on the cutter end-face. In addition to a burr-free feature, the method also showed the feature of less cutter wear when compared with conventional slitting, as shown in Fig.20.

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117

The reason for this would seem to be that less slippage of work material occurs on the end-face of the cutter during the slitting operation, since, as shown in Fig.21, a quite different characteristic of slippage between Roll Slitting and conventional slitting was observed in the cutter wear test using Bainite hardened steel coil with a blue finish [7]. In Fig.21 (b) it is clearly seen t h a t in the case of conventional slitting there is a trace of heavy slippage toward the cutting edge over the end-face, which is believed to have occurred due to a strong spring-up movement of the material when severed through, whereas in the case of Roll Slitting, Fig. 21(a), which has no sever-through during the first stage, just the state of carbides in mild relief was found in place of heavy slippage.

Fig.21. SEM photos showing the end face of worn cutter-tip portions after 10000 integrated cutter revolutions, (a) Roll Slitting, (b) conventional pull--cut type slitting (work material: Bainite hardened steel coil of 0.3-ram thickness).

9. Conclusions (1) A burr-free slit product can be obtained by subjecting the coil stock material to partial-shearing action by means of rotatable circular cutters as the first stage and then by flattening the thus partially-sheared material to its original thickness by means of cylindrical rolls as the second stage. (2) For this burr-free m e t h o d -- called the "Roll Slit" m e t h o d -- to be successful, it is necessary in the first stage to select a cutter overlap which is near the m a x i m u m shear force point, with the cutter clearance being preset preferably at, or near to, zero. (3) Poor squareness of the cut-off face and larger deviation of breadth in the slit product can result as the clearance value increases in the negative direction, even though the successful overlap range tends to increase.

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(4) Roundness of the cutting edge of the cutter disc has little disadvantageous influence on the quality of the product. (5) Aluminium and its alloys, as well as silicon steel, all of 0.3-mm thickness, were successfully slit w i t h o u t burr, using a proto-type practical slitter using the Roll Slit method. (6) The method seems to be particularly suited to material having a good shear ductility. (7) A 0.3-mm thickness oriented silicon steel coil with thin insulation films on both surfaces -- which shows a premature burr growth during conventional slitting -- was successfully slit without burr employing the Roll Slitting method for up to 15000 integrated cutter revolutions, using a mini-slitter with cutter discs of 20-mm diameter. Acknowledgements The authors are very grateful to Dr Takeo Nakagawa of the University of Tokyo for his invaluable suggestions and discussions. The authors would also like to express their deep gratitude to Mr Sachihiro Iida for his indispensable efforts in carrying out experiments. The authors' thanks are also due to many manufacturing companies which kindly offered various assistances and support for this research as well as to Dr Kiyota Yoshida of The Institute of Physical and Chemical Research for his encouragement and Mr. Isamu Aoki of the University of T o k y o for his valuable information and discussion. References 1 2 3 4 5 6 7

J. Brockhaus and H. Singer, BLECH-Rohre, 8 (1970) 16. T. Maeda and M. Murakawa, J. Jpn. Soc. Technol. Plasticity, 18 (1977) 114. T. Maeda and M. Murakawa, J. Faculty Eng., The Univ. of Tokyo (B), 34 (1977) 377. T. Maeda, Science of Machine, 10 (1958) 140. I. Makino, Press Technique, 13 (1975) 93. M.D. McShurley, Canadian Pat. Spec. No. 502, 628 (1954). T. Maeda, M. Murakawa and S. Iida, J. Jpn. Soc. Technol. Plasticity, 18 (1977) 553.