|Kiwi Bloke 1||18/08/2018 23:40:08|
|88 forum posts|
The parallel gibs on cross-slide and top-slide on one of my lathes are particularly difficult to adjust properly, and produce inconsistent results. I'm planning to check, scrape and dowel the strips (GHT-like) and to attend to the adjustment screw seatings on the strips, but I'm not sure what's the best solution.
The gib strips are located at present by a single dimple, mating with the end of the adjustment grub screw. It's the usual problem of an inadequately-located gib strip, moving relative to the screw. The other screws bear on the plane face of the strip.
An additional problem is that the screw responsible for one slide's adjustment causes the gib strip to bind against the 'roof' of its female dovetail. One might expect the screw to 'push' the strip 'down' the face of the dovetail, but, clearly, the strip's dimple and the end of the screw bind, pushing the strip 'up' the slope.
I've read previous threads on gib strips, but still would like to ask questions.
I assume that the long sides/edges of gib strips should never be forced into firm (and therefore friction-ful) contact with the dovetail 'roof'. Ideally the gib strip should contact only its adjustment screws, dowel, if present, and moving face of dovetail. Correct?
Ball-ended screws, or balls between screw and strip, bearing on the plane surface of the strip, have been advocated, but is this sensible, since it offers no gib strip location or restraint? Would dimples help? If so, what shape?
Dog-point screws bearing in larger-diameter square-ended 'pockets' - if the strip is thick enough for this?
One central dowel, or two, each towards either end?
I've seen surface grinder slides in which the gib strip adjustment screws bear perpendicularly on the face of the strip. This seems a better idea than the customary oblique bearing. Might one consider conical-point screws, such that there is (hopefully) line contact between gib strip and the side of the cone?
Other/better ideas? Yes, I know, taper gibs...
2956 forum posts
I found on our ML7 that rounding the ends of the standard gib screws into a hemisphere (simply by spinning in the lathe and filing) made an improvement.
I put pins in the gibs of my old Drummond some years ago, GHT-style, but did not notice any particular difference in their performance.
One narrow edge of the gib strip is usually in contact with the top surface, as well as the main wide surface. You might try knocking the corner off the gib strip with a file so it does not jam in the corner but bears on the flat surfaces.
1046 forum posts
Just been through all this myself after making a longer and beefier cross slide for my Denford Starturn 8 CNC lathe.
Keep the gib strip free from touching/binding on its other unused faces.
I drilled & reamed the adjuster screw holes and made new pushrods with 60 degree end faces to match the angle of the dovetails.
I also pinned the gib strip at both ends of the slide via each of these pushrods by first drilling into the end face for the insertion of the dowel before cutting the 60 degree angle - That is: drill the hole, cut the angle and then glue in the dowel. This gives good support to the whole gib and the angled pushrod ends give a larger surface to bear against.
Once you've made your new gib strip, bend it into a slight bow (say 0.5 - 1.0mm over a 300mm length) such that it contacts the middle of the saddle dovetails first. Then adjust from the inside to the outside. The bow acts like a spring to ensure the fixed dovetails are always pulled into contact with one another, even when slightly worn.
I also did away with locknuts in favour of a touch of thread-lock.
Edited By blowlamp on 19/08/2018 00:36:20
Edited By blowlamp on 19/08/2018 00:43:37
|David George 1||19/08/2018 07:40:16|
502 forum posts
Hi Kiwi Bloke I have replaced many gib strips and would make a strip to have dimples like a drill point to suit a grub screw with a point to suit. I would make a new strip with the angles on by clamping the strip of gauge plate in to the slide with a piece of round steel along the length using toolmaker clamps and or bolts from the other side when one edge has been cut debur to fit slide rotate and cut other side same to come flush or just under. Whilst still clamped in the slide spot the dimples with small drill the same size as thread taping size. Take out of slide mark one end to enable to get in correct way round and clamp to a strip of steel tip the head of mill drill over to angle of the dovetail. Using a stiff short spoting drill drill a spot to same diamiter of screw. I always take the very sharp point of the grub screws which suit the drill point angle.
|Ketan Swali||19/08/2018 09:44:26|
|1010 forum posts|
Hi Kiwi Bloke,
Before you consider scraping or making something else, consider making vertical slots at the places wherever your dimples on your current gib strips are. Most of the time this solves misalignment issues. If it is a newish lathe, better to avoid OTT on scraping... the gibs... as most of them look like crap and they are meant to.... and they will bed in over time. However if you want a bedded in feel, just go over it with some wet and dry Emory mounted on a block and rub. Also don’t over do it. Try the simple solutions first, is would I would suggest.
