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Ground Thread Tap Limits for KATO STI Taps

The KATO Tech Group gets many inquiries from our customers asking us about “H Limits” on our unified size taps. These are defined as Ground Thread Tap Limits by the tap manufacturers, not to be confused with the class of fit designations on metric threads. Sound confusing? This is understandable.

Due to variations in materials, machining conditions, etc., taps very rarely if ever cut to their exact dimensions. Therefore, in the case of KATO STI taps for free machining materials, the tap manufacturers have established dimensions for the taps that they feel will produce the required tolerances for Class 2B and 3B STI tapped holes. The pitch diameter of the tap is defined by its H limit or high limit, which is the tolerance range above the basic pitch diameter. Each “H” number is .0005 inches over basic. We rely on the expertise of the tap manufacturers to design and select the best H limits for the most popular materials being tapped for insert applications Perhaps the illustration and example below will provide some clarity as to how H limits tie the pitch diameter of the tap to that of the tapped hole.

Figure 1: Pitch Diameter Tolerance .375-16 UNC STI Tapped Hole

STI

Pitch Diameter Tolerance of STI Tap

Example: .375-16 3B STI Tap = H3 per Tap Manufacturer

“H” number × .0005 = Max. Pitch Dia. over Basic Pitch Dia. of Tapped Hole

Tolerance = +0.0000
-0.0005

H3 = 3 x 0.0005 = 0.0015
Basic P.D. = +0.4156
Max. P.D. of Tap = 0.4171
Tolerance = -0.0005
Min. P.D. of Tap = 0.4166

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