2b vs gh3 tap
Can someone please tell me the difference between a "2b" and "gh3" fit of the thread on a tapped thru hole? i know it has something to do with a roll form thread and this is a very specific protocal?
2b is a thread fit class that is not related to tap nomenclature. gh3 indicates a ground (g) thread tap that is .0015" high/above/over (h) nominal pitch diameter. The class of fit obtainable from any given tap depends upon a great number of variables and can't be guaranteed by the tap specification alone.
Get your hands on a Machinery's Handbook or go to a major tap manufacturer's website and all the info you are looking for will be there.
DFW is correct, and I would like to add my own thoughts. Most inch cutting taps from #10 to 1/2" seam to be sold having a H3 limit. But other limits are available. Form taps use a higher limit, I presume because the material will spring back after forming. With my 1/4-20 taps for example, cutting are H3, and roll are H8.
Machinery's Handbook will give the max & min pitch diameter for a given internal thread. The pitch diameter is determined by the H limit on the tap. A tap with a H3 limit, for example, might give a different pitch diameter in different materials. I find that many plastics shrink, so you might need a higher H limit to get the 2B specs. How much difference is determined by trial & error.
Sometimes in production tapping, you start off with the highest H limit that meets specs, and as the tap wears, you will still get good holes for a while.
Metric taps use D limits, but the concept is the same.
Balax & OSG have excellent technical sections in their catalogs.
For H-form taps, the number is the oversize in half-thousandths increments, so an H3 will be .0015" over, an H5 will be .0025" over, etc.
While most tap sizes are available in several fits, there's a "standard" or customary fit for each size.