Cutting an unlisted thread on colchester student

How do I cut a 1.75mm or 3.5mm thread on my imperial colchester student

The thread plate lists 1.25...1.5...2....2.5...3...4...5.....6..

So how do i calculate a different gear train.... Which i dont have any extra gears for yet

If for example i divide 25.4 (1 inch)by 7.5 (tpi) i get 3.3866 where the thread plate lists that setting as 4mm.

is that the metric/ imperial lever that adjusts that?

Hello Ian,

One suggestion is to set up for 2.5 mm cutting but interpose intermediate gearing for 35/50 in the gear train. You surely have those gears in your selection. That will give you 1.75 mm; 70/50 will give you 3.5 mm

I would do an 'air pass' first to check the result before you commit to metal

Regards

Brian

Edited By Brian Wood on 02/11/2018 10:36:50

Brian

i dont have any extra gears

just the driver and driven with a spare driver 42 twice the size for the smaller tpi's the gearbox covers almost anything i've ever needed

I had to do something similar with my Craftsman lathe in order to cut 1.75 [12mm] and 3.5 [30mm ]. As far as I remember,I set up for some speed but used a different setting of the levers on the gear box.

Sounds iffy,I know,but I made a note of what I did,for the next time around.

Hello Ian,

You asked how to calculate a gear train to suit these different threads and implied you would be getting the gears needed. I have simplified my thinking, you just need the two gears, 35 and 50 T . To cut 3.5 mm threads you select the gearing for 5 mm with those interposed.

That is what the maths shows in it's simplest form, I can't do magic.

Regards

Brian

Ian, would this forum post assist you at all?

Failing that I've finally got a new metric lathe arriving in Buxton on Monday

As you know from my one trip into your cellar,  I've not used a Student, but but I assume yours is the one with the imperial only gearbox, labelled up as the Dominion on Tony's web site. 

There is also some info on gears required in the manual from p 17 on, which is available on VintageMachinery.org

127 teeth are lots of money from HPC, but you could try Bede Tools

Good Luck

Bill

Edited By peak4 on 02/11/2018 11:35:48

Edited By peak4 on 02/11/2018 11:45:57

Right I'm still confused about what to do

whilst it has a imperial leadscrew theres a lever on the gearbox to switch between metraic and imperial threads

this is my gearbox table

As standard the gearbox is driven by a 21 tooth or a 42 tooth driving a 35 tooth on the gearbox input.

on my lathe i use timing belts so to keep the same ratio its a 24 or 48 driving a 40.

Can anyone with better maths brains tell me what gears i need or new pulleys to acheive a 1.75 and 3.5 pitch screw

Edited By Ian Parkin on 02/11/2018 12:41:53

You would need to change the 42T to a 49 and select the 1.5mm setting for 1.75pitch and the 3.0mm setting for 3.5 pitch

1.5 x 49/42 = 1.15

Edited By JasonB on 02/11/2018 13:04:52

Thanks Jason

30T on the gearbox driven by the 42T should do the same thing and may be easier to source

A 56T pulley driving your 40T pully will also work

Edited By JasonB on 02/11/2018 13:23:34

Jason

your sum above should be presumably

1.5 x 49/42 = 1.75

not 1.15 ?

As its easier to source pulleys than gears what size should the driven one be as i cant fit anything larger than a 48 tooth as the mainshaft is close

yes should have read 1.75, Think my fingers were thinking about the 1.16666666 increase in gear box speed you needed.

Won't work keeping the 48T pully unless you can find a 34.286T pully to go with it. Other pully combinations are 42-30 same as the gear option or 49-35 if you can squeeze it in. You need any combination that gives 1.4

Ian,

You didn't explain originally that you were using toothed belts on this lathe, I was working things out for a gear train, as you requested.

To return to your problem

Another combination that gives you the multiplier of 1.4 that Jason speaks of is 35 driver--25 driven.

I do have the original gears (only 3 )but usually leave the belts on as less noise and no need for setting clearances.

so basically I need to set the gearbox to say 1.5mm or 3mm and apply a 1.4 increase to the gear train or pulley ratios

When I do the calculations again, using a 42 driver and the gearbox settings you will be using, I find that you will need a 36 gear on the gearbox

The sum is 42/36 x 1.5 = 1.75 mm and likewise 42/36 x 3.0 = 3.5 mm

Edited By Brian Wood on 02/11/2018 17:53:58

Edited By Brian Wood on 02/11/2018 17:55:15

No you need a 1.16666666 increase to the existing which gives a ratio of 1.4, the old ratio from your belts or the gears was 1.2

Hello again Ian,

Sorry for resurrecting all this again, but I was away for the weekend at a family wedding and to while away the sleepless hours in a strange bed I fell to thinking about your metric gearings from a different approach altogether.

Now back home I was able to check them through. If you gear the lathe as 35/42 rather than the normal way, you get some rather unlikely 'imperial ' pitches, such that setting up for 12 tpi threading will give you a metric pitch of 1.764 mm; only 0.014 mm in error of the 1.75 mm you sought.

And, setting the gearbox up for 6 tpi theading will give you a metric pitch of 3.528 mm, this time in error by 0.028 mm

The multiplication factor 35/42 is the inverse of 1.2 [the normal arrangement of 42/35] which is 0.83333333; you just multiply all the pitch values by that amount.

I leave you to explore the new possibilities this little piece of thinking outside the box gives you

Regards

Brian

Edited By Brian Wood on 04/11/2018 19:20:41

Thanks Brian

i have ordered some pulleys so I can hopefully cut my 3.5mm pitches

the student handbook does go through how to cut different than what’s in the gearbox but it assumes that you also have a complete set of change gears and the instructions slightly gloss over how to calculate metric pitches

Actually I must try the setting 2 across from 3mm pitch and see what results could there have been a brainstorm in the Colchester factory and they left off 1.75 and 3.5 off the chart which surely were widely used in the 60’s

Ian

Sorry double post

Edited By Ian Parkin on 04/11/2018 19:37:23