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Problem with tapered roller bearings fitted to mini Lathe

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Patrick Galvin16/09/2020 19:19:18
12 forum posts

Hi All,I have an Amadeal CJ18A mini lathe in which I damaged a plastic Hi/Lo gear and needed it replaced.I decided to put in metal gears and at the same time replace the bearings with tapered roller bearings.It took a lot of work as there were quite a few issues when I opened the machine but nothing serious.I lapped the shaft where the bearings were fitting and installed everything.The rear bearing was definitely tighter on the shaft than the front bearing.I tightened everything up and reassembeled the lathe and started it up.Everything ok.I tested runout with a clock at the chuck end,very good.After runing the machine for half an hour at different speeds it got hot but not that you couldn't hold it.I tried cutting metal DISASTER turns out like a hammered finish on longitudinal cuts,ok on end cuts.I thought I had a bearing too tight so I set up a jig to pull the bearings a bit but it wouldn't budge.Is a bearing faulty or is it stuck on the shaft please?When I spin the chuck by hand it seems tighter than I think it should.In high gear at top revs with nothing in the chuck it will spin for a few seconds and cut out.I presume because too much friction?

Patrick Galvin16/09/2020 19:47:58
12 forum posts

I loosened the two nuts completely and ran the machine hoping it might relax a possibly tight bearing but no such luck.When the lathe did warm up after first turning it on it was the back end that heated first.

Pete Cordell16/09/2020 22:39:46
11 forum posts

I think you need to someone with press to remove that spindle hopefully without cracking the headstock
My thoughts on the bearings is stick with the ball bearings or the angular contact ball bearings

Ady116/09/2020 22:54:32
3810 forum posts
519 photos

Loosen it all up and shove a tailstock live centre against the spindle nose to support it properly

Then try it out a bit at a time, starting with the rearmost bearing being properly secured and located

At no point in time should there be a problem turning the shaft by hand


edit: it sounds like you misaligned the bearings and they may be knackered, overtightening is another  possibility

Edited By Ady1 on 16/09/2020 22:58:13

Patrick Galvin16/09/2020 23:36:55
12 forum posts

Hi Ady1 I'm having trouble getting any movement freeing it up and I'm afraid I'll break something if I put any more pressure on it.That sounds like a good deduction,if the back bearing was misaligned by the time I put the chuck on the front I would be moving further away from the front bearing and into chatter at the cut.Your right Pete I need to find someone with a press.Thanks.

Mike Poole16/09/2020 23:41:02
2699 forum posts
64 photos

After slackening the preload it may be worth giving the spindle a tap to resettle the bearings. Was everything perfectly clean before assembly as a tiny foreign body can cause problems.


Patrick Galvin17/09/2020 12:08:24
12 forum posts

I completely cleaned and dried everything before assembly.I've just remembered I put a light greasing between both parts of the bearings.Probably a mistake?Anyway I set up a puller using 12mm all thread through the bore but I can't get any movement out of the bearings.They're stuck on rock solid.

Edited By Patrick Galvin on 17/09/2020 12:10:22

Pete Cordell17/09/2020 16:34:30
11 forum posts

Patrick check your inbox

J BENNETT 117/09/2020 18:38:04
44 forum posts

The bearing is stuck on the shaft. With taper roller bearings the second bearing to go on the the shaft needs to be a sliding fit on the shaft. This should be a very close fit but you should be able to just slide it by hand. The first bearing on the shaft is normally a press fit. In this way when you slacken the adjusting nut the shaft can slide through the rear bearing a little and the front bearing can separate a small amount. You can then re-adjust.

Without the rear bearing being able to side it becomes very difficult to adjust the bearing - which in itself is a black art - it also makes assembly and dismantling straight forward.

I converted my very early model Warco 250 Lathe to taper roller well over 10 years ago and after a little running-in and final adjustment it has been fine ever since.

Very recently I have been rebuilding the spindle assembly on my old Warco X2 mill, as part of my very very long running project to convert it to CNC. I have replaced the original morse taper spindle with an R8 version and fitted taper roller bearings. The lower bearing on the R8 spindle has a larger internal diameter than the top one. This lower bearing is a tight fit on the shaft and I spent a few happy hours making a "puller" to pull the bearing on. The puller consists of a bottom cup which fits over the lower end of the spindle, a ring piece which slides over spindle and engages with the inner race of the bearing, a length of 50mm steel tube which engages with the ring and extends beyond the top of the shaft and a top cap which fits on to the tube. The bottom cup and the top cap both have a 12mm hole. A length of 12mm studding is then passed through the assembly. A pair of nuts are locked together at the lower end and a large well oiled washer and a single nut to the other.

This "puller" worked extremely well and I was amazed at how little torque was required to smoothly pull the bearing down the shaft and on to its seat. In practice two of the "ring pieces" are required, one to fit the upper part of the shaft and part counter bored to the larger diameter to fully seat the bearing a little way on. The second "ring" is bored all the way through at the larger diameter to push the bearing all the way down on to its seat.

Before fitting the lower bearing I carefully measured and polished the the upper part of the spindle for a very close sliding fit of the upper bearing. The main point I found here was the need for a very small amount of lubricant when test fitting the bearing. Although a "sliding fit" had been achieved, with spindle and bearing fully degreased I found it almost impossible to slide the bearing on without it jamming and sticking. Spraying a little WD40 on to a paper towel and then lightly wiping the spindle and bearing allowed it to just slide on.

