Chicago screws require a 3/16th inch hole, and at least .280 thick material to be used.
This means 3 layers of .090 will not cut it! This is therefore mostly used when you have one of those heavy duty factory made belt clips to add to your project. I have also used them, but added extra material just over the screw holes to use as a filler. I've also used them with small stainless steel washers when i was joining 3 layers of .090 kydex. That gave me the extra hundreth of an inch i needed to use them.
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