Corbetts Little Jim Lathe restoration - newbie needs advice

Hi y,all.

As posted in my newbie introduction I have bought a small 3 1/2" lathe as pictured below. The lathe is a Corbetts "Little Jim". (All the pictures are in an album "Little Jim Lathe" which I think anyone can see.) I know little about lathes although i have read the recommended books, so I need advice and direction.

There are problems and I thought it best to tackle these step by step in this new thread. There is clearly a great pool of experience and talent here so I would really appreciate any advice and this may perhaps be useful to other newbies getting started.

I stripped and cleaned the lathe. Pictures of the bed are attached. I adjusted the gib strips and can eliminate movement, but yes this makes saddle very tight at the tailstock end. I also adjusted the solid headstock bearings and the spindle feels like it has no lateral movement but has some axial slop. How important are these issues? I am more after something to learn on than high precision.

I think the first three issues to address are
- the suitability and speed of the motor.
- stripped threads in the crossslide
- damaged backgear.
If there is something more important to look at first please let me know. I will park the problem with screw gear for now.

Next post will describe the motor issue.

The motor is pictured below. The name plate has been removed. Can anyone guess at the age and horsepower. By the time to wind 30m of thread onto a small spindle I figure it must be a 4 pole motor running at 1500 rpm. Insulation readings are good.

What should I do to service this motor. That capacitor must be pretty old. Should I dismantle and clean? I can see the rotor when looking in the bottom vents and this has two tranverse springs. I thought an induction rotor would be solid so what are these springs for and what if they are old and weak? How do I grease or check the bearings - there is screw on the front above the front bearing, is that for oil/grease.

The 1500rpm at the motor is slightly reduced by the current pulley setup. Is this still too fast for solid bearings, especially since the backgear is not working. What are my options:
- fix the backgear - yes - subject of a later question.
- make a countershaft - any ideas on how/layout would be very welcome.
- VSD - I could be tempted if it is cheap and it would also run my crappy wood lathe. Could a VSD really turn down the speed to something workable.

Any thoughts?

Given the motor has a capacitor it'll be single phase. A VFD will not work with a single phase motor. They are designed to work with 3-phase motors. Cheap VFDs operate with constant torque as the frequency (aka motor speed) decreases. Since the speed decreases and the torque stays constant the power available decreases.

Andrew

Hello Andy,

Most of your turning will be done near the headstock, so don't worry too much about the gibs being tight at the tailstock end. It's a small lathe, so you don't need a huge motor (1460rpm, 1/4hp will be more than enough) but you will still need a countershaft - which is not hard to make if you can get your hands on some pulleys...

This is the "temporary" one I made for my EW over a decade ago - It's two bits of square tube, bolted together on a flat steel plate and hinged at the bottom (a heavy door hinge) to a wooden motor plate. Couple of pillow block bearings and a suitable shaft and you will be good to go. I'm still using a version of it...the wiring is a bit neater these days though...

You need the bearings to turn freely but not be too loose. Be careful when adjusting the headstock, you can crack the housing. Can't tell from the photo but it might be worth checking if you can fit new flanged Oilite bearings - in which case, I would not spit them but just use as supplied.

Have fun, take it easy and make sure nothing sticks out too much (the work, the tool and your fingers! )

Regards,

Ian

Edited By IanT on 05/07/2021 23:11:29

Thanks Ian.

What tensions both belts? Is the motor and the countershaft all one unit that is hinged and the weight of the motor makes tension.

Since my motor is so big and heavy I was hoping to bolt the motor to the bench or something solid to avoid vibration. I can make a hinged shaft like yours but unsure how to provide tension to both belts.

What range of spindle speed should I aim for.

Thanks Andrew -

Had to research a bit on single phase vfds. I have seen ads for some from ATO but it is more complicated than I thought and I still don't understand. Depens on how the motor capacitor is wired. I guess single phase vfd is not commonly used or available.

Will look at a countershaft.

P.S. Do you think this motor safe to use and should I do anything to service it.

Hi Andy,

I have a 9" Southbend lathe.(4.5" swing and built in the early 1950's) and was originally fitted with a 1/4 hp motor which I replaced it with a 1/2 hp. The bed is worn at the head end. As most of the work is done at the head end, I adjust the gibs to suit and re-adjust if I have to work at the tail end.

It also has solid bearings which are part of the head stock casting and has little wear (amazing how good cast iron is for bearings if it is well lubricated). Axial clearance is adjusted by a nut thrust washer and nut on the end of the spindle. Not so much of a problem when turning between centres but more so when mounting the job in a chuck and face cutting.

The support for the intermediate shaft and motor is pivoted at the bottom. Tension for the motor belt is adjusted by moving the motor in slotted mounting holes.

A push rod, on a toggle arrangement tensions the drive belt to the lathe. The push rod is a turnbuckle with left and right hand threads and is used to adjust its length.

