Gib screw spacing too wide?

[Bob Farr]

I've been fixing up an older Atlas/Craftsman 12" and noticed today that when the tool post is about midrange the two end gib screws are not pressing the gib against the dovetail. For about 3/4-inch of travel the two outer screws are just "floating" off the ends and the only gib screw with a bearing is the center one:



This permits a little slop unless that center gib screw is snugged right up. It seems like two more intermediate gib screws would be a good idea, but mine can't be the only one like this, right? Am I fussing too much? Maybe I should just shut up and cut some metal.

[Forrest Addy]

So add some. I suggest you use allen head seet screws. Those socket heads are bulky and collect chips. I prefer to use a little snug fitting bevel ended slug between set screw and gib. If your set screws have cup points, flatten them.

The traditional setscrew gib uses jam nuts to hold the setting. If you use one set screw for a clamp those sexy adjustable level handle goodies work real slick. You do have to balance bulk with convenience.

http://www.carrlane.com/Catalog/inde...3C3B2853524758

[lane]

If you don`t like it fix it . We can all improve something. Any thing to make our machinery better is worth the trouble.

[GadgetBuilder]

Forgive me if you've checked all the obvious things below but it took me quite a while to understand how to make the gibs in my machine work properly (because these checks weren't obvious to me).

What happens if you adjust the end gib screws so they touch the gib, does it then not move freely for the full travel?

Is the gib straight if you take it out and lay it on a flat surface? Does the gib fit the opening closely? Does it move front to back when you change direction of movement?

If you measure the center dovetail (using two hardened pins so you can use the caliper), are the sides precisely parallel? Is each side straight? Does the dovetail make contact properly (judged by bluing or inking)?

Getting the gib and the dovetail just right can take some fussing but I'd be inclined to check the fit carefully before adding more adjusting screws.

In "The Model Engineers Workshop Manual" by Geo. Thomas he has a nice description of making and fitting the gib to his retracting tool holder for screwcutting.

John

[J Tiers]

gadget....

I think the question is not "why does my compound shake like a leaf"....... but rather "does this look like a good idea?"

His point is that IF the end gib screws are off the end in some positions, the compound may be prevented from rotating a bit ONLY by the springiness of the unsupported gib, and the presumed fact that the other side is on a solid dovetail.

I don't think it is as bad as it seems, because the other side IS on a solid dovetail...... There is effectively 3 point support*, and any twisting would be resisted even by the single screw backing up the gib. of course the lever arm is less than it would be, but appears pretty substantial even so.

The screw and thin gib as a gib is definitely inferior to any of a dozen or so other designs of gib. But it works, and in any case is your only option in the case of the Atlas, unless you want to mill it tapered, and put in a scraped-to-fit tapered gib. That would offer positive support full length if properly scraped.

* so long as the compound etc isn't worn hourglass.

[lazlo]

The screw and thin gib as a gib is definitely inferior to any of a dozen or so other designs of gib.

Mike and I were wondering this last weekend: what's the "best" gib design -- a tapered gib that's locked with opposing adjustment screws, or a plain gib that pinned every X inches?

I hate plain gibs -- they're such a pain to get adjusted correctly -- when you tighten the jam nuts, it distorts the screw, throwing the gib out of whack. So you end up getting the set screws close, then dork with the locking screws.

My Clausing 5914 (lathe) uses tapered gibs everywhere, and they're so much nicer...

But on the other hand, I notice the high-end Gilman industrial dovetail slides come with plain gibs. I'm guessing that's because you can micro-manage the fit of the gib all along the bearing surface, while on the tapered gib everything has to be perfectly flat?

[GadgetBuilder]

J Tiers,

My error, I misunderstood the question. When he said the screws weren't pressing the gib against the dovetail I thought he observed a space between the screw and the gib such as might occur if the gib were bent. I tend to be pretty conservative about drilling holes in my machines so I didn't consider that the OP might modify things based on theoretical considerations rather than operational difficulty.

John

[Bob Farr]

Gentlemen,

Thank you for all of the tips. I had considered drilling the toolpost to add two more gib screws, but I don't think I will now. I disassembled and checked the fits, as suggested. The dovetails are parallel and straight, and the gib is flat. Furthermore, while the gib runs off the ends of the dovetail quite a bit, the points at which the outer screws meet the gib are barely off the ends of the dovetail for a moment, traveling either direction:



I had not considered that the center gib screw still triangulates with the opposite dovetail even with the outer gib screws off the ends, so I really don't think there is much to be gained by drilling the toolpost casting for two additional screws. Maybe the engineers at Atlas were pretty smart after all!

The allen head cap screws were temporary while I was fiddling with this, but I like them enough that I will probably modify the tips to fit the gib dimples. I hate messing with lock nuts on set screws, and I've found (in vibrating Harley applications at least) that a small dab of non-hardening gasket sealer on the screw threads will keep them from wandering on their own but still allow easy adjustment by hand.

Thank you,

Bob

[CCWKen]

I made a couple of gib screws that I use to lock the cross slide and compound. Notice anything different about your Cross Slide? I think your's is off a 6" lathe.

[Bob Farr]

That is a nice gib screw idea Ken, and maybe I'm overlooking something but I don't see a difference:



I'm missing my cross slide screw cover, and I have an early one-piece apron/carriage without power cross feed or way wipers, but both of our cross slides have four gib screws and both tool posts have three gib screws.

I wouldn't be surprised if my machine is some hybrid of gathered parts, but I must be overlooking something that you see.