Corbetts Little Jim Lathe restoration – newbie needs advice

[IanTParticipant @iant]

It could be a 3/8th 10 TPI ACME thread Andy – so you can get a tap to do that.

Unless the threaded part is 'pinned' in someway, you should be able to pull it out using a simple puller. So with a bit of planning, you could make a replacement part (and the puller) before you take the original nut out. Normally, the feed nut (being softer) will wear a bit more than the threaded steel rod – so you might be lucky and find that a new feed nut will be good enough…

Regards,

IanT

[RedsetterParticipant @redsetter]

Looking at the cross slide photo, there seems to be a lighter spot on the flat surface above and just to the right of the brass feed nut. It may just be discolouration, but is it a retaining pin? I doubt whether the nut is just pressed in.

Edited By Redsetter on 07/07/2021 07:15:12

[Andy CarlsonParticipant @andycarlson18141]

Hi Andy,

I've been following this thread over the past couple of days. Looks like you are getting some good advice.

On the subject of the cross slide nut, I replaced the one on my Faircut lathe last year. Mine looks like a very similar design to yours. With the Faircut I did find it necessary to finish the job by tapping in situ.

Mine was a square thread which made life more tricky – I had to do most of the cutting with a single point tool on the lathe and also make my own tap to do the finishing cut. If you don't have a working lathe you will need a different approach to mine but if you intend to replace the feed screw too then you can pick whatever thread you like so that should help.

It was a fairly long job with a lot of uncertainty along the way. The words are on a couple of threads on here…

https://www.model-engineer.co.uk/forums/postings.asp?th=166091
https://www.model-engineer.co.uk/forums/postings.asp?th=144221&p=5#2638709

Good luck!

[Andy Thompson 3Participant @andythompson3]

Thanks guys

Pete – not really enough meat left for a new thread plus must be a stepped hole as little brass showing on inside.

Ian – not acme but square thread and I don't think you can get taps for that see pic. But moving to a 3/8 acme thread may be the solution. There are taps and dies in RDG tools for £20 each. Would a hand die cut such a long screw from scratch.

Red – you have eagle eyes and I have been blind. See pic. There is indeed a pin. Do you foresee any issues with drilling this out? Need to make sure new pin is below level.

Andy – that very similar to my problem. I need through through your thread again. I know I don't have your skill and tools to repair in the same way

[RedsetterParticipant @redsetter]

Andy, the pin is probably parallel and goes right through the casting. If so, it can be persuaded out with a light hammer and a suitably sized pin punch. It should not be too tight.  Can you see the other end?

 

 

 

Edited By Redsetter on 07/07/2021 12:20:34

[IanTParticipant @iant]

That does look like a pin to hold the feed screw in place Andy, in which case it would need to be drilled out to remove the nut. However, this would probably be easier than working on the nut in-situ I think.

A 'square' lead screw of that length really needs to be screw-cut and would not be something I'd recommend you try at this (or any) point in your hobby machinist career!

You can however purchase ACME threaded rod and also a matching nut – although it may have to come from the US. Over here a similar (metric) thread is called a 'trapezoidal' thread and might be easier/cheaper to come by. It should be a much simpler job to modify commercial threaded parts (to fit your lathe) rather than making them from scratch…

Regards,

 

IanT

 

Edited By IanT on 07/07/2021 12:30:37

[Andy Thompson 3Participant @andythompson3]

Redsetter – you are a star. Knocked the pin out. Tapped a dodgy M10 thread and used large socket a puller. So the sucker is out.

Now – any idea where I could get a nut 3/8 -10 nut made. O.D. from my vernier is 12.8mm. Don't think I need huge accuracy, just something to get me going. The hole is off centre slightly but I think centred would be fine – surely I could just drill the faceplate hole larger if necessary. Happy to pay!

Ian – I hear what you say and will look at this if I cannot get something made.

[Andy CarlsonParticipant @andycarlson18141]

Hi Andy,

I would not suggest that you go down the same route as me – I had a working lathe to make the nut and tap and it was still a lengthy job that took multiple attempts. Square threads are a pain to get right. Hopefully some of my posts will still help you though.

