For drilling an accurate hole in overlapping sheets. The inverted type duplicator allows you to locate a hole from the back of an overlaping sheet. #10 #21 #30 #40 1/4" drill sizes, 6-1/4" or 13-1/4"
PN 12-01507 is pictured.
My fault for not looking at the picture more carefully before buying. The description even says INVERTED TYPE. The tool is quite well made but not what I expected or wanted. I have not come across a use for this type of strap duplicator yet but for the price of the tool relative to the shipping I will just keep them and see if a need ever comes up.
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Instrument screws are usually a 6-32 so you would want a #35 bit. I would go with the number 30 duplicator to make the initial hole and then follow up with a #35 drill.