March 13, 2003
If you're a die designer, a standard precision progressive die can present countless challenges for you. Some of these dies have to produce thin slots, small holes, or tricky coins.
Another design challenge is the thin-material, precision progressive die for achieving consistent bends of more than 90 degrees. These dies have material less than 0.020 inch thick and can achieve a tolerance of ±1 or 2 degrees for an angle on the final part.
Any tool in any shop is quoted to run at a certain speed and produce a part within the tolerances documented on the customer's part print. However, if a part has a bend that is more than 90 degrees and the tolerance is fairly tight, running the die at the quoted speed could be a concern. A standard 90-degree bend is bad enough; at more than 90 degrees, a bend can really challenge you.
This illustration shows the side view of the tab (left) and the front of the die (right). The grain is parallel to the bend.
When looking at a bend of more than 90 degrees, you have to ask yourself a few questions before finishing the strip layout:
Following are descriptions of some example bends for which these questions can be considered.
Example 1 shows a tab on a part bent 100 degrees (see Figure 1). The material is 0.016-in.-thick brass with a temper of 1/2 hard. The most important question in this case is grain direction. A bend parallel to the grain direction makes it easier (see Figure 2). This can be done with a preform followed by a final form.
The diagram's side view shows how the position of the preform is forward of the final bend.
The whole trick to this method is the preform. You must position the preform at the right location to form the farthest portion of the material used in the total bend (see Figure 2). To do this, calculate the position in the ideal condition with zero springback, and then estimate how much springback will be in the final part. In this case of 0.016-in.-thick 1/2 hard brass, a good estimate would be 3 percent.
The total bend is 100 degrees. With a standard formula for calculating the material usage in a bend, use 0.044 in. with an inside bend radius of 0.020 in.
The standard formula used to calculate this is a = 0.017454 (r + n) x100
If you apply numbers to this formula you have a = 0.017454 x(0.020 + 0.005328) x100 = 0.0442 in., which can be rounded off to 0.044 in. That means the 100-degree bend will use up 0.044 in. in the flat blank. If a preform is 45 degrees, use the first 0.022 in. in a 45-degree bend. To overbend the form, you can move the preform out a few thousandths of an inch. For example, try using 3 percent springback (3 percent of 0.044 is 0.00132 in.). Try moving the preform out an additional 0.002 in. This means positioning the preform 0.024 in. out instead of the 0.022 in. as the formula states.
If your final part is open by 2 degrees, you are at the high limit of the tolerance, and that's not good. You need to move the preform out the amount of material you would use for a bend that is 2 degrees larger—maybe another 0.001 or 0.002 in. You can leave the final form alone, moving the preform one way or the other to adjust the final bend.
The side view shows how high a part must be lifted to feed the strip forward.
Example 2 is the same part, but its grain direction is perpendicular to the bend. This makes your job a lot more difficult. Assume you decide to run the part so the bend is leading (see Figure 3). The final bend will require either a lifter or a slide. Example 2 includes the lifter, while Example 3 includes the slide.
The main consideration when using a lifter is how high you need to lift and what happens to the strip as you are forming. In this case, the part needs to be lifted over the top of the final forming station, 0.150 in. off the die surface. This is not a great condition, but it can be done successfully.
Since the tab is bent downward, you can simply push the part down to the die surface before the forming punch makes contact. This can be accomplished using a spring stripper. If you use a tunnel stripper, you must incorporate a pressure pad. It also is wise to look at the stations before and after this lifter to see if you need to add pressure pads and to keep the strip as level as possible.
Example 3 uses the same forming sequence as Example 2, except it uses a slide. Many shops dislike lifting the strip and then pushing it back down. In this case, the slide seems to be the better method. Lifting the part that high is problematic. Slides cost more to make and they take longer to design than a simple lifter; however, in this case, the end result will be a better-running tool.
First, you need to look at the action needed to perform the final forming operation. In this situation, the main reason to use a slide is to get the forming arbor out of the way so you can advance the strip into the next station without a lifter. The slide is the forming arbor. So you need to advance the strip into the forming station, actuate the slide forward, and stop slide movement. Then the final forming punch can come down and form the tab. In reverse, the forming punch then comes up, the slide moves out of the way, and the strip advances forward to the next station.
This slide does not need any force. All you have to do is get the forming arbor into position in time to form the tab, and then move it back. This is the perfect situation for an air cylinder. You can time the cylinder to actuate forward at the end of the feed cycle and retract the air cylinder at 270 degrees or on the way up, before the feed cycle starts again.
When using a slide, it is always a good practice to make the fit between the slide and the slot as close as possible. You do not want any rocking or flexing during forming. To move a slide, even a couple thousandths of an inch, can be expensive, so you need to leave the final form alone and make the adjustments with the preform.
This is the same part with a form like in Example 1, and the grain direction is parallel to the bends. The only difference is the material, which is full hard brass. The forming sequence in Example 1 might work, but there will be considerably more springback.
Example 4 shows a different situation. It is the same part with a form like in Example 1, and the grain direction is parallel to the bends. The only difference is the material, which is full hard brass. The forming sequence in Example 1 might work, but there will be considerably more springback (see Figure 4).
