Mini-Lathe Repair

Thanks Jason, thats helpful I will give it a try without pre cutting and see how I get on.

Made up a spindle lock for when I cut keyways on the mill. Odd looking mounting plate..? well it was staring at me from the odds box, a couple of quick mods then turned up a locking pin. I used the guard mounting holes to fix it in place, certainly locks the spindle well.

I have another issue with the lathe, I am now struggling to produce a good surface finish with it. I never had a problem with it previously, I could always get a an excellent finish so something has changed and I am wondering what..?

This morning I spent some time re-adjusting the gibs, setting them up as good as I possibly could and there is no play in both the top and cross slides. There is no issue with the carriage either so all seems to be good from the bed up to the tool post.

I re-sharpened and tried two HSS tools and also a brand new carbide insert but there was no improvement. The finish is not horrendous but its not as good as it should be or as good as the lathe gave before.

Unless you guys can think of any other cause I am wondering if the head spindle bearings are on their way out. The lathe is less than a year old so that shouldnt be the case but if I am unlucky and they are dodgy bearings..? Having said that I cant feel any play or end float in the spindle but if they are starting to become an issue I can imagine them first showing up in the cut finish.

For £20 I can get a pair of angular contact type bearings as replacements, they are said to be an improvement over the originals anyway. I cant think of anywhere else to look for the problem.

Have you under or over adjusted the spindle bearings, when you reassembled the spindle did you run it for a while to see if it got warm/hot and maybe it went slack when everything settled down and needs slight adjustment. I would clock it to see if you have any axial or end float movement.

Martin P

Presumably, you are using the same material as before? ie comparing apples with apples.

Martin, I did run it in after changing the gears but I didnt check it for temperature. Clocking the spindle is a good idea as I clocked it when the lathe was new so have something to compare it with as well as checking it for movement..

Ega, yes the same material free cutting steel and 6082 alu.

Ron

Ron

As a matter of interest I changed the headstock bearings for the angular contact ones which you can preload and will run better than the originals. It is a bit of a faff stripping the headstock and you should be aware that the bearings are a tight fit in both the headstock and on the spindle, I thought it a good idea to polish the spindle for the rear bearing so that there is a possibility of movement when adjusting the preload. Having said that I'm into the nth mod to get the whole thing usefully productive, never again, nor would I recommend a mini lathe they are just not well enough made.

Regards

Martin

Thanks Martin, angular contact and preload,? can you explain a bit more. Also the tight bearings..? did you manage in the shop or did you have the use of a press.

Ron

Hi Ron

I'll do my best. The bearings fitted at manufacture are standard ball bearings, undoubtedly as a cost measure since they're cheap, are ideal for highish speed radial loadings, however I'm sure you'll appreciate that the action of a tool into a workpiece provides a significant axial loading which they are not so good at. This is the cause of chatter because the cage and balls are forced away from the path they would naturally wish to follow. Angular contact bearings use inner and outer races that are sections of a cone and rely on preload to ensure that thay are running true before the action of an axial load. The idea two c nuts on the spindle rear are in effect a preload adjustment and a locking nut, and ideally you want a situation where there is sufficient preload to make the bearings run correctly without too much that would cause them to overheat.

When I changed mine tapping the spindle out took the front one out at the same time, the rear one required some suitably sized tube and since they were scrap I just battered it which worked, The new bearings permit by design installation of the outer races into the headstock by virtue of careful alignment and bits of wood, the front inner race used a combination of tubes and the rear inner was almost a tight sliding fit with everything else in place. Adjusting the preload was pretty much common sense and gut feel rather than anything more technical, difficult to describe but if you've any feel for mechanical stuff, probably intuitive. Best I can do, hope it helps.

Martin

PS Arc have a downloadable bit on their website with pictures that may help.

Edited By Martin Shaw 1 on 23/01/2019 21:30:16

Martin, I think you mean taper roller bearings. Angular contact bearings are similar to conventional ball bearing races but they're designed to take axial loads.

Arc's guide to fitting angular contact bearings. **LINK**

Brian

Chris

Your quite correct, some vin rouge had been taken.

Martin

Thanks Martin and to Brian for the link.

I am thinking it may be as well to replace the bearings with angular contact type regardless though it would appear that the original bearings are ok, well as far as I can tell.

This morning I backed off the two spindle locking nuts just in case I had over tightened when I changed the gears. I mounted a 20mm steel bar in the 3 jaw to check for play or end float but found none. With the lathe in neutral avoiding the drive assy the bearings felt smooth and free, no roughness.

I clocked the edge of the spindle chuck location flange and it was just the same as when the lathe was new 0.01mm for the edge and 0.015mm on the flange face, which is excellent I doubt it gets much better than that on a mini lathe.

So I am at a bit of a loss where the lack of a good finish is coming from, it would appear not the bearings and having checked and adjusted from the carriage up I,ve run out of ideas.

The one thing I do know its damn frustrating.

Ron,

Can you pinpoint when the finish changed? Coüld it coincide exactly with when you changed the gears? I think you did say the lathe was slightly noiser now than previously. Could this be another effect of the gear material? Ie the steel gears giving a different / higher torsional vibration at the spindle? No idea if this would be the case, pure speculation.

Paul.

Hi Ron

what sort of bad finish are we talking , vibration marks or surface tearing of the material ? , as the lathe was doing good finishes before the mods you have done to the drive recently may have introduced some vibrations that were not there previously or surface tearing suggests a tool angle or cutting clearance problem, when I grind a tool I get the magnifying glass on it to check the cutting edge as its easy to get in particular the side rake a bit too vertical so then it wont cut well or give a good finish , also after grinding its usual to stone the tool to finish it and that can also lose the sharpness and clearances if not done with care.

regards Geoff

Hi Paul,

Thats a good point, though I have had some good surface finish since I changed to metal gears but has something changed now the gears are bedded in..? Although I can find no play or end float in the spindle and the bearings feel smooth by hand I did wonder that if they have deteriorated a touch is that showing up when running..? I dont know.

Ron

Edited By Ron Laden on 24/01/2019 10:04:32

Thanks Geoff,

I will try and get a close up picture of the finish, you guys can probably identify the type of problem.

Ron

A photo or two of setup will help us old wise guys.

Plus one on the above. I very much doubt the problem is to do with bearings; summat else has changed. A picture and description of the 'poor' finish will definitely help narrow things down.

Andrew

May also be worth turning it over with the belt off but in gear to see if you can feel any tight spots in the new gears.