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View Full Version : Gib screw spacing too wide?



Bob Farr
12-20-2008, 06:22 PM
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:

http://i58.photobucket.com/albums/g272/frankenglide/Toolpost1.jpg

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
12-20-2008, 07:27 PM
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/index.cfm/27025071F0B221118070C1C512D020609090C0015482013180 B041D1E173C3B2853524758

lane
12-20-2008, 08:27 PM
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
12-20-2008, 09:33 PM
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
12-21-2008, 01:50 AM
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
12-21-2008, 09:12 AM
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
12-21-2008, 11:31 AM
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
12-21-2008, 01:08 PM
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:

http://i58.photobucket.com/albums/g272/frankenglide/Toolpost2.jpg

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
12-22-2008, 12:08 AM
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.

http://img.photobucket.com/albums/0903/CCWKen/Tools/Craftsman%20Lathe/GibLocks.jpg

Bob Farr
12-22-2008, 05:54 AM
That is a nice gib screw idea Ken, and maybe I'm overlooking something but I don't see a difference:

http://i58.photobucket.com/albums/g272/frankenglide/Toolpost3.jpg

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.

JCHannum
12-22-2008, 07:13 AM
I don't see anything different with Bob's cross slide either. The 6" was much smaller.

The 12" Craftsman lathe was basically the 10" Atlas lathe raised one inch. The headstock and tailstock castings were raised at the base. The apron and cross slide were not changed, the additional inch of height was achieved by raising the height of the compound casting.

The power cross feed was available on some lathes and not others, as was the tapered or babbit bearing headstock.

J Tiers
12-22-2008, 08:24 AM
The 12" Craftsman lathe was basically the 10" Atlas lathe raised one inch. The headstock and tailstock castings were raised at the base. The apron and cross slide were not changed, the additional inch of height was achieved by raising the height of the compound casting.


That would explain the weird "tower" that contains the t-slot..... That just looked "wrong", and I wondered why it was done, thinking it was to save a few ounces of cast iron..... and in a way, I guess it was.

CCWKen
12-22-2008, 05:57 PM
Ops. Yeah it looks the same to me too. :D Looking at the picture in post #8, the cross slide looked short. It's obvious now that it has four gibs. The compound is the same as mine. It overhangs when centered. I usually run with the front (toward center line) and center gib on the V. The locking screw was moved to the center on the compound. (Shows front in pic.) I moved it to the center so it's a more positive lock no matter where the compound is located.

Sorry for the confusion. Mine that is. :o

By the way, my A/C is ~1939 Deluxe. That could explain the wippers and power cross feed.

Bob Farr
12-23-2008, 04:14 AM
*** By the way, my A/C is ~1939 Deluxe. That could explain the wippers and power cross feed.

Mine is about the same vintage, but there's nothing Delux about it :)

By the way, it was your "Cleaning an old lathe" thread from a couple years ago that encouraged me to try this resto. Nice job, sir, and I think the chips and oil make that pretty Ford-blue paint look even better!