Page 55 - CITS - Fitter - Trade Theory

FITTER - CITS

Calculation involved in finding out drill size (metric and inch)
What is tap drill size?
Before a tap is used for cutting internal threads, a hole is to be drilled. The diameter of the hole should be such
that it should have sufficient material in the hole for the tap to cut the thread.
Tap drill sizes for different threads
ISO metric thread
Tap drill size for M10 x 1.5 thread

Minor diameter = Major diameter - (2 x depth) Depth of thread = 0.6134 x pitch of a screw
2 depth of thread = 0.6134 x 2 x pitch
= 1.226 x 1.5 mm

= 1.839 mm
Minor dia = 10 mm - 1.839 mm
= 8.161 mm or 8.2 mm.
This tap drill will produce 100% thread because this is equal to the minor diameter of the tap. For most fastening
purposes a 100% formed thread is not required.
A standard nut with 60% thread is strong enough to be tightened until the bolt breaks without stripping the thread.
Further it also requires a greater force for turning the tap if a higher percentage formation of thread is required.
Considering this aspect, a more practical approach for determining the tap drill sizes is Tap drill size = major
diameter - pitch

= 10 mm - 1.5 mm
= 8.5 mm.
Compare this with the table of tap drill sizes for ISO metric threads.
BSW inch (1”) threads formula 1” = 8 T.P.I
Tap drill size = Major diameter - No.of threads per inch 1 inch = 8

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CITS : CG & M - Fitter - Lesson 7