Mill tuning – thrust bearings?

[Iain DownsParticipant @iaindowns78295]

HI, all.

Today I stripped my small mill (CMD10) table down and tuned it up a bit. I'm pleased with the result, but I wonder if I can do a bit better.

The lead screw is effectively held firm by the handwheels which turn it. there is a locking nut which screws down and presses the handwheel against the mount.

I discovered that one of the reasons I had silly backlash on the cross table (X?) was that I'd not tightened it down hard enough.

The problem I'm seeing is that the resistance to turning is not even through the 360 degrees of the wheel. half turns well and half binds. If I tighten it up then it is hard to turn, but there is little movement or backlash. If I loosen it, the wheel is easy to turn but the backlash is more significant.

I had in mind to see if I could some thrust bearings and interpose between the wheel and the mount (or possibly the lead screw and the mount on the other side. I could thus tighten the wheel down a bit more but still have reasonably free turning.

Of course this is just hiding the ultimate root of the problem, but I'm not sure I'm up to discovering that and fixing it. It is a relatively small backlash and about as good as the mill has been since I got it.

Is this a sensible thing to do or should I be satisfied with what I've got?

Iain

[Iain DownsParticipant @iaindowns78295]

[Tim StevensParticipant @timstevens64731]

It may be that your lead screw is slightly bent. This happens easily as the handles stick out and can be used (by non-engineers) for lifting the whole thing during storage or delivery. Mine was bent between the handle itself and the beginning of the thread, and it was fairly easy to straighten once I had worked out what the problem was and how to remove the part.

While you have the bits on the bench, you can check for other faults, swarf, poor machining, bearings not quite home, etc, none of which are unknown.

Hope this helps

Tim

[Iain DownsParticipant @iaindowns78295]

A naïve question, but how does one detect a slight bend in a leadscrew? it really is a small difference. And how do you straighten it? Using the conventional engineers favourite, or is there a more subtle way?

Iain

[Ian PParticipant @ianp]

For the same amount of work required to fit thrust races to the feedscrews you may as well fit pairs of ballraces.

I modified all three axis of a CMD10 and fitted 8mm bore 22 OD ballraces (Very cheaply available as skateboard spares).

I turned a close fitting parallel stub mandrel that I pushed the mounting block on (the mandrel had a slightly increased diameter at the chuck end) I bored one the bearing recess and then turned the block end for end to do the other recess. I took light cuts and just relied on the friction grip.

Once the recesses are bored enlarge what is left of the original bore to allow a little clearance for the diameter of the spacer that will sit on the shaft between the two ballraces. By adjusting the length of the tubular spacer its possible to slightly preload the ballraces. Probably easier to do than it is to describe but the end result will be a big improvement.

Ian P

[Michael Cox 1Participant @michaelcox1]

Hi Iain,

I have basically the same mill as yours but sourced from ArcEuroTrade. I replace the feedscrew mounts with new ones incorporating ball bearings and the difference is amazing. Very smooth, low torque movement is now possible.

Further information here:

http://mikesworkshop.weebly.com/mill-feedscrew-bearing.html

Mike

[John FieldingParticipant @johnfielding34086]

Hi Iain,

The bearings mentioned by Ian Phillips sound like the 608 series, very cheap and readily available. They are 8mm bore, 22mm OD and 7mm thick. I use them in several places and they last forever. The 608ZZ is a metal dust shield version but the ones I can get at a bargain price is the 608RS which is a fully sealed type with rubber oil seals on both sides. The plain version is simply the 608 which has not seals at all and is designed to run in oil.

A slightly smaller bearing is the 626 series, these are 6mm bore, 19mm OD and 6mm thick and also cost next to nothing. They are the cordless drill front bearing type so are also freely available from a bearing stockist. Again they come in ZZ and RS version as well as the plain 626.

[Iain DownsParticipant @iaindowns78295]

That's really helpful. I was astonished that you could buy these bearings for pennies. I've ended up buying some green ones at £8.60 for 16 which works at a tad over 50p each. There were postings at much less than that!

Ian – I get the basic idea. You will have a stub sticking out of the chuck and you push the mount onto it. What I'm not sure about is how to lathe the recess. I can see how to do this if the mandrel wasn't in the way (it's just extending the bore), but I've not had much luck with cutting recesses on the end of a bar. What's the trick? Or do I just make the mandrel so that it is 7mm short of the depth of the hole (sounds challenging to get that right!).

Mike – I like the idea of doing a casting and have dreams off adding some heat treatment to my workshop, but sadly that lags behind sorting out my current projects and learning to weld (stick). I notice that Arceurotrade have spare bearing blocks for £4.50, which strikes me as similar to the materials cost for casting a new one in aluminium. Hmmm…….

All I was actually thinking of doing was putting a 2mm deep thrust bearing either side of the block. Might need changing every few months as it gets gummed up, but may also do the job (whilst I get round to the more complex bearing replacement!

Iain

[Iain DownsParticipant @iaindowns78295]

Actually, I've just gone an ordered a pair of cheap 8x21x2mm thrust bearings for 4 quid. Might as well try it out.

(Also the skateboard bearings – maybe for later!)

Iain

[KWILParticipant @kwil]

John F says the bearings last forever, I changed a set a few years ago that had been in a fan motor that had been running continuusly for 24 years!

[John FieldingParticipant @johnfielding34086]

OK Kwil,

Maybe not forever, but 24 years is pretty good going I would say. All Inknow is that in 24 years I probably won't be turning metal into swarf anymore!

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