Unimat 3 mods.

[david bennett 8Participant @davidbennett8]

I have ditched the belleville wahers on my Unimat3. The 28 lbs. or so of thrusts seem excessive to me, so I have gone for a solid preload.I had a spare headstock, so single pointed a 17 x 1 mm. thread on the spindle (between centres for a special long nut, which abuts the bearing, and a locknut. The bellevilles have been replaced with a 6mm wide spacer against the bearing. The small external circlip has also been ditched.

 

 

I had to drill another tommy bar hole, and the automatic feed is disabled. I have cut a variety of spacers for any future experimnts.

So far , the results are promising, if not dramatic. That annoying squealing when you pause a longitudinal cut is gone. Blue pivot steel cuts better,so does parting off. It is early days, but there is no apparent downsides. Everything is reversable except the extra thread and tommy bar hole, and it was all done on the u3. I can post further details if anyone is interested.

dave8

[Michael GilliganParticipant @michaelgilligan61133]

Bring it on, David

… always interesting to see experiments like this.

MichaelG.

[SillyOldDufferModerator @sillyoldduffer]

Delighted to see this report.  The important part for me is Dave experimentally confirming there’s an improvement.   One hidden thing to look for though is excessive wear, likely to take a few hundred running hours to appear.

My guess is the lathe designer chose a Belleville washer as a cheap simple way of managing bearing life.  I believe it’s a compromise: if the bearing is left loose, the lathe performs badly.  Therefore tight is better.   But, as tighter causes more wear and tear on the machine, it’s important not to overdo it.

Worst case, the lathe performs brilliantly for a few hours, then seizes in a cloud of smoke.   More likely, the extra wear and tear will be  slow, perhaps taking years to become obvious.

This kind of problem is often detected in critical machinery by periodically sampling the oil and having it analysed for metal content.   Gives a good insight into how fast wear is happening, and often which parts are failing fastest.   Used to improve safety by taking rapidly wearing engines out of service early, and to save money by deferring servicing of engines in good condition.      Analysing the oil feels over the top on a Unimat though!

Dave

[david bennett 8Participant @davidbennett8]

Dave, my bearings are adjusted to have a small positive clearance. I consider this is less likely to produce wear than 25lbs. of thrust. That’s what made me do this.

dave8

[david bennett 8Participant @davidbennett8]

PS. I note the Emco Compact 5 uses just one wave washer for thrust.

dave8

[PeteParticipant @pete41194]

That type of pre load with belleville washers also allows the spindle to float a bit and expand or contract due to temperature variations. And for the Compact 5 lathe, it’s why the front bearing is a tight press fit and the rear has a slip fit. Going to a solid preload might exert far more on the bearings up or down than advisable as the spindle grows or shrinks in length. I’m not a spindle or bearing expert, so I could also be wrong. But as an example and using imperial measurements, a steel lathe spindle will lengthen or shrink by approximately 6 millionths of an inch, per inch of length, for every degree of temperature change. So a 6″ long spindle with a 50 degree change in temperature is going to have it’s length increase or decrease by about .0018″.

[JasonBModerator @jasonb]

With the U3 having a head that is 60mm wide we can bring that down to 0.0007″. Less if measuring between the inner bearing faces.

Then add in the fact that the whole head gets warm when running for some time not just the spindle in isolation so that also means the bearings will move to take up some of that 0.0007″ and you may be down to two tenths

This problem does not seem to get mentioned for mini mills and mini lathes that essentially have a similar setup of preloaded non taper bearings.

Interestingly when the CNC has been running non stop for a couple of hours at 5000rpm the spindle is noticably tighter than when cold so if spindle expeansion were the only factor then it should be looser not tighter. Ketan says this is fine and he knows a thing or two about bearings.

[JasonB]

On JasonB Said:

[…]
This problem does not seem to get mentioned for mini mills and mini lathes that essentially have a similar setup of preloaded non taper bearings.Interestingly when the CNC has been running non stop for a couple of hours at 5000rpm the spindle is noticably tighter than when cold so if spindle expeansion were the only factor then it should be looser not tighter. Ketan says this is fine and he knows a thing or two about bearings. […]

Forgive my ignorance please, Jason … but aren’t the mill heads cast iron ?

whereas Dave’s U3 looks very much like an alloy die-casting.

[ I have no idea whether it is though ]

MichaelG.

[JasonBModerator @jasonb]

They are indeed Michael but you are thinking the same as me that it is not just the expansion of the spindle taken in isolation that matters it is the rate that other parts expand (or contract) as well that should be taken into consideration. As well as where the heat is coming from eg is it from cutting through the chuck and into the spindle, the bearings or the motor heating the whole assembly.

Also as you know preload is best set on a machine that has been warmed up not one that has stood in a cold shed during winter for a week without running so components are at their running temperature and therefore size.

The CI mill/lathe heads tend to put the bearings further apart than the little alloy U3 so the actual change of preload is probably similar eg longer distance x smaller differential change = shorter distance x larger differential change.

[Michael GilliganParticipant @michaelgilligan61133]

Thanks for the clarification, Jason

MichaelG.

[JasonBModerator @jasonb]

Just did some numbers.

Differential movement on the U3 between head and spindle at 50degF change would be 0.0009″ over 2.5″

On a Minilathe/mill with say 4.5″ over the bearings in a CI head then differential would be 0.0003″

I have not taken into account any angular position of the balls which may affect any percieved change in gap

But if preload is set on a warmed up machine it would not be worth worrying about as there won’t be any significant differential change. This is why commercial CNC machines are put through a warming up cycle at the start of the day before being put to work.

[david bennett 8Participant @davidbennett8]

Interesting observations. I find it a bit academic, as I expect the slip fit of the rear u3 bearing to accomodate any likely expansion problems.

dave8

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