Another option you could consider would be to use a vee thread form. If your feed screw is 3/8 diameter then you could use 3/8 BSF which has 20 TPI so every turn would be 50 thou. That might make it easier/cheaper to source threaded rod – GWR Fasteners do it in mild steel for example. Going metric would expand your options further but you may wish to keep things Imperial.

Some folks have approached this job by making a flanged nut and drilling and tapping a couple of holes on either side of the nut to hold it in place – this will allow you to side step any questions of press fit tolerances and that retaining pin. The downside is that you will lose a little cross slide travel to accommodate the flange thickness.

[Andy CarlsonParticipant @andycarlson18141]

Posted by Andy Thompson 3 on 07/07/2021 13:16:10:

So the sucker is out.

Now – any idea where I could get a nut 3/8 -10 nut made. O.D. from my vernier is 12.8mm. Don't think I need huge accuracy, just something to get me going. The hole is off centre slightly but I think centred would be fine – surely I could just drill the faceplate hole larger if necessary.

Glad you have the nut out.

I'd be a bit cautious – the cutout for the retaining pin has broken through into the bore so you may not have much room to play with if you want to carry on using the pin to secure the new nut.

I'm sure there are plenty of folks on here willing to help once you know for certain what you are going to do with both the nut and the feed screw.

Whereabouts are you located BTW?

[Howard LewisParticipant @howardlewis46836]

If the OD of the Leadscrew is 12.8 mm, it is a 1/2"

For a machine of that age, I would expect everythging to be Imperial, so securing threads are likely to be Whit form, BSW for Coarse, and BSF for the fine ones, possibly with BA for the gib adjusters.

You mentioned damaged gears. Most likely caused by engaging Backgear with the pulley locked to the mandrel, to lock it while a stuck chuck was freed.

Moral DON'T DO IT!

There are methods of unsticking a chuck without endangering the gears.

When you bare looking for replacements (possibly from another machine, and require modification – boring or bushing )

The gears need to be the same DP and pressure angle.

Count the teeth on one gear, and measure the OD in INCHES. (Teeth + 2 ) / OD = DP. For a machine of that age, the Pressure Angle is likely to be 14.5 degrees.

Modern gears are almost invariably 20 degrees, so will not match..

If you cannot adjust out the end float, while the lathe is functioning, you should, be able to produce a solution.

measure the end float and turn up a washer, (Ideally phosphor bronze, or brass ) to fill the gap,

Then remove the mandrel and reassemble so that end float is eliminated.

HTH

Howard

[Andy Thompson 3Participant @andythompson3]

Thanks Andy. Not sure the best way to go.

The simplest for me would be get someone to make a nut, I could send them the rod. 10 tpi does sound like quite coarse contol – 3.6 deg for a thou

I like the idea of 20tpi but can the slide be driven by any thread form. I thought square or ACME was necessary.

Could I use an M10 rod. I see GWR have M10 stud connectors – apparently with an OD of 13mm? Perhaps I could file/turn this down to fit 1/2" hole. I think it is still OK to use the pin. Need to drill through with insert in (or maybe mark insert take it out and file a groove. I think the pin penetrated the bore because of my mangling – even if it does then just run a tap through it. Perhaps a grub screw would be better?

[Andy Thompson 3Participant @andythompson3]

Thanks Howard. I measure the screw at .35" with vernier so presumably 3/8" with clearance. The nut is 12.8mm so as you say 1/2".

Since you bring up the backgears let me add that story.

The driven pully on the spindle has a grub screw to secure it to the spindle. The lathe has an eccentric lever to engage the backgears. The grub screw in the spindle pulley would then be moved to the big backgear bullwheel.

See picture of the large backgear. The hole for the grub screw is broken. I think you have explained how this happened. It was like this when I got it and yes the chuck is stuck. How can I fix this.
– mount wheel at angle on my pillar drill and drill opposite to the break. Would this be safe?
– find someone to braze up the broken bit? (Is gear bronze?).

And how do I free the stuck chuck – how do I grip the spindle.

