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Gib Adjustment - how tight is too tight?

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Beagle15/06/2011 13:01:50
21 forum posts
I gave my Speed 10 a strip-down and clean a few months back with a view to keeping everything smooth and running nicely. As part of that job I obviously adjusted the gibs on all the slides. Now my cunning plan at the time was to mount a bar in the toolpost with a mag-base DTI on the bed - this would allow me measure any free-play movement of the saddle / crossslide / topslide relative to the bed, and thus take out movement as required.
 
I worked one slide at a time, locking off the others, and then applied gentle twisting loads by hand to each slide looking for movement on the DTI / toolpost test bar. Where I found it to be excessive I carefully adjusted the gibs a little tighter, attempting to get an even pressure along the gibs. In the end I happily got what I felt was a 'just' backlash-free saddle / cross / top slide assy. I put a good quality slideway lube on and figured all would be well.
 
A few months later and the slides still seem snug, but I am a little concerned that they may be too snug - the main saddle is beginning to leave darkened slideway oil on the bed, which suggests to me that I've got some metal-to-metal contact somewhere, and a lack of an oil-film - despite regular oiling. The main saddle handwheel is relatively 'snug' to turn - not tight - but not easy to get a fluid longitudinal movement of the saddle. I had put some of this down to the high gearing ratio between the handwheel and saddle movement, and the small radius handwheel needing a higher torque to move everything, but with the greyed-oil, I'm now thinking it's a smidge too tight.
 
Do people concur with this view, and what advice can anyone provide on optimum setting of gib strips?
 
Logic tells me that any backlash in the slides is probably bad, hence my attempts to take it all out - or do I have that wrong? (The Speed 10 is relatively low use, so I'm not convinced anything is particularly worn)
ady15/06/2011 13:14:04
612 forum posts
50 photos
Experience is the best teacher, but things should usually move quite happily.
I started off looking for the nirvana of zero backlash etc but it's not worth the effort until you have an upcoming job which makes it worthwhile.
And tighter jibs mean more wear on your leadscrew nut.
 
It depends on the job for me.
I don't bother when I'm roughing down, and have a wee check, nipping and tweaking things when I've got a critical bit to do.
 
Hobby lathes are a compromise and you shouldn't expect one to behave like a 3 ton toolroom lathe kinda thing.
 
Tapered gibs are the best/simplest system to maintain, but only certain machines have these.

Edited By ady on 15/06/2011 13:15:50

Roderick Jenkins15/06/2011 14:36:35
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2138 forum posts
586 photos
Beagle,
 
As I'm sure you are aware, backlash is caused by the loseness of fit of the screw in the nut so adjusting the gibs to tighten up the movement will not eliminate the backlash, it will just make it more difficult for cutting pressures to move the slide until the nut bites on the other flank of the screw. I suggest therefore that you tighten up the gibs so that they eliminate any rock but allow free and easy movement of all slides. Any backlash issues should be addressed, if necessary, by locking the slides when making a cut. I have replaced one of the middle gib adjustment screws with socket heads and without a lock nut on both cross and top slides so that I can nip them up with an allen key which I keep handily by the lathe. Having given that council of perfection, I do tend to keep my topslide just a lttle bit stiffer than the saddle and cross slides but I don't tend to use the topslide as much as the other two.
 
Rod
Beagle15/06/2011 15:25:56
21 forum posts
'I started off looking for the nirvana of zero backlash etc but it's not worth the effort until you have an upcoming job which makes it worthwhile. And tighter jibs mean more wear on your leadscrew nut.'
 
Ady - Thanks, that makes good sense. I'll probably back them off a smidge then just so things free up a touch and to avoid premature wear. I guess I optimistically expected zero free-play, and fluid movement at the same time. Thinking about it more, I guess this is not an easliy realiseable situation - esp. when I'm deliberately measuring movement on a test bar which is prob 150mm away from the centre of rotation and thus magnifying the 'problem' perhaps worse than it is.
 
Rod - Perhaps my mis-terminology, but I'm refering to backlash between the saddle and bed caused by the gibs not being tight enough - what you call rock; rather than the longitudinal backlash between leadscrew and half nut. I'm trying to eliminate all the rock, whilst keeping free-and easy slide travel. What you and Ady have pointed out though is that this may not be possible (esp. on smaller machines), so I'm best off loosening the gibs a touch and then locking as and when machining permits / dictates the need.

