SC3 gib strips

After all the useful posts I have been playing around. I managed to mill a fraction off the tip of the gib which made it fit snugly into the dovetail, but somewhat surprisingly if I reversed the gib it fitted perfectly which suggest it isn't a parallelogram in section. Either way nipping up the cone point screws did what was needed, the gib sat exactly where you would expect it to.

However I still couldn't get a satisfactory compromise where rigidity was maintained but ease of operation enabled, so I tried deepening the points in the gib, bad mistake, now the adjustment tipped the gib out of alignment. I think that this a partly due to drilling into a surface at an angle and on reflection it is the pointed dimple surrounding the cone point that maintains the correct angle to the dovetail. The brass gib now had so many wrong holes in it that the scrap bin beckoned. I reverted to the original steel gib but the other way around. This worked as described above so I thought maybe a centre punch would be adequate to maintain alignment of the gib, it does but by no means ideally and I have come to the conclusion that you should do no more than screw the adjusters tight to form a dimple and then adjust for operation.

The net outcome of this is that I will obtain a replacement brass gib from Arc, BTW you do have to polish out the machining marks to have a chance, mean while the steel one will do, but the optimum is not really satisfactory, rigidity is compromised by operability. I think the truth of the matter is that the manufacturing tolerances are sufficiently lax for the odd rogue to slip through, maybe I really am expecting too much. Thanks though for all the help.

Regards

Martin

One tip that I learned from the Minilathe user group many years ago was to slip a small ball bearing (about 2mm) into the dimples on the gibs and use cup point grub screws as the adjusters.

Along with gently lapping the bearing surfaces with a fine diamond file and dressing the (warped) gibs flat this really improved the cross slide/ top slide smoothness and rigidity. Also the brittle iron saddle gibs only lasted a few months before they cracked but the brass replacements have lasted ten years.

* Danny M *

It's interesting that not all mini-lathe owners encounter this problem, at least not in the severe form Martin and others describe. I didn't for one. It's true that finding the sweet spot took time, but I managed despite being cack-handed. My gibs show no signs of trying to rotate, and the action was smooth. I got a slight improvement by polishing the steel gib but I don't think it was worth the effort, and I never bothered trying brass.

I wonder if the shape of the gib screw end, and the shape, position, angle and depth of the dimples, and the shape of the gib all interact. Perhaps slight differences between these explain why users report good, acceptable, and bad performance. I suppose it confirms what we already know, which is that mini-lathes are an inconsistent breed. As such, it would be instructive to compare a 'good' mini-lathe side by side with a 'bad' example to see what the causes are. I'm out of the game unfortunately: my mini-lathe had to go when I upgraded.

Dave

typos!

Edited By SillyOldDuffer on 03/08/2017 11:20:58

Edited By SillyOldDuffer on 03/08/2017 11:21:18

I though you might appreciate an update. Having stripped the complete saddle assembly it was obvious that both the dovetail of the cross slide and the surface of the gib strip were somewhat rough so the brasso came out and much rather mundane to and fro has polished up the rubbing surfaces nicely. They felt quite sweet when dry so with lubrication they are much improved. This was all done by hand without the leadscrew in place, and what remains is a slight tight spot which suggest a machining problem that I haven't quite polished out.

Having put the leadscrew back in and adjuted the nut properly it is much improved with almost complete lack of movement of the crosslide, except of course where it's meant to. I am much heartened by this, and if I can get the top slide as good it will at least make the lathe reasonably usable. At some point I am going to make a new gib strip that fits accurately into the space, whereby the adjustment screws will just be to snug it up and the strip won't then be able to rotate at all..

Regards

Martin