Patrick Galvin17/09/2020 20:06:24
12 forum posts

I polished the shaft before fitting the bearings because there were some score marks underneath where the front bearing sits.Not from anything i done but from the factory.The back bearing was very tight to get off so I lapped that point a good bit but it was still tight when I fitted the new bearing.It had to be pressed on.The good news.I managed to free up the stuck bearings and slowly began retightening the nuts to torque the bearings.The chuck is spinning nice and free now and as I go I'm doing test cuts.Much better,the finish is improving every time I tighten the bearings more.The wrench I have is a home made affair with about a 3" handle so I can't tighten it much more though I feel it will take a bit more.Is there any way of knowing when to stop.The finish at the moment is fairly good on free machining steel using HSS cutter but not as good using carbide.

Patrick Galvin17/09/2020 20:26:19
12 forum posts

Hi Pete Cordell while waiting for access to a press I managed to free it up myself thanks.If the bearings were damaged I was going to change back to ordinary bearings as you suggested.These are going in the right direction so I'll persevere some more.Thanks

old mart18/09/2020 14:25:00
1911 forum posts
151 photos

The rear bearing is more likely to be damaged than the front, but if you can get the rear one to be a light fit on the spindle, so it can move without being tight, it would be best to give them a very good clean and regrease them and try again. The grease should not take up more than half the volume between the spindle and housing, this allows the grease to be displaced from the bearing races as they turn. If the grease has nowhere to go, as in overpacking, it will be churned around by the rollers and overheat. Having a DTI helps when adjusting the bearings, you can measure the end float on the spindle as the bearings are tightened, and know exactly the point where the preload begins. A tiny ammount of preload is all you want, and the standard method with taper rollers is to run the spindle at top speed. Check the housing temperature stays luke warm after 20 minutes. The DTI can then tell you if there is any radial play when you try lifting the front of the spindle. 0.01MM, or 0.0005" is a good ammount to start with, you have to understand that there will be some movement detectable even if the bearings are tight.

I would give the taper rollers another chance, unless there are visible indentations showing on the rear bearing outer ring.

Patrick Galvin18/09/2020 22:01:37
12 forum posts

I got the bearings moving again.I didn't remove the bearings to check for damage as it would require a complete strip down so I decided to try adjusting them and see what sort of finish I'd get. I have them adjusted to give me a decent finish on brass and steel.Though I'm getting a fairly good finish on Delrin its not as good as I was getting with plain bearings.

Patrick Galvin18/09/2020 22:26:30
12 forum posts

I'll check the spindle for radial play tomorrow.Thanks.

Patrick Galvin24/09/2020 12:25:57
12 forum posts

Radial play almost zero.Everything running great.I had a screw loose under the saddle which was causing slight vibration but once tightened everything is good.Just as an observation these machines from Amadeal have a grub screw and nut missing from underneath the saddle from the factory because it would interfere with the gear and prevent meshing with the leadscrew.A design fault.Thanks for everyones help.Pat.

Neil Wyatt24/09/2020 13:05:46
18135 forum posts
713 photos
77 articles

HI Patrick,

If you use the google search box on our home page and look for roller bearing preload you will learn a lot.

It sounds like your inner races were tight on the shaft and you ended up applying too much preload.

It is OK for roller bearings to run warm, but they should stabilise at 'warm'.

As for grease, they should be about 30% full, more than that and the grease won't flow.

My experience of taper rollers in my Mini Lathe was 100% positive, although the preferred solution these days is tending towards angular contact bearings.


Patrick Galvin24/09/2020 23:16:07
12 forum posts

Hi Neil,thats exactly what I did,over tightened them.The back bearing was the one giving me trouble just a bit too tight and difficult to adjust but after 4 efforts I got it spot on.I had to loosen the bearing by giving the end of the shaft a slight tap and then recommence adjusting.I marked the nut and thread as I tightened it with a white pencil so to know where I reached the right spot.I overshot it a few times but had my reference marks to go back to on the next attempt.After running for half an hour at top revs the casting is barely warm.If I ever I have to remove the shaft again I will lap it more under the back bearing to make it easier fitting.I've learned a lot during this exercise and still learning.I'll check out the home page google search . Thanks Pat.

SillyOldDuffer25/09/2020 08:56:20
6188 forum posts
1345 photos

Threads like this are jolly useful.

Changing the bearings on a mini-lathe isn't a matter of simply slipping the old ones out and popping in a couple of replacements! It would be straightforward except the bearings are a very tight fit. When I stripped the internal gear on my mini-lathe I got the spindle bearings in and out with brute force and ignorance. I was seriously concerned the bearings would be damaged, and I'd have to buy new and try again.

Got away with it, but reading this thread first would have saved a lot of bother. It was before I discovered this forum and how good everyone is at answering questions.

Thanks for sharing Patrick, and well done for fixing it.


Patrick Galvin25/09/2020 21:23:05
12 forum posts

The bearings in my lathe were mismatched.The front one was a sealed bearing but the back was not sealed.I damaged the back one taking it out.I was relieved when the new bearings got stuck that I hadn't damaged them.Thanks Pat.

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