Ask the Stamping Expert: Overcoming deformity by design
The importance of keeping the die strip level through progressions
Achieving typical goals in stamping, such as zero development and fewer labor requirements, is made easier when the progressive die strip is kept level through the entire progression. Keeping the strip level can eliminate wave problems caused by incorrect pilot/lifter use occurring in punch forming.
One of the goals of any die design should be to create a tool that minimizes the need for highly skilled labor to fabricate and build it. Most if not all tooling should be designed for manufacture on precision CNC or wire electrical discharge machining equipment.
One of the goals of any die design is zero development—no design alterations after the die has been built. This sometimes can be more costly than the die build itself. When you add up the hourly costs of the toolmaker, managers, and engineers; figure in rework costs both for scrapped components and replacements; and then top it off with press time used and the lost revenue from tying up that press in development, you can easily approach $25,000 in costs over a 30-day period.
Of course, zero development is easier said than done, but following a few fundamental rules will help you make it achievable. One of the most important rules is to keep the progressive die strip level through the entire progression. It's always frustrating to go to development with a newly built die only to have dimensional issues with the strip or parts caused by very slight bends or deformities.
Pilot Holes and Lifters
For instance, waves between the pilot holes on the strip from a progressive die are a problem and will affect part quality. While carriers are made thin to minimize material usage, the carrier is not the problem. Instead, the root cause might be that, by design, the strip needs to be carried on lifters and the pilots are designed between the lifters, not on top of them. As the stripper contacts the material, the lifters push the material onto the face of the stripper.
As the pilots enter the material, if nothing is pushing the material in the opposite direction, tight pilots will deform the carrier down toward the die between the lifters because of some entrance resistance.
Whether the pilots are snug by design, the feed progression or pilot release is slightly off, or the entrance force required to align the pilots and carrier holes is too strong, the carrier will bow if not supported underneath. The thinner the material, the bigger this problem is. An important rule to remember is never to allow any action on one side of a die strip that is not balanced by an action on the opposite side.
The solution to the wave problem is to design for keeping the die strip level through the entire progression. Place the lifters under the pilots with receiving holes in them, and use rectangular lifters with large entrance ramp lead-ins under the rails. Envision in the design how the lifters will push the material onto the pilots on contact and how the strip will be held parallel to the face of the stripper as the face contacts the material and closes to die level.
Do not skimp on spring pressure. If you use a dozen 1-in.-diameter die springs under your stripper plate with a 1/8-in. preload, use 3/8-in. die springs under the lifter. No waves will appear in the carrier if the lifter springs are of sufficient strength.
Spring-loaded Punch Forming
Another common problem is that forming with a spring-loaded punch can cause waves, bumps, and stripper cavity marks on strip from a progressive die. Punch forming is required sometimes to meet stringent requirements for material usage, burr direction, and so forth.
For instance, spring-loaded punch forming would be required if you have to bend a 1/2-in.-long leg and want to bend it up into the stripper. This way you don't have to lift the strip more than 1/2 in. above die level to progress it or cam the form in and out on the die side for every stroke. You can mill clearance in the stripper for the leg to clear it in the horizontal axis and progress uninhibited. You also can minimize lifting strips, and subsequent die stroke, to help make the tool run more stable.
Failures commonly occur because the spring pressure is not balanced with the bending forces, so the strip does not remain level through the stroke. As the stripper contacts the material and all pilots are in, and as the stripper drives the material down to die level, you need to push the material into a form on the die side with a spring-loaded punch.
The forming punch under spring pressure should do the work until the form is complete; then as the die closes on the bumpers, the form punch backs up under spring pressure and hits home at the bottom of the cycle.
Most of the time the form punch preload pressure is not enough to overcome the bend force, so the form punch does not stay flush with the stripper face. This causes the base material to bend into the stripper cavity as the form punch backs up. Usually this is so insignificant—as little as 0.001 to 0.005 in.—it goes unnoticed. Then at the bottom of the stroke, the form punch hits home and flattens out the deformity. However, the strip has been deformed, and hitting home on dead bottom stroke only masks the problem, leading to dimensional instability.
Again, the solution is to design for keeping the die strip level through the entire progression.
On contact at high speed, the form punch will want to bounce back a little to overcome the stagnant inertia of the strip. To counteract this and for safety, the forming punch must have a minimum preload force of two times the calculated force it takes to bend the material. Nitrogen springs are a good choice for this.
The goal, again, is to keep the die strip level through the entire progression. If you have a 1/4-in.-high form die and need to ride the strip on 1/4-in.-high lifters to clear the form as the strip progresses, you then need to have a pressure pad under the strip in the forming station at the 1/4-in. height to hold the strip level as the stripper contacts it. This ensures the strip is piloted and securely clamped between the stripper and die pressure pad, so it will not slide during the forming.
Preloading the form punch to two times the forming force, plus the lifter preload force, and setting the punch at face or 0.001 in. above the stripper face (0.001-in. safety margin) keep the strip clamped and level on contact before any work is done.
Tips for Improvement
The general idea is that anytime tooling comes down to the raw material strip plane as the die closes, something should be directly under the strip that will push it in the opposite direction onto the face of the stripper. The only exception is if the die has no lifters and the strip rides on the die chase.
All spring-loaded punches must have enough preload force to prevent them from backing up. Otherwise, material will be allowed to deform slightly into the stripper cavity.
Also consider these tips to improve results:
- Balance the springs and calculate the spring forces.
And above all, keep the die strip level through the entire progression.
Thomas Vacca is director of engineering at Micro Stamping Co., 140 Belmont Drive, Somerset, NJ 08873, 732-302-0800, firstname.lastname@example.org, www.microstamping.com.
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.