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Die Science: 4 causes of poor strip feeding in progressive dies
- By Art Hedrick
- March 6, 2017
- Article
- Bending and Forming
Most stamping plants have experienced poor strip feeding in a progressive die. Not only can this problem cause great frustration, but it also can lead to die and press damage, poor quality, scrap, safety hazards, and downtime. Four potential causes usually are to blame.
1. Die Alignment
The centerline of the pilots in the die must run parallel to the coil feed direction, and the edge of the coil must be aligned with the side or guide rail of the die. Because coil feeders and straighteners typically are in fixed positions, alignment is achieved through positioning the die in the press within the correct boundaries.
One way to ensure proper die alignment is to key the die to the bolster plate, assuming the edge of the bolster plate or press bed is parallel to the coil edge. Small blocks of precisely machined steel, called keys, are fit in keyways machined into the die and bolster, aligning the die. Other ways to achieve die alignment are positive stops or locator pins. Regardless of the method used, the die must be parallel to the coil feeder.
2. Feed Line or Feeding Height
Avoid feeding strip into the tool at an angle. Instead, set up the feed line height to the die’s proper feeding level (see Figure 1). This keeps the material straight and flat, giving it stiffness and making it less likely to buckle during feeding.
Position the feed table as far from the die as possible to prevent the strip from kinking when a lot of vertical travel is required, such as in deep drawing. If you can’t place the table a reasonable distance from the die, you might need to provide a way for the table to move up and down with the vertical travel of the strip.
3. Coil Camber
Coil camber is the lack of a straight coil edge. Camber can be present in the received coil—caused by the slitting process—or it can be induced into a strip by the coil feeder, leveler, or straightener. Severe camber can cause poor feedability into a progressive die.
Avoid setting the stock guides too tightly against the edge of the coil. Remember the function of the guide rails is to guide the strip to a position where the pilots can locate it. Because the strip varies in width and camber, you can’t expect the guide rails to locate the strip precisely. The rails and guides often are set so tightly that any amount of camber or strip width variance will result in poor strip feeding.
4. Pilot or Feed Release Settings
The pilot release, or feed release, function on a coil feeder allows the strip being fed into the die to be released so that the pilots in the die can locate and register the strip properly.
Most coil feeders use feed rollers to index the material inward. These feed rollers clamp onto the strip and rotate a given 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 cause the strip to be pulled out of position by the weight of the take-up loop between the feeder and coil. The feed release must be timed so that the bullet nose of the pilot is entered into the strip for the full pilot diameter. You can find this point by inching the press downward until the very tips of the pilots begin to enter into the strip.
When you’ve reached this point, note how many degrees into the press stroke you are and set the pilot release to let go of the strip at that point. Adjust your pilot release so that the material or strip remains fully unclamped (feed rollers open) until all of the work has been performed in the die and the strip has been brought back up to the feed line height.
Once the strip is at feeding level, the feed rollers can clamp the strip and feed it forward one progression. Monitor this point during the press stroke one more time and adjust the feed release with respect to the degrees during the crank stroke. The correct adjustment will allow the material to move inward linearly and away from the die during the vertical travel of the lifter bars.
This procedure is critical, especially for progressive dies making deep-drawn parts that require a lot of vertical lift to feed forward (see Figure 2). Improper feed release setup can cause misfeed; pilot hole elongation in the strip; bent, broken, or galled pilots; and poor location and gauging.
The culprit for poor feeding in a progressive die also could be poor strip design, incorrect strip lifter selection, or poor strip-suspension points. The key is to find the root cause of poor feedability and correct it.
About the Author
Art Hedrick
10855 Simpson Drive West Private
Greenville, MI 48838
616-894-6855
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The Fabricator is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The Fabricator has served the industry since 1970.
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