CNC Lathe: Pipe Thread Programming

Dear Answer Man,

Could you help me with a program example for my SL-30? I want to use G76 for a tapered pipe thread without a thread chamfer. It’s a 1.25 NPT thread, 0.875″ deep. Thanks!

Larry Cho

Dear Larry:

The taper is 3/4″ per foot, so:

1) 0.75 ÷ 2 = 0.375

2) 0.375 ÷ 12 = 0.03125

3) 2nd tan–1 of 0.03125 (need a calculator for this) = angle of 1.7899 degrees. This is the taper of the thread. Now determine the minor diameter, which is X1.161 at Z0 (the face of the part). Determine the X position at Z0.5, which is X1.1785, and the X value at Z-0.5, which is X1.1435. Then:

1.1785 – 1.1435 = 0.035

0.035 ÷ 2 = 0.0175, which is your positive I value. The rest should be easy.


O00005 (1-11.5 PIPE THREAD)



S1000 M03

G00 X0.95 Z0.5

M23 (or M24 for chamfer off)

G76 X1.1785 Z-0.5 I0.0175 K0.03

D0.01 F0.0869




You can use a CAM system or the calculator on the Haas Lathe control to make points, lines and angles with which to determine X values.

Sincerely, Answer Man

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