surface grinding near San Diego?

You can measure flatness of the table first, Bill, without grinding it. But knee-type milling machines are subject to several types of inherent, geometrical traverse errors, so it's more likely that your problem lies there. Somewhere there is a good, illustrated explanation of this. Maybe someone knows where it could be found online.

Do you have a precision straightedge and a dial indicator?

Most likely the way slides are worn, not the table itself. There are several things that all need to be right for the machine to cut square. Assuming this is a knee mill like a Bridgeport, both the top of the knee and the bottom of the saddle need to be straight. Second, the bottom of the saddle and the top of the table need to be parallel. Between these, there is also the X axis slide, and there could be wear there too that is throwing the Y out of alignment.

Jon

I have a dial indicator that does .0001", which is what I measured with earlier today. No precision straightedge, but what made me suspect the table surface is this: I have an optical flat disc salvaged from an old instrument for I know not what purpose, but sliding the disc across the table in "Y" direction, there seem to be some height changes at the edge of the table slots. I can more feel it than see it, so maybe not enough to worry about? This is a pretty old machine, a Gorton O16-A, built in

1954 it looks. Could indeed be just wear everywhere! Any pointers to measurement "how to" info would be appreciated!

Bill

I wish I could help with the measurement, and I have two books on the subject of rebuilding and tuning machine tools, but they're in my storage and I haven't looked at them in 20 years.

I'm hoping someone will have a link to a very nicely illustrated explanation of such errors -- I remember seeing it in print a few decades ago.

An optical flat might be hard to read, as sensitive as they are. And a little rounding at the edges of the T-slots is no surprise in an old machine.

By the time they had the sag errors all figured out, sometime in the 1930s, builders were building-in some correction factors. Sometimes they overcorrected. It's not completely obvious, and you can really chase your tail trying to figure out where the problem is.

A good straightedge would really help -- either the kind used by machine tool rebuilders or just a knife-edge precision blade. I have one made by Starrett, 18" long.

If you're ambitious, you can get any old straightedge quite close to straight by laying it on a piece of craft paper, tracing the edge with a razor-trimmed chisel-edge pencil, and then carefully flopping the tool over its own edge, and tracing again. File or stone the high spots. When you get too close for the pencil marks to be clear, use a razor to cut a fine line in the paper, in place of the pencil. You can get surprisingly close if you have patience and you're careful.

Ok, you gave me motivation...I checked out my 12" Mitutoyo metal ruler on a small surface plate, and it seems pretty decent for straightness. So I put it across the mill table, with a light behind it, and do NOT see light leaking under the edge of the ruler. So sounds like the table is flat enough for most purposes, certainly mine. All of the ways on this machine, old as it is, still show a lot of scrape marks. Could be I just need to make allowances for how it is... not a big problem for most things I might attempt.

Bill

Bill Martin fired this volley in news:inb2hj$pik$1 @dont-email.me:

Bill...

that's not un-intuitive, it's exactly what's to be expected.

It isn't the table's surface that is worn, but the ways. Near the center, where most of the work has taken place, they're worn down considerably. Near the ends, they are not worn; thus, they are "higher".

Don't grind the table, scrape the ways.

LLoyd

That's the way it often goes with old machines, particularly knee mills. It's not generally much of a hardship. In the really old days, no one considered a mill to be a precision machine, anyway.

In place of a straight edge and indicator, you can also use an optical technique. Mount a mirror on the side of a flat-bottomed block, and look at the change in vertical angle of reflected light as it is moved down the table. This is the technique used to calibrate granite surface plates. Usually an autocollimator is used to measure the angle, but a poor man's substitute is to look at the height of a reflected laser pointer on a far wall. Of course this method measures change in tilt versus position which then requires a little math to convert to change in height of the table. The goverment has a publication on how do do this:

Yeah, that makes sense. I did notice that the vertical change was not exactly linear with position, which is very consistent with your diagnosis. Not likely that I'm going to be able to scrape ways, will just have to live with the known error, I can keep it under .001" by staying away from the outermost table position.

Bill

Yes, I'm not overly concerned, just was wanting to understand what I was seeing.

Bill

Wow. That is some little bit of math.

Bill

I didn't see what kind of mill this is but it sounded like the problem you have might possibly be corrected by shimming the base or adjusting the feet (if so equipped) or adjusting the knee lock (if it is a knee mill) . You could easily induce (or remove) a .0015" bend or tilt in the Y axis by how the casting components are stressed under load.

I did attempt to adjust the knee lock, only made it worse. Induced an even bigger change going from unlocked to locked. I just set it back to mildly snug top and bottom, and now there is almost zero change from unlocked to locked. That never was the issue though, so I didn't really fix anything there.

Bill