Colchester Student mk1 change gears

[Drew MasonParticipant @drewmason67976]

[Drew MasonParticipant @drewmason67976]

Hi all,

Firstly, glad to finally join the forum after years of lurking! I found myself tonight questioning a situation with my student lathe and thought 'right, its time….;

The lathe was an eBay purchase and definitely has a 'history'. I've put off doing any screw cutting with it until tonight as I have access to better machines at work on the rare occasion I need to do any. At some point I had to have a crack and see how it got on, but to my frustration (definitely not suprise), I found that it didn't cut a TPI as I wanted it to. After stripping off the change gear cover I found that is runs a 24T drive gear, through a 120T intermediate gear, to a 45T driven gear, with no other spare gears at all. Balls.

My first question to myself is, why? The lathe has a very nice selection of pitch options as standard! It just seems like hard work working anything out now. I see companies selling aftermarket gears give plenty of options so I must be missing something here (usually the case).

My second question is, would anyone out there have a set of original 42/35/21T gears that they would part with? Or even just a 21T gear?

Many thanks in advance!
Drew

[JasonBModerator @jasonb]

As to why, a previous owner probably wanted a pitch that was not in the gearbox (possibly metric) so altered the gears to suit. Come sale time the other gears were lost or sold on by a dealer as the individual gears probably fetch more sold as spares that included with the machine.

Did that 24tpi thread come out a bit finer? 25.6tpi by any chance

Edited By JasonB on 03/03/2020 07:25:08

[DC31kParticipant @dc31k]

Posted by Drew Mason on 02/03/2020 21:13:25:

My second question is, would anyone out there have a set of original 42/35/21T gears that they would part with? Or even just a 21T gear?

If the gears used on this lathe are hard to come by, you can help yourself by familiarising yourself with gearing nomenclature and finding gears of the same tooth specification. You need to know Diametral Pitch (DP) and Pressure Angle (PA). Most easily digestable reference is Ivan Law's book in the Workshop Practice series.

If you report these details here, we can tell you which other machines share the same tooth specification.

If the centre detail does not match your existing ones, this is normally easy to modify (could be as simple as pinning the new gear concentric to one of the old ones).

There is a seller on eBay whose user ID is you_engraving who makes delrin change gears at very reasonable cost.

[DC31kParticipant @dc31k]

Posted by Drew Mason on 02/03/2020 21:13:25:

Or even just a 21T gear?

Sorry to follow up so soon, but there is evidence that the 21t gear is for additional or extra or less-used pitches.

The standard gear is the 42t gear.

https://www.model-engineer.co.uk/forums/postings.asp?th=116473

https://www.mig-welding.co.uk/forum/threads/colchester-student-screwcutting.80489/

(Above link has pdf manual for download)

https://www.practicalmachinist.com/vb/general/colchester-student-dominion-original-change-gear-question-360779/

[Ian ParkinParticipant @ianparkin39383]

 

Dc31k

Certainly on my Colchester student the 21 t is the driver for 4 rows of threads and feeds

the 42 is for 1 row of faster feeds and lower tpi’s

this with a student with a gearbox on as the op’s

Edited By Ian Parkin on 03/03/2020 19:12:37

Edited By Ian Parkin on 03/03/2020 19:13:49

[Drew MasonParticipant @drewmason67976]

Jason – The thread I was aiming for was 16tpi, but with the 24 / 200 / 45 tooth arrangement that is fitted to the machine it came out way off, I didn't measure it as I knew whatever the case, it wasn't right. Someone probably had a method to it, but its turned out a bit of a pain and expense to me!

DC31K – Hopefully I can sort out some original gears (and I think I have a lead on what I need which would be nice, but its going to hit me nearly £100). I'll certainly pursue your suggestion should i find myself in a pickle though, thanks for that! With regard to your 2nd post, the manual says the original set-up is 21T driver, 200T intermediate and 42/35T driven gears.

Ian – my plate isn't the same as yours. Mine is all in one block (no extra line/mention of 42T)

Thanks for the input!

[DC31kParticipant @dc31k]

Looks like there were a couple of different versions produced. The Student is quite a long-lived machine so changed through the generations.

Drew's plate is the same as the bottom four lines of Ian's plate, so is missing some of the finer threads.

It is interesting that Colchester did not provide for standard M12 1.75mm pitch threads on either machine. The same omission is made on the Chipmaster.

[Simon Williams 3Participant @simonwilliams3]

Drew:

Not sure I can resolve the complications of the screw cutting chart as above, but I might be able to help with a 42 Tooth gear. I say "might" – here's why.

I have a selection of gears squirrelled from various sources, one of which advertised then as suitable for Bantam (which is what I've got). But some are 16DP (which fit correctly and mesh with the originals) and some are 14DP (which don't).

One of these days I'll get a set of 16DP cutters and use the 14DP gears as handy blanks..

In the meanwhile one of these 14 DP gears is 42 teeth. So we need to determine if it will fit with the gears you already have.

Can you measure the outside diameter of one of your gears, and count the number of teeth. It's probably got the number stamped on it, but it's worth checking carefully that someone hasn't done what I'm proposing to do and that you do have the number of teeth specified.

You said, I think, you have a 45T gear. A 45 tooth gear in 14 DP will measure 47/14 = 3.36 ins OD (n + 2 divided by the DP). OD is measured over the tops of the teeth, so it's the diameter of the gear blank.

A 45 T gear in 16 DP will be 2.94 ins OD, so it should be possible to tell the difference.

If the 14 DP gear is of use to you, read on.

That deals with the outside dimension, now to consider the bore. I'm not familiar with the innards of the Mk1 Student, but I'm assuming it uses the Colchester 6 spline drive which others of the marque use. I measure the spline as 6 teeth, 23.00 mm major diameter, 19.75 mm minor diameter.

Gear thickness seems to be a fairly nominal 3/8 inch.

If the 14DP gears are of use to you let me know and we can sort something out via PM.

HTH and best regards Simon

[Simon Williams 3Participant @simonwilliams3]

Good evening again – just a minor clarification.

I'd always assumed that the centre spline was standardised, but looking more closely this isn't right. The 14DP gears I've got have a 6 spline centre but the 16DP gears have an 8 spline centre.

HTH Simon

[Alberto BeggioParticipant @albertobeggio46704]

Good morning Ian,

I’m am new on this forum and I saw the gears table of yours Colchester Student imperial lathe, I’m try to making a reply aluminium table for a friend (maybe in Photoshop), that have the table in not good conditions; could you give me, please, the dimensions of the plate and the thickness too ?

Thank you in advance,

Alberto.

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