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View Full Version : Help, top slide re-assembling too tight



tmc_31
10-16-2011, 03:21 PM
Hey guys,

A few months ago, I crashed my lathe (Jet GHB1340A) and destroyed the top slide ( the one the tool post bolts too). I ordered a new top slide from E replacement parts ( if they tell you they have it in stock, don't believe it). I built a tool block to use in the interim and it works well, it is much more rigid than the compound (thanks Sir John). I finally got the cross slide from Jet via e replacement parts.

I have been trying to re-assemble the compound with the new cross slide. It is very tight assembly, way too tight.

My question is, do I file the gib so that it fits smoothly or work on the dovetail?

Tim

Rustybolt
10-16-2011, 03:33 PM
Rule of thumb. Wreck the cheapest part first. play around with the gib. hell, Just make a new one.

tmc_31
10-16-2011, 03:48 PM
yeah, that makes sense

thanks,

Tim

SGW
10-16-2011, 04:05 PM
What Rustybolt said....

winchman
10-16-2011, 04:06 PM
Isn't the gib adjustable? Perhaps you can do something to increase the range of adjustment.

tmc_31
10-16-2011, 04:18 PM
Isn't the gib adjustable? Perhaps you can do something to increase the range of adjustment.

Winchman, yes the gib is adjustable within a fairly narrow range, I don't think I can increase the range of adjustment, but I might be able to file the gib to make it fit within it's range of adjustment.

Anybody know what a gib is made of?

Tim

macona
10-16-2011, 04:29 PM
Should be cast iron.

tmc_31
10-16-2011, 04:34 PM
Should be cast iron.

Makes sense, I don't think CI to CI will have the galling problems that you would have if you had similar steels or aluminum rubbing together.

Tim

DFMiller
10-16-2011, 04:52 PM
Too tight is much better than too loose.
A bit of fitting is required.
Dave

Rosco-P
10-16-2011, 05:24 PM
New top-slide and old gib? A bad combination, should have ordered a new gib to be fitted to your machine. Is the old gib perfect flat? Have you checked on a surface plate to be sure it hasn't become banana shaped from age/stress?

DR
10-16-2011, 05:40 PM
Should be cast iron.


Hmmmm.........

I don't recall ever seeing a CI gib. Steel, brass...yes.

What type of gib is this? Flat or tapered? Tapered can be kind of tricky to make, flat are simple.

lane
10-16-2011, 08:01 PM
The new parts are not going to just fit and work . You are going to have to scrape them in to fit correctly. Any replacement sliding part for machinery will have to be hand fitted to the specific machine.

tmc_31
10-16-2011, 08:35 PM
New top-slide and old gib? A bad combination, should have ordered a new gib to be fitted to your machine. Is the old gib perfect flat? Have you checked on a surface plate to be sure it hasn't become banana shaped from age/stress?

Is that as in Roscoe P Coltrane?:D

You are correct Roscoe, the top slide is new, the gib is old. I do believe that it is CI after looking at it a little closer. I did check it on a surface plate and found it to be a little banana shaped, but not much. The lathe has less than a hundred hours on it and the original scraping marks show no wear to speak of. It is a tapered gib, tapering .049" over 6"

Since I have no experience with scraping, I think what I will do is sand one side down using a granite flat as a backing plate until it fits snugly ( as in finger tight) at it's loosest adjusting point. That should leave me some adjustment for wear.

If I screw it up, I can always buy a new gib

Tim

macona
10-16-2011, 10:10 PM
Hmmmm.........

I don't recall ever seeing a CI gib. Steel, brass...yes.

What type of gib is this? Flat or tapered? Tapered can be kind of tricky to make, flat are simple.

The ones in my 10EE are cast iron. Heard stories of broken gibs in mills? They are cast iron too. Cast iron is used because it has one of the lowest coefficients of frictions of a metal. Way lower than steel and brass.

They may be cast iron and you never know it since if it was made from a piece of durabar you will never know until you try an break it.

darryl
10-17-2011, 12:51 AM
Though I'm in general agreement with the gib thing, I would have suggested first to find out why it's tight. If you know it's the gib, great, you know what area to work on.

Here's something I've found a couple of times- dovetail surfaces that aren't completely flat. During the machining, a bit of a burr might be raised on one edge. You can do a test and simultaneous cure with a file. Holding the file flat to a mating surface, give it one or two strokes. If it skates across easily, there probably is no burr, but sometimes you'll feel it catch on something. You can play with gibs all you want, but if anything is preventing mating surfaces from having good area contact, you're probably just going to screw up the gib.

One I haven't seen, but is possible, is that the mating parts aren't machined perfectly. One or both parts may have a slight twist. I would do a careful cleaning, oil lightly and wipe clean, then check for any rocking. Once the fitup is good, then play with the gib.

