Mini-Lathe Repair

About a month ago I stalled the lathe twice whilst parting of some steel, it survived the first time but I lost the low speed range on the second time. I knew the high/low range gears in the head are plastic (nylon) and it seemed pretty obvious that a gear had stripped. The high range was still ok so I have been using the lathe since.

I finally bit the bullet and stripped it down to remove the head and as you can see in the picture the small gear is well and truly stripped, it is also split. The gears are one thing and easily replaced but what annoyed me more is that the inside of the head is bone dry. There is not a single smear of grease or a drop of oil anywhere, it was assembled in the factory with no thought to adding any lubrication. I know some argue that the nylon gears dont need any lubrication so maybe thats what the factory work to but you would have thought they would lube the gear change assembly and its operating lever.

I checked the rest of the drive and parts for damage but it all seems ok.

My mini-lathe (Chester) has instructions how to lubricate the head, a bit hap-hazard but there all the same. Check your instruction set.

Mind you, a good stall can do all sorts of damage if you're unlucky.

Edited By Brian Sweeting on 06/12/2018 20:19:39

Part of the fun with these lathes is deciding if the makers know what they're doing or not!

The defence would point out that some plastic gears shouldn't be lubricated because oil either damages them directly, or traps muck and becomes abrasive. (Plastic gears don't like abrasives because they're relatively soft.) Likewise, the high-low mechanism has a tiny duty cycle and could last donkeys years even completely dry.

The case for the prosecution is these lathes are built down to a price, with a simplified design, and - as we know - they're not always put together properly in the factory.

If it's any comfort I don't recall seeing lack of lube in the headstock being mentioned before in the 20+ years mini-lathes discussion on the web. As it doesn't seem to cause a problem in practice I guess it's value engineering and the shortcoming doesn't matter (much).

Dave

Are we looking through the bottom and is there a cover plate, greasing plastic gears means you can trap dirt, swarf etc which would then become a grinding paste, I think you have a catch 22 regarding lubrication.

Martin P

And it costs money to add grease.

Andrew

The view is thru the bottom of the headstock which sits directly in the lathe bed....no covers, no means of adding lube of any description

A drop of oil couldn't hurt Ron, but I also see some issues with the design of the parts.

1. the gear that broke is a small dia pitch gear so after cutting a keyway in it, there's not much meat left. Weak construction, bad design.

2. If the keyway was made with sharp, not radiused corners, there will be a concentration of stress at the sharp corner and I would expect a plastic gear with this condition to break right where it did. The key top edges look sharp, so I suspect the gear's keyway was also.

3. If I'm seeing the photo correctly the gear tooth at left on the broken gear looks like it has a wear mark from about the pitch line to the tooth tip (right to left in photo) but it looks like the engagement (top to bottom in photo) is only half the tooth. The fact that there is a wear mark indicates this gear is seeing highish load. Probably needs to be a better grade of glass or carbon fibre filled acetal plastic or Nylatron rather than regular grade acetal / Delrin. (or better yet, a cast iron or steel gear)

4. The selector fork tips are dead square. You can see where they rub on the bigger gear at rear. If these sharp faces engage the teeth edges of the small gear huge forces will result, contributing to breakages. As they are, eventually the rear gear's face will wear away. This condition could have been easily avoided at the factory at virtually no extra cost by making the forks a little longer and grinding a 15 degree lead-in on the end faces, so they glide over any teeth edges they contact rather than grabbing an edge. Again a little oil would reduce wear here.

Sorry you are having trouble with your machine. Is it too late to get a refund? Pointing out some of the above to the sellers might give you some ammo to use in a "goods fit for use" argument.

Edited By Jeff Dayman on 06/12/2018 20:41:09

A couple of years after I got my mini-lathe the back drive bearing on the lathe went, I thought I had completely f**ked it, the plastic gears stripped and it stopped (not necessarily in that order),.

I replaced the plastic gears with metal ones. As someone said at the time, next time something else will go.

One suggestion at the time was to use a hole, probably for the earth connection behind the control electronics to inject lubricant. On my lathe the only access was a couple of screw holes, other lathes seem to offer better access. Another suggestion was whilst you are there to drill a couple holes so you can have a look and or lubricate without having to take the headstock off. I did neither, just applying a bit of grease and putting it back together.

Whilst I was at it I replaced the ball bearings with roller bearings. David Fenner´s "The Mini Lathe" book was useful. Arc-Euro trade has some nice illustrated articles (lots of pictures) on the taking apart and rebuilding. Now they seem to advocate other bearings.

Dick H

Thanks guys,

I am starting to wonder if changing to the metal gear set would be a better option, they are said to be noisy compared to the plastic but if they provide a more reliable drive I could live with that. I have only hesitated in considering the metal gears as the plastic ones are thought to be a safety valve/fuse which avoids damage elsewhere.

Ron

The metal gears are more noisy ( i  have  just removed mine and went to belt drive for that reason ) they will need regular lubrication , one way is to drill a hole through the side of the head next to the shift lever and use a spray lube like lithium grease sprayed in theough the hole with the extension tube that comes with the spray can . Some use a combination of metal and nylon as it is usually the small gear on the input shaft that dies so the just replace that unit and leave the nylon one on the spindle , this negates the need to strip the headstock completely . 

Edited By XD 351 on 06/12/2018 23:39:03

Edited By XD 351 on 06/12/2018 23:42:57

I'd argue that if the plastic gears used in a lathe headstock had an issue with either oil or grease, the choice of that particular plastic was a major design flaw.

I changed the plastic gears for metal ones earlier in the year. The noise is bad, especially over 500/600 rpm in high speed. I'm seriously considering changing back to plastic ones, material and design faults not withstanding. The original plastic gears lasted close to 15 years and only broke when I was doing a fairly aggressive interrupted cut. Whilst it's a right royal pain in the neck to replace them, realistically it only takes a couple of hours.

cheers

Best of luck!!

Bill

Has anybody ever had the plastic gears wear out rather than just substitute for a shear pin?

Brian

You do need to bear in mind that the gears are used as the weak link should major jam ups stall the machine as soon as you start changing them for something more durable you then move the weal link elsewhere such as the more expensive motor or board.

If putting metal gears inside I would be looking at making one of the belt pullies the weak link either with a shear pin, slipper clutch or similar with the added advantage that the weak link will now be where you can easily get to it rather than needing a complete strip down.

Thanks guys,

Jason, thats the worry I have with metal gears, thinking about it, although its a pain having to strip down to remove the head its really not such a big deal. This is the first time I,ve done it and its not difficult, as Bill says it is probably a couple of hours work especially after you have done it the once.

Ron

That's accidental rather than by design. Plastic gears are used because they're dirty cheap to make, once you have the mold tool.

Andrew

Yes probably more by luck than design as it is the worst place to put the item that will fail first. Fact still remains that changing them for steel will make something else fail the next time the machine is overloaded.

An adjustable slipper clutch should not be too hard to make just needing some hardened washers and ball bearings, or change from timing type belt to multi vee so you can set that a bit slack and have it slip if there is a jam would be the best thing to do at the same time as changing to metal gears.

Jason, the multi vee belt could be a good idea for the plastic gears also, using the plastic would save the noise of the metal ones and the bit of slack in the belt move the weak link from the gears to the belt...what do you think..?

Sounds like a plan.

Not sure on the "engineering", but to cut the noise, could the small gear be replaced by a steel one, and the larger driving gear be plastic.

What is the primary drive on these lathes, is there a belt from the motor to a jack shaft? Or is the gear fitted to the motor shaft. The belt would solve a lot of problems.

Ian S C