Cheers

[Howard LewisParticipant @howardlewis46836]

Hi AT3!

Yes, you could well set up the gear and drill and tap another hole in the boss. As close as possible to square to the bore, would be my advice.

You will have lost a little bit of bearing are when using Back Gear, but probably not enough to cause any disasters

You are in the "Chickem and Egg" dilemms. The really nice thing to do would turn away the boss where it bis broken and then to make up a Top hat bush, ready drilled and tapped for the grubscrew, and Loctite that into a slightly enlarged bore in the gear. But for that you need the facility of another lathe! Or someone nearby who can do those jobs for you.

As to removing a stuck chuck, there are are various slight variations on a theme.

Firstly, tighten the belts for motor and countershaft, setting up for maximum Chuck speed.

1 Grip a piece of hexagon bar in the chuck.Choose a sensible size, and for which you have a 1/2" drive socket.

(3/8 might do but 1/2 will ,be stronger )

2 Put a long power bar into the socket.

3 Hit the end of the power bar, HARD, with a mallet.

4 Alternatively, Apply an Impact wrench to socket, in the hope that it will hammer the chuck free.

or

A Grip a piece of sturdy bar (preferable to wood ) through the chuck jaws.

B Whack end HARD

Either way it might need more that one bash to free the chuck.

You may find that warming the body of the chuck with a heat gun will help it to unstick

In either case, hopefully, the inertia of the motor will be such that the chuck unsticks. Once it has moved a few degrees, the grip should decrease dramatically.

Place a board across the lathe bed, in case it fall!

For the future:

You already know not to engage Back Gear to lock the mandrel

Clean both internal and external threads.

DON'T run the chuck on under power, it will tighten under cutting loads.

HTH

Howard

[Andy CarlsonParticipant @andycarlson18141]

Posted by Andy Thompson 3 on 07/07/2021 15:20:38:

Thanks Andy. Not sure the best way to go.

The simplest for me would be get someone to make a nut, I could send them the rod. 10 tpi does sound like quite coarse contol – 3.6 deg for a thou

I like the idea of 20tpi but can the slide be driven by any thread form. I thought square or ACME was necessary.

Could I use an M10 rod. I see GWR have M10 stud connectors – apparently with an OD of 13mm? Perhaps I could file/turn this down to fit 1/2" hole. I think it is still OK to use the pin. Need to drill through with insert in (or maybe mark insert take it out and file a groove. I think the pin penetrated the bore because of my mangling – even if it does then just run a tap through it. Perhaps a grub screw would be better?

Take your time, applying additional coats of thinking is usually a good plan.

No particular thread form is necessary provided that the screw and nut match. Each form has its pros and cons of course but a new thread of any form will work better than what you have now. Plenty of lathes are designed with vee form cross slide screws – the Stringer/EW 2 1/2 inch for example.

Both square and ACME tend to be quite a coarse pitch in order to achieve a sensible thread depth. All types will work… you just need more turns on the handle with a finer pitched thread.

M10 would work but the usual pitch for M10 is 1.5mm which would be less convenient for mental arithmetic when using the lathe. You can get finer pitches (i.e. 1mm) but they are less common (and probably more expensive). It's also 0.5mm bigger diameter than 3/8 inch so you will be losing some wall thickness on your nut… plus it will be 0.25mm more likely to foul the retaining pin. On balance sticking to 3/8 might be better. I've been trying to remember where I saw someone who had metricated their (worn out) cross slide screw but I can't find it now… probably one of the many lathe restoration videos on YouTube.

Can you turn something (anything) yourself ? Maybe… it depends on whether you can get this lathe working at least in a basic way so that it can mend itself. You could perhaps lock the cross slide by tightening the gib screws and then set the compound to 90 degrees and use that to put the cut on… just while you get the cross slide screw fixed up.

[Andy Thompson 3Participant @andythompson3]

Thanks Howard, Andy,

You have given me plenty to think about and a few more coats of thinking is better than stripping back. I will order up some stuff and experiment and get back to you.

