Drilling addition holes for gib screws has caused distortions in the slide

Hi. Whilst I had my carriage apart I thought I'd add extra gib screws as this seems to be a common modification. I used a pillar drill and sharp bits and all went well but I've just noticed that the bearing surface of the carriage is no longer flat where the holes have been drilled.

Have I done something wrong?

In one of the images below you can see the distortions where I've started to flatten them with a flat stone.

Edited By Philip A on 12/11/2020 15:45:20

Looks like compound slide? Three of them look OK.🙂

Tapping the threads so close to the edge probably caused the metal to bulge slightly where it isn't constrained.

The question is; why on earth would you want to drill and tap the extra holes? It might be a common modification, but I've never heard of it before. I don't see that the extra screws add anything. The slide is fully constrained with three screws, assuming the outer ones might go past the end of the fixed slide. The top slide on my lathe seems quite happy with three screws.

Andrew

Bad luck, I would never have guessed that cast iron would do that. The light stoning that you have carried out should have remedied the problem, though.

If you have a surface plate, a sheet of 600 wet and dry and a couple of rubs will sort it out easily. Before getting the 24" cast iron table, we used a glass plate made from thick float glass. It is about 12" square and is an excellent poor mans plate. The one we have is still in use for smaller jobs.

I did this mod on my SC4, well worth the effort

Jim

I think the more usual reason for drilling extra holes would be for dowels or clamp screws ie not adjustment screws.

I suppose if the gib is on the thin side extra adjustment screws might be useful.

The originals

My couple pence worth, I am not a pro, you,

You may have gone undersize on the holes, I'm not blaming anyone

I think the idea for the extra fixings is to clamp up the compound without having to adjust the other three. I get a perfectly acceptable finish with the three. However I have found with a GIB type tool post and solid base I get far more rigidity, better depth of cut, chatter and surface finish. I only use my compound when I need it which is not that often, heck with the solid base I thread with none of the 30degree nonsense, it chunks out. A far superior mod.

Woody.

Edited By woody1 on 12/11/2020 21:37:48

Exactly this. I always got away with it in the past.... force of habbit, oh well, lesson learnt.

Something that I've mentioned before in the context of bed warping is that cast iron behaves very like chipboard . Tight hole close to an edge , in goes a screw , result bulge on the edge.

Following what Lee says above you should insert the screws before finalising the corrective action.

I don't think any harm has been done, I would not have been content with only three screws. The high points are easy to deal with and its a compound, which gets less used than other parts of the lathe and can be kept a little on the tight side anyway. This NOS compound I picked up recently has six, and also two dowels, right near the edge.

Edited By old mart on 13/11/2020 17:51:16

May I ask the purpose of the dowels please?

Also are slot fixings better, I find Allen grubs strip of there hex pretty easily.

Edited By woody1 on 13/11/2020 18:29:03

.

They provide longitudinal location ... which prevents the gib ‘riding up’ on the tips of the screws, which would effectively tighten the gib.

Very nicely explained by the much-revered Geo. H. Thomas

MichaelG.

I took that compound to bits to replace the 70 year old oil/grease and the gib was securely attached to the body by the dowels. There is only about 0-0001" backlash throughout the travel, but I will be tightening the gibs as the whole assembly is too free moving for my taste. Apart from the dowel holes, there are no indentations for the screws present.

Smart & Brown gib fixings are flush with the surface, the nuts sit in counterbores. I have a tubular spanner for the nuts and use either a little screwdriver or a hex key on the gib screws.

Edited By old mart on 13/11/2020 21:05:52

Sorry excuse my ignorance,

So if I locate my gib with said dowels I will gain smoother travel throughout my compounds travel?

How do I install the dowells? Big hammer (joke). I'm flumxed on how to do.

Thank you old mart to for your reply and replies on my other threads.

Sorry for hijacking this thread.

Woody.

I don't think dowels will make much difference to the working of a gib. There is usually some method to stop the gib sliding endways, such as a countersink in line with one of the screws and a matching conical end on that screw. You would have to dismantle the compound to find out.

The dowels are stoned to a slight taper so that they are tight in the body of the slide but a sliding fit in the gib. Thats how I did my Myford topslide, you only need two.

regards Martin

I've read about the dowels but never seen a picture or anything to show what they look like or how they work.

I cannot tell you how tight the dowels are that Smart and Brown fitted to the model A are, I have taken two compounds and the crossslide to pieces and have never disturbed the gibs.

 You would have to get suitable diameter and length dowel pins, then drill and ream the slide with the gib secured to it for exact alignment. The reaming would have to allow a sliding fit for the dowels which could be Locktited into the casting. If the gib is hard, then a solid carbide drill the same size as the dowels could be used without reaming. Seems like a lot of effort to me.

Edited By old mart on 14/11/2020 17:25:37