Screw cutting 1.25mm pitch on a Colchester Student.

[Kevin NichollsParticipant @kevinnicholls15835]

Hello to all , I have a Round top Colchester Student lathe rebuilt it some years ago only used it for general machining as I have lovely Exe 3.5 ” lathe that I have also rebuilt that I use mainly all the time as its so handy ,a good friend is trying to make a replacement spindle nut for his Abora milling machine the nut is 62mm x 1.25mm which he cannot cut on his lathe with out buying a 127t gear, looking at my Colchester the screw cutting chart states that it can cut a 1.25mm pitch thread by using a 42t gear on the headstock and a 21t gear on the leadscrew with the gearbox levers set at D&A and the tumbler shifting arm inline with the 1.25mm pitch on the gearbox setting chart on the gearbox with the machine set at these positions I tried to cut a thread it was miles out measuring the thread it was 4.5mm pitch I have re checked the setting and are correct to the Colchester manual this is the first time I have attempted to screw cut on the machine so if anyone has any ideas what I am doing wrong they would be greatly appreciated.

Regards Kevin Nicholls.

 

[sprocket 3Participant @sprocket-3]

Hi,Kevin. A big discrepancy there at 4.5 mm pitch instead of 1.25mm.

Is your Colchester an imperial or metric machine as supplied from the factory?

Is your main thread cutting lead screw imperial(TPI) or metric pitch?

 

[DC31kParticipant @dc31k]

There will be a relationship between 1.25mm, 2.5mm and 5mm pitch.

Try one of the other two and see if the thread produced bears the same relationship to the 4.5mm pitch as when attempting 1.25mm.

If the results are consistent, something is wrong with the setup you have chosen (perhaps because you have misinterpreted the instructions). In particular, have you set lever 3 for metric?

Please link to the manual you have so we can all read and hopefully come to the same conclusion. The one I see only uses the 42t gear as a driver in position C/A for 4mm, 5mm and 6mm metric pitches. It says that 36 pitches can be cut using the 21t driver, but the remaining 9 (at the coarser end) need the larger driver. This makes logical sense. It does not make sense to use a large driving gear for a fine pitch thread.

You would find further assistance on the Colchester group at groups.io

[sprocket 3Participant @sprocket-3]

reading into your post a bit more and I am wondering about that gear train you mention.

If your lathe is imperial wouldn’t it require a compound conversion gear train to cut the metric thread ranges? I ask this because it seems from your post that you have got it set up with just a simple 3 gear train and perhaps what is required is a compound train with 2 gears on the idler stud giving you the conversion ratio for metric pitches.

I have also heard of scenarios with some Colchesters where there is a front gear lever selection option for cutting worm gears and if that is selected in error then the pitch generated is a long way out from the normal metric screw pitch you’re trying to cut….perhaps worth checking that out.

[sprocket 3]

On sprocket 3 Said:

reading into your post a bit more and I am wondering about that gear train you mention.

 

In order not to wonder, but to actually know, it would be a good idea to consult the manual for the machine, which can be found within two minutes using a search engine.

In that way, reading the manufacturers own instructions, it would prevent throwing irrelevant and unhelpful red herrings here and yonder speculating about conversion gears and worm gears.

[larry phelan 1Participant @larryphelan1]

DC31k, you dont muck about, do you !

[Clive FosterParticipant @clivefoster55965]

As I recall things the usual reason for wrong pitch problems when cutting metric threads on an imperial round head student is not setting the lever on the upper right of the gearbox to the correct position.

It has three settings imperial, metric and screw disengaged.

Most folk seem to leave it in the imperial position all the time ignoring the manual advice to disengage the screw except when screwcutting.

Clive

[JasonBModerator @jasonb]

The way I read the manual is that you do not need the 42T for 1.25mm pitch, only the pitches on the bottom row of the chart need the 42T which as it replaces a 21T will double the other pitches from the CA range.

There is also no mention of changing the gear on the leadscrew , only changing the 21T driver for the 42T. But as you don’t need the 42 just leave as it was.

[JasonBModerator @jasonb]

As said lever 3 needs to be in the metric position otherwise you will get the pitches shown on the imperial lines of the chart

[bernard towersParticipant @bernardtowers37738]

Absolutely Jason, just how I set mine and have had no need for the gear to be changed in the 35 year ownership of my Student.

[Kevin NichollsParticipant @kevinnicholls15835]

Hi to all , Many thanks for your thoughts on my screw cutting issues the lever to the right is set to the metric position as per manual before I changed the position of the gears the machine was left as it came before I rebuilt it which was 21t on the headstock and 42t on the leadscrew with the 120t between the two  I am wondering if some one has had the gearbox apart and the ratios are not correct to the chart ?

 

Regards Kevin Nicholls .

[JasonBModerator @jasonb]

Are you sure it is 4.5mm pitch that you are getting and not 5mm?

Simple maths of the 42spindle/21screw and 21spindle /42screw will give a pitch 4 times coarser

I have a suspicion that the 35T should remain on thegearbox imput and that the 42T is just stored there when the 21T is being used a sthe driver, Bernard should be able to confirm. That is why just changing the headstock gear from 21 to 42 doubles the available pitches from the line above.

[bernard towersParticipant @bernardtowers37738]

Yep you are right mine is 21/120/35 and I also have a 23/24/26/27/28/42/48/127 in the drawer which is where they’ve been since i bought the lathe.

[JasonBModerator @jasonb]

Thanks for confirming what I thought.

So geartrain shoudl be 21/35, that equals 0.6

OP has 42/21 which equals 2

2 divided by 0.6 means the leadscrew will be turning 3.33333 times as fast as it should

3.333333 x 1.25mm will give a pitch of 4.167mm

[larry phelan 1Participant @larryphelan1]

Ah, these Higher Maths go over my head, so they do !, and just when I thought I knew how to cut threads !

Just goes to show how easy it is to make a mistake [not that I ever do of course ]

Still, I do have a few pieces in my scrapbox with some rather unusual threads on them, no idea what they are or how I managed to produce them.

I put it down to ONE OF THEM DAYS ! [Those days when NOTHING goes right, which become more common as one grows older ]

Am I alone ?

[bernard towersParticipant @bernardtowers37738]

Larry, It proves you at least try!!!

[larry phelan 1Participant @larryphelan1]

Thank you Bernard for those kind words, so badly needed !

Just goes to show how easy it is to miss something simple and go mad trying to see what you did wrong. The good thing is that once you cop on, you dont make the same mistake twice [well, not too often anyway ]

Fortunately there are other folk able and willing to assist.

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