handbook of die design 2nd edition phần 5 pot

Suchy_CH06.qxd 11/08/05 10:54 AM Page 287 BLANKING AND PIERCING OPERATIONS BLANKING AND PIERCING OPERATIONS FIGURE 6-42 287 Chips within the die The punch may often need to be made slightly smaller than the required size of the hole, as the opening often closes up on its retrieval Approximately 0.001 in (0.03 mm) per diameter reduction in punch size is advisable Where rough edges of the punched opening are obtained, a shaving operation may be needed Sometimes an alternative material-removing method should be chosen to alleviate such a problem Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH06.qxd 11/08/05 10:54 AM Page 288 BLANKING AND PIERCING OPERATIONS Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH07.qxd 11/08/05 10:55 AM Page 289 Source: HANDBOOK OF DIE DESIGN CHAPTER BLANK CALCULATION OR FLAT LAYOUT 7-1 THE IMPORTANCE OF FLAT LAYOUT OR BLANK LAYOUT The importance of an accurate flat layout has been stressed throughout the preceding text In die work and any sheet-metal work in general, the importance of an accurate and dimensionally correct flat layout cannot be overemphasized Many problems may be avoided if a full-sized or scaled layout is made first and the part’s manufacture is evaluated on the basis of it, instead of referring to the bent-up drawing, which, after all, may or may not be manufacturable 7-1-1 Flat Layout Development and Calculation When making a flat layout of a complex part, it always helps to start from a certain side, one that seems to be basic or the most complex, and unfold the remaining portions bend after bend A bracket, shown in Fig 7-1, is simple enough to serve as an example of the unfolding technique First, the A flange may be flattened out to become flush with the vertical portion This should be done visually, just by looking at the illustration and imagining the flange rotating around its pivoting point or an axis of rotation, as if retained by hinges Such axis of rotation is always located in the center of bend radius Next, the whole vertical segment should be folded down, to become a flat continuation of the horizontal section, as shown in Fig 7-2 Such a flattened portion may be sketched, adding other segments to it as they are unfolded To provide the flat layout with dimensions, we start off a single corner and all dimensioning with regard to that location (for a dimensioned drawing, see Fig 7-3) However, when checking the numbers later we should calculate them off another location and see if the results will be the same A sample of flat layout for the bracket is included in Fig 7-4 When calculating the dimensions in flat, flange A may be assessed off the top of the vertical section (see Fig 7-3) by subtracting its height from the overall dimension, or 3.875 − 2.25 = 1.625 in [98.43 − 57.15 = 41.28 mm] This dimension is included in Fig 7-3 as a reference, added for flat layout purpose only On an actual drawing such a dimension will not be appropriate, as it is already expressed by the difference between 3.875 and 2.25 in [98.43 and 57.15 mm] 289 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH07.qxd 11/08/05 10:55 AM Page 290 BLANK CALCULATION OR FLAT LAYOUT 290 FIGURE 7-1 CHAPTER SEVEN Sample bracket Since the depth of relief slots is not indicated, it is probably not overly relevant, and in such a case we can make these cuts as deep as necessary Usually, a depth equivalent to the distance measured off the outside surface to the center of the bend radius plus one material thickness, with up to 2t will suffice as shown in Fig 7-5 The width of the relief slot is not specified either, which similarly allows for a variation Usually a size of at least one material thickness may suffice, preferably with 1.5t to 2t FIGURE 7-2 Flat layout preparation Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH07.qxd 11/08/05 10:55 AM Page 291 BLANK CALCULATION OR FLAT LAYOUT BLANK CALCULATION OR FLAT LAYOUT FIGURE 7-3 FIGURE 7-4 291 Bent-up bracket, with dimensions Bracket, flat layout Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH07.qxd 11/08/05 10:55 AM Page 292 BLANK CALCULATION OR FLAT LAYOUT 292 CHAPTER SEVEN FIGURE 7-5 Bend allowance determination The depth of the relief slot’s bottom must therefore be calculated by using the dimension 1.625 + OuterRad + 1.5t = 1.625 + 0.093 + 0.093 = 1.812 in [41.28 + OuterRad + 1.5t = 41.28 + 2.36 + 2.36 = 46.00 mm] The length of the flange can be calculated by adding the depth of the relief slot measured off the top of the flange (= 0.187 in or 4.75 mm) plus the length of the flange (= 0.812 in or 20.62 mm) and subtracting one