Page 65 - MetalForming October 2009

Q: I just finished reading your article on pressure strippers in the July/August 2009 issue of MetalForming. The illus- tration in Fig. 2 shows two screws mount- ed with the heads down and they are labeled as nylon screws. Is this correct? Is it safe to use nylon screws in a stripper?
A: You are an astute observer. The screws in the illustration are labeled incorrectly. They should have been labeled “Nylock” screws. A nylon screw would not provide the strength required in a stamping die and should never be used.
The Nylok Blue Patch is a self-lock- ing element permanently spray-bonded onto the threads of a fastener. When the mating threads are engaged, the tough, resilient blue nylon patch element is compressed and a counterforce is creat- ed to establish a strong metal-to-metal contact and positive resistance to vibra- tion and loosening. This eliminates fas- teners from falling out and dropping onto the lower die during die operation. And, they’re reusable.
Q: I read your article on the topic of forming stainless steel (February 2009). I found the article to be very interesting and pertains directly to my company.
We have recently received several projects where the specified material has been stainless steel, which is not a common material for us. We have been making good parts and maintaining the tool fairly successfully, except for the first draw-form tool. The inserts in the lower form are the issue. We have tried several different types of tool-steel inserts with different types of coating. The result has always been the same— galling is almost immediate. We have had some success with the aluminum- bronze inserts you talked about in your article. The bronze does not gall at all, but is does wear out rapidly, causing us to have to recut and shim this station quite often. Any suggestions?
A: After reviewing the pictures sent, it is clear that the problem lies in an area on the part where material thickening is
to be expected when deep drawing. Additional punch-to-die clearance must be provided in this area to avoid imme- diate galling. Austenitic grades similar to what you are using can increase in thick- ness as much as 30 to 40 percent, depend- ing on the amount of deformation. This is a much greater percentage than you may be used to with low-carbon steels. You must verify that the die clearance is correct, or correct the deficiency, before attempting any other solutions.
Next, make sure that the draw lubri- cant you are using is designed for stain- less steel. Forming stainless steel gener- ates very high friction where the sheetmetal and die surfaces meet. This friction is converted into high heat ener- gy capable of breaking down draw lubri- cants commonly used to stamp low- carbon steel. In your case, an extreme pressure (EP) lubricant with very high film strength and super wetting char- acteristics developed specifically for stainless steel may be required. MF
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