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Gib Screw Locking Methods

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Dr_GMJN31/10/2020 23:18:43
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1225 forum posts

I’m doing a lot of fettling on gib strips on my ML7 at the moment - saddle, cross and top slides.

With nothing else to interfere with the feel of the slide (felts, leadscrews and rack etc removed), the tolerance between acceptable and locked is so fine that the action of locking the nuts is more than enough to totally knacker the adjustment. It’s literally any further movement of the screwdriver clockwise after contact will lock the slide (especially with 4 or more screws).

Despite the method being used forever, it seems to me it’s an unnecessarily difficult way of doing it: For a start, the pitch of the screws seems far too coarse for the job, and the locknuts constantly work against you. It invariably turns into some kind of balancing act of torque with a spanner and screwdriver, and I’m always wondering if it’s the optimum setting.

What’s the issue with simply coating the screws with threadlock, setting them and waiting until it sets - job done?

Or use bolts that can be expanded like a mandrel with a central Allen grubscrew? In fact any method that doesn’t involve almost certainly accidentally rotating a carefully adjusted coarse screw would be good.

I actually can’t think of a worse method than a locknut.

Steviegtr31/10/2020 23:23:02
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2244 forum posts
311 photos

Have spent much time doing the adjustments on the super 7. It is fiddly but doable.

Steve.

Ady131/10/2020 23:49:18
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4740 forum posts
715 photos

I've recently started trying out finger tightness adjusting

I unscrewed my locknuts on my drummond until they were at the top, where they got tight, and now use them as finger tightening adjusters for my grubscrews

Got fed up of constantly tweaking with tools

Ady131/10/2020 23:53:35
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4740 forum posts
715 photos

The best system of course is a tapered gib

If you can get a spare saddle casting you can make your own

Hopper01/11/2020 01:14:09
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5505 forum posts
137 photos

I've never found it to be a problem. Maybe you are trying to set your gib clearance too tight? There is supposed to be clearance there, a tiny bit.

It helps to make sure the ends of the gib adjusting screws are a nice smooth half-round dome shape where they bear on the divots in the gib strip. Sometimes you have to spin them in the lathe and file them to shape if they are standard pointed or dog point ends. And make sure the divot drilled in the gib strip has not had a groove worn in it over the years by the end of a pointed screw. Redrill until the groove is gone if so.

GH Thomas advises in one of his books to drill and dowel the gib strips with two tiny dowel pins that are a press fit into the casting and a neat sliding fit in the gib strip but I did that and could notice no difference in operation so dont think its worth the bother.

You shouldnt have any trouble if you follow the standard procedure. Adjust and lock up the two end screws first until you get the movement you want. Then adjust the middle screws to just come in and touch the gib strip and lock them up.

You have to hold the screw with a screwdriver while doing up the lock nut to prevent further movement. A right-angle screwdriver works best for this, giving finer adjustment by benefit of the shank sticking out at 90 degrees to the screw so a little movement at the far end of the shank gives very fine movement at the screw.

If you continue to have trouble with the gib screw tightening up with the locknut, set the screw a tad loose and let it tighten up to the final setting when you nip up the lock nut. Note too that the lock nut only needs nipping up, not 100 foot pounds of torque etc.

It's the same procedure used for setting tappet adjuster screws and locknuts on millions of car and motorbike engines over the past 100 years or more and it works well if done properly. Just takes a bit of practice maybe.

My old Drummond (M-type) does not have lock nuts on the gib screws so I put teflon thread tape on them to hold them in position after being set. I definitely would not put Loctite etc on there as you then cant make further adjustments as needed from time to time.  Locknuts are the best solution, which is why the later ML7 has them.

Edited By Hopper on 01/11/2020 01:18:34

oldvelo01/11/2020 05:59:56
276 forum posts
54 photos

Cup point set screws with a ball bearing <o> . Tighten all Grub screws remove one dress up the dimple in the jib with an egg beater hand drilll. Repeat on all screws then dismantle and clean up. Reassemble with the ball bearing in first sitting on the jib. Adjust to suit.

 

Edited By oldvelo on 01/11/2020 06:00:37

Edited By oldvelo on 01/11/2020 06:01:50

Dr_GMJN01/11/2020 09:41:25
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1225 forum posts

Thanks guys. The screws have hemispherical ends. Some seat in conical drillings, other (adjacent to the seated ones) don’t.

