Screwcutting on a Colchester Triumph

Yesterday we encountered a problem whilst attempting to cut a 4.5 tpi thread. the machine is an old round head Triumph with an imperial / metric quick change gearbox (selectible via another lever) and a 4tpi leadscrew.. When the appropriate selections from the chart were made, A and C on the two no, two position levers and the last but one position to the RHS on the sliding lever the thread was not 4.5 tpi.

The stud gear is 18T, the idler 120T and the screw gear is 36T.

Can anyone confirm whether this is the correct gearing? We don't have a manual for this lathe unfortunately.

Thanks

Dave

Corrected the idler though this doesn't influence the ratios.

Edited By David Davies 8 on 08/09/2023 21:29:34

I think this might be the manual for the Clausing version of the round head Triumph:

http://vintagemachinery.org/pubs/detail.aspx?id=3846

For that, and for 4.5tpi, it says (on page 14/15) the 36t gear should be the DRIVER (I think that is what you call the stud gear above) and is used in place of the normal 18t gear. The DRIVEN gear (what you call the screw gear) should remain as 42t.

What tpi did it actually cut with the set up you describe?

Can you get it to cut 9 tpi or 18 tpi successfully? If so, 4.5 tpi will be one or more multiples of two different to those.

Thank you DC31k for taking the time to reply and providing the link to the lathe manual.

It is now obvious what went wrong, we retained the 18T gear as the driver and substituted the 36T as the driven.gear.

We will try this out when me meet at the 'man cave' next Thursday.

BTW, Stud and Screw gears are the terminology used by Boxford on their machines.

Cheers

Dave