Re: TPI discrepancy

m. allan noah

The leadscrew is 8 too, not 9. The gearbox input gear is 56 tooth, not 40.

On Sun, Jul 26, 2020, 5:42 PM John Dammeyer <johnd@...> wrote:

Threading isn't rocket science.  To me the 9 TPI leadscrew doesn't sound right but I don't know.  Maybe South Bends did come with that.


I'd put a dial indicator on the ways and then turn the lead screw by hand one turn with the half nut engaged and the backlash removed by first turning the leadscrew a few turns in the same direction.  If it's 8 TPI it would 0.125". If it's 9 TPI it's 0.11111111111111111111111…" which is not really logical.  Where if 10 TPI it's 0.1  Generally leadscrews are set up to not have infinite repeating decimals.


Next forget about the pitch you want and if the leadscrew is really 9 TPI then set up to cut 9 TPI.  Now the lead screw will turn exactly one turn for each turn of the spindle.  If that doesn't work again take a look at the gears.


John Dammeyer


"ELS! Nothing else works as well for your Lathe"

Automation Artisans Inc.

www dot autoartisans dot com






From: [] On Behalf Of Michael R via
Sent: July-26-20 2:31 PM
Subject: Re: [SouthBendLathe] TPI discrepancy


Yep, using the half nuts for threading.

The gear configuration is as follows:
Top stud gear - 40t (according to my parts manual it should be 20t)
Idler gear - 80t
Stud gear to lead screw - 40t (this is correct as it's indicated on the gearbox thread chart)
Lead screw - 9p

Another issue with this machine is that the longitudinal power feed spins way too fast, not delivering a good finish.

Wondering if the top stud gear could be the culprit for both problems?

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