Hi folks - total newbie - forgive me. With standard / imperial system, anyone know why small taps don't use fractional measurements (e.g. 10-32, 8-32, etc.), and then at 1/4-20, it switches to fractional, which is much more logical to me? I am setting up my shop and organizing taps, dies, drills, reamers, etc. and it's a bit of a mystery to me. Thanks, looking forward to engaging in this awesome hobby!
Welcome aboard, Sir. It takes a newbie to upset the apple cart on occasion so this subject can be come to the front. Machine screws are sized with an arbitrary formula. I have a few suspicions about where that formula came from, but no real facts. I suspect way back in the dark ages of machine tooling, an attempt was made to match the metric system for fasteners. Machine screw sizes below 1/4 inch get into 1/32s, 1/64s, and even 1/128s. Quite frankly, most folks don't have a good grasp of fractions and really don't have time to fool with them. Just how big would a screw be that was 3/128 inch?
Start with a screw size of 0.060 inch. That's almost but not quite 1/16, .0625. Since only people that are OCD worry about the fourth decimal place, call it .062. So, .060 is a
size 0. Now, add (or subtract) the odd size of .013 for each number. So that .060 plus .013=.073, size 1. .060+(2x.013)=.086, a size 2. Do this up to Nr 14, .060+(14x.013)=.242. With 1/4 being .250, it now is large enough to transition. The same system works down as well. 0.060-.013=.047, which is a size 00 or 2/0, .060-(2x..013)=.034, size 000, or 3/0. It goes further, my supplier has taps down to 5/0. I suppose for use by opticians and clock makers. 3/0 is the smallest I personally use.
Now thread pitches, actually TPI, run in lots of 8. If you track through the system, TPIs will step by 8 TPI. As it happens, in the 1/4 inch arena, several screws fall into place. Starting with 1/4-20 at .250, then Nr 14-24 at .242. And finally M6 at .238 by 25.4 TPI. There are other numbers that fall very close, 3/16, Nr 10, and M5 another common point. Nr 10 Vs 3/16 is so close they are interchangable, with M5 fitting tightly, but fitting.
Screws do go fractional down to 1/16 inch. I have taps and dies for 1/16, 3/32, and 1/8. Mostly as curiosities, they aren't really useful. Since I retired, I work with models at 1:87 scale. Early in my career, ca. 1970s, I worked with instrumentation for process control, where small and odd sized fasteners were common. When silicon semiconductors became more common, mechanical solutions quite simply got too expensive. What one man could build in a day, ten thousand silicon devices came off the press in a couple of hours. I made the transition from electro-mechanical to solid state because of the connection between my hobby and my profession.
The fastener sizes, both "machine screw" and fractional, are listed in Machinery's Handbook. But the background information is only good for historical purposes and is heavy reading. Add in metric sizes, and the confusing becomes unintelligable.