bend allowance, or 0.187 + 0.812 − 0.093 = 0.906 in [4.75 + 20.62 − 2.36 = 23.01 mm] Bend allowance may be taken off the charts in Chap Table 7-1 is an additional bend allowance chart, widely used in sheet-metal fabrication, where press-brake bending using V-dies is prevalent The chart is added to permit a wider range of comparison of dimensions in flat, which sometimes are advisable to calculate using several different techniques To use this chart, outside dimensions of each bend must be added up, with one bend allowance per bend subtracted from the total Where the material thickness or bend radius are not included, such information may be calculated by using the formula π BA = 2t −  (0.45t + BR) − BR   2  (7-1) For a so-called sharp bend, the formula will become π BA = 1.5t −  (0.50t + BR) − BR 2    (7-2) where BA = bend allowance BR = bend radius t = material thickness 0.45t = 45 percent shrink allowance for a radiused bend 0.50t = 50 percent shrink allowance for a sharp bend Continuing with the evaluation of a flat layout shown in Fig 7-4, some dimensions may be found to double each other by expressing the same information calculated off two different ends of the part It sometimes pays to include these on the flat layout, especially Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website 0.015 0.018 0.024 0.031 0.036 0.047 0.054 0.062 0.078 0.093 0.109 0.125 0.140 0.156 0.171 0.187 0.203 0.218 0.234 0.250 0.026 0.030 0.037 0.047 0.053 0.067 0.076 0.087 0.107 0.127 0.147 0.168 0.187 0.208 0.228 0.248 0.269 0.288 0.309 0.330 0.015 0.033 0.037 0.044 0.053 0.060 0.074 0.083 0.093 0.114 0.134 0.154 0.175 0.194 0.215 0.234 0.255 0.276 0.295 0.316 0.337 0.031 0.046 0.050 0.058 0.067 0.073 0.087 0.096 0.107 0.127 0.147 0.168 0.188 0.208 0.228 0.248 0.268 0.289 0.309 0.329 0.350 0.062 0.059 0.063 0.071 0.080 0.086 0.101 0.110 0.120 0.141 0.160 0.181 0.202 0.221 0.242 0.261 0.282 0.302 0.322 0.343 0.363 0.093 0.073 0.077 0.085 0.094 0.100 0.114 0.123 0.134 0.155 0.174 0.195 0.215 0.235 0.255 0.275 0.295 0.316 0.336 0.356 0.377 0.125 0.086 0.090 0.098 0.107 0.114 0.128 0.137 0.147 0.168 0.187 0.208 0.229 0.248 0.269 0.288 0.309 0.329 0.349 0.370 0.390 0.156 0.100 0.104 0.111 0.120 0.127 0.141 0.150 0.160 0.181 0.201 0.221 0.242 0.261 0.282 0.301 0.322 0.343 0.362 0.383 0.404 0.187 0.113 0.117 0.125 0.134 0.140 0.154 0.163 0.174 0.194 0.214 0.235 0.255 0.275 0.295 0.315 0.335 0.356 0.375 0.396 0.417 0.218 0.127 0.131 0.138 0.147 0.154 0.168 0.177 0.187 0.208 0.228 0.248 0.269 0.288 0.309 0.328 0.349 0.370 0.389 0.410 0.431 0.250 Bend radius 0.140 0.144 0.152 0.161 0.167 0.181 0.190 0.201 0.221 0.241 0.262 0.282 0.302 0.322 0.342 0.362 0.383 0.403 0.423 0.444 0.281 0.153 0.157 0.165 0.174 0.180 0.195 0.204 0.214 0.235 0.254 0.275 0.296 0.315 0.336 0.355 0.376 0.396 0.416 0.437 0.457 0.312 0.167 0.170 0.178 0.187 0.194 0.208 0.217 0.227 0.248 0.267 0.288 0.309 0.328 0.349 0.368 0.389 0.410 0.429 0.450 0.471 0.343 0.180 0.184 0.192 0.201 0.208 0.222 0.231 0.241 0.262 0.281 0.302 0.323 0.342 0.363 0.382 0.403 0.423 0.443 0.464 0.484 0.375 0.194 0.198 0.205 0.214 0.221 0.235 0.244 0.254 0.275 0.295 0.315 0.336 0.355 0.376 0.395 0.416 0.437 0.456 0.477 0.498 0.406 0.207 0.211 0.219 0.228 0.234 0.248 0.257 0.268 0.288 0.308 0.329 0.349 0.369 0.389 0.409 0.429 0.450 0.469 0.490 0.511 0.437 0.234 0.238 0.246 0.255 0.261 0.275 0.284 0.295 0.315 0.335 0.356 0.376 0.396 0.416 0.436 0.456 0.477 0.497 0.517 0.538 0.500 10:55 AM (Continued ) 0.220 0.224 0.232 0.241 0.247 0.262 0.271 0.281 0.302 0.321 0.342 0.363 0.382 0.403 0.422 0.443 0.463 0.483 0.503 0.524 0.468 11/08/05 Inches Material thickness TABLE 7-1 Bend Allowance Chart for Sheet-Metal Material, V-Die Bending∗ Suchy_CH07.qxd Page 293 BLANK CALCULATION OR FLAT LAYOUT 293 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website 0.84 0.97 1.14 1.36 1.62 1.87 2.13 2.39 2.91 3.17 3.94 4.20 4.85 5.49 6.14 6.79 7.43 8.08 9.37 1.16 1.29 1.46 1.68 1.94 2.20 2.45 2.71 3.23 3.49 4.26 4.52 5.17 5.82 6.46 7.11 7.76 8.40 9.70 1.50 1.59 1.72 1.89 2.11 2.37 2.62 2.88 3.14 3.66 3.92 4.69 4.95 5.60 6.25 6.89 7.54 8.19 8.83 10.13 2.50 2.02 2.15 2.32 2.54 2.80 3.05 3.31 3.57 4.09 4.35 5.12 5.38 6.03 6.67 7.32 7.97 8.61 9.26 10.55 3.50 2.23 2.36 2.53 2.75 3.01 3.27 3.53 3.79 4.30 4.56 5.34 5.60 6.24 6.89 7.54 8.18 8.83 9.48 10.77 4.00 2.56 2.69 2.85 3.07 3.33 3.59 3.85 4.11 4.63 4.88 5.66 5.92 6.56 7.21 7.86 8.50 9.15 9.80 11.09 4.75 Use the outside dimensions of each bend and subtract the bend allowance shown 0.73 0.86 1.03 1.25 1.51 1.77 2.03 2.28 2.80 3.06 3.84 4.09 4.74 5.39 6.03 6.68 7.33 7.97 9.27 0.75 2.88 3.01 3.18 3.40 3.65 3.91 