One method to achieve the desired results is a qualifying slide to tap the tab in place. You can start with the same forming sequence and then move the piece over in front of a slide with the correct angle on the face. In this case, you need to look at the motions needed to get the results you want.
First, advance the strip in front of the slide and then hold down the strip. The slide comes in, bottoms out, and immediately comes back. In this case, an air cylinder wouldn't be the best choice. Instead, use a cam from the top, actuating the slide forward and a spring to pull it back. The travel on the slide is very short. Try using a 45-degree angle on the cam and the slide. This way, if you need to go in 0.001 in. more, you can move the cam down 0.001 in. Any move up or down relates to the exact amount of movement in or out (see Figure 5).
This side view of the slide and cam arrangement shows it in the closed position, illustrating how a downward motion is translated into a sideward motion. If a 45-degree angle is used, a downward move of 0.001 in. becomes a 0.001-in. move inward.
These examples have been very basic. Both of the following example parts are from a tool that is running quite successfully. The part is 0.013-in.-thick brass, 1/2 hard, with 2 percent springback.
In Example 5, the end of the part is curled around onto itself while a tolerance of ±0.003 in. is maintained on the overall height and the gap. This is done by bending around a horn, plus using several slides in the forming sequence. The completed form then is bent around another horn with another slide and a final slide to qualify the gap and height (see Figure 6). This looks complicated at first, but if you break it down into the component parts, it makes much more sense.
Let's look at each component individually. The first step is to draw the forming sequence along with the springback (see Figure 7). Take a look at the forming sequence and notice the springback. The material is 1/2 hard brass and has a fair amount of springback. Springback, like other errors in a progressive die, can accumulate. After the fourth form, the springback can be substantial. At the third form, wrap the end around a horn.
This forming sequence is used to form the tab over a horn. To close up the final gap, the first preform can be moved. To increase or decrease the length of the overhanging tab, the second form can be adjusted.
The one thing you always need to consider with any slide is the timing. When the strip feeds forward into the station that has the slide, you must understand what happens. The slide chosen in the first forming sequence comes in as the die closes and hits home at the bottom of the stroke. The only thing you must do is to make sure the strip is in the right place before the slide gets there.
In this case, a spring-loaded pressure pad is used to ensure the strip is held down on top of the die surface before the slide comes in to wrap the bend. This is the station where all that springback will really show up. That's why you must use the springback in the forms before the slide, as well as in the slide itself.
The next two stations are the best, in the author's opinion. Stations 4 and 5 simply bend the formed part over an arbor. It's amazing how small they are. The slides are noncontact. Air cylinders are used so the timing can be just right.
The two forming sequences are shown superimposed on each other, illustrating how the position of the preform can control the final form.
Again, a spring-loaded pressure pad is used to get the part in the proper position for the slide. In this case, the pressure pads start the bend in motion. If the slide comes in when the part is straight, there's no guarantee the slide will pick up the form and bend it properly. Bending just the tip around a little bit presents the part to the slide at the proper angle. The slides just have to come into a certain position and go back again. The position is controlled by a stop block mounted on the slides.
Example 6 shows how you can use a preform to bend a part well past 90 degrees (see Figure 8). In this example, the grain direction is with the bend. In other words, the view shown here is the front of the die. The part can't be lifted without jeopardizing its integrity. Slides are used in the second and third forms. The final form has to end up with a gap of 0.010 in. The maximum gap is 0.014 in., and the minimum gap is 0.010 in.
There really is no way to qualify the gap. It must be good in the final form. With this part, the maker started with the final form. The slide is designed to allow the part to close completely. With springback, the 0.005- to 0.010-in. gap should be fine. The previous station is there to form the farthest part of the bends and to wrap the final form around the slide.
In this illustration of the sequence used to form the gap (left), the preform is superimposed on the final form to illustrate the relationship among the three forms.
If the gap is oversized, move the preform out accordingly. If the gap is undersized, you can move the preform in a bit. These slides move in together and must be in position and stopped before the forming punch comes down. They also will need support, or they will snap off on the first shot. To add support on either side of the part, bridge the clearance slot in the die.
The timing for these two slides is different from that for the first two slides. These slides cannot move until the feed cycle is finished. Then the slides move in completely and stop as the die is closing, but before the punches or any spring-loaded pressure pad makes contact with the top of the strip. Then the slides must move completely out of the part before the die opens enough for the feed cycle to start again, but after the spring-loaded pads are up off the strip.
This is very simple if you use air cylinders with solenoids that are timed with the crankshaft of the press. You can actuate the cylinders by hand while baring or inching the press around. This way the timing will be perfect.
Once you've verified and set the timing, you're ready for a tryout. There is nothing more exciting than seeing your design run in the press with slides moving, air blasts going off, and parts dropping out the bottom of the die. That's what makes this job so much fun!
Chester J. Punicki has 31 years of stamping experience. He can be reached at Cjp13@optonline.net.
STAMPING Journal® is the only industrial publication dedicated solely to serving the needs of the metal stamping market. In 1987 the American Metal Stamping Association broadened its horizons and renamed itself and its publication, known then as Metal Stamping. Print subscriptions are free to qualified stamping professionals in North America.