Metric thread cutting on a Colchester

[Dave Green 4Participant @davegreen4]

Headache

[Dave Green 4Participant @davegreen4]

Hello all. The name is Dave. I've just got myself a Colchester mrk1 lathe (I think) just trying to get to grips the the thread cutting thing. I'm tying to cut metric threads but I think the gears on the back could be the wrong way round. As in number of teeth. Can someone clarify this

At the top is a 42t the big one is 120t. That meshes into a 35t

[DC31kParticipant @dc31k]

Colchester is a brand of lathe, just as Ford is a brand of car. Ford made Mark 1 Cortinas, Escorts and Capris.

You need to find out exactly what machine you have and obtain a manual for it. Read that manual very well.

From the tooth numbers of the gears you give, it is possible that you have a Colchester Student. If so, kindly read this thread:

https://www.model-engineer.co.uk/forums/postings.asp?th=150056

[Bernard WrightParticipant @bernardwright25932]

Hi Dave,

The 42t gear is usually used for the larger pitches of thread, if you look on the gearbox plate it illustrates at the bottom as needing the 42t to be used for those pitches.

Normally it would have a 21t as the driver, 120t is only an idler and the 35t is the driven (on the gearbox).

This actually leads me into a query of my own, I presume your gearbox plate shows Imperial in black, with the only Metric pitches available in red.

There is no 1.75mm and 3.5mm pitch showing, the manual gives a formula to create gearing, for odd Imperial pitches and Metric.

I followed these rules and produced a 24t Driver and a 29t Driven for a supposed 1.75mm pitch using the 20 plunger hole and A-D selector levers.

Needles to say I didn't get 1.75mm pitch more nearer 2.1mm pitch.

Has anyone on here overcome this problem, I would really like to hear from you.

[DC31kParticipant @dc31k]

Posted by Bernard Wright on 16/01/2021 01:15:20:

Has anyone on here overcome this problem, I would really like to hear from you.

Could you please mention the make and model of lathe to which you are referring.

If you could post a link to the manual, then you will have a lot more chance of success.

Right now we are left to guess what lathe you have and what formula you have used.

[Bernard WrightParticipant @bernardwright25932]

Sorry missed that bit of detail, My lathe is a Colchester Roundhead Master 6-1/2" Mk1.5 Mach No: F 3/63579 X.

It's more of a Mk1.75 as I've modified it quite a lot, I'll post a photo shortly.

[DC31kParticipant @dc31k]

It is difficult to find a manual online for the Master. The lathes.co.uk site says it was identical to a Student in all but centre height.

Could you confirm that the manual here:

http://manuals.chudov.com/Clausing/Colchester-Student-Lathe-Manual.pdf

has the same gearbox layout and formula (page 17) as your own.

[Ian ParkinParticipant @ianparkin39383]

Bernard

i have the same lathe and come against the problem of cutting a 3.5mm thread

a student though marked as 6 inch usually is 6.5 inches so the same as a master..most were built like that according to lathes.co.uk

look through my posts for the solution

Ian

[Ian ParkinParticipant @ianparkin39383]

Bernard i use a 35 tooth driver and a 21 driven levers at DA lever into imperial and the gearbox lever at the position corresponding to 1.5mm and don’t use the thread indicator…reverse the lathe between cuts

this is for a 3.5mm thread which i cut a lot of

Edited By Ian Parkin on 16/01/2021 11:00:21

[DC31kParticipant @dc31k]

The following is worked out using the formula given in the Student manual linked above. It may or may not apply to your machine.

The manual notes the following limitations on the gear tooth sizes: maximum driver 60t, maximum driven 64t.

For 1.75mm pitch, use AD (Y=2) and for 3.5mm use AC (Y=1)

Best to worst, written as [X, driver, driven]

[18, 32, 43] [30, 31, 25] [20, 43, 52] [26, 43, 40] [16, 41, 62] [28, 44, 38] [19, 22, 28] [22, 30, 33] [24, 63, 64]

[Ian ParkinParticipant @ianparkin39383]

dc31k

what does all that mean in practice?

if i want to cut a 1.75mm thread what do i need (best)?

the levers set at AD….what’s Y ?

use a 32 driver and a 43 driven ? What’s the x (18)?

which position is the gearbox set to?

and do you use imperial or metric on the lead screw lever?

[Bernard WrightParticipant @bernardwright25932]

Hello again Folks,

I can confirm that manual in that link looks identical to my 'Official Master Manual'.

I've also had some progress from my earlier posting, I took the gears I'd made off the machine (24t & 29t).

I then replaced the 35t original gear on the gearbox, but tried the 24t on the drive splines instead of the usual 21t.

I had a go at threading with the plunger in 20 hole, and levers in the same A-D position, and the selector on Metric, what do you think?

I ended up with very close to 1.75 pitch, unbelievable and very relieved.

I'll post some images in my album to insert in my next posting.

Ian, I think you'll need a 48t Driver with a 35t Driven and the levers/plunger/selector same as above to give 3.5p, really hope that helps. Bernard.

[DC31kParticipant @dc31k]

Posted by Ian Parkin on 16/01/2021 12:11:21:

what does all that mean in practice?

Let me preface this by saying I do not have either a Student or Master lathe. Everything I have written is as a result of reading the manual linked above. If you also read it, you will see where the terms come from.

Let us go back to your original post. You say you have a way to cut 3.5mm pitch using 35t, 21t, DA, and 1.5mm gearbox setting. So if you want 1.75mm, that is half (or double) 3.5mm. The easiest thing to do is to change the arrangement of the levers that give you the two letter combination. This is exactly the same as changing the 1.5mm gearbox setting to 0.75mm (or possibly 3mm).

From the manual above, there are two levers, one with A, B and the other with C, D. That gives four possible arrangements of levers. On page 17 of the manual above, it gives a value to be used with each combination. The values are 1, 2, 4 and 8. From page 17, DA corresponds to a value of 2 so for 1.75mm pitch, you would need to try either 1 (AC) or 4 (BC).

The reason I am hesitant to commit myself is that I always get confused over the halving or doubling, especially when metric (pitch) rather than threads per measurement unit are involved. It is a case of try both options: one will be correct.

As to my post above, it is just the results from using the formula on page 17. X is the gearbox position, shown in the manual as (16, 18, 19, 20, 22, 24, 26, 28, 30). To find the 'best' ratio, you have to try all nine possibilities.

The formula tells you to convert your desired metric pitch to an equivalent teeth per inch. If you do that, you end up with a set of nine driver/driven ratios all with a multiple of 127 in their denominator. And you cannot have that because the manual also tells you that the biggest driven gear can only have 64t (and the biggest driver 60t).

So you then find a handy online continued fractions calculator and ask it to give you the closest fraction it can to the exact one, with a maximum denominator of 64. You then back-calculate the teeth per inch each of these nine approximate fractions give and sort from best to worst.

The best, [18, 32, 43] above, gets you 1.7502mm pitch (14.5125 tpi). The worst, [24, 63, 64] gets you 1.736mm pitch (14.6286 tpi).

Edited By DC31k on 16/01/2021 15:02:27

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