Colchester change gears?

From memory a 21 on the output shaft from the headstock and a 42 on the input shaft to the gearbox

Sorry Ian, by my reading of the manual that is the wrong set-up which is what Alex probably has.

Standard set up is driver 21 teeth, intermediate 120 teeth, gearbox driver 35 teeth. The 42 teeth gear normally sleeps on the gearbox shaft outside the 35 teeth gear doing nothing. Swopping the 21 teeth driver for the 42 teeth gear gives an extra 9 thread TPI / pitches additional to the standard range of 36. Basically it halves the TPI on any setting.

Usual reason for odd threads is that the 42 tooth gear has been used on the gearbox input shaft instead of the 35 tooth one. That makes the end train gear ratio 0.5 instead of 0.6 which will make any screw-cut thread "really odd" but isn't far enough out to be immediately obvious before you start.

Clive

Yes clive i remember now

I have converted my lathe some time ago to belt drive and my ratios are 24 to 40 and place a 48 on the headstock end when i want the fast threads 4,5,6 tpi

sorry for my crap memory

No worries. 21 going down to 35 always seems odd. Especially if you've not got the manual. Somehow one expects a factor of 2 when driving for TPI. Not having a Colchester I had to cheat by reading the manual!

Clive.

Hi Alex, I have a Colchester student manu8al on PDF if you would like one, phil dot hermetic at langtoft dot net.

Hi, Thank you all for the replies. I have been into the workshop and managed to make a real mess!

I have the 42 tooth gear, the 120 tooth gear and a 33 tooth gear. 33 was close enough that I thought I had managed to cut a 12 tpi thread. When I tried to use the thread dial things went badly!

I then read Clive's post and realised my mistake. I will have to try and track down a 21 tooth and 35 tooth gear. I am suprised that they are not with the lathe.

Phil, thank you, I will send you an email.

Thanks Again,

Alex

Hi Alex, I actually have a 35Tgear that you can have, and I need a 42T. If you would like to I would be very happy to make a swap. Either way, you can have the 35T.