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Tap Drill Size Formulas:

To Calculate the Tap Drill Size for Inch Cutting Taps, use the Following Formula:

Inch Cutting Tap Drill Size =

Basic Major Diameter of Thread - {(.0130 x % of Full Thread) ÷ Threads Per Inch}

Example:

To Find the Tap Drill Size for a 1/4-20 Tap with 75% Thread:

Tap Drill Drill Size = .250 - {(.0130 x 75) ÷ 20} = .201” = #7 Drill


To Calculate the Tap Drill Size for Metric Cutting Taps, use the Following Formula:

Metric Cutting Tap Drill Size =

Basic Major Diameter of Thread - {(% of Full Thread x Pitch in Millimeters) ÷ 76.98}

Example: To Find the Tap Drill Size for a M6 x 1.0 Tap with 70% Thread:

Tap Drill Size = 6.0 - {(70 x 1.0 ) ÷ 76.98} = 5.09067mm = 5.1mm Drill


To Calculate the Tap Drill Size for Inch Fluteless Form Roll Taps, use the Following Formula:

Inch Roll Tap Drill Size =

Basic Major Diameter of Thread - {(.0068 x % of Full Thread) ÷ Threads Per Inch}

Example: To Find the Tap Drill Size for a 1/4-20 Roll Tap with 75% Thread:

Tap Drill Size = .250 - {(.0068 x 75) ÷ 20} = .2245” = #1 Drill


To Calculate the Tap Drill Size for Metric Fluteless Roll Form Taps, use the Following Formula:

Metric Roll Tap Drill Size =

Basic Major Diameter of Thread - {(% of Full Thread x Pitch in Millimeters) ÷ 147.06}

Example: To Find the Tap Drill Size for a M6 x 1.0 Roll Tap with 70% Thread:

Tap Drill Size = 6.0 - [(70 x 1.0 ) ÷ 147.06] = 5.524mm = 5.5mm Drill


Tapping Feed Rate Formulas:

Inch Taps Feed Rate Formulas:

To Calculate the CNC Mill Inch Tapping Feed Rate in Inches Per Minute, use the Following Formula:

TPI = Threads Per Inch

Feed Rate in IPM (Inches Per Minute) = {(1 ÷ TPI)  x Spindle RPM}

Example:

You are Tapping a 28 TPI (Threads Per Inch) Thread on a CNC Mill at 1500 RPM

1 ÷ 28 = .0357

.0357 x 1500 = 53.55 Inches Per Minute Feed Rate


To Calculate the CNC Lathe Inch Tapping Feed Rate in Inches Per Revolution, use the Following Formula:

TPI = Threads Per Inch

IPR = Inches Per Revolution

Feed Rate in IPR (Inches Per Revolution) = 1 ÷ TPI  (Threads Per Inch)

Example:

You are Tapping a 20 TPI (Threads Per Inch) Thread on a CNC Lathe

1 ÷ 20 = .05 Inches Per Revolution Feed Rate


Metric Taps Feed Rate Formulas:

This Formula Converts Metric Taps to an Inches Per Minute (IPM) Feed Rate

To Convert a Metric Tap to an Inches Per Minute Feed Rate for a CNC Mill, use the Following Formula:

Multiply the Metric Tap Pitch by .03937 then Multiply it by the Spindle RPM

Example:

You want to Tap a M5 x .75 Metric Tap at 1000 RPM

.75 x .03937 x 1000 = 29.5275 Inches Per Minute Feed Rate


This Formula Converts Metric Taps to an Inches Per Revolution (IPR) Feed Rate

To Convert a Metric Tap to Inches Per Revolution Feed Rate for a CNC Lathe, use the Following Formula:

Multiply the Metric Tap Pitch by .03937

Example:

You want to Tap a M3 x .5 Metric Tap on a CNC Lathe Using an Inches Per Revolution (IPR) Feed Rate

.5 x .03937 = .0197 Inches Per Revolution Feed Rate


Tapping Formulas:

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All of the Tapping Formulas Calculations can be easily done in the Web Machinist Software Program’s Tapping Calculators Module (below) Available Here