4.17 4.43 4.95 5.21 5.98 6.24 6.89 7.53 8.18 8.83 9.47 10.12 11.41 5.50 3.20 3.33 3.50 3.72 3.98 4.23 4.49 4.75 5.27 5.53 6.30 6.56 7.21 7.86 8.50 9.15 9.79 10.44 11.73 6.25 3.52 3.65 3.82 4.04 4.30 4.56 4.81 5.07 5.59 5.85 6.63 6.88 7.53 8.18 8.82 9.47 10.12 10.76 12.06 7.00 3.95 4.08 4.25 4.47 4.73 4.99 5.24 5.50 6.02 6.28 7.05 7.31 7.96 8.61 9.25 9.90 10.55 11.19 12.49 8.00 4.27 4.40 4.57 4.79 5.05 5.31 5.57 5.82 6.34 6.60 7.38 7.63 8.28 8.93 9.57 10.22 10.87 11.51 12.81 8.75 4.59 4.72 4.89 5.11 5.37 5.63 5.89 6.15 6.66 6.92 7.70 7.96 8.60 9.25 9.90 10.54 11.19 11.84 13.13 9.50 4.92 5.05 5.21 5.43 5.69 5.95 6.21 6.47 6.99 7.24 8.02 8.28 8.93 9.57 10.22 10.87 11.51 12.16 13.45 10.25 5.24 5.37 5.54 5.76 6.01 6.27 6.53 6.79 7.31 7.57 8.34 8.60 9.25 9.89 10.54 11.19 11.83 12.48 13.77 11.00 5.67 5.80 5.97 6.18 6.44 6.70 6.96 7.22 7.74 8.00 8.77 9.03 9.68 10.32 10.97 11.62 12.26 12.91 14.20 12.00 6.10 6.23 6.39 6.61 6.87 7.13 7.39 7.65 8.17 8.42 9.20 9.46 10.11 10.75 11.40 12.05 12.69 13.34 14.63 13.00 10:55 AM ∗ 0.40 0.50 0.63 0.80 1.00 1.20 1.40 1.60 2.00 2.20 2.80 3.00 3.50 4.00 4.50 5.00 5.50 6.00 7.00 0.50 Bend radius 11/08/05 Millimeters Material thickness TABLE 7-1 Bend Allowance Chart for Sheet-Metal Material, V-Die Bending∗ (Continued ) Suchy_CH07.qxd Page 294 BLANK CALCULATION OR FLAT LAYOUT 294 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH07.qxd 11/08/05 10:55 AM Page 295 BLANK CALCULATION OR FLAT LAYOUT BLANK CALCULATION OR FLAT LAYOUT FIGURE 7-6 295 Support bracket, bent-up part where certain locations will have to be constantly recalculated off different ends These dimensions may also serve as an efficient way of checking the flat layout, especially where the same person who drew and calculated it will have to check it and manufacture the part The flat length of the flange B should be calculated the same way as described previously Here, the difference between the outer edge of the horizontal portion and that of the flange should be figured starting off the bottom of the relief slot, which should be established first To verify the calculation, dimensions off the left edge of the blank should be used The bracket, shown in Fig 7-6 presents a slightly different problem Here, the flange A contains an oval indentation for strengthening Side flanges B1 and B2 show sharp-cornered cutouts of considerable size, which may cause problems in bending, because their bottom edge is located only 1.5t off the center of bend radius The dimensioned bent-up drawing of the part and its flat layout are included for study and comparison of results (see Figs 7-7 and 7-8) 7-1-2 Phantom Areas A part in Fig 7-9 containing so-called phantom areas is shown for evaluation Phantom areas are portions of unavailable material, not obvious from the bent-up drawing (see Fig 7-10) Only on observation of the flat layout (shown in Fig 7-11), it becomes clear that it will be impossible to obtain the shaded corners of the two side flanges from the material given A change of either part’s manufacturing process or that of its outline will have to be made If the outline must be kept, the manufacturing process can have the two corners welded on later and sanded smooth—an operation quite expensive, unpractical, and cumbersome Some may volunteer to weld both sides to the flat bottom, which will totally defeat the practicability of die-manufacturing process A folded-up drawing of the part with dimensions and its flat layout are included for personal comparison and study Note the width of the bottom flat portion being made 0.81 + 0.00/−0.01 in [20.6 + 0.0/−0.2 mm] on the flat layout, in congruence with the folded-up drawing shown in Fig 7-10, where a material-thickness wide gap is shown between Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH07.qxd 11/08/05 10:55 AM Page 296 BLANK CALCULATION OR FLAT LAYOUT 296 FIGURE 7-7 CHAPTER SEVEN Support bracket Bent-up part with dimensions the bottom and inner walls of the enclosure The tolerance range in this case depends on several variables, such as • Material thickness and its tolerance, especially the increase in thickness • Tolerance requirements for the 1.0 in [25 mm] overall width of the part, as shown in Fig 7-10 FIGURE 7-8 Support bracket, flat layout Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH08.qxd 11/08/05 10:57 AM Page 344 BENDING AND FORMING OPERATIONS 344 CHAPTER EIGHT FIGURE 8-23 Rotary bending process using Ready Benders® (Reprinted with permission from Ready Technology, Inc., Dayton, OH Patent Number 5,404,742.) Ready Benders® regulate the springback of the material by overbending, rather than coining As a result, a