[How do I?] Fixing A Wedge-shaped Milling Table?

therbig

therbig

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How can I correct a milling table that is slightly wedge-shaped?

Running the table in the X-axis under a dial indicator shows that one side is about 0.003 higher than the other. Given that the saddle and the indicator are staying put, this means that the table is slightly wedge-shaped between the ways and the surface. I am taking the measurement with the gibs tightened, so it's not that the table rides up and down on the dovetails.

I have been compensating for this by putting a shim under one side of the vise. But a shimmed joint is never as rigid as a surface joint. And it doesn't help when I work on long parts directly on the table.

I don't have a surface grinder, much less one with a 20x7 capacity. But I do have carbide insert face mills that might take out the wedge. They get a very good finish on steel, too.

How stupid would it be to mill the table flat? Sure, it would not look nicely ground anymore, but who cares? Would the rougher surface give me problems? Is milling cast iron more difficult than steel (I have never done it)?

I would appreciate any advice. Thanks,
Tom
 
Are you positive it's not just sagging as one end gets so far off the center? I've had that problem with my bp, and messed up some long involute splines before I realized it. They would not mate up with the ID splines they were supposed to slide in. Turned out they were "curved" you might say. In my case, it was exacerbated by the weight of the dividing head on one end of the table. But table weight alone can do it. When you have the table at the extreme end of travel where you see the 0.003, can you lift the table by hand and change it?

Oh, and if you're thinking of milling it in situ, I sort of doubt you have the travel to cover the entire table. Otherwise, you'd be no worse off I would imagine. Then you could lap it nice and flat, shiny and new. But check your travel. What kind of mill is it?
 
Thanks, Tony. I will check the sag explicitly.

The travel isn't quite enough except on the side that's high. So I could do it by skimming in from the low side. How do I lap something this big? A large piece of glass?

It's the Grizzly version of a Sieg SX3.
 
therbig said:
Thanks, Tony. I will check the sag explicitly.

The travel isn't quite enough except on the side that's high. So I could do it by skimming in from the low side. How do I lap something this big? A large piece of glass?

It's the Grizzly version of a Sieg SX3.
Click to expand...
You could use a big flycutter rather than a face mill. That would give you a bit more reach. Cast iron machines very nicely (but messily).
 
Mapping the table by using a stationary indicator and running the table underneath it shows you very little about the geometry of your table. The table could be curved upwards or downwards over its length like a banana and the indicator could possibly stay on zero. The only real way to test that table is to put it on a 'known to be in calibration' surface plate, and test the height all over, turn it over and do it again from the other side, and also test for bend and twist. A precision scraped camelback straightedge might also be used for part of the testing. Many milling machines have been ruined by doing the simple and dirty machining you are considering. If you want to know how to test it and repair it, read and understand Machine Tool Reconditioning, the bible for that work:
http://www.hobby-machinist.com/threads/connelly-on-machine-tool-reconditioning.41802/ Warning, this is not a one evening read...
It will recommend scraping the machine if it is out of factory limits to correct geometry, in a specific sequence that will NOT start with the table. That requires skills and understanding that take time and practice to develop.
 
I have a large (sort of) cast iron vee block, about 6 x 9 that I use for a lap. Sometimes it's not aggressive enough so I put a big slug of steel round in it for weight, but then it gets too tiring. And if you do mill the table, it's entirely possible that it will come out worse, so you spends your time and takes your chances. With only 0.003 to work with, I would just mark it out and lap it.

Bob, he's not addressing a machine rebuild or recondition....merely one flaw. I understand perfectly what you are saying and why, but without evidence of additional issues with the machine, and the capabilities to address them.....he can gain some accuracy by simply resurfacing the table. It won't be the same as a rebuild, but he's not trying to rebuild. Crawl before walk before run.
 
Is your machine in tram? If it is and yet the table rises .003 as it travels along the x axis the plane of the ways cannot be perpendicular to the spindle axis.

I'd want to do more measurements before milling the table.
 
Is this a knee mill or a bed mill?
 
Could be the gib also, I think. It is probably not consistent in form, and not precise. As the table moves and the gib doesn't, the clearances are bound to change and this will cause a shift in the table. Might be up, down, ot the front or to the rear....or some combination. I think you need to do a lot more checking before cutting anything, for sure. It's hard to put back what you cut off.
Or you could put a piece of 1" aluminum tooling plate on top of the table and mill to your heart's content. If that makes it flat, great. Leave it and make use of a sacrificial piece. Or if you're convinced enough, lap or cut the table. But be sure before you cut the table. Bob brings up great points, but unless you have the means of checking it properly, you'd just be guessing at what you think you find wrong.
 
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