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Technical Formulas: Machine Screw & Fractional Sizes

This document provides formulas and examples for calculating thread percentages for machine screws and metric sizes. It also discusses how to adjust tap selections to account for plating or coating thicknesses. The thread percentage calculation determines the relationship between the thread's major and minor diameters. For plating adjustments, the standard tap size is increased based on the plating thickness to ensure proper threading.

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31 views1 page

Technical Formulas: Machine Screw & Fractional Sizes

This document provides formulas and examples for calculating thread percentages for machine screws and metric sizes. It also discusses how to adjust tap selections to account for plating or coating thicknesses. The thread percentage calculation determines the relationship between the thread's major and minor diameters. For plating adjustments, the standard tap size is increased based on the plating thickness to ensure proper threading.

Uploaded by

Dave B
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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Technical formulas

Thread percentage calculations


The following formulas can be used to calculate thread
percentages in tapped holes for the following conditions.
1. Cold formed threads: The after-tap minor diameter
is created by the relationship between the pre-tap hole size
and the cold forming tap D or H number, and is measured
using cylindrical pin gages or other means.

2. Cut threads: The after-tap minor diameter is created directly by


the drill and is measured using cylindrical pin gages or other
means.

Machine Screw & Fractional Sizes


Thread Percentage =

(Thread Major Diameter [in.] Minor Diameter [in.])

x TPI

.01299
Example: 1/4-20 thread with .201 inch minor diameter.
Thread Percentage =

(.250 .201)
.01299

x 20 = 75.44%

machine screw major diameters


machine screw #

Major Diameter (in)

10

12

.060

.073

.086

.099

.112

.125

.138

.164

.190

.216

Metric Sizes
Thread Percentage =

(Thread Major Diameter [mm] Minor Diameter [mm])


.01299 x Thread Pitch [mm]

Example: M8 x 1.25 with 6.80 mm minor diameter.


Thread Percentage =

(8.0 6.80)
.01299 x 1.25

= 73.90%

How to Adjust for Plating Thicknesses


Standard cold forming or cutting tap H or D numbers must
be increased to accommodate for the coating thickness.
Platings or coatings that are very thick are not always uniformly
applied to the threads and may cause gaging problems

regardless of the tap oversize condition. Heavy plating or coating


thicknesses are especially difficult for fine pitch threads because
they tend to fill in the thread profile rather than coat the thread
flanks evenly.

inches

Mils

Microns

number of h or D numbers to
add to standard tap selection

0.000125

0.125

3.2

0.000250

0.250

6.4

0.000375

0.375

9.5

0.000500

0.500

12.7

0.000625

0.625

15.9

0.000750

0.750

19.1

0.000875

0.875

22.2

0.001000

1.000

25.4

TECHNICAL INFO

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TECHNICAL INFO

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