J Tiers
10-17-2011, 10:05 AM
On that note......... Check to be sure the "points" of the dovetail (if present) are not interfering with the other part..... You can just file them blunt if so, they are not important and really shouldn't be pointy.

tmc_31
10-17-2011, 01:08 PM
Darryl,

Thanks for the input, I checked the topslide dovetail surfaces this morning with a file, there was a slight burr where a hole was drilled through for a gib tensioner. I smoothed it off with the file and also filed the corners so that i was sure that I had flat surfaces. When I reassembled it, the gib went in about 1/8" further than it did before filing. The gib still needs to go in about another 1/2". All the dovetail surfaces seem to be flat and true under visual inspection using machinist squares as a reference.

J Tiers,

I am not sure what you mean by "points" of the dovetail. I looked for some points this morning, other than the corners I saw nothing like that. A little further explanation would help this feeble mind and be greatly appreciated sir.

As Lane pointed out, I think this assembly just needs to be hand fitted. As I see it now I just need to scrape, file and/or sand the gib down to where it fits. It could have been worse, the topslide could have been machined oversized to where the gib fit too loose:eek:. easier to make it smaller than bigger:) .

Thanks all,

Tim

lane
10-17-2011, 07:24 PM
One other thing not mention and I have seen it a number of times before over the years . Check that the new part and the part it fits to has the same dove tail angle . I have replaced cross slides on lathes and ordered a new suppose to fit slide and have the dove tail on the new slide be 55ºand the part on the lathe be 60º are some other angle . Call the factory they say ( O you have to fit them your self.)
Another question is it a tapered gib are a flat gib with set screws to adjust . that can make a big difference too.

tmc_31
10-17-2011, 09:03 PM
One other thing not mention and I have seen it a number of times before over the years . Check that the new part and the part it fits to has the same dove tail angle . I have replaced cross slides on lathes and ordered a new suppose to fit slide and have the dove tail on the new slide be 55ºand the part on the lathe be 60º are some other angle . Call the factory they say ( O you have to fit them your self.)
Another question is it a tapered gib are a flat gib with set screws to adjust . that can make a big difference too.

Lane, The gib is tapered. Just by eyeball, it seems about the same angle as the dovetail. I will check it closer tomorrow morning when I get back out to the shop.

Thanks,

Tim

lynnl
10-17-2011, 09:12 PM
If it's tapered and you do some sanding/grinding/scraping on it, just remember that a little bit goes a long way in fitting tapers. Be sure and check the fit often once it starts getting close.

Of course it takes a lot of sanding to actually remove much metal.

darryl
10-17-2011, 09:31 PM
I had forgotten about checking the angle- I've been caught in that myself. I actually machined up a 'mating' part, only to find that the angle wasn't 45 degrees, which is what it looked like. Next time I'll check first, even if I think I know what the angle is.

I used an X-Y table for parts for another machine, and I found also that the dovetails on one part had a rounded corner where it met the flat. I had to relieve the edge of the mating dovetail so they could get good contact.

J Tiers
10-18-2011, 12:55 AM
The "points" are just the sharp edges....

Looking at the end, a male dovetail has the "points" on the widest part of outside, a female has the "points" on the narrowest part of inside.....

If they are left "sharp", they may bind up in the corner of the mating dovetail.

darryl
10-18-2011, 01:48 AM
Hmm- if the parts seem to go together well, but the gib doesn't go in far enough, that suggests that the top slide dovetails are too close together or that the top slide is too thick. You don't want to mess with the mating surfaces unless there's good reason to, but suppose you did remove a few thou from the flat- the top slide would become lower, the dovetails effectively further apart, and the gib would go in further. It would seem offhand that you got the part in a condition which would allow for scraping and fitting. Maybe you should blue it up and check to see how close it actually is. Even if you do some minor scraping, you're going to affect the fit of the gib, since as others have mentioned the shallow taper is going to mean that even with a small amount of material removal, the fit will change quickly.

Another thought I had was that since you crashed the lathe, it's possible that the bottom part of the compound has been 'altered'. Bent slightly perhaps. It would be good to ensure that those surfaces are truly flat still. If they aren't, then no amount of fitting of the top slide will make it work properly. You'll get tight spots, then loose spots, and if you adjust for the tight spots, there may be too much play in the loose spots. If the fit-up is right, you should be able to adjust the gib to give the top slide a fairly even drag over the whole range of adjustment. An indication of a good fit is if there's very little range in the gib adjustment screws between having drag and having very free movement. There should be almost no 'mushiness' in the gib adjustment.

tmc_31
10-18-2011, 12:06 PM
Thanks J for clearing that up for me. I think I am ok in that regard.

Darryl, the top of the compound (or "swivel slide" as it's called in the parts manual) doesn't seem to be damaged under visual inspection. I will look at that a little closer. It may well be that the new part was intentionally supplied with a little extra meat to allow for hand fitting. As you say, it shouldn't take much off the taper to make a large difference in how far the gib goes in.

Thanks,

Tim

Rosco-P
10-18-2011, 01:10 PM
It may well be that the new part was intentionally supplied with a little extra meat to allow for hand fitting. As you say, it shouldn't take much off the taper to make a large difference in how far the gib goes in.

Thanks,

Tim

Before taking anything off the replacement parts (other than removing burrs and sharp edges), I'd take measurements of the compound and the mating base dovetails to determine the clearance between the two. If the original gib is bowed, the slight spring in it might be preventing it from sliding all the way in.