Ask the Stamping Expert: What’s the best way to feed light-gauge metal?

While doable, push-feeding stainless steel and other light-gauge metal through the tool can be tricky. Stamping expert Tom Vacca explores an alternative—pull feeding—and the die equipment that is available. Getty Images

Q: I recently read your article “Stamping thin, hard stainless steel” from 2015. In the article, you mention the challenge of feeding thin material, but I still have some questions. Can you provide more explanation?

A: Speaking from a progressive die build standpoint, push-feeding thin material through the tool is quite tricky. Steel that is 0.002 to 0.004 inch thick will buckle against the slightest resistance. The material guide liners must support the material top and bottom very tightly, with only a small space for applying lubricant before the material enters the die.

Here are two important tricks to reduce material resistance:

1. Relieve the surface area of the plates with a series of undercuts along the length, called scalloping. When completed, less than 1% of the total surface area of the plate should touch the material.
2. Drill and tap a hole in the center of the bottom liner to connect an air line so that an application of light air pressure will cause the material to float.

Push-feeding light-gauge material is very doable, but there’s also the option of pull feeding. Let’s take a look at the three basic types of raw material feeds.

Servo Feeds

If you need one feed that will handle all materials, I recommend a servo feed, although it can be the most expensive option depending on the model.

Servo feeds can handle a variety of stock thicknesses and widths, and they can be programmed for individual jobs. You pull up the program and the pitch (the length of the coil stock fed on every press cycle), and the pilot release is preset. These feeds are fast, handling 600 to 800 strokes per minute (SPM) on very short pitches.

Our jobs are relatively small, and so are the servo feeds we use, so we designed a bracket to mount a servo feed on the exit side of the press for pull feeding. If we need to run a push-feed job and the servo feed is not in the way, we leave it as-mounted and use the standard feed mounted on the entrance side. If the servo is in the way, we can remove it within 30 minutes.

Servo feeds have a nearly infinitely adjustable feed cycle. For example, mechanical feeds usually have a set feed cycle of 120 or 180 degrees of the press cycle. If the strip is only free to move for, say, 90 degrees of the press cycle because you are doing in-die tapping, which can require much more of the press stroke, you can program this on the servo feed.

One of the biggest benefits of a servo feed is that it senses the force required to feed the material. When it senses a spike in that force, the servo will stop and pull back to the previous pitch. This prevents an off-pitch misfeed in case the press does not stop before making the next hit, saving many thousands of dollars in broken tooling. I’m not sure if this would work on very thin material, but using pull feeding would increase the odds for success.

Air Feeds

Air feeds are a cheap, reliable option. I have run coil stock from 0.005 to 0.060 in. thick and 0.375 to 5.0 in. wide very successfully up to 400 SPM, and their limits are even broader. Air feeds are not as accurate as servo or mechanical feeds, but the stamping tool pilots often make up for slight variations in feed length. This tends to occur when the force to pull the coil stock varies because of changing loop control conditions at the infeed.

The biggest challenge with air feeds is timing them, as there’s an inherent lag because they are air-driven. As you speed up the press, you have to advance the signal to feed. I have run jobs at 400 SPM with my signal to feed set just before bottom dead center of the stroke—when the punches were still in the raw material. Because of lag time, feeding occurred at about 270 degrees on the press cycle.

While timing an air feed at high speed is an art, it’s also quite a challenge to lace up at low speed. The feed is so advanced that it wants to drive the coil stock while the tool is still closed.

Mechanical Feeds

In general, mechanical feeds are supplied by the press builders. These feeds are, by far, the fastest and most accurate. They can run at more than 2,000 SPM in the right conditions.

In addition, with mechanical feeds, you can customize the feed grippers with a clearance slot if the strip you are pulling has raised features.

Remember that regardless of the type of feeder you choose, lubrication can create a lot of drag between the raw material and the liners, so proper lubricant application is critical. High-end lubricators are available that allow very precise control of a light mist of lubricant, then suction off the excess, leaving a perfect, even coating on the material.