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Tapping and Threading Formulas

Tap Drill Calculation Formulas

Inch Cutting Taps

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:

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


Metric Cutting Taps

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:

Drill Size = 6.0 - [(70 x 1.0 ) ÷ 76.98] = 5.09067 = 5.1mm Drill


Inch Fluteless Form Roll Taps

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 Fluteless Tap with 75% Thread:

Drill Size = .250 - [(.0068 x 75) ÷ 20] = .2245” = #1 Drill


Metric Fluteless Roll Form Taps

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 Fluteless Tap with 70% Thread:

Drill Size = 6.0 - [(70 x 1.0 ) ÷ 147.06] = 5.52400mm = 5.5mm Drill



Use the Following Formulas to Calculate the Tap Drill Sizes for Inch Taps and Metric Taps Both for Cutting and Fluteless Taps.

Tapping and Threading Feed Rate Calculation Formulas

Inch Taps:

IPM = Inches Per Minute  (Mill Tapping Feed Rate)

IPR = Inches Per Revolution (Lathe Tapping Feed Rate)

TPI = Threads Per Inch

IPM = RPM ÷ TPI

IPR = 1 ÷ TPI




Metric Taps:

(These Formulas Convert the Metric Taps / Threads to Inch Feed Rates)

To Convert a Metric Tap to Inches Per Minute Feed Rate for Mills:

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

Example:

We want to Tap a M3 x .5 Metric Tap at 1000 RPM

IPM (Inches Per Minute) Feed Rate = .5 x .03937 x 1000 = F19.685


To Convert a Metric Tap to Inches Per Revolution Feed Rate for Lathes:

Multiply the Tap Pitch by .03937

Example:

We want to Tap a M3 x .5 Metric Tap on a Lathe

IPR (Inches Per Revolution) Feed Rate = .5 x .03937 = F.0197

A Very Simple Tap Drill Calculation for Any NC / NF or ISO Metric Cutting Tap Basic Size Thread.

This Formula will Calculate a Tap Drill Size for an Approximate 75% Thread.

Formula: Major Diameter - Thread Pitch = Tap Drill size


Inch Tap Example:

Calculate Tap Drill for a 3/8-16 Tap

Note: Inch Tap Thread Pitch = 1 ÷ The Threads Per Inch

1 ÷ 16 = Thread Pitch = .0625

3/8-16 = .375 - .0625 = .3125 Tap Drill


Metric Tap Example:

Calculate Tap Drill for a M10 x 1.5 Tap

M10 x 1.5 = 10 - 1.5 = 8.5mm Tap Drill




“Machining Formulas” Reference Guide.  Contains 30+ Formulas for the Machinist!

Machining Formulas Reference Guide Web Machinist Software

Above is an example of a custom inch cutting tap calculation.  And below is the resulting screen for the custom tap drill calculation:

The Web Machinist Tapping Calculator provides the tap drill sizes for both inch and metric taps.  Cutting taps, form taps, roll taps and even helicoil taps.  Feed rates and spindle speeds are also calculated and you can even calculate your own custom tap drill sizes! (See below:) Very nice!

Also included in Web Machinist is a Threading Calculator:

As you can see, in the Web Machinist Unified Threading Calculator (above), not only are you provided with the thread dimensions in inch and metric but also feed rates and 3 wire pitch diameter measuring factors.  Very useful!

Web Machinist Software:

Both the Tapping Calculator and Threading Calculators are included in the Web Machinist Software Program below:

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“Tapping Calculator” Calculate the Tap Drill Size, Speed and Feed Rates for both “Inch” and “Metric” Cutting and Roll Forming Taps!

Tapping Calculator Web Machinist Special Deal!

From the Threading Main Menu (above) You can choose to calculate Unified Threads, Metric Threads, Acme Threads or choose the Tapping Calculator, Tapping Formulas Reference Guide or Thread Measuring using the 3 Wire Thread Measuring System.

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As you can see, in the Web Machinist Metric Threading Calculator (above), not only are you provided with the thread dimensions in millimeters and inches but also feed rates and 3 wire pitch diameter measuring factors.  Very useful!

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Also included in the Web Machinist Program is an Acme Threading Calculator (above), not only are you provided with the thread dimensions but also the 3 wire pitch diameter measuring factors.  General Purpose and Stub Acme Threads are supported.  Very useful!

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