The angled lever in the RH photo releases the push rod for changing the belt position for different speeds.

Paul

All my motors are old clunkers and I run them on a RCD circuit which keeps you "safe"

Obviously if you upgrade to a decent motor the improvement is huge, variable speed and reverse make a big big difference

Andy

Ain't you lucky. I had a dead pre-war Myford 30 years ago that I resurected. It became a nice and well used lathe. I learnt a lot from it.

My advice, for what it is worth, is

1. Sit the lathe on a steel plate at least 1/2" thick. It should provide additional stiffness
2. Use the existing motor. Motors are expensive and if it is safeand works you can spend money else where. There is no reason why you need variable speed (I have a feeling it is now used because it is cheaper than a set of pulleys).
3. Fit a smaller pulley on the motor. You want your lowest speed to be about 100rpm (below that back gear is used).
4. Consider using a counter shaft. It will give you more flexibility with the speed. You are unlikely to use the lathe above 800rpm. Remember that HHS tools will cut very successfully at speeds well below the so-called recommended speed. Mount the motor well below the counter shaft. I put the motor and counter shaft on hinged boards that could be jacked up and down to tighten the belts.
5. Paint it gaudy colours.

All the best with machine. Have fun and it should do you good work.

JA

Just reading Ady1' s comments - I have never needed to use reverse on any of the four lathes I have owned. I think the only time I have was at night school when cutting a 26tpi thread with a single point tool.

Second edit - sorry for the duplicate - fingure problems.

Edited By JA on 06/07/2021 09:21:47

As others have commented, the motor/countershaft belt tension is fixed by having a slotted base and sliding the motor back & forth. The tension of the countershaft/lathe is set by the swing of the hinge and mine is simply set by a square wooden block under the motor that adjusts the angle used (and also takes the weight of the motor). I don't have my belts too tight, it causes excessive wear and if you have a dig-in (and you will) it's much nicer to have the belt slip. It's a small lathe and on my EW I only use HSS tools. If they are sharp, they will cut.

I've never actually worked out my EW speeds - but the motor-c/s pulley ratio is probably just under 3 to 1 (so somewhere in the 550rpm range) and the countershaft has three settings, the middle one being roughly neutral and the other two stepping up or down by about 200rpm I'd guess. I have a back-gear fitted but rarely use it on the EW.

You can make a more elaborate countershaft set-up with a toggle lever, which can also act as a simple clutch but the simple design works well enough and will get you turning sooner.

Regards,

IanT

Just to clarify; a VFD can be single phase or 3-phase power in, but is always 3-phase out. So a VFD can be used to run a 3-phase motor from a single phase supply.

A single phase induction motor is not self-starting as there is no rotating magnetic field. So capacitors/start windings are added to give the motor a nudge on start up. If a single phase motor is run on a variable frequency the parameters of the capacitors and extra windings will be incorrect. Single phase motors are one huge fudge.

On the assumption that a true 3-phase supply is not available I'd use a 4-pole single phase motor along with a countershaft and belt drive.

Andrew

No-one knows! On the plus side, old motors are robust and unlikely to fail dangerously: there's a good chance it's OK. But approach with caution! Old motors are old. The bearings may be worn out or gummed up, connectrions corroded, insulation damaged, capacitor(s) sick, or centrifugal switch jammed.

Old motors can be serviced, but much depends on what's wrong. Replacing bearings and removing grot is easy enough; after that it gets complicated. It's possible to rewind a motor yourself, but so tedious and time consuming it's better to have the job done by a specialist. Replacing the motor is likely to be quicker, easier and cheaper.

Ancient electrics should always be approached cautiously. Older machines may be wired with rubber insulated wiring: as this perishes, replace it. They often had no emergency stop or No Volt Release, and perhaps were controlled from the mains socket. This is OK for initial testing, but not good for regular use. As combined NVR/Emergency stop switches are highly affordable, there's no reason not to fit one. Anyway:

• Check the wiring for damage and loose connections.
• Before proceeding further, consider your personal level of understanding, technical aptitude, and willingness to learn. For many hobbyists, there is no great mystery about domestic electricity; they understand the risks. Others focus on the mechanical side and have zero understanding or interest in electrics; they are much more likely to be shocked! Avoid accidents due to ignorance by understanding your limitations.
• Important! To avoid electrocution, confirm continuity from the earth pin on the plug to all the exposed metal work on the lathe: motor case, headstock, and bed etc. Buy or borrow a multi-meter.
• Make sure the motor turns freely by hand before applying power.
• Check the fuse is the correct size: 5A should do
• Without touching the lathe or motor, switch on and see what happens! There's a very good chance the motor will spin up without drama. Let it run for 15 minutes to ensure it doesn't overheat and turn off if any trouble emerges. Bad things (post again if they happen):
  • Plug Fuse blows and/or house RCD pops
  • Smoke, sparks, loud buzzing, or sizzling noises
  • Motor doesn't start
  • Motor runs erratically
  • Motor slowly overheats
• Tips.
  • As most 250Vac shocks are relatively mild, it's easy to become gung-ho about their dangers. Don't.
  • An electric shock received whilst standing on a concrete floor is more severe than what happens on a wooden or carpeted floor. Worse if the concrete is damp.
  • Shock is proportional to contact area. Pressing a hand flat onto a live motor case is far more dangerous than a tapping it with a finger tip.
  • Shocks are much worse when sweaty.
  • Chaps trained to work on live equipment are told to always keep one hand in a pocket. This prevents current passing across the heart, which is stopped by a few milliamps.

