On 24 May 2005 21:29:34 -0700, "
wrote:
I have an Atlas 10 in. lathe with the Pick-o-Matic gear change setup.
There is a wheel on the inside cover that has a listing for the various
feed rates and TPI available. My problem is that I now need to do some
metric threads, 1.0 and 1.25 mm pitch. Does any one know if this
possible on this lathe?
The lathe dates from 1945 or so and has worked well so far. But perhaps
obsolescence has overtaken it.
Thanks
Dave Scott
This question comes up every so often here. To generate an
absolutely correct metric thread [assuming a perfect inch lead
screw] a 127-tooth change wheel is indeed required. The problem
is this is a large gear [relative to the other change gears] and
may not fit.

Because the thread engagement of most lathe parts is only two or
three diameters, a thread that is "close enough" is all that is
required.

The ideal metric/inch ratio is 127:100 or 1.27000

Other smaller gears can be combined to give very close
approximations of the ideal ratio.

For example

47/37 = 1.27027023 or +0.02128% lead

80/63 = 1.2698127 or -0.012498% lead

There are of course other combinations.

An additional benefit is that generation of a 127 tooth gear
requires differential/compound indexing which is beyond the
capabilities of most inexpensive dividing heads, while 47,37,and
63T gears can be generated with the typical home shop equipment,
and you are likely to already have an 80T gear.

Remember if you are using a metric/inch change gear, you *MUST*
keep the half nut engaged as the thread indicator does not work
with these odd thread fractions. A spindle crank can be a
lifesaver if you need to thread to a shoulder.

GmcD