And then add British Standard and Whitworth and you got a real collection of taps. Like my '28 Chevy banger with 1/2-12 head bolts. That is a Whitworth size. What is it doing on a Chevy? Did you ever notice British wrenchs are marked with the diameter of the bolt. Not the bolt head. So a wrench marked 1/2 fits a 3/4 nut on a 1/2 inch bolt.
If it makes you feel any better, British Whitworth(BSW) and British Standard Fine(BSF) size designations make even less sense.
Within all the systems are threads that exist, but aren't generally used. A Machinist's handbook shows lots of different U.S. ones.
Interesting that we both posted essentially the same thin thins at exactly the same tome, 3:00. I remember the same size wrench is called a different size depending on whether the bolt is BSW or BSF. For example, even though the head is the same size, 3/8 for one and 5/16 for the other. And in spite of the fact the country uses the inch/fractional system, British bolt and nut hex sizes are not even fractional numbers like in the U.S., so standard sized fractional wrenches won't fit on most of them.
Machine screws start at "0" which is .060 diameter and increment up by .013 for each number. From http://en.wikipedia.org/wiki/Unified_Thread_Standard The following formula is used to calculate the major diameter of a numbered screw greater than or equal to 0: Major diameter = Screw # × 0.013" + 0.060". For example, a number 10 calculates as: #10 × 0.013" + 0.060" = 0.190" major diameter.
Lots of good info here, as well as the usual HAMB ********! Got another one for you....why don't they make a "tap" for sheet metal screws. Often times, when going through thick sheet metal, say, 16 ga. (there's another " how do they size it" thread) with stainless sheet metal screws, they often break. Why can't I get a good, hardened tap to prepare the hole for the sheet metal screw?
Pffffttt... all I need is 2 spackle buckets, filled to the brim. I could prolly toss most of it and get it down to a single bucket. Bob
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