lesser amount of material from the radius area becomes relocated, which is the reason for a greater bend allowance than that of wipe bending The general formula for the bender’s bend allowance is BAbender = 0.01745PA(PR + 0.43PT) (8-9) where all values are as shown in Fig 8-24 Ready Technology Co has another tool called a Ready Hemmer®, which can form a 90° bend completely flat in a single stroke of the press, as shown in Fig 8-25 This tool can be used not only for flat hems, but also when forming over an insert, as shown in Fig 8-26 The advantages are easier handling of the part in the die, and reducing a hemming operations to two press strokes versus the normal three press strokes with a conventional hemming tool Producing stiffening ribs with rotary benders leaves the parts free from galling or distortions (Fig 8-27) When compared to wipe-produced ribs, the benders not need the FIGURE 8-24 Bend allowance in rotary bending (Reprinted with permission from Ready Technology, Inc., Dayton, OH Patent Number 5,404,742.) Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH08.qxd 11/08/05 10:57 AM Page 345 BENDING AND FORMING OPERATIONS BENDING AND FORMING OPERATIONS 345 FIGURE 8-25 The Ready Hemmer® and its function (Reprinted with permission from Ready Technology, Inc., Dayton, OH Patent Number 5,404,742.) FIGURE 8-26 Flat hem and a hem formed over an insert (Reprinted with permission from Ready Technology, Inc., Dayton, OH Patent Number 5,404,742.) FIGURE 8-27 Edge-stiffening ribs produced with Ready Benders® (Reprinted with permission from Ready Technology, Inc., Dayton, OH Patent Number 5,404,742.) Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH08.qxd 11/08/05 10:57 AM Page 346 BENDING AND FORMING OPERATIONS 346 CHAPTER EIGHT amount of maintenance a wipe punch would require and the consistency of the forming process is greater 8-6-4-1 Bending With Rotary Inserts This type of bending is actually an alternative of U-die application, with rotary inserts placed at the corners of a die (Fig 8-28) The inserts are spring-loaded, allowing the material to land upon them, and retracting under the press force On release, inserts force the part up One advantage of this process is the possibility of overbending the flanges as a protection against spring-back occurrence 8-6-4-2 Bending With a Pivoted Roller The roller is attached to the punch plate by a pin As the upper section of the die slides down, the roller engages the material, forcing it down and under the nose in the die block (see Fig 8-29) 8-6-5 Bending With Flexible Tooling Bending with flexible tooling utilizes rubber or urethane forming pads instead of hard-metal tooling (Figs 8-30 to 8-33) Its advantage lies in the possibility of forcing the flexible tooling material to fill gaps and undercuts, taking the in-between sheet-metal strip along Forces necessary for bending with flexible tooling are higher than those needed for conventional bending methods These differences have not yet been assessed because of the great amount of variables involved The strain-hardening tendency of the material will have to be evaluated in comparison with the elastic properties of the flexible forming material, aside from other influencing aspects With V-die bending utilizing elastic material, the bending process is rather different from the one using hard tooling First, the sheet-metal material is compressed by the nose of a tool, elongating tangentially under its pressure (see Fig 8-33) The pressure of the elastic pad is restricted to quite a small area immediately surrounding the tool impression The radius of the bent-up part usually ends up being larger than that of the tooling Enlarging the radius of the V-die does not readily solve the problem, as a considerable enlargement is needed for the part to follow the shape of the punch However, there are no directions to follow in such an undertaking, as all the work on a subject has been arrived at experimentally, with not enough data collected yet FIGURE 8-28 Bending with rotary inserts Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH08.qxd 11/08/05 10:57 AM Page 347 BENDING AND FORMING OPERATIONS BENDING AND FORMING OPERATIONS FIGURE 8-29 347 Roller-type bending die 8-6-6 Forming With Cams Cams are unique arrangements, which, as driven by the power of a press, can produce a form of the side of a part, pierce openings from various directions, and even produce spring-back-free bends Cams achieve these tasks by transforming the vertical motion of the press ram into a horizontal or inclined motion of the cam slide Of course, this all is done at a cost The cost of cam mechanisms