Feedscrew wear vertical mill

After many months of fiddling about getting my Centec 2A prepared I have finally got round to cutting some metal with the vertical head and started to cut some tee nuts. Taking a light cut across the top surface all was well but when I was cutting along the side all was not well, am feeding against cutter rotation but I was able to see the table juddering. All slides etc are adjusted correctly and other two axis are locked when cutting. Mill is from the 1940s and I just happened to secure vice in centre of table, no doubt the most used portion. At this point I can push table to and fro slightly, taking up wear between pitch, when I move table to the far end of its travel there is no movement so I am assuming this points to feedscrew wear and not the half nut? For the smallish parts I intend to make I can just use the unworn section of screw/table but in the long term a new feedscrew will be needed I assume? John

Have a look at Automotion Components web pages. Lead screws and nuts of all sizes and pitches are available.

I have just bought a 1 metre length of lead screw for about £16 plus nut for similar money all plus tax/delivery.(longer lengths up to 3 metre available)

A bit of simple turning to suit the ends and I will be up and running. No connection just a happy customer.

**LINK**

Thanks for suggestion and links, feedscrew is 1/2" and 10 tpi and they all appeared to be metric? Have a bit more investigation to do, John

**LINK**

John

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Checking the lathes.co website it also appears to be square thread and not acme! Thanks for links though John

If you are changing a pair of nut/leadscrew it does not matter if thread form differs. Just pick correct pitch, their imperial pitches are listed as acme. For my lathe cross slide I went up from 15 to 16 mm diameter to suit availability of pitch. Chris.

If you are replacing both the feedscrew and nut (and I can't think why you wouldn't), it doesn't matter what the threadform is, surely? Your main problem would seem to be finding an imperial one, although Ajohnw's link solves that as long as the matching nut can be modified to fit. Good luck with the search and let us know if you find a new supplier. It's always good to get a new source of parts.

John

I'm not so sure leadscrew backlash is the problem. Did you try a cut at the end of the table where there is less freedom? Could be belt slipping under load or bearing adjustment. Also blunt endmill or > 1/3 sideways depth of cut.

Changing nut as well would make sense but the form on a centec uses a half nut that pivots in/out of engagement for a fast traverse?

Bazyle, ran out of time to try cut at end, that's tomorrow's job, but with cutter stopped and table traversed to end there was no movement when pushing the table. Also chatter/judder starts in same place each time. Mill is probably bit under powered, couldn't see belt slipping but could definitely see table move. End mills were just what came with machine so will get some new ones also. John

John, Automotion's site gives the nut dimensions. It may be possible to split one or ask them or another similar supplier to make a special to your requirements.

John I have to agree with Bazyle, leadscrew backlash is probably not the cause. The more likely cause is wear in the table ways. I concur that the most wear is in the centre of the table, the machine could well have been used and abused during the course of its life. Try the following check, remove the leadscrew completely, now set the table up and push it from one end to the other. This should not be an easy sliding fit but should be quite stiff and require some effort to move the table. This should tell you where or if there is any wear in the slides. You may be able to reduce this wear with careful adjustment. Regarding wear on the screw the most likely place for the wear would be the nut as you are using the same area constantly whereas the screw is constantly moving, so the same small area is not being used. If the nut is worn it should be very obvious. One other check you can do, again with the screw removed. Set up a clock to touch the side of the table when it is extended to its limit, make sure the clock is rigid and ideally not connected to the machine. Get hold of the end of the table and push and pull it sideways, you will get some movement of the clock if it is more than .005" .006" thou then your table is not tight enough. Make sure all the locks on the knee are done up, but not on the table itself. Blunt cutters will exacerbate the problem.

Thanks for advice, will try some checks later today. The Centec has two table feeds, the normal fine feed leadscrew and a quick rack feed so I will try a cut isolating the fine feed and if vibration is present in same place then it confirms its not the leadscrew? John

Well the vibration was still there with the feedscrew disengaged so it does point more to wear in the table as you have suggested. Will remove screw and set up again to check for wear/adjustment. Will just have to use the lathe for milling at present and hope that doesn't throw up any problems! John

Well I took the table off today and no obvious wear visible, will clean and reassemble without the leadscrew and adjust gib strip up tight and that should show any table wear along its travel? Pivoting half nut looks slightly worn but again nothing major, is there a quick way of checking a square thread to tell which part if any is worn? If it was acme I could get a die and put on to check but don't know for square thread. Cheers John

If you adjust it correctly with the table in it's middle position you will probably find it goes tight pushed either way. Table wear.

If while it's in the middle you put a dti on the side at end and push on the side of the table and there is movement either way they are still too loose or the mating part has worn to a sort of round shape.

Another check is to rest the tip of a dti on the table in the centre and then move the table as far as it will go each way. It's difficult to set this but the weight of the overhang will cause the end of the table to rise slightly.

In practice as there usually is some wear on older machines. It's a case of trying to balance all of these out as well as they can be.

Setting the knee slides is even more fun. Only way I have found is super tight and backing off. It's usually just possible to feel some resistance to movement via the handle and I should say the table just about falling under it's own weight or not quite doing so.

John

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Edited By Ajohnw on 11/03/2016 17:31:14

Check the thickness of the thread's stiicky out bit on the male at both ends and in the middle of the screw with a vernier. Angle the vernier at right angles to the lead angle. i.e, not axially

Frank

You can check the wear in the leadscrew and nut with the scales on the dials. The leadscrew is unlikely to be worn at the limits of travel so measure the back lash in the centre and at either end. The significant wear is all most bound to be in the nut but if your expecting zero back lash you had better fit ball nuts and a lead screw to match. Even if you fit new parts there will still be some backlash.

The time to worry in my view is when the backlash gets to 30% of the leadscrew pitch but what ever it is will still be usable. 50% is much more of an oh dear. It's a good indication of how much use machines have had. Nothing to do with abuse just use.

John

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