I'm trying to decide the correct drill bit size for a hole I have to make to fit my thread.
I measure the (major) diameter of the threaded section as 0.5325 inches. The threads are not supposed to go into the plastic.
I looked at the chart provided by Bosch, and assuming I have a 9/16 thread, the drill bit size should be 31/64. I tried 31/64 drill bit and drilled a test hole to see if it fits and it did not. I don't have 33/64 bit, but I tried 1/2 inch and it barely fits.
What is the correct method to determine the drill bit size from the measured threaded diameter? Also, since I am drilling into plastic, what is the correct way to determine the correct pilot hole drill bit?
Best Answer
When you look up a chart of drill sizes, you want clearance sizes. As an example, this drill chart has the drill size used to create holes that will be threaded with a tap, as well as the larger size you would use to create a hole that will clear the given threaded fastener. It actually gives two clearance sizes for that. You might want to use the larger if those two sizes for non-precision work that has more than one fastener, otherwise you could end up with alignment issues between pieces, where the holes don't line up.
Here is a wikipedia table of the same thing.