is always higher than the cost of regular punches and FIGURE 8-30 Forming with flexible tooling Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH08.qxd 11/08/05 10:57 AM Page 348 BENDING AND FORMING OPERATIONS 348 CHAPTER EIGHT FIGURE 8-31 Bending with flexible tooling dies, which is the reason why die designers turn to the cam-involving solutions only when everything else fails The two cam dies shown in Fig 8-34 represent two types of cam movements: (a) that with a spring-operated return of the slide, and (b) that with a cam-operated return In each case, the punch only rough-forms the part, while the cam-driven slide finishes the forming The movement of the cam in the first illustration is guided over adjustable and replaceable inserts, which, when attached in strategic locations, are capable of prolonging the life of the cam mechanism In cam design, a 45° minimum and 50° preferred angle of inclination of driving surfaces is important In Fig 8-35, the piercing punch is guided by a horizontally-oriented guide FIGURE 8-32 Complicated forming and trimming, flexible tooling Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH08.qxd 11/08/05 10:57 AM Page 349 BENDING AND FORMING OPERATIONS BENDING AND FORMING OPERATIONS FIGURE 8-33 FIGURE 8-34 349 V-bending with elastic tooling Two types of cam movement Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH08.qxd 11/08/05 10:57 AM Page 350 BENDING AND FORMING OPERATIONS 350 FIGURE 8-35 CHAPTER EIGHT Side-piercing cam arrangement bushing, which is spring-loaded to retain the pierced part during the retrieval of the punch The punch is equipped with a spring-loaded central pin for a positive ejection of the slug The whole punch assembly is tied together and it is returned into its original location via spring force on retrieval of the cam driver The opening for slug removal may need to be inclined in some cases, while a straight hole will suffice in other situations The surfaces of the slug-removing path should be flame hardened for greater resistance to damage 8-6-6-1 Cam With a Dwell Cam with a dwell is sometimes used to prolong the forming operation, or where additional procedures are to follow and a time for their accomplishment is needed (Fig 8-36) This type of a cam can be implemented in cases where its path of travel does not need any further adjustment FIGURE 8-36 Cam with a dwell Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH08.qxd 11/08/05 10:57 AM Page 351 BENDING AND FORMING OPERATIONS BENDING AND FORMING OPERATIONS 351 During the dwell period of the cam function, the slide is remaining pushed against the formed part motionless, even though the press ram is still descending Care must be taken when calculating the height of the assembly, so that a wider shank of the cam does not hit the slide The tip of the cam should not come into a contact with the die shoe either, and where a questionable situation exists, a relief opening in the shoe must be provided Movement-wise, a calculation of the actual movement of the slide with a provision for variation in the stock thickness must be performed It is often advisable to provide a largerthan-required gap for the formed material in order to account for all its differences in thickness Fine-tuning of the distance traveled can be accomplished by shimming the assembly where appropriate 8-6-6-2 Miscellaneous Cams Standardized cam units can now be purchased for various uses As a sample, an aerial cam slide unit is shown in Fig 8-37, and a die-mount cam slide is in Fig 8-38 These cam assemblies have soft mounting surfaces for standard or custom applications, plus hardened, self-lubricating wear plates Standard angles are available in the range from 25° through 50° and special angles can be ordered upon request A slide movement producing a curl is shown in Fig 8-39 Here the material to be formed is nested in the die block, where it is further secured in its position by the approaching slide When the exposed edge of material encounters the beginning of a radius in the slide, it starts to follow its shape, forming a curl The location of a burr on the sheet-metal material is of importance in this process, as it should always be positioned away from the forming surface of the tooling Flipping the burr to face the slide may obstruct the curling action and produce deformation of the part instead In some cases, it may also scratch the surface of the tool’s curling section 8-6-7 Bending of Miscellaneous Materials Various bending calculations that not fit into any described category are presented here for possible evaluation and use They have proved quite accurate for certain materials and applications FIGURE 8-37 Aerial cam slide unit (Reprinted with permission from Danly IEM, Cleveland, OH.) Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH08.qxd 11/08/05 10:57 AM Page 352 BENDING AND FORMING OPERATIONS 352 CHAPTER EIGHT FIGURE 8-38 Die-mount cam slide unit (Reprinted with permission from Danly IEM, Cleveland, OH.) Bending of soft copper and soft brass The formula to calculate bend allowance is Eq (8-10) Its application is the same as previously described BACu = CRin + (0.55t) (8-10) where Rin = inside radius of the bend t = material thickness C = constant, depending on the angle of the bend For 90° bends, this value is 1.5708 For bends of different angularity, see Table 8-7 Further calculation of the total length of the part Ltotal is performed the same way, as shown in Eqs (8-5a) or (8-5b) FIGURE 8-39 Curling cam operation Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH08.qxd 11/08/05 10:57 AM Page 353 BENDING AND FORMING OPERATIONS BENDING AND FORMING OPERATIONS 353 Bending of half-hard copper, half-hard brass, and half-hard steel The formula to obtain the bend allowance is BA1/2Cu = CRin + (0.64t) (8-11) To calculate the total length, use the bend allowance obtained this way in Eqs (8-5a) or (8-5b) Bending of hard copper, bronze, cold-rolled steel, and spring steel The bend allowance formula is BAspring = CRin + (0.71t) (8-12) Use Eqs (8-5a) or (8-5b) to calculate the total length of the part 8-7 SPRINGBACK Springback is the amount of elastic distortion a matrial has to go through before it becomes permanently deformed, or formed It is the amount of elastic tolerance, which is to some extent present in every material, be it a ductile, annealed metal or a hard-strength maraging steel In ductile materials, the springback is much lower than in hard metals, with dependence on the modulus of elasticity (also called Young Modulus) of a particular material The amount of springback increases with greater yield strength or with the material’s strain-hardening tendency Cold working and heat treatment both increase the amount of springback in the material Comparably, the springback of low-strength steel material will be smaller than that of high-strength steel and springback of aluminum will be two or three times higher yet Springback occurs in all formed or bent-up parts on release of forming pressure and withdrawal of the punch The material, previously held in a predetermined arrangement by the influence of these two elements, is suddenly free from outside restrictions and immediately makes an attempt to return to its original shape and form (Fig 8-40) FIGURE 8-40 Springback Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH08.qxd 11/08/05 10:57 AM Page 354 BENDING AND FORMING OPERATIONS 354 CHAPTER EIGHT The angle of the bent-up flange aB is greater than that altered by springback aS The same way, the radius RB increases on becoming affected by the springback RS However, the lengthwise portion W, which is the length of the arc, remains the same Its relationship to other areas of significance is given as t t W = α B  RB +  = α S  RS +  2 2   (8-13) From this relationship, a spring-back factor K can be obtained: t K= t RS + RB + (8-14) Usually springback can be found between 0.9 and 1.0 for bends, using small bend radii Equation (8-14) was proved true for bends with large bend radii or for those with small bend angles However, with small bend radii, it may be considered valid only if the bend angle has a greater than 45° bending angle For small bending angles and sharp bend radii, the spring back is usually quite large Values of spring back for steel are shown in Table 8-8 Shown in Fig 8-41, the yield stress of material is exceeded at a certain point, at which moment the whole deformation so far attained is elastic, or a springback Should we release the pressure at that moment, the material will return to its normal shape However, we continue to exceed the material’s elastic limitations, as we arrive at a point “A.” A line parallel to the material forming line can be drawn from this point and its horizontal difference from the point “A” is the value of springback Additional forming causes the material to become work-hardened, which moves us to the point “B.” Here, the material’s springback is greater, enhanced by work hardening qualities of the steel The slope of the material’s forming line is dependent on the Young modulus It is therefore pertinent to always specify the steel (or any material for that purpose) to be ordered within the same yield strength range A difference in yield strength will definitely produce variations