Die Science: Reading progressive die strips — Part I
Looking at the results of a process—in this case, the strip produced in the die—certainly can lead to good data indicating the root cause of a problem. Distorted, elongated pilot holes, mismatched cuts are common defects. Distorted pilot holes can be the result of a poor carrier design. An improperly set up feed release can cause misfeeds, pilot hole elongation, and bent, broken, or galled pilots, as well as poor part location and gauging.
Editor's Note: This is Part I of a two-part series discussing causes of distorted pilot holes and mismatched cuts. Part II, which discusses hard marks, poor die design, and coil-related problems, will appear in the June issue.
A reader contacted me last year and requested that I address how to read progressive-die strips. By "read" I believe he meant analyzing the strip to determine corrective action.
Looking at the results of a process—in this case, the strip produced in the die—certainly can lead to good data indicating the root cause of a problem. Keep in mind that each die is unique, and data other than that about the strip may be required for a thorough analysis. However, looking at the strip defects is a great place to start.
Distorted, Elongated Pilot Holes, Mismatched Cuts
These most certainly are common defects. Numerous factors can cause distorted pilot holes, including a poor carrier design. In addition, if the pilot holes are distorted, the parts are likely to be located inaccurately in each station. However, keep in mind that minor distortion of the pilot hole may be acceptable if the die is dependent on the part's form to act as a nest or part locater. Three causes—and ways to remedy them— are:
1. Poor Carrier Strip Design. It is generally acknowledged that for progressive dies, the distance between the station centerline and part centerline must remain constant. This distance is referred to as the pitch or progression. Each time the press cycles, the strip will be indexed that precise amount.
When a drawing operation is performed in a progressive die, the blank's edge flows inward toward the part. This disrupts the relationship of the distance between the station centerline and part centerline.
In addition, the pilot holes will most likely move as well. Also keep in mind that when you perform drawing or simple bending, you may need to position the strip to be at different levels during the opening and closing of the die.
To compensate for this inward metal flow and allow the strip to be at different heights during forming and cutting, often it is necessary to incorporate a web feature, commonly referred to as a stretch (flexible) web or carrier, which is an extra loop of material. This stretch carrier deforms as the metal is formed, maintaining an equal distance between the stations (see Figure 1).
Drawing in a progressive die without a good stretch web design is very risky. Figure 2 shows a poor carrier design for drawing in a progressive die. This type of carrier is commonly referred to as a solid carrier. Solid carriers do not allow the metal to flow inward or allow for differences in vertical part positioning. In addition, a solid carrier should remain flat throughout the entire opening and closing of the die. To minimize pilot hole distortion, you may need to hold the area of the strip surrounding the pilot with a pressure pad.
2. Improper Feed / Pilot Release Timing. Pilot release, or feed release, is an important part of setting up a progressive die. The pilot release function on a coil feeder releases the strip that is fed into the die so that the pilots in the die can properly locate and register the strip.
Most coil feeders use feed rollers to index the material inward. These feed rollers clamp onto the strip and rotate a specific amount, causing the strip to feed one progression forward. For the pilots to position the strip correctly, the feed rollers must unclamp the strip before full pilot entry. However, letting go of the material too soon before the pilots partially enter the strip can pull the strip out of position by the weight of the takeup loop between the feeder and the coil.
The feed release must be timed so that the bullet nose of the pilot enters into the strip but not the full pilot diameter. This point can be determined by inching the press downward until the very tips of the pilots begin to enter into the strip. Then note the distance, in degrees, during the press stroke that the pilot is at, and set the pilot release to release the strip at this point (see Figure 3).
Program or adjust the pilot release so that the material or strip remains fully unclamped (feed rollers in the open position) until all of the work has been performed in the die and the strip has been brought back up to the proper feeding level, or feed line height. Once the strip is at feeding level, the feed rollers can clamp the strip and feed it forward one progression.
Once again, carefully monitor this point during the press stroke and adjust the feed release according to the rotation distance, in degrees, during the crank stroke. Doing so correctly allows the material to move linearly inward and away from the die during the vertical travel of the lifter bars.
This is critical, especially for progressive dies that are making deep-drawn parts and require great vertical lift force to feed forward. An improperly set up feed release can cause misfeeds; pilot hole elongation; and bent, broken, or galled pilots, as well as poor part location and gauging.
3. Insufficient Cutting and Holding Pad Pressure. During metal cutting and forming, often tremendous side forces are generated. If the pressure pad that is intended to hold the metal in place does not have sufficient holding force to keep the metal from moving, the pilot holes may elongate, especially when a solid strip carrier is used.
One good rule of thumb is to always keep a reference strip for comparison. Most stampers keep an original strip from when the die was new, and also keep the most recent strip from the last production run.
Until next time Best of luck!
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