Class 2G Acme Tap

Can anyone explain to me how the "Class" system which includes "Class 2G" works in reference to Acme taps? I looked in Machinery's Handbook and found a reference to it but couldn't find an explaination.
Thanks

There are two Acme thread standards.

General purpose (G) Acme thread is available in three classes 2G, 3G, and 4G. The 2G class is the most common and the least expensive. The higher classes offer reduced backlash.

The other Acme thread standard is called Centralizing (C) Acme thread, and is often referred to as precision Acme thread. It is also available in three classes – 2C, 3C, and 4C.

The 2G class of fit is also referred to as Standard or General Purpose.

Barry Milton

Thank you Barry for the info. Why do they call it "Centralizing"? What do the numbers (2,3,4)indicate?

A G class Acme relies on the nut floating with respect to the shaft, or otherwise having the two very well aligned and fixed. One thread can move laterally with respect to the other and cause binding when the two try to wedge together. On a C class or centralizing thread, the crests and roots have less clearance between then, so if one thread moves laterally a bit its crest makes contact with the root of the other one before any wedging can happen, hence the name centralizing, because the tighter crest and root tolerances keep one fairly closely centralized with respect to the other. Even though both could be produced to the same lead accuracy, the C will produce more accurate positioning, since the lateral wandering of the G causes axial differences in movement for a given amount of rotation due to the fact that, as they wander they're moving on an inclined plane.

Thank you metlmunchr for the great explaination. Can you shead any light on what is the difference between a C2 and C3 and C4?

The tolerances get tighter as the class number increases. For example, on a 1/2-10 Acme, a 2C external has .0025 tolerance on major diameter, while a 4C has a total tolerance of .0007 on the same feature. The tolerances get correspondingly tighter on the mating internal threads too, so as the class number increases there's less maximum clearance between the mating threads. The tolerances are only tightened on one end, so a class 4 would actually meet the spec for a class 2 or 3 as well, but the reverse would not be true. There was also a C5 and C6 with even tighter tolerances but they're not used any more. I'd imagine this is mainly due to the use of ballscrews in high accuracy applications instead of ground acme's.

Metlmunchr,
I don't understand the following statement you made:

"The tolerances are only tightened on one end, so a class 4 would actually meet the spec for a class 2 or 3 as well, but the reverse would not be true."

Could you explain it further?

I need to make 3/4 6 ACME threads through the hub of a few handwheels to accommodate some threaded rod.

I'm real dumb, so bear with me.

I assume since the threaded rod is RH, that I also need a RH tap.

I see there are combination rough and finish taps.

The handwheel is CI.

I need to know the best source for a tap, how many flutes, or other specifics on the tap, fluid and such. It will be hand tapped.

Any hints, tricks and tips?