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Unimat 3 milling spindle bearing upgrade

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Barrie Lever10/08/2019 17:46:14
325 forum posts
2 photos

,In a recent message thread there was discussion about poor surface finish using a Unimat 3 milling attachment.

JasonB and myself independently of one another carried out some tests and obtained acceptable surface finishes, however another contributor to the discussion pointed out that Unimat 3 milling spindles often have slack in the bearings.

Unimat 3 milling spindles have two phosphor bronze plain bearings, with axial play and thrust be handled at the spindle nose bearing.

I checked my spindle and sure enough there was worrying amounts of slack in both axial and radial directions.

I decided that I would look at upgrading the spindle to ball races. Upon careful checking of the spindle the radial slack was in the pulley end bearing, in effect the spindle was see sawing about the spindle nose bearing, the slack in the pulley end bearing was probably caused by drive belt tension.

I stripped the quill out of the housing and removed the shaft from the quill to take measurements for possible bearing replacements.

I found a cycle bearing that had a 28mm OD and 15mm ID, this looked like a good option as the spindle has 15mm bearing journals, 28mm OD would mean boring out the quill to accept the bearing but there looked to be plenty of material to enable this.

I also realised that there was room at the top of the quill to take the new bearing and actually leave the old slack plain bearing in place.

Currently my biggest lathe is an EMCO Compact 5, so all mods would have to be carried out on this machine.

I carefully setup the quill and supported it with a fixed steady, runout at this point was 0.01mm, bored the quill to take the bearing as a light push fit and pressed the bearing home whilst the quill was still mounted in the Compact 5's chuck.

I decided to take up the axial end float in both the plain bearing and the ball race via a belleville washer pressing on the ball race inner, the tension being applied by a sleeve pushed by the pulley as it was tightened down it's thread.

After machining the assembly went very nicely, but the acid test is does it run well and is there a noticeable improvement in surface quality.

Well I am pleased to say that there was a marked improvement in surface finish, also notice able was a great reduction in the spring cut.

I think that the photos explain the mod quite well but if there are any questions then fire away.




Edited By Barrie Lever on 10/08/2019 17:47:10

Kiwi Bloke11/08/2019 03:01:31
290 forum posts
1 photos

Well, I'm full of admiration for someone who fixes a problem so swiftly! In disgust at their bearing slackness, I packed away my Unimat milling heads and tried to forget about them. Perhaps motivation will now return...

When you had the quill apart, did you investigate whether a standard-sized ball bearing could be fitted into the nose end of the quill as well? Presumably, it would need to be bored out, as at t'other end.

Barrie Lever11/08/2019 08:54:55
325 forum posts
2 photos


Yes I did look at using the same bearing both ends, the problem is that the rack teeth for the quill movement are quite deep and this prevented a safe fitting of the 28mm x 15mm bearing at the spindle nose end.

The wear in my quill was definitely at the pulley end.

I have a spare bearing and plenty of the Belleville washers if that will help your motivation.



Edited By Barrie Lever on 11/08/2019 08:55:54

Kiwi Bloke11/08/2019 11:05:59
290 forum posts
1 photos

Barrie: thanks for your encouragement and offer. I've no idea when I'll get one of those round tuit things... Where did you go for Bellville washers?

I see that Simply Bearings (from whom I've had excellent service) have 15mm ID bearings in 20, 21, 24, 26 and 28mm OD. Perhaps this makes a nose-end bearing swap practicable. What do you think? My spindles have a lot of radial play at the nose end, so I'd want to attack that end as well.

Barrie Lever11/08/2019 12:46:02
325 forum posts
2 photos


The quill bore was a weird size at 25.7mm, so the 26mm OD bearing would work in there but I think you would have to leave the bronze bearing in place to provide additional shoulder support as 0.15mm is not a realistic shoulder to push the bearing back against.

Alternatively drift out the bronze and press a steel piece into the quill and machine into that for the shoulder.

I got the Belleville washers from Ebay, I have 19 left, if you or anyone else wants one just PM me.



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