Ask the Stamping Expert: Which equation adds up to smooth strip feeding in progressive dies?

Q: Many people believe that the feed height = passline height. But the way I learned it, feed height + lift height = passline height.

So which equation is correct?

A: I would say that the feed height = passline height + lift height, with lift height being how high the material lifters in the tool are above the die chase level.

For the strip to feed efficiently, it must not be obstructed while progressing. This means the material should not have to overcome the pressure from the lifters pushing the strip up and creating a bow-up condition during the feed cycle between the feed and the die chase.

When the pilots enter at the end of the feed cycle and the feed goes into pilot release, the pilots need to be able to do their job and pull the strip into position. This pull should be no more than a few thousandths of an inch. I understand the concern here, specifically on thick, heavy stock. The pilots and the press should be designed to be robust enough to withstand the side load. Surely the side load from pulling the strip will be much less than the load of actually stamping and forming thick, heavy stock.

This is most important on thinner-gauge material. If the strip gets pushed back and sticks on the liner plate or the oiler, it will accumulate as wavy stock over several pitches, and at some point it will snap flat, resulting in an over feed and possible misfeed. However, if the strip gets pulled into position, it will always be taut and cannot accumulate any error.

With any strip lift in the tool, there will be downward movement in the strip as the stripper or punches make contact with the strip. Depending on material thickness and lift, this down movement needs to be absorbed without creating a kink in the strip at the tool entrance. This can be achieved with an opening between the feed and the tool where the stock enters the die. This opening needs to be big enough to allow the strip to bend the distance of the lift without overcoming its yield and creating a kink or bow.

If the tool has an excessive lift height due to the nature of the stamping, and there is limited space for an opening or the material thickness won’t allow a large gap, the feed height can be put halfway between the die chase and the lift height. Keep in mind this may introduce a problem feeding the strip.

We have been very successful following this method over the years running various types of materials in thicknesses from 0.003 to 0.080 in. at speeds from 60 to 1,200 strokes per minute, forming anything from a flat blank to a deep-drawn cup.

In most progressive stamping operations, the feed unit is followed by some sort of liner and strip lubricator, which usually is solid-mounted to the press or the tool. In most of our operations, we can use a roller applicator for the strip lubrication that is mounted to the liner after the feed, and we can create a gap right after the roller. The radius of the roller eases the downward movement of the strip, helping to prevent the strip from kinking as it moves up and down on the lifters through the stamping cycle.

This gap also can act as an opening to detect a buckle if there is any obstruction while feeding the strip forward. It is better to have buckle outside the tool than to double or, in most cases, triple the material thickness inside the tool with a jam-up. That never ends well.