Not saying it can’t be done, just that it seems a needlessly painful way of doing it. The thread pitch alone seems wrong for achieving fine adjustment.

Hopper01/11/2020 09:49:06
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5505 forum posts
137 photos

Fine threads in cast iron tend to crumble and strip so there's a reason they use coarse pitch screws.

SillyOldDuffer01/11/2020 09:50:41
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7562 forum posts
1680 photos

Another of those apparently simple jobs that take practice. Starting out, bright-eyed, bushy tailed and inexperienced I found adjusting gibs difficult just as Doctor G and Steve describe. Obviously the problem was my crude lathe. Now I'm with Hopper - what problem? It's easy.

Same lathe, same spanner, same screwdriver - what changed was me. Setting gibs is a matter of feel and, like using a micrometre properly, it only comes with practice. Some are better at it than others.

Rather a lot of machining in home workshops requires the operator to 'play' machines like a musical instrument. First his lathe has to be 'tuned' by setting the gibs, and rpm etc, then operated with more or less finesse to make the 'music', ie produce acceptable objects. The operator is important. Skill compensates for many machine shortcomings, which is why good work is done on worn lathes and basic equipment. Buying a Stradivarius won't make me a concert violinist, and when it fails because I'm not a musician, nor will upgrading to a Guarneri.

Dave

 

 

Edited By SillyOldDuffer on 01/11/2020 09:51:41

Clive Foster01/11/2020 10:04:43
2837 forum posts
103 photos

I'm surprised that Hopper found dowelling the gib strips, GHT style, made no difference to performance as I've always found it effective as part of "Clives patent older slide refurbish" programme. Its perhaps indicative that my Smart & Brown 1024, a lathe of the very highest constructional quality, uses conventional gibs with ball ended adjuster screws and twin location dowels on both cross and top slides rather than the taper gib more commonly found on high end lathes of similar vintage.

I think the adjustment sensitivity is due to minor wear on both gib screw points and gib recesses. Probably not helped by manufacturing tolerance variations in the relative positions of screw and gib recesses. As built there is a little clearance so the gib can find its natural position, effectively floating on the adjuster points. Because the gib is floating oil drag when the direction of travel is reversed causes it to shift sideways by a teensy amount at each reversal. A tendency exacerbated by any inexactitude in the relative positions of gib strip recesses and adjuster screw points. Over many years of operation the multiple reversals eventually wear both gib screw point and recess a little so, eventually, they only fit really well in that one position. Hence clean, accurate adjustment is rendered impossible over that last fraction of a thou. Basically the difference between smooth, shake free movement and tight is around half the thickness of an oil film. Not much.

I first became aware of this when sorting out a ShouthBend whose gib strips and gib recesses hand been badly mullered by ham fisted attempts to adjust it involving severe over tightening of the screws. The on the bench the gib could be seen wobbling around as the screws were turned.

My method involves making a new gib as thick as can be got into the gap. I carefully fettle the upper edge so its upper edge sits in intimate contact with the slide casting. Clamping the whole slide assembly together with the gib pushed tight up against the lower surface of the slide dovetail I use a sharp slocombe centre drill to make conical recesses for new adjusting screws using the existing tapped holes as a guide. This jig drilling approach ensures everything is lined up properly. The dowel hole gets drilled at the same time.

Like Hopper I use ball ended adjuster screws, I imagine the oldvelo cup point screw and ball is equally effective on a doweled slide where side to side float is impossible. Maybe the two components could theoretically create a smidgin of clearance for float as compared to the one piece screw but whether that is of any consequence I know not.

Done with care the ball in cup recess system makes it much impossible to lock a lubricated slide without going well into the scary torque range of adjusting screw tightness. Ball ended screws in conical recesses tend to push the gib up into its recess against the underside of the slide so everything is much more rigid. I imagine the extra contact area of ball in cone and careful fitting is why Hopper didn't need the dowel.

The standard floating way works well enough until things wear and is much cheaper to make. Smart & Brown could afford individual fitting, Myford couldn't.

I always adjust things with the screw removed. Pushing back and forth by hand gives a much better feel for setting and helps spread the oil. Several sweeps between adjustments are desirable to let things settle. 5 to 10 usually. As is re-oiling. Which makes for a messy job. Well worth taking the time to get it just so right from the start as, properly set, the slides will go a long time before re-adjustment is needed.