Ketan at ARC
|Douglas Johnston||19/08/2018 09:54:54|
514 forum posts
On my Myford Speed10 lathe I used the dowel in each end approach and found it very good at keeping the gib properly located. I took the cross slide off, clamped the gib in place making sure that it cleared any rubbing at the top, then drilled and reamed a 3.5mm hole through the cross slide and gib at each end. The only reason I used 3.5mm was because I had a stock of 3.5mm roller pins from an old bearing.
Edited By Douglas Johnston on 19/08/2018 09:58:11
|Clive Foster||19/08/2018 10:06:12|
|1473 forum posts|
Having done the GHT ball ended screw and dowel located gib strip modifications half a dozen or so times on various machines I can confirm that it works very well indeed.
Before starting work on the modification its important to clean all surfaces properly so that bare metal shows, including the officially non mating ones. On an older machine, especially one subject to infrequent lubrication, often with improper lubricants, there will be significant build up of crud on all surfaces. As the build up is uneven, generally being greater over the unswept and less regularly swept portions of slide and gib, you will never be able to get things sorted properly until its all removed. Not as easy as one might hope as the stuff tends to be very difficult to shift with no universally effective solvent. No substitute for elbow grease. Just keep going (and keep cursing) until its all gone. I reckon that on the older machines I've done, say 30 + years, about half the "wear" was uneven crud! I sometime think that if machine tool makers had ever figured out how to get an even deposit of 30 year aged crud stuff hardened beds and chrome ways would have been considered both unnecessary and inferior.
I always run a hacksaw blade into the corners and ensure that there is clearance when trial fitting.
Dog point screws won't work properly. Spherical ended ones as specified by GHT working in conical sockets are correct. The top of the gib must contact the slide surface. If it doesn't the system is not stable. Especially if you only use one dowel. On my 1024 VSL Smart & Brown used spherical ended screws and made the gibs to contact on the slide surfaces despite using two dowels which, theoretically makes surface contact unnecessary. I figure if its good enough for Smart & brown its good enough for anyone. Its important to remember that the objective is adjustable equivalent to a solid surface. If the gib is free floating as with standard point and dimple construction it can tilt which isn't good fro strength. Only allows adjustment to take up clearances. Simple spherical end screws in conical recesses aren't much better as there is little leverage against tilt.
The main disadvantage to having contact between gib and slide is that it takes much more effort to get adjustment right. The screw has to come out, everything needs liberal lubrication, and you need to push the slide back and forth by hand many times during the adjustment process until its smooth the whole way. I'd be unsurprised to be told that I use 30 or more strokes. Once the job is done its done. So long as you lubricate regularly with eh correct oils it won't need touching for years. Done right clearances can be made very small without things seizing up. I did this mod to an Abwood vice which on acquisition boasted possibly the world champion worst ever bodger made gib strip. A section out of the welded side of ERW square tube, bent and with no straight edges! Having made nice new steel gib strip of the correct thickness, just slim enough to slide in with maybe 2 or 3 thou clearance, I found it essentially impossible too get the gib tight enough to lock the vice up.
The GHT thumb screw locks are great. Do fit them. One day my 1024 will get a set.
Its worth remembering that by the time you decide that something must be done about not so good gibs things will be in such a bad way that anything half sensible and carefully done will be an improvement. After all, despite admittedly being less than idea, simple pointed screws in dimple gibs work at least adequately for a fair few years from new.
|Kiwi Bloke 1||19/08/2018 11:15:33|
|88 forum posts|
Thanks everyone for helpful tips and good ideas. I think non-tapered gib strips are a triumph of practicality over theory. Perhaps that could be said of a lot of engineering. The many detail design variants suggests that the search for the perfect solution continues... I hope discussion on this thread helps anyone who is struggling with the wretched things.
