Bantam - warped compound gib

I wasnt completely happy with the feel of the compound slide on my colchester bantam, so disassembled it for cleaning and adjusting. It is in excellent condition with one exception, the gib is slightly warped. I assume this happened during manufacturing as a result of machining the angle with residual stress in the material causing it to distort. Surprised it has been working as well as it has, although it wasnt evident until I loosened the bolts.

What are my chances of straightening it? Haven't yet measured it but suspect .005", possibly as much as 0.010" over 6". The main bit that concerns me is that it has 4 large holes in it to bolt it to the compound with cap screws so worried it wont bend evenly.

.

Very small, I would think

Probably better to re-work it, now that the stresses have relieved.

MichaelG.

Have you tried applying a straight edge againsst the light so as to see whether the bend is a uniform curve or localised?

I would think that with a good vice with straight jaws and an understanding of the bend profile, you've got a good chance of improving it considerably, even if you can't annihilate the bend completely.

I think its made from cast iron so you would probably snap it if you try to bend it.

Alan

Hello Mark.

My knowledge of Bantam compound slides is minimal so my reply is based on three assumptions.

First, I assume the gib is trapezium shaped in cross section. Second, The gib is cast iron as suggested by Alan Jackson. Third, You have access to a surface plate.

I would suggest that you scrape the gib flat. You don't say whether it is bowed along the length of the sliding angled face or the gib to compound slide mating face. Either way, I would suggest that both faces are scraped to flatness, then there should be no other stresses or distortions introduced on re-assembly.

It is a suitable method for both cast iron or steel (whatever your gib is made of) and as a slow but 'gentle' process for correcting the distortions, you are in control of the process all the way through, whereas machining the distortions out is a little more 'aggressive' and may not remove the bow completely.

One other thing you may wish to consider - if the gib is BENT straight - is it possible that the one or two of the gib fixing holes will then be out of alignment with the mating hole in the compound slide itself.

Best regards to all.

Alan

Thanks all

I think it is steel, but could be wrong.

It has 3 square sides and one angled, as opposed to two sets of parallel faces. I suspect it has bowed in 2 axis, not just the one. I have a surface plate but have yet placed on it to measure it. May do that this evening.

The mounting holes are out of alignment now (although not so much that the screws wont go in), bending it back would put them back in the right position.

Is the gib concave or convex to the dovetail? I would think that the screws would tend to straighten it whichever way round it is. 0.010" out of straight in something so thin and flexible is probably nothing to worry about, and might even be an advantage, especially if it is touching the dovetail at the ends first.

Edited By old mart on 11/11/2019 16:02:05

Convex (touches the slide in the middle and the stationary bit only at the ends). It is anything but thin and flexible. The only way the screws will straighten it is to wind in the adjusters at either end, which will place pressure on the middle until it bends, causing it to go very tight in the process. In fact it is so tight in the middle even without adjusting the screws that i cant reassemble it.

It looks like you will have to take a bit out of the middle. Try rubbing it on some 180 paper on a flat surface, holding the gib in the middle. This is where a surface plate, or a piece of float glass is very useful. Keep an eye on the rub marks made by the paper. If the gib is 6" long, then rubbing sideways on a 2" wide strip of paper would leave the ends untouched until it got noticeably straighter. A sheet of paper held down with double sided tape will flatten anything rubbed on it to within 0.0005" per 20 inches length.

Truing to straighten it might well break it.

Hi Mark,

It would probably be quicker to make a new gib strip than to try to straighten your bent one. You could try heat and cold to straighten it, heat with a blow torch on the concave side then cool on the convex side with a sopping wet rag; a bit of a dark art which I have only seen done once.

Hope this helps.

Peter.

I've almost managed to straighten it (G clamp). It was out by 0.015" in the centre and was bowed in both axis and even has a slight twist. It was more like 7.5" in length, not 6". Still some tweeking and no doubt some lapping/sanding, but getting there.

A new one was best part of £100 plus p&p and plus vat. I should be grateful it's still an option I guess.

Edited By Mark Davison 1 on 12/11/2019 18:41:38

If you have pretty well straightened it, you might as well see if the twist can be reduced, by using a vise and an adjustable spanner. The only way to get it settled completely, assuming it is steel and not brittle, is to go a tiny bit past straight, and then push it back by hand.