Andy – a while ago I bolted up the cross slide and used the top slide to turn some wood and a brass ferrule. So I could try again, however this is the thread that keeps on giving. I just checked the top slide thread and eek – see picture.

This is the same 3/8×10 screw but this time the casting is threaded. And the casting is cracked and the threads are nearly gone. I guess I can try the same approach for the topslide, maybe turn down a M6 stud connector and make a new screw.

I guess I may need a new topslide. The slots in cross slide appear to be 57mm or 2 1/4" apart. Is this fairly standard and will it be easy to find. I don't really understand this top slide anyway. There is only one bolt to hold it down and this bolt is on the right. Given the downward force from turning surely it would be better on the left?

[RedsetterParticipant @redsetter]

I am thinking that a Myford ML7 feed screw and nut might be adaptable to the cross slide. The nut has a flange with two screws so it should be attachable, and the hole can be opened out if necessary to clear the body of the nut. Not sure about other dimensions, but they can't be far off, as it is doing much the same job. Maybe not the cheapest solution but they are readily available.

[Andy CarlsonParticipant @andycarlson18141]

Posted by Andy Thompson 3 on 07/07/2021 19:22:49:

Andy – a while ago I bolted up the cross slide and used the top slide to turn some wood and a brass ferrule. So I could try again, however this is the thread that keeps on giving. I just checked the top slide thread and eek – see picture.

This is the same 3/8×10 screw but this time the casting is threaded. And the casting is cracked and the threads are nearly gone. I guess I can try the same approach for the topslide, maybe turn down a M6 stud connector and make a new screw.

Hmm – that looks pretty terrible trying to squeeze a female thread in where there isn't room. I wonder if maybe someone fitted a larger diameter screw at some point in its history because designing it like that would be shocking.

In terms of repair I think I'd be looking at somethinng with a flange and screws into the face of the casting because you dont want anything forcing that dovetail angle open – it might end badly. Several prewar lathe designs have compound slides pivoting on a single bolt like that and they do crop up for sale from time to time. The heights may differ between lathes though so it might be a bit of a lottery.

[Andy Thompson 3Participant @andythompson3]

Hi Guys. Sorry for the slow progress but strange how so many other jobs pop up when the weather is good.

I have made progress on the following issues.
– stuck chuck
– back gear
– countershaft
– screws for cross and top slide
– tail stock clamp.

I will outline these solutions below to get your comments on any improvements and possibly help future newbies like myself.

I have queries on
– spindle and bearing condition
– is this lathe really useable
– what further modifications
– what gear to buy

Perhaps we can show that a cheap old lathe is a viable way to get started.

[Andy Thompson 3Participant @andythompson3]

For the stuck chuck.

I removed the chuck body and put WD40 on the threads. I put the chuck back in the same position so I hope this dies not affect its accuracy.

The best way to grip the chuck seems to be stilsons on the back of the back plate. This must be less damaging than a bar across the jaws.

To grip the spindle, I used the old belt which needed replaced. I twisted this with a lever and also levered the lever down hard to maximise the grip/friction – see pic. (I guess you could do this with any old separate belt by looping it double under the spindle without removing the spindle.) A whack on the stilsons and the backplate came free.

I put the chuck back in the same position so I hope this does not affect its accuracy.

[Andy Thompson 3Participant @andythompson3]

For

the back gear,

the issue was the broken hub around the grub screw. Pic shows it mounted on the pillar drill with a hole drilled. Despite the apparent angle in the picture it is on straight and opposite the old hole. Biggest problem was getting the hole started without slipping off – bit of grinding and chiselling to flatten.

Back gear works well and surprisingly quiet – see video. (Tried for ages to upload this video but I gather this is not possible unless loaded elsewhere. Can this be done from dropbox)

The shaft for the backgear is only located with a grub screw and a ball bearing that engages with a dimple – no shoulder – not very solid. See video of how it rocks and twitches. This may be because the ball bearing does not fully fill the dimple? Any improvement – drill out and replace with full grub screw?

