What is 7/32"x ** tpi tapping drill size?

[Overs Pianos]

Carl and all,

Carl wrote;

>If someone has a chart listing 1/4-36 tap drill size I'd be interested to
>know what it is.
>
>Thanks
>
>Carl Meyer

If you dealing with a strange thread, with an unusual number of 
threads per inch (TPI), a handy formula I use for calculating the 
minor diameter is;

(All measurements are in inches)

Minor diameter = Major diameter - (1.220/TPI)

Example 1. If you wish to know the minor diameter of a 1/4" (0.250") 
agraffe thread with 32 threads per inch.

Minor diameter = 0.25 - (1.220/32)
		= 0.212"

Since it is good engineering practice to allow for some clearance 
between the drill size and the minor thread diameter, a 7/32 (0.218") 
drill would be a suitable drill size for tapping a 0.25" by 32 TPI 
thread.


Example 2. If you wish to know the minor diameter of a 1/4" (0.250") 
agraffe thread with 36 threads per inch.

Minor diameter = 0.25 - (1.22/36)
		= 0.216"

A 5.7 mm (0.224") drill would be a good drill size for tapping a 
0.25" by 36 TPI thread.

This formula, which I worked out many years ago, holds for all 60 
degree thread forms. It can be used for metric threads (which are 
also 60 degree threads) provided that the pitch and diameter of a 
given metric thread is converted to inches first (1 mm = 39.37 thou). 
Since I have imperial drills in 1/64" steps, and metrics in 0.1 mm 
steps to 10 mm. Using the above formula I can quickly determine a 
suitable drill size without bothering to open my Machinery's 
Handbook. It's also very handy when cutting threads on the lathe.

Ron O


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Website:  http://www.overspianos.com.au
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