Ask the Stamping Expert: How do we prevent buckling in thin material during strip feeding?
A: This is a very common problem with a couple of very achievable solutions, each with its own benefits and drawbacks:
- Mount a pull feed on the exit end.
- Use a push feed.
Pull feeding is more expensive, because you will need to invest in a feed—most likely a servo-driven hitch feed. You also will have the expense of mounting the feed on the press and installing the electronics required for the feed to communicate with the press.
In addition, you will have to design and build the brackets and stamped raw material webbing guides between the die and the feed, and then determine what to do with the webbing. If volume makes it cost-effective, you could invest in a scrap chopper. We have done this by purchasing an off-the-shelf scrap chopper and building a frame. The frame accommodates the chopper sitting on top of a 55-gal. steel drum. When the drum is full, we lift it off and replace it with an empty drum.
I estimate you will spend about $30,000 on a pull feed, $15,000 on electronics (you will need a resolver), $25,000 on a PLC interface, and $10,000 on engineering and fabrication of miscellaneous brackets and liners. If you add the scrap chopper, you have a $100,000 project. It will work very well, and if you can justify the cost, this is the way to go. Pulling the material is much easier than pushing it, without a doubt.
The other option is push feeding, which is the type of feed you have. This feed uses partial roll segments rather than a full-diameter roller. The upper and lower roller segments basically rock back and forth to feed the material.
The space between the feed rollers and the die entrance must be filled with a liner to contain the material—basically two plates of hardened steel with a groove ground between them to accept the raw material. The groove should be 0.005 to 0.010 in. deeper than the material thickness. In this case, start with 0.010 to 0.015 in. and grind deeper if needed. This is critical to prevent the material from buckling.
Next, test the liner by hand with a piece of cut strip stock. This will help you develop a nice slip fit of the strip in the liner, but not so sloppy as to allow the material to ripple within the liner. This can cause slight variations in feed length in the tool as material accumulates in the liner and then springs forward.
There are three keys to making this work:
- Bring the top and bottom liner plates to a knife edge so they can be positioned as close as possible to the feed rollers and end as close as possible to the tool entrance.
- Scallop both plates to relieve the surface area. We would grind a series of 1/16-in.-wide grooves, 0.010 in. deep at 0.072-in. centers, along the entire length of the material liner grooves. This leaves a series of 0.010-in.-wide bearing surfaces. Be sure to polish them to prevent scratching of the raw material.
- Drill and tap a hole to accept an airline fitting in the center of the bottom liner. Feed in 5- to 10-PSI compressed air to float the coil stock as it passes through the liner. Try it on the bench while adjusting the air to feel for the optimal pressure that yields the best results. You will be amazed! The strip will virtually float and move through the liner with very little resistance. Cut out a small window across the liner wider than the raw material, about 3/16 in. wide, to allow mounting of a mist spray lubrication unit to oil the top and bottom of the stock.
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