Acceptable wear limits on a milling machine

[Paul MajorParticipant @paulmajor25237]

Hiya,

got a thread running over in the manual machine tools section detailing the rebuild of my Beaver mill.

As I am stripping it down I am noting some wear on the ways for the knee, saddle and table.

Its a big old beast and I didn't have the chance to see it working before I got it so can't tell how much movement there was on the different axis.

So, in general, what sort of wear can you get away with on the relevant surfaces? Whats the best way to measure the wear and what would you do to fix it?

Any advice gratefully received

Thanks,

Paul.

[Paul MajorParticipant @paulmajor25237]

What is acceptable wear on the mating surfaces and how to measure

[davebParticipant @daveb17630]

I refurbished an old Bridgeport, the worst wear was to the saddle slides, about 20 thous dip in the middle of the knee. I attacked it with files and scrapers, a straight edge and a surface plate, took about ten days to get it to an acceptable condition. Very satisfying but I wouldn't do it again. There was some wear in the table slides but I learned to live with it. Regrinding is possible but you have to decide how much you are prepared to put into it. The trouble is, there is often wear everywhere, screws, bearings etc. Once I had the worst sorted, I treated it as an ongoing project and dealt with things as they became necessary (essential).

Dave

[Paul MajorParticipant @paulmajor25237]

Thanks Dave,

been reading up on scraping, may end up having to do some but was trying to figure out which bits would need it most as I don't really want to spend days scraping every surface

Any advice on how to measure wear on the different surfaces?

Cheers,

Paul.

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi Paul ,

I afraid that you would need an enormous surface plate , dial indicators ( with swing around and stand off jigs ) , a precision block square and some specially made test strips to actually measure and correct the wear properly in all the machine components .

Probably much better to do functional tests by milling sample blocks and bars , infer where wear is worst and do corrections piecemeal until machine works to your chosen standard .

A specific problem with knee mills is that they change shape depending how much the table is extended .

This is known as table droop and is a nightmare sometimes .

Old long table versions of Bridgeports are notorious for this and at worst extension with heavy workpiece in unfavourable place droop can be more than 15 thou .

I only mention this because it is very easy to be misled sometimes into believing that you have a bad wear problem rather than an intrinsic design problem made worse by a quite small amount of wear .

Simple milling tests and traming as described many times will probably give you all the measurements you really need for correction .

It’s a philosophical point I know but setting up a milling machine by test and traming is intrinsically more meaningful than setting up by having each individual machine component finished to perfection .

There is a mathematical reason too .

Regards ,

MikeW

Edited By MICHAEL WILLIAMS on 05/01/2014 13:19:07

[Paul MajorParticipant @paulmajor25237]

Thanks guys, off to read up on tramming

Also need to understand better how much wear you can take out with adjusting the gibs. The way on the edge of the knee where the saddle gibs clamp onto has a slight groove worn in it about 5 thou deep, need to figure out how much this will affect things.

cheers,

Paul.

[davebParticipant @daveb17630]

Paul, I your photo show a groove in the surface, is it possible to make a wider gib so that it runs on the unworn part?

I used a 12" X 2" X 1" straight edge and 18" surface plate for the top of the Bridgeport knee. If you have an adjacent flat (reference) surface it's possible to do a quick check with a sensitive DTI and stand. It is most important to establish a reference surface (you measure from this to the part you are working on) before removing any material, if there is no adjacent or opposite surface you will need to work from a surface plate. without the reference surface, your part may finish up flat but could be tilted in any direction. I found it best to mark out the high areas with a felt tip pen and remove the material with a file, grinder or whatever means is quickest, do a little at a time and check often to ensure you don't take too much off. Once you have the area reasonably flat, you can start with scrapers and blue to get a bearing surface. Carbide scrapers are best, you can make them, you can also make them from old files but will spend as much time sharpening them as using them. If the part is worn hollow in the middle but not at the ends, keep a small area at each end intact and use these as your reference, remove the ends when you have got the middle part flat and then scrape to finish. Don't waste too much time on surfaces with scratches or gouges, if the rest of the surface is OK, just dress the scratches flat with a smooth file.

Dave

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