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View Full Version : How to cut 27 tpi, 30 tpi, and other weird threads

SGW
05-16-2006, 02:15 PM
The question of how to cut 30 tpi came up a couple of weeks ago.

explains how to set up South Bend lathes to cut 27 tpi. In the process, you also get 30 tpi and other things.

Here's a simple-minded Liberty Basic program that figures out what you get for any screw and stud gear. By way of explanation, the "stud gear" is the gear in the gear train that's on the end of the tumbler reverse. The "screw gear" is the gear on the imput to the QC gearbox. For my South Bend 10K, the standard stud gear is 20 and the standard screw gear is 56. You can modify the program to suit whatever your lathe has.

10 input "Stud gear (Standard = 20): ";t
15 if t = 0 then t = 20
20 input "Screw gear (Standard = 56): "; x
25 if x = 0 then x=56
30 dim G(8)
31 g(1) = 8
32 g(2) = 9
33 g(3) = 10
34 g(4) = 11
35 g(5) = 11.5
36 g(6) = 12
37 g(7) = 13
38 g(8) = 14
50 print
60 print "Stud gear = ";t;" Screw gear = ";x
70 for r = 0 to 4 step 1
80 for c = 1 to 8 step 1
90 print using("###.##", (20/t)*(x/56)*g(c)*(2^r));" ";
100 next c
110 print
120 next r
130 print
150 goto 10

Here is what you get if you use a 42-tooth screw gear. Unfortunately, the display seems to strip out extra spaces so the table format is messed up....

Stud gear (Standard = 20):
Screw gear (Standard = 56): 42

Stud gear = 20 Screw gear = 42
6.00 6.75 7.50 8.25 8.63 9.00 9.75 10.50
12.00 13.50 15.00 16.50 17.25 18.00 19.50 21.00
24.00 27.00 30.00 33.00 34.50 36.00 39.00 42.00
48.00 54.00 60.00 66.00 69.00 72.00 78.00 84.00
96.00 108.00 120.00 132.00 138.00 144.00 156.00 168.00

LES A W HARRIS
05-16-2006, 06:51 PM
The HF12x36, actualy not all that Horrible or Frightening, many extra threads are available but are not documented.