Mill Gibs

[seal killer]

pete and All--

About that long parallel; maybe not. Think over $700 at McMaster-Carr.

--Bill

[Harold_V]

pete and All--

pete, first, a correction: I wrote "knee" but should have written "cross feed."

That's (properly) called the saddle, Bill.

I loosened both gib screws a bit, then tightened one. I never felt any difference in the feed. I kept screwing with it. Eventually, I decided to switch ends by screwing the other end in to tighten it and then locking it down with the opposite screw.

I would suggest that the outer end would be the large side of the gib. Otherwise it couldn't be inserted after the slide was in place. So then, you'd tighten the outer screw, while backing off the inner screw (the side near the column) to lessen clearance of the saddle slide. I would recommend you do that until you feel a definite tightening of the slide, then back off a tiny amount. A slide that feels tight when not loaded may still move under pressure of roughing cuts.

H

[Bill Shields]

And since it is cross slide...your knee could be out a bit..which affects the cross slide

Doe the 'problem' get better or worse if you tighten the knee?

Is the one problem with a knee mill..if knee is off.. everything can be wonky

[pete]

You really need to post a couple of pictures of exactly what you have Bill. Even a scan of a page from your user manual that shows the gib and it's adjustment parts would help. At this point the rest of us are just going by your rough description it's what you have. I'd much rather it's 100% sure than guess. But as Harold said, even getting towards a proper adjustment, there will be a definite tightening feel as you move the gib IN to tighten or lessen the clearances. Tapered gibs work exactly like an adjustable parallel with matching but opposing angles. As the gib moves in it gets moved sideways as well, that reduces your clearances. Because I don't own your model of mill, there's no way to be exactly sure of what it's design is compared to mine. So explaining exactly where and what side my gibs are and which is the fat end might be pointless.If you have to, use a drop light, flash light, or other to visually check each end of your gib. One end will be noticeably fatter than the other. That's the end that has to be adjusted IN to start taking up the clearance, the screw at the other narrower end can be used to re -adjust the gib back out a bit if you get the adjustment a bit too tight. But mostly it's meant to lock your final adjustment so the gib can't then move on it's own in either direction. Set screw gib adjusters have the same with those lock nuts. And since I don't know your exact mill, the knee gib should be on the same side the knee locks are and it's fat end "should" be at the top side of the knee.

At this point it's pretty plain your probably multiple years since adjustments should have been made to both the gibs and nut back lash if yours are adjustable. They may not even have been correct from the factory. As an example that might make you think about it, mine were at best ok before I pulled it apart, the X,Y table and knee were usable I guess, but not nearly as smooth as I thought they should be. And mine had a disappointing .008"-009" on average back lash on the X,Y dials. So while it was apart I did all that detail cleaning on the ways, screws & nuts. Re-lubed and then readjusted the gibs and nuts. Then it started to feel exactly how I thought it should. The small bit of grinding dust, a few chips from machining and all the dragon fat the factory slathered on as an anti rust preservative prevented them from adjusting mine to where it should have been. Then after the above, it did start feeling like I thought it should from playing with other mills at a few dealers. Tight but still much much smoother than it was. I also got the X,Y down to about .004" back lash with no tight spots. (very important nothing is overly tight). And .004" is what Bridgeport themselves guarantee there brand new machines have.

Maybe not right now if your really busy, but if it were me, I think I'd move that pulling the table, cleaning, re-lube, gib and nut adjustment to my, I need to get to that real soon list. Since it seems it's never been done, I think you'll then see a vast difference in how the machine feels, and it's over all accuracy because it can then be adjusted to the proper and tighter condition. That even gets you better surface finishes. I'd bet it will feel a lot better and be more accurate than when you bought it. And doing so also lets you visually check those oil galleys I mentioned or any possible unusual wear patterns starting like way surface gauling when there's no oil getting to those surfaces. From my perspective, my mill was maybe more than I could afford at the time. It really hurt to save up for and buy. But I wasn't willing to settle for less. So I do what they do in the open pit mines with real expensive equipment. The machine gets all the regular preventative maintenance and lubrication it requires. If that takes a bit of work and time I still do it. Automotive and machine tool oil sumps are supposed to get oil changes after X number of miles or hrs. Machine tool slides etc are obviously different, so you do have to go in there by whichever method and do what oil filters and oil changes accomplish as well as those regular adjustments to take up the inevitable wear. It's a simple fact more machine tools will see much more wear from lack of or no preventative maintenance, incorrect amounts or incorrect/no oil, and lack of basic cleaning than ever wear out just machining parts.

A fancy super accurate straight edge won't really allow you to run any checks more than the parts on the machine already do for just how tight it's slides may or may not be. Now if you want to check it's actual geometry that's different. I won't bother detailing where to place your indicators magnetic base so it's independent from the slide your checking, you already know that. Once the nut/gib adjustments are done to where the machines tight, still smooth, low back lash but no tight spots, first check your back lash numbers on the dials for X,Y. Write that down for both. Those are your base line numbers. To check the Y axis clearance, put the indicator tip against something like the side of your mill vise. On this axis the indicator plunger needs to be at least visually square to the object it's on while horizontal in both Y and Z. Physically pull and push hard on the table in X. Write down your total movement, then subtract your X axis back lash number. That's your clearance on the Y axis dovetails. They'll always be some because it has to be there. But you want a minimum number without tight spots. That's a factor of any wear you now have, surface finishes on the parts, and just how accurate it was made to be. To check the X axis, put the indicator tip on maybe the center of either the front or back vertical table surface. Again at least visually square in X,Z. Push and pull hard on the table by maybe using the vise but not on the table ends along the Y axis. Subtract that total movement from your Y axis screw back lash. That's your total clearance number for the X axis. Also check the X,Y movement as there locks are applied. You'll see at least some movement even from the oil thinning out. But you want to know on average how much it is because that might be important for high accuracy work, and maybe even more so with a dro. Then check the table front with an indicator tip on it as the knee locks are applied and released. With a slightly loose knee gib you might see the table rise a bit with the locks tight. There's as I said a lot of weight so it might not show at all. That's why I use that prying with a 2"X4" I mentioned to be sure.

