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### Tap Drill Size as Percentage of Engagement Calculator

ANSI Hardware Design Guide

Metric Hardware Specification Charts and Tables

Tap Drill Hole Size based on Precentage of Full Thread Equation and Calculator

The size of the tap drill hole for any desired percentage of full thread depth can be calculated by the formulas below. In these formulas the Percent Full Thread is expressed as a decimal; e.g., 75 percent is expressed as .75. The tap drill size is the size nearest to the calculated hole size.

For American Unified Thread form:

$H}_{s}={B}_{md}-\frac{1.08253\cdot {P}_{ft}/100}{{N}_{ti}$where

H_{s} = Hole Size drilled (in)

B_{md} = Basic Major Diameter (in)

P_{ft} = Percent full thread engagement (%)

N_{ti} = Number of Threads per Inch

For ISO Metric threads

H_{s} = B_{md} - ( 1.08253 · P · P_{ft} / 100 )

where

H_{s} = Hole Size drilled (mm)

P_{} = Pitch (m)

P_{ft} = Percent full thread engagement (%)

The constant 1.08253 in the above equations represents 5H/8 where H is the height of a
sharp V-thread. The pitch is taken to be 1.

Reference:

- standard ANSI/ASME B1.1,
- ISO 68 and ANSI/ASME B1.7M-1984

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