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Class of Threads, H Limits

Classes Of Threads
There are (3) established Classes of Thread, designated in the unified series by adding: "A" for Screws and "B" for Nuts (or other intenal threads) to show definite limits and tolerances.

Class 1B Thread
Is wherea 1A screw can run in readily for quick and easy assembly. The hole is classified as 1B. The fit is a 1B thread, (very seldom used in modern metal working)

Class 2B Thread
Consists of a 2A screw in a 2B hole. 2B thread has wide applications. It is used to accomodate plating, finishing and coating to a limited extent and threfore, has fair tolerance allowances.

Class 3B Thread
3A screw in a 3B nut or internal threaded hole, used where tolerance limits are close.

GH Numbers
GH Numbers are listed below. "G" designates Ground Thread. "H" designates the pitch diameter is on high side of basic. These two letters (GH) are followed by a numeral indicating the Tolerance of Pitch diameter oversize.

H1 = Basic to Plus .0005    
H2 = Basic Plus .0005 to Plus .0010    
H3 = Basic Plus .0010 to Plus .0015    
H4 = Basic Plus .0015 to Plus .0020    
H5 = Basic Plus .0020 to Plus .0025    
H6 = Basic Plus .0025 to Plus .0030   H=Above Basic
H7 = Basic Plus .0030 to Plus .0035   L=Below Basic

Relation Of Tap Pitch Diameter to Basic Pitch Diameter
American Tap Manufacturers use a series of tap pitch diameter limits. These limits feature a .0005 tolerance in tap sizes #0 Thru 1" and a .001" or greater tolerance in tap sizes above 1" thru 1 1/2" diameter.
Example: 1/4-20. Relationship between Tap Pitch diameter limits and basic nominal pitch diameter.

Pitch Diameter: .2175
  1. A tap cannot produce a class of thread it can produce a tapped hole within specific product limits.
  2. Since the tap is used only in tapping a hole or producing an internal thread, a tap has no control over the fitting properties of the mating external thread.
  3. To produce what is commonly referred to as a class of thread both external and internal threads must be within their respective product limits. Only when both members of a thread assembly fall within their desired class limits can the proper fit be assured.
  4. The acceptability of any class of threaded hole is determined only by an accurate "G0" or "HI" Thread plug gage of corresponding class. The acceptability of the male part with an external Thread is also determined by a corresponding "GO" or "LO" Thread Ring gage.
  5. Tap limits refer to the various sizes of tap manufactured. A tap whould be selected which will produce an internal Thread within the desired product limit. Tap limits are designated as L1, H1, H2, H3 etc.
  6. Although ground taps are produced to precision tolerances under closely controlled manufacturing processes and are guaranteed for accuracy of individual elements, there is always the possibility of the presence of unknown factors which can be a detriment to good tap perfomance.