Threads, Taps, and Tapping - 4:Tap Grinding Codes
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WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.
As noted earlier, there are three tolerance class of fit, 1= lose, 2= medium, and 3= tight.

Taps are rarely listed in a tool catalog by the tolerance class but rather by a grind number (G).

You must refer to Machinery's Handbook to find a table that will give you the correspondence between the desired tap size and the grind number.

In the manufacturing process the tap is ground on the High or the Low side of basic pitch diameter and shown in place of tolerance class as GH or GL.

These two letters are followed by a number showing the tolerance of pitch diameter. The "G" is normally understood and not always shown in the alpha-numeric code, for example 1/4 20 GH3 or 1/4 20 H3. The table bwlow shows you an example of recommended tap for a 1/4-20 thread.

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