in forming, in work hardening, and in the final outcome of metal stamping process TABLE 8-8 Springback Values for Steel Material Ratio R/t Angle of the bend Condition of material 30° 30° 60° 60° 90° 90° 120° 120° Annealed Hard Annealed Hard Annealed Hard Annealed Hard 10 0 0 −1 −1 −1 1 0 0 14 ˇ Source: Svatopluk Cernoch, Strojne technická pˇ cka, 1977 Reprinted with permission from SNTL ríruˇ ˇ Publishers, Prague, CZ Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH08.qxd 11/08/05 10:57 AM Page 355 BENDING AND FORMING OPERATIONS BENDING AND FORMING OPERATIONS 355 FIGURE 8-41 Force-elongation diagram showing the effects of work hardening and springback (From: MetalForming Magazine®, January 1999, by Stuart Keeler Reprinted with permission from PMA Services, Inc., Independence, OH.) 8-7-1 Springback Removal There are several methods of springback removal in bending, most of them utilizing either overbending or coining In Fig 8-42, the formed part’s sides are secured in their location after forming by a punch or die section, which most often coins the material The bending method shown in Fig 8-42a counts on the die’s curved bottom to return the formed part to its flat position on retrieval of the bending forces This will also force the sides toward the center, eliminating the springback effect In Fig 8-9 shown previously, a method utilizing ironing action against the bent up sides of the part is depicted Selective coining of material can be used for other purposes as well (see Fig 8-43) For example, by strategically coining strips of bent-up U-shapes, a bow, (i.e., camber) and perhaps twist as well, can be removed from the material The effect of the coining process is that of interruption of the flow of stress lines that would normally be present there, a residue from bending operation Bending of U-channel shapes presents more problems, though Another often-encountered flaw is the emergence of vacuum, which may develop during the downstroke of the press At that moment, the material is fully retained between the forming punch and die and may sticks either to the punch, or the die, in the latter case being difficult to eject Coating of critical surfaces, relieving some areas of contact, bending in two strokes, or increasing the spring pressure—none of this can help where vacuum tends to develop Air holes through the tooling, punch, die, and the spring pad, are still the best solution to this problem Production of large radiused (shallow) bends is yet another area of problems waiting to be solved Large radiused bends tend to spring back enormously, and are absolutely Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH08.qxd 11/08/05 10:57 AM Page 356 BENDING AND FORMING OPERATIONS 356 CHAPTER EIGHT FIGURE 8-42 Methods of springback control in bending (Pictures d and e are from: Practical Aids For Experienced Die Engineer, Die Designer, and Die Maker 1980 Reprinted with permission from Arntech Publishers, Jeffersontown, KY.) unpredictable when it comes to their shape retention, especially where large segments and softer materials are used Several preventive methods can possibly be used, some of them shown in Fig 8-44 Here the shallow form has its sections ironed right after the bend radius, to secure the bend formation The upper flanges are slanted, and additionally, the bottom of the part can be coined for further security of the shape Not always are all these remedies needed, but a combination of some may often be found beneficial FIGURE 8-43 Coining for removal of stresses in strips Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH08.qxd 11/08/05 10:57 AM Page 357 BENDING AND FORMING OPERATIONS BENDING AND FORMING OPERATIONS FIGURE 8-44 357 Springback control in a shallow-radiused part 8-7-2 Residual Stresses As already mentioned elsewhere in this publication, every forming operation consists of two types of deformation: elastic deformation and plastic Where the plastic deformation produces permanent changes in the part, that is bending, forming, and drawing, the influence of elastic deformation is but temporary On cessation of forming force, it allows the formed segment to almost completely negate its effect and return back to its preelastic shape and location This is a springback Yet, there is a portion of elastic deformation that cannot be totally released this way and which remains trapped within the material These small pockets of elastic stresses are called residual stresses and they can be found throughout the part’s geometry, locked in by the changes due to plastic deformation Evenly distributed residual stresses may cause but a slight dimensional