Clive

Dr_GMJN01/11/2020 10:14:47
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1225 forum posts

Thanks Clive - I’ve just bought new gibs for the ML7. The saddle gib isnt like the original, it has no drilled dimples, so this mornings job is to do that. It’s a flat surface, not a dovetail. The new gib has two small holes in it though. No idea what they’re for.

Only two of the 5 screws have dimples in the gib (on the original). I would expect that unless they are aligned with the screws with absolute precision, you’d be fighting movements between those screws too.

The new cross slide gib strip is dimpled, and the dimples seem to have been drilled at an angle to suit the dovetail.

so, we’ll see how it goes.

John Hinkley01/11/2020 10:28:58
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1187 forum posts
391 photos

If you fancy an experiment and your eyesight is better than mine, try the trick of cross-drilling the adjustment screw and inserting a piece of nylon cord - such as is used in strimmers.  Cut overlength it acts like a nyloc screw. Leave the locknuts out of the equation. It worked for me on the retaining screw on my radius turning attachment. Use new screws and replace the old ones if it doesn't work out.

John

 

Edited By John Hinkley on 01/11/2020 10:30:18

Bazyle01/11/2020 10:44:21
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6041 forum posts
220 photos

A right angled screwdriver as mentioned by hopper is helpful. Best is socket head screws and the long tail of the allen key. Make a note of the position of the allen key as if a clock minute hand before you loosen them. Then take each one in turn up to a gentle lock and note that time. The difference shows you the 'slack ' you had and you can calculate that into thou if you like. Then you can either back off you a known amount.

ega01/11/2020 10:54:27
2265 forum posts
188 photos
Posted by John Hinkley on 01/11/2020 10:28:58:

If you fancy an experiment and your eyesight is better than mine, try the trick of cross-drilling the adjustment screw and inserting a piece of nylon cord - such as is used in strimmers. Cut overlength it acts like a nyloc screw. Leave the locknuts out of the equation. It worked for me on the retaining screw on my radius turning attachment. Use new screws and replace the old ones if it doesn't work out.

John

Edited By John Hinkley on 01/11/2020 10:30:18

Or use a simple drill jig.

Martin Connelly01/11/2020 11:03:42
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1899 forum posts
205 photos

I did what Bazyle suggested and replaced slot headed screws with hex sockets (including the tailstock adjustment) so that a long series hex wrench gave repeatable positioning when loosening and tightening lock nuts. In the case of the tailstock adjustment two hex wrenches makes adjustment easy compared to the two simple slots the original screws had.

Martin C

Robert Atkinson 201/11/2020 11:38:06
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1090 forum posts
20 photos

+1 for using Hex socket grub screws and holding with allen key.

You could go one stage further and put a metric thread in with a solid thread insert. his could be one desigined for CI or a repair type.

Robert G8RPI.

Douglas Johnston01/11/2020 11:53:39
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760 forum posts
34 photos

+1 for dowels to locate gibs, it made a huge difference on my Speed 10 lathe. Hex grub screws to replace the standard screws and use some method of eliminating the backlash on the threads of the screws so you get very precise movement of the screws. I used a small dab of hotmelt glue on the threads of the screws to eliminate the backlash but there are other techniques for this. Provided the anti-backlash technique makes the screw movement a little stiff you don't even need locknuts.

Doug

not done it yet01/11/2020 12:27:52
6343 forum posts
20 photos

If using hex grub screws, do allow for sufficient stick-out past the locknuts - the ends are the weak point.

I had one break off flush (well, slightly below flush) when loosening the adjuster’s locking nuts.🙁 Not my doing - it was the first time I had wanted to dismantle the machine. It had never needed adjusting while in my possession.

Dr_GMJN01/11/2020 14:05:10
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1225 forum posts

I’ve got hex. Socket grub screws on the mill. It’s the same issue, but using different tools. In fact I found it’s even worse unless you use hex tool bits - a standard Allen key has so much movement in torsion it’s not even funny.

I’d have thought using a hex grub screw as the expander in a split bolt would be the easiest method - a bit like an expanding mandrel; you’re locking the thread without pulling it back.

old mart01/11/2020 16:27:46
3347 forum posts
208 photos

This is how Tom Senior mills lock some of their gibs. It would work with the multi screw type, but not a tapered gib for obvious reasons. Drilling the gib could be a problem if it is hard and you don't have any solid carbide drills._igp2678.jpg

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