Most single-phase motors are unsuitable for electronic speed control.

In my opinion, all single-phase motors are an inferior complicated compromise. Several things to go wrong, poor starting torque, they dislike being stopped and started, difficult to speed control, mildly inefficient, and they vibrate. Their only advantage is they run off ordinary domestic electricity; still useful today, and there were no affordable alternatives to them in 1947! In 2021 it's worth upgrading to a 3-phase motor with VFD if the motor is shot. Though if the old motor works, will be 'good-enough'.

Speeds. Top speed at the chuck is restricted by the lathe's plain bearings. Assuming they're in good condition I suggest no more than 1000rpm, 800 if nervous. Middle pulley, about half that, slow pulley about 100rpm. (All approximate.) The rpm needed depends on the diameter of work being turned because it's the speed of the cutter at the surface that matters. Small diameter work requires high speed, big diameters need lower rpm to get the same surface speed. VFD's are handy because they allow operators to tweak speed for best results. Backgear to get slower than 100rpm, is good for threading, winding coils, and any job requiring extra torque (turning power).

Dave

This is one of the problems that I found in the lathe motor when I got it home. The internals were not much better and one of the reasons that I decided to replace it.

Paul.

The tailstock clamp is wrong, it would not have come with a bolt head to fit a dovetail, using it may damage the slide for future use, cast iron can get nibbled by that setup.

You need another matching cone pulley on a layshaft driven by a 10" or 12" pulley and something like a 2 or 3" on the motor to get the speeds decent as others have said. Current speeds are a bit woodworking.

Thanks guys very helpful.

Andrew - I guess vfd is a non starter! My motor has four teminals and a label - input to 1 and 4 and capacitor to 2 and 3. I guess there are two coils. Out of curiosity, would a vfd with two outputs at 90 deg phase connected to each coil and remove capacitor not work? Any idea why I can see springs on the rotor?

Duffer, Ady1 - I have done domestic electrics and repaired things for decades so reasonably confortable. Earth continuity is fine, insulation is fine and circuit is RCD protected.

Ian, JA, - thanks for pic of your shaft! I understand now. Not sure about those colours JA!

So I am convinced about the need for a shaft and have been out measuring what pulleys I have. With one 4 stepped pulley I think I can get 963/620/349 rpm and 236/151/85 with backgear. That uses 2" pulleys on the motor and smaller on the shaft. I wonder if the belt will slip. It would be better if I could source a 7 or 8 or 9" pulley on a 1/2" boss but that seems to cost a fortune at bearings UK. Their plummer blocks are also very expensive - any better source? Do I really need self aligning bearings? Where did you get that threaded rod to tension?

That motor is so big and heavy that I would like it firmly bolted down. I an thinking of mounting motor and shaft on a skid. The skid can be bolted down but then loosened and moved to change the spindle belts. Does it matter how close the drive and slave pulleys are if I can get a belt short enough? I will experiment and let you see.

Thanks Dave.

Yes that tailstock clamp is very Heath Robinson. Any suggestion to improve. I was just going to angle a piece of 12mm steel and try to drill a bolt hole in just the right place. Will that work?

For others who may not have seen it on my intro thread, the tailstock is pictured below. The upper tailstock is held by the pin you can just see an one bolt. So it can be rotated to parallel the bed but not adjust laterally. Any issue?

Correct, there will be a main winding and a start winding connected to the capacitor. The springs will be associated with a centrifugal switch which disconnects the start winding once the rotor reaches a set speed. Connecting a VFD as suggested will not work. As discussed VFDs create a 3-phase output, so the phases are inherently 120° apart. Since the output is 3-phase there is no neutral connection, so you couldn't run the windings phase to neutral, only phase to phase. Many industrial systems do not use neutral. All my ex-industrial machines are connected with 4 wires - 3 phases and earth.

Furthermore the outputs of the VFD are not sine waves, but are pulse width modulated signals where the pulse width is changed cycle to cycle to simulate a sine wave.

Andrew

Edited By Andrew Johnston on 06/07/2021 15:53:59

Thanks Andrew - I am still learning.