The other day while doing further work on my son's Subaru, I drilled a hole in a piece of angle iron then started using my 1/4-20 tap to cut some threads in the hole. The idea was to have a threaded hole which I can screw a bolt into to secure the upper portion of the vehicle's new radiator. During the process, I broke my tap. That really sucks and I wondered why I might have done it. I started thinking I may have not drilled the correct size hole prior to starting to run the tap. It may have been too small and therefore might have caused the breakage.
My question is: What is the proper sized hole to drill prior to using a specific tap? I was using a 1/4-20 tap, but I'm sure it may be useful to have a list from a 1/4" up to 1" tap size. I wouldn't think it would matter between coarse or fine thread. Also, if someone were really industrious, they could align it with the metric equivalent sized taps as well.