On Feb 18, 12:53*pm, wrote:
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 *,,

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