Jumpin' Gibs

Hi friends,

I got me a case of the jumpin' gibs,

I'm making a miniature milling machine and the vertical plane uses a dovetailed slide which holds the knee by a gib strip, this has 4 grub screws to nip up the slide, i spotted through with dimples on the strip and rounded off the screw head, but whenever i tighten them up, it tries to hop out!

Perhaps the screws are touching close to the edge of the gib and it's throwing it off. I'm thinking of placing two hold down screws ontop of the exposed area of the gib to keep it in it's place but, do you think that will work?

Michael W

Are you sure that the gib strip is clearing the V's that make up the slide - the sides usually need angling.

John

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The late great GHT used to recommend dowelling the gib strip to prevent unwanted movement.

Russell.

I have a vertical slide on my Stent T&C grinder but it doesn't do that. The Model Engineers Workshop Manual, George Thomas describes retaining gibs (albeit horizontal) from lateral movement with a pin as I recall to improove the action of the slide.

There is no reason they should move other than the one I mentioned and having the dimples in the wrong place. That aspect needs to be rather accurate.

John

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Make your dimples deeper. Once the point of application is more than half way through, the gib will behave.

Neil

Ah it's ok i've fixed it.

The dimples are nearly completely aligned, i sandwiched the whole thing together with a G clamp to spot through and make sure they were right in place.

Here is a crude illustration to show you whats going on;

The writing is a little small but i hope you can see it,

The gib is dislodged from it's seat whenever i tighten up the grub screws (it's pushed up and away).

My solution was to make two steel "finger plates" as i call them. with 1 hole in the middle and another at the top. One is tightened right up against the body with a socket screw and the other is left to apply pressure to the top of the gib to keep it in place. Unorthodox but seems to be working and i can't think of another way to make it stay there.

Michael W

Ps. i only just caught sight of your post Neil, but it makes sense i dont know why i didnt think of that. I'll know in future what i should do!

Edited By Michael Walters on 13/04/2016 13:33:52

My lathe cross slide gib is dowelled as others have suggested. Smart and Brown original design. Dowels are about 2.5mm.

Martin

It is not clear from your sketch, but the usual arrangement would be, that the shaded part "labelled cast iron body" was carried over to slide on the other part (with the red arrows on it), so the gib would be surrounded with metal and unable to rotate on the set screws.

Frank

If your gib arrangement is actually as drawn, the screws are pushing the gib up an "open" slope - I would have expected to see the screws on the LHS as drawn, pushing the gib into the "closed" dovetail ?

Nigel B

I agree with you Nigel, it's the wrong way about.

Mark

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I also agree.

This image is from Boxfords site. The image is from a lathe but the principle is the same.

Bigger image here **LINK**

Nick

You're all right, and you've identified the source of the problem, thank you.

But the solution isn't quite so easy, the gib is pushing onto an open planed vee, the cross slide image shows the Vee ways both closed. Im pretty sure i've made the drawings to spec, why they have designed it this way i'm not sure, it isn't possible to insert screws on the left hand side because the column is dovetailed both sides. I'm coping on my solution for now.

I think what you mean is that the form of the bed dovetail needs to be reverse so that the gib is pushing onto a closed surface.

For eg. On the same model, there's a small bed stop that needs to be 20.54mm long, with a slot most of the way down the body to 17.75mm, then an M5 hole needs to go through below that, but i've only got less than 3mm of room left? it's not a critical part but the measurements just don't add up, it's very frustrating when the way it's been drawn just doesn't make sense in reality and you end up scrapping something.

Michael W

Edited By Michael Walters on 14/04/2016 09:51:04

Perhaps you could show us the original drawing to clear up any confusion. Where did that design come from?

Russell;

I suspect the issue isn't helped because the grub screw taper angle is similar to the angle of the dovetail. What hole shape would you need to ensure a good mate between the grub and the dimple? Also, there are various grubs available, with conical, parallel pin and flat ends. Perhaps the best solution would be to machine a flat face on the gib (perpendicular to the grub) before then dimpling it?

As suggested above, a couple of location dowels or (even better) grub screws with a pin end would at least stop the gib sliding under pressure.

Another way to locate a gib is used on the Colchester cross slides, shown below for interest. The gibs are items 9, adjusted with grubs 59-356 and held / locked with items 45-204.

An additional advantage of this approach is that you can lock the gib in place after adjustment using the vertical cap head bolts. Conversely, if you don't loosen said bolts, I can tell you from experience that trying to adjust the gibs can be frustrating!

Yeah i think a pin would be a good idea, my sherline machine has a similar arrangement for the gib strips, with no dimples though, just an interference fit, made out of some black polymer plastic. They say by the time it wears out it would've had years of service out of it and cheap to replace.

I think i wouldn't be allowed to put the drawing on here as its a hemingway kit so i dont think it'd be a good idea.

Michael W

Other people have posted an image of the related parts of bought plans etc on here Michael so can't see it being a problem.

I still think your dimples don't line up 'cause the arrangement should push down not up if it does anything at all. There should be clearance at the bottom of the gib and then the screws keep it in place via the dimples.With the usual style of cross slide which this isn't there would be a need for clearance at the top as well. If the screws aren't near ball ended the dimples would need angling to suite as well.

John

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