To add a little detail about my 'problem' machine. It's a well-respected, very small lathe, manufactured far from the orient. I won't identify the maker because I don't wish any implied criticism to tarnish their reputation. The machine is unworn and crud-free. I can't access it at present, but, from memory, the vertical height of the top-slide gib strip is in the order of 5mm, and the adjusting screws are M3, so we're dealing with awkwardly small stuff.
Thinking theoretically about gib strip location, stability, etc. makes me wonder the things ever work acceptably. It seems wrong for adjustment screw forces to act slantwise (at an angle to the surface of the strip), rather than normal to the strip. If the screw bears into a dimple, the force on the gib will be deflected 'down the slope' as the screw is tightened, won't it?. A ball-ended screw, or an interposed ball, bearing on the slanting, plane back surface of the gib should apply (and react) force along the sphere's radius, at the point of contact, and hence normal to the gib. Isn't this what we want? Are dimples a bad idea? Perhaps I'm confused - which way is the strip pushed when it reacts sideways slideway forces - 'uphill'? The more I think about this, the confuseder I get...
It seems to me that the gib will tend to be deflected (or simply just fall) 'down the slope', so should its lower edge be arranged to bear 'properly' on the moving slide? In other words, should it be properly finished as a bearing surface?
2956 forum posts
If it's not, you might was well let the big secret out of the bag so those with previous experience on such machines can share it with you. Otherwise, yet once again, we are reduced to groping in the dark hazarding best guesses on a Top Secret project.
|Kiwi Bloke 1||19/08/2018 11:56:24|
|88 forum posts|
Hopper, I sympathize with your comments about groping in the dark. I'm not going to be drawn on the lathe's manufacturer because I feel protective about decent manufacturers who face competition from the criminal junk-producers of the orient. My comments about its design could be construed as criticism, and I don't wish to sully the manufacturer's excellent reputation.
I'm not suggesting that all engineering equipment from the orient is junk - far from it. The problem is identifying and avoiding the junk, before beer tokens are lost. What with VW and its neighbours, and Optimum, and others, you just don't know who to trust, do you?
My questions, whilst applicable to the particular lathe, are intended to be general as well. Gib strips are a problem for many people, on a disparate range of machines.
3328 forum posts
I've got tapered gib strips on a cross slide from a pultra and they really are way better than the silly system of multi tweeking bits on a standard straight jib
Yet another to-do job *sigh*
|Peter G. Shaw||19/08/2018 14:06:27|
911 forum posts
My milling machine had absolutely awful gibs - rough, triangular dimples which didn't seem to be aligned correctly, so I set about improving them.
Here's what I did:
Made new strips out of mild steel. Cut to approximate size and then scraped what was to be the sliding surface to be as flat as you want. There is no need to do both sides, although depending on the steel, one needs to guard against warping. I then used the female part of the slide mounted in the vice as a guide. I mounted the embryo gib against the female side of the gib with a rod and two clamps to hold it all in place. I then fastened 2 sheet steel covers over both the horizontal sliding surfaces of the dovetail, about 1mm thick and filed across both to get the angle. Having done that, I turned the stip over and added additional packing between the already filed edge and the bottom (top?) of the dovetail thus ensuring that the subsequent filing would ensure that the strip was indeed less than the total space of the gap - if that makes sense.
Next I clamped the new strip in place, and drilled through both the female part of the dovetail and the strip and fitted a pin, as per the GHT idea (I think).
For the adjusting screws, I inserted ball bearings between the screws and the gib strip. To do this, whilst the strip was clamped up as above, I used a hollow adjusting screw to guide a suitabe sized drill and created V shaped dimples in the strip thus ensuring that the new dimples were correctly aligned. New adjusting screws with a flat end to bear on the ball bearing and that was that.
It all worked very nicely. Was it any better? Dunno, but at least I tried, having followed all the info I'd gathered from this hallowed forum.
I seem to think that the idea of using brass has been raised. I've a vague thought that somewhere I found that provided all the above was done, and the dovetail itself was cast iron, then brass probably won't give much of an improvement.
Peter G. Shaw
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