[Andy Thompson 3Participant @andythompson3]

Spindle and bearings

So while all this was going on I took photos of the spindle and bearings. See pictures. The spindle is worn, the bearings are scored but perhaps not badly. All reinstalled but I worried as I adjusted up the bolts in case the casting broke – how likely is this.

I tightened them until shaft turned easyly but I could not feel any play. I don't have a DTI yet to check for movement.

Someone above mentioned replacing these bearing with new ollite ones. Bearingboysuk have them – should I consider these or wait and see.

[Andy Thompson 3Participant @andythompson3]

Countershaft.

Made something from 40mm section that I had left over, budget plummer blocks and one new pulley.

This gives me a rpm range of
618
410
227
151 with backgear
100
56
Is this a suitable range.

With an extra belt I could also get
841
525
345
206
128
84
Is this necessary/useful.

See pic. The motor is mounted to the sled. The whole sled slides and hinges to change belts. Then it is clamped down to the table. Given the size and weight of the motor I wanted it securely clamped. I will add a horizontal M12 bolt to press against lathe and help tighten the belts.

In hindsight I would do this differently. I am a bit worried that the 2 and 2.5" pulleys will slip – but that is maybe not a bad thing for a newbie. I would also maybe mount the shaft higher and try to squeeze things narrower – i was trying to keep it low and compact so that i could hinge a belt guard off the wall.

[Andy Thompson 3Participant @andythompson3]

For the slides

The issue was that the nuts were knackered. The aim was to get something that worked and let me make something better.

Thanks to you guys I found out how to remove crosslide nut and decided to go with standard thread. For cross slide I ordered a M12 stainless stud connector that was only 0.3 mm bigger than the hole. Forgive me – but I mounted it in my pillar drill and applied a file to it until it just fitted. The side of insert should be drilled and held with a pin. However I thought a drill would just slip off and chew up the softer casting. So I marked the position for the pin, withdrew it and filled a slot. Put it back in and finished with a drill. Pinned and it was fairly aligned but not perfect. Will maybe redo with lathe and change to fine pitch of 1mm – coukd easily find rod and connector.

M12 is too large to go through the hole in the original faceplate of the slide. Rather than drill this I made up a new face from something agricultural that was dug out of my garden. Locked nuts either side hold it but it really needs thrust bearings to stop backlash and make it smooth. Forgive the handle made with a grinder – making new handles may be first job.

Top slide was the same except I used M6 rod and connector. The insert was an easy push fit – the casting is weak. If necessary i will drill and pin this also.

[Andy Thompson 3Participant @andythompson3]

Tailstock clamp

The issue was that this was missing a proper means to clamp it. For want of anything better – again I used a bit of something dug out of the garden. I don't know what this is made of but it was horrible to cut and drill and I just could not tap it for a stud.

Tough to drill it in just the right place to keep it snug. It needed the right size shim to keep it from tipping and only contacting on top edge – worried about damage to dovetail.

With hindsight although the dovetail angle is good the plan shape should have been a wider triangle. I will remake this if I get a deent bit of steel. Should I cover the contact face with copper?

[RedsetterParticipant @redsetter]

The countershaft looks very practical, and you will probably think of refinements as you go along.

The 56 to 618 speed range is probably the more useful of the two options but it is not super-critical and depends a bit what work you are doing. With the 84 to 841 range you may find that you have to change into back gear rather often in normal use, which is a nuisance. It should be possible to get a wider range with different pulleys, e.g. ML10 has 280-490-840 in direct drive, and 48-85-145 in back gear. But see how you get on.

Yes, you can crack the headstock bearing castings if you overtighten them, so you are right to be cautious, and the bearings themselves don't look too bad so I would leave them alone for now – there is no guarantee that current standard sizes will fit. If it ain't broke don't fix it, and the main thing is to get it all working at this stage.

Good work on the cross slide.  It is worth adding some sort of dial if you can, so that you know where the cut has got to.  The tailstock clamp again looks fine to start with, perhaps an improvement would be a captive stud as it is convenient to be able to adjust it with one spanner.

Edited By Redsetter on 22/07/2021 19:06:18

Edited By Redsetter on 22/07/2021 19:15:41

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