There's way more to do checking your machines basic geometry I'm sure not going to get into until you have your mill adjusted to where it should be, and not unless you want to know how to do so. But buying a good hardened and ground cylindrical square or even making one on your lathe will make those basic checks a whole lot easier. Unless you have a larger fixed square with it's known inaccuracy to 10ths level already. Cylindrical squares are easier to check there accuracy, more stable in use, and no re-postioning while checking X,Y directions. I don't even own any fixed squares yet, but do have a couple of those cylindrical squares. For shorter fixed squares, I just use my 123 or 246 blocks instead.

[John Hasler]

What size cylindrical square do you find most useful?

[pete]

I have a B&S 558 cylindrical square that's about 6" long and an SPI 12" model John. The B&S can do what most other CS's can't, and it can actually measure how far something might be out of square. Or on items less than about .002" out it can. And sometimes knowing how much it's out is more important than knowing it is off, but not exactly by how much. Yes feeler gauges could be used to get you close, but afaik there not available in 10ths increments, and if they were you'd sure need a lot of them. So the B&S is pretty handy. If there's out of square variations along an items length, that can also be measured just by rotating the square until the lights gone and then reading the measurement marks on the square. The 12" is depending on the size of your machine tools obviously better over those longer distances. Checking almost the full travel of a BP type mills knee as one example. If I could only have one, then probably I'd pick the 12" because it can still be used to preset a dti/surface gauge combination to check exact amounts of out of squareness. It's pretty heavy though. But even without a proper cylindrical square, someone here some years ago and it might have been Glenn? showed a really great method of very accurately checking the knee travel with a dti and a simple bored hole in a piece of plate in the mill vise. It was one of the most clever alignment checks and tests I've seen anywhere ever. Absolute genius level thinking for sure. But that unfortunately can't check the tables true and exact X - Y squareness to each other. But that plate/bored hole trick could check the spindle travel and for out of square bearing bores to the spindle length.

[John Hasler]

Thanks, Pete. What diameter are your cylindrical squares?

[seal killer]

pete and Harold and Bill--

Ok. This is a lot to process but I am eager to do it. More for the sake of learning than the health of the mill.

My original issue was that as I slid the table in and out, the indicator I had on the workpiece showed that the table was being tilted left or right by ~0.002" depending on whether I was moving it out towards me or towards the column. Adjusting the gib has eliminated that problem.

But does that elimination apply to all knee heights? I dunno. In my thinking, it should. However, as so often proven on this forum, my thinking is often informed by my ignorance . . . not a good combination.

Pictures will be forthcoming.

My time this weekend has been absorbed by a forthcoming trip to the (semi) Big City (Springfield, MO.) But I will be back on this as soon as I can. Thank you so much for your interest in helping me.

--Humble Bill

[pete]

4" on the SPI John, memory dimensions only on the B&S (it's packed up) I seem to recall about
2 1/2"- 2 3/4". To be perfectly honest, I think a completely usable CS could be made using schedule 80 pipe. 160 would be even better not only for the heavier walls but more weight, probably real hard to find in short lengths though. There's numerous threads around about how others made there's. It needs a lathe in half decent shape and tuned well enough to turn parallel to close limits. Maybe a hard sanding block and high numbered emery to do some fine adjusting for size if the exact diameter along it's full length isn't quite what it should be. I'd also probably recess a couple of end caps and maybe epoxy bond some additional weight inside it. Afaik there probably all turned between centers and certainly ground on them. But for a straight machined square, it's all simple turning and facing. Just a bit fussier. With a unhardened mild steel one you'd still have to be quite careful not to create any dings on the ends though. The B&S works as a measurement tool because one end has a very fine angle purposely ground into it and ball mill dots are used in a non straight line as the measurement increments. Flipping it over onto the other end gives you a standard CS. Whoever first figured out this type of CS was far smarter than I'll ever be.

[Harold_V]

Ok. This is a lot to process but I am eager to do it. More for the sake of learning than the health of the mill.

I agree with your assessment. Knowing and understanding how and why these things behave as they do goes a long way in helping diagnose problems when they arise.

My original issue was that as I slid the table in and out, the indicator I had on the workpiece showed that the table was being tilted left or right by ~0.002" depending on whether I was moving it out towards me or towards the column. Adjusting the gib has eliminated that problem

I'm not sure where the discussion of the knee gib got introduced to this discussion, but from your quoted comment, above, it's clear that the gib concerned is the saddle gib. There would be no reason for the table to shift side to side by moving the knee, although it might experience up and down motion if the column gib (knee gib) was not adjusted properly.

But does that elimination apply to all knee heights? I dunno. In my thinking, it should. However, as so often proven on this forum, my thinking is often informed by my ignorance . . . not a good combination.

It should make no difference, as the knee slide (column) is not related to the saddle slide. Each should display unique problems, not at all confusing with one another.

H

[seal killer]

Harold--

The knee gib came into the conversation from my original error. I do not know why I said "knee." Old age? (I don't drink . . . somewhat sad to say.)

--Bill

[Bill Shields]

Drinking make little difference as we get older...unless you over do it