distortion of the part However, if unevenly dispersed and acting in different directions, these stresses can produce warping, twisting, oilcan, and other defects It may sometimes happen, as a part is formed, that it comes out of the die almost perfect But an additional operation, be it piercing, trimming, additional forming, restriking, or welding, may suddenly produce an unexpected amount of distortion and the previously perfect part ends up in a scrap bin This is due to residual stresses, introduced into the material during its fabrication In welding, the immediate vicinity of the weld becomes stress relieved This in itself can have a profound effect on the part either immediately or later in service Then, due to cyclic loading, all defects tend to become emphasized with time and may cause the part’s collapse and perhaps total destruction Both residual stresses and springback can bring about a host of unexpected problems and a sound part design, along with a good tool design, combined with a good manufacturing practice cannot be overemphasized There are certain features encountered in sheet-metal parts that almost always produce greater than necessary stresses in formed parts Such features consist mainly of sharp corners, sharp bend radii, greater differences in height, to name but a few 8-8 SURFACE FLATNESS AFTER BENDING In bending, as in drawing, it is sometimes quite difficult to produce a flat surface on a part Especially with larger flat areas, these can easily become warped, bowed, inclined, Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Suchy_CH08.qxd 11/08/05 10:57 AM Page 358 BENDING AND FORMING OPERATIONS 358 CHAPTER EIGHT FIGURE 8-45 Straightening of sheet metal or otherwise distorted, but not flat Such a surface, when pressed upon by hand, snaps back and forth, like an oilcan, from which this occurrence took its name: an oilcan effect In order to prevent this from happening, ribs, joggles, and other strengthening indentations were introduced, to provide for the dimensional as well as functional stability of parts Objects that have to stand on a flat surface, like containers or cans, have their bottoms either bent or drawn in, keeping but a narrow rim of flat surface to stand on Some products have their contact surfaces offset, with a small ridge to provide for the necessary flatness Where the supposedly flat surface of an object is distorted or warped, no amount of hammering or presswork will make it straight again This is due to the mechanical properties of the material, which does not allow for any permanent alteration unless the elastic limit of the material is exceeded Part a in Fig 8-45 cannot be straightened by any feasible amount of pressure applied from above In order to flatten this surface, it must be reversed and supported on two extreme ends, as shown in part b In such a position, even a minute force will produce the flattening effect Another way known, a part such as this can be straightened, is when submitted to pressures excessive of its modulus of elasticity Using hydraulically produced forces these methods are sometimes resorted to, lately Of course, the cost of the necessary equipment can bring this solution out of reach of most manufacturers Annealing of the part may be found of help sometimes, provided there are no excessive residual stresses within its structure If such is the case and residual stresses will become relieved by the annealing process, severe distortion may result Localized annealing by a torch was usually not found effective Often, where parts cannot be formed to hold their shape, a double bending method or reverse forming is used (Fig 8-46) In such a case, the bend is first produced in the opposite direction to that which is desired The bend is then reversed until a correctly shaped product is obtained FIGURE 8-46 Method of bending and rebending for accuracy Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website ... 0.1 25 0.187 0. 250 0. 250 0. 250 0. 250 0. 250 0.3 75 0.3 75 0.3 75 0.6 25 0. 750 Millimeters 0. 75 1.00 1. 25 1 .50 2. 25 2. 75 3. 25 4. 75 6. 25 6. 35 6. 35 6. 35 6. 35 9 .50 9 .50 9 .50 15. 75 19.00 A careful study of. .. 0 .52 3 0 .53 1 0 .53 9 0 .54 7 0 .55 6 0 .56 3 0 .57 2 0 .58 0 0 .58 8 0 .59 6 0.604 0.613 0.621 0.312 0 .54 7 0 .54 8 0 .55 1 0 .55 5 0 .55 8 0 .56 3 0 .56 7 0 .57 1 0 .58 0 0 .58 7 0 .59 6 0.604 0.612 0.620 0.628 0.637 0.6 45 0. 653 ... 0 .51 5 0 .52 4 0. 250 Bend radius 0.449 0. 451 0. 454 0. 458 0.460 0.466 0.470 0.474 0.482 0.490 0.498 0 .50 7 0 .51 5 0 .52 3 0 .53 1 0 .53 9 0 .54 8 0 .55 6 0 .56 4 0 .57 2 0.281 0.498 0 .50 0 0 .50 3 0 .50 6 0 .50 9 0 .51 5 0 .51 8