Edited By Beagle on 15/06/2011 15:29:46

Pat15/06/2011 16:12:10
94 forum posts
1 photos
Hi Beagle
 
Play in the dove-tail slides leads to poor surface finish and chatter marks. The slides do indeed require a spacial oil - my preference for small machines is Rocol Ultraglide - it is expensive - some of the other slideway oils are too viscous for small machines. I have no connections to Rocol other than being one of their satisfied users!
 
To add to Rod's tip replace all the gib screws with grub screws with the cup ends and drop a single steel ball down the tapped hole so that the ball distributes the force evenly through the dimple in the gib to the dovetail slide. I use lock nuts on all the screws and use a knurled socket with the allen wrench to lock and unlock each screw during adjustment. I have a saddle lock that is independent of the gibs for use when I want to keep the saddle stationary.
 
If you use two hands to apply the feed then the Gibs can be reasonably snug but you will not be able to spin from one end of the lead screw to the other easily. IMHO the traverse from one end to the other of the lead screw with no feed applied should be just possible but requiring effort.
 
The use of tapper gibs is full described in detail on the web and is one of those enhancements that can wait to another day or when I have nothing better to do!
Regards - Pat
jomac16/06/2011 11:58:18
113 forum posts

Its a good idea using ball bearings next to the gib strip, but on some lathes there is just not enough thread left to fit a round ball as well. As for a gib lock, George Thomas describes in one of his books, how he made a very good lock screw, I adjust my gib screw, first by tightening the screw with a very small screw driver and only just past finger tight, then using a larger screw driver to stop the screw turning any further, nip up the lock nut, do these one at a time, lastly with the cross slide screw disengaged see if the slide moves easily and smoothly, (some lathe will not allow you to do this), if it does not slide evenly, start again, by loosening one gib screw at a time to find the culprit. Its a slow process, but you only have to do it once in a blue moon.

John Holloway.

Beagle17/06/2011 12:45:30
21 forum posts
Pat, John,
 
Re: your comments about a BB between the gib adjust screw and the gib - how / why does this improve things? Common sense is telling me it probably should, but I'm just missing that vital link somewhere!
 
My Myford has the standard gib screws fitted (whatever Myford supply). Now I've not checked the ends of these yet to see the shape, but I'm guessing they'll be the commonly-available conical (pointed) or flat, rather than domed. You mention that a BB against the gib distributes the force better, but is it not still approximately similar to the pressure that a grub screw applies? - afterall, in relation to the whole gib area, the contact points we're talking about must be little more than 1-2mm^2 in either case?
 
I did try adjusting the gibs the other evening (although I was short of time so will try again properly soon). What I noticed was that the rock felt by hand / seen by eye increased from barely observable to a noticeable movement just through backing off gib screws by a minute amount - maybe only 1/20th of a turn. This itself seems over-sensitive to me, so I do wonder it the end profile of the grub screws is non-uniform, so that'll be my next check.
 
I may have some small bb's knocking around, so I'll perhaps try your tips and see if that improves adjustability.
 
Thanks guys,
- Clive
blowlamp17/06/2011 13:07:29
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1530 forum posts
98 photos
You can fiddle with the screws 'til the cows come home and still not be happy.
The best way of adjusting the slides is to nip the adjusting screws down so that movement is just a little tighter than you need and then tap the slide (on the same side as the adjusting screws) with a mallet or hammer + wood etc to reseat everything until you get a nice even movement.
 
It's how it's done in the factory
 
Martin.
Roderick Jenkins17/06/2011 14:29:33
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2138 forum posts
586 photos
Clive wrote: My Myford has the standard gib screws fitted (whatever Myford supply). Now I've not checked the ends of these yet to see the shape, but I'm guessing they'll be the commonly-available conical (pointed) or flat, rather than domed.
 
Not on my S7, they are beautiful black screws with the end reduced to the core diameter finished in a hemisphere. This thread has shamed me into taking out my locking screw and shoving a BB down the hole. There's quite a bit of inverse snobbery about Myfords on this site but it's little touches like this that make me grateful to have been able to buy a good s/h one.
 
 
Rod
blowlamp17/06/2011 15:26:36
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1530 forum posts
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Sorry chaps, but I can't imagine what improvement you could expect by putting ball bearings in there.
 
If you're sure the lathe isn't worn, then I'd suggest you keep it simple and adjust the play out with the screws as designed and provided by Myford.
 
