Formula for Changing thread gears

I'm using Rajkot conventional lathe , i want the formula for changing thread gears for required pitch,
for example 20 mm pitch and 4 TPI lead screw what are the changing
gears ,,
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On Feb 18, 12:53pm, snipped-for-privacy@gmail.com wrote:

You start with the thread that 1:1 gearing between the spindle and leadscrew will cut, in this case 6.35mm pitch, then find a gear ratio that is some multiple of 20 to 6.35, like 400 / 127. You can compound the 400 but 127 is almost essential in metric conversions unless you REALLY like working with messy fractions. Try Boston Gear. Since 20mm is coarser than 6.35mm, you need to arrange them to speed up the leadscrew.
I handled this for my 8 TPI leadscrew by writing a spreadsheet that recreated the quick-change gear chart. The formula accessed an external cell in which I tried 80/127, 100/127, 120/127, and so on until the pitch I needed popped up in the chart somewhere.
Jim Wilkins
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