Because I find locking nuts infuriating, the one thing I do on my own stuff is remove them and apply a bit of that blue thread lock to the adjusting screws. That way they become stiff to turn, but remain easy to adjust.
 
 
Martin.

Edited By blowlamp on 17/06/2011 15:32:31

Roderick Jenkins17/06/2011 15:57:59
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2138 forum posts
586 photos
I guess it might avoid distortion of the gib by having the screw bite into it but probably it just makes the adjustment smoother.
Pat18/06/2011 16:24:42
94 forum posts
1 photos
Hi Martin
 
The ball at the bottom of each gib adjusting screw makes a better match with the dimple in the gib strip and that makes the adjustment a little easier particularly if cup ended grub screws are used that is all. I also think that if the gibs are just drill point dimples then they are a lot better than using screw points.
 
My personal preference is to pin the gibs in two places with loose dowels and use with plain gibs with no dimples. I also replace any gib that causes concern with hard steel ones and rely on the dowels to hold it in position and the balls to exert pressure from the grub screws. IMOH the advantage is in that the dimple and the screw point or hole and parallel shank on the screw approaches let the gib float about too much. .The nuts on the gib screws can be half nuts not full nuts and by using a modified box / socket to fit the nut it is easy to nip the screw with the nut.
 
Regards - Pat
Beagle21/06/2011 19:14:25
21 forum posts
Posted by Roderick Jenkins on 17/06/2011 14:29:33:
Clive wrote: My Myford has the standard gib screws fitted (whatever Myford supply). Now I've not checked the ends of these yet to see the shape, but I'm guessing they'll be the commonly-available conical (pointed) or flat, rather than domed.
 
Not on my S7, they are beautiful black screws with the end reduced to the core diameter finished in a hemisphere. This thread has shamed me into taking out my locking screw and shoving a BB down the hole. There's quite a bit of inverse snobbery about Myfords on this site but it's little touches like this that make me grateful to have been able to buy a good s/h one.
 
 
Rod
 
Rod - I stand corrected! After your comment I pulled the gib screws on mine - they are indeed the dome-ended black finish bolts as you describe. I backed them off a smidge (maybe only 1/20th turn) but the difference between tight-saddle and sloppy-saddle is sooo fine.
 
Think I'll try Martins tip on blue thread lock and a subtle mallet 'whomp' next time!
 
I've alleviated the problem slightly by fitting a marginally larger diameter handwheel over the weekend - this allows me to keep the saddle snug(ish) but without making smooth one-handed end-to-end feeding too difficult.
 
Cheers guys.
 
 
,21/06/2011 21:07:15
41 forum posts
1 photos
Hi
On my Boxford the grub screws tighten up on little domed head spacers and the dimples in the gib strip that they fit into appear to be made with a ball ended slot drill of the same diameter. This is obviously a variation on the ball bearing idea.
I have no idea why they make them like this but they are certainly easy to adjust with plenty of 'feel'
 
Phil
John McNamara22/06/2011 13:21:04
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1331 forum posts
122 photos
Hi All

Hi Beagle

Grey oil?. Unless the lubrication film is perfect; and this is almost Impossible, there will be metal to metal contact from small asperities; (projections on the bearing surfaces) rubbing against each other causing wear. We have to live with this with all machinery. However when I have adjusted a slide for a good fit and the load increases without me changing the gib, it is a signal that all is not right. Is it because the bearing surfaces have galled, raising up the surfaces via small scratches, or could it be dirt has got in and is jamming and possibly damaging the surfaces? In both cases it is worth investigating. Most likely it is fine particles of metal that have worked their way in with the oil.

When you take the slide apart rub a white cloth along the way, you are almost certain to find small silvery flakes.

On The subject of oil a slide way oil containing Teflon works wonderfully Molybond makes one, I am sure there are others. I use it on my lathe particularly on the bed ways it’s a big old lathe and it makes working it much easier. And it does not wash off with coolant although I don’t use coolant on the lathe.

A while back I developed a CNC stitching machine. It made quilted bedding, we had a constant problem with the rotary hook (That takes the thread from the needle) galling due to continuous operation and overheating, several hundred dollars to replace. Teflon Oil in this case light water white machine Oil with Teflon added saved the day. We never had a problem after that. Where before the hook used to get roasting hot, hot enough to evaporate the Oil it never got above warm. The Teflon forms a coating on the surface, I got it from an industrial sewing machine supplier.
 
Cheers
 
John

Edited By John McNamara on 22/06/2011 13:31:38

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