Lathe facing convex or concave

What I want to know is how all this will affect the 2mm step on the shaft that I am making? After all most of us only have a model making lathe of approx. 3.5" swing and seldom cut big diameters. I can see that as a test it can prove something so that remedial action can be taken. 2 x the error, its concave or convex and measurable as an error surely.

Clive

This explains it.

Gary

Thank you very much Gray for letting me know how to check the straigthness, I can blame my instructors for not letting me do that test, but they are now all long gone. Problem is the longest rule I use is 50cm and I only have a small surface table of also unknow accuracy. Niko.

Edited By speelwerk on 25/03/2014 09:31:52

Thanks again Graham, I am sure that that will be more than accurate enough for the work I do, sadly I do not have acces to such equipment. No doubt someone on the forum can sort it out, but I do not think he lives near me in the Netherlands. Niko.

Edited By speelwerk on 25/03/2014 10:06:40

Niko,

You mentioned, earlier, that you had bought several of the rules/straightedges, so ...

A simple check would be to put any two of them edge-to-edge and check for gaps.

Repeat this for every combination/permutation and you should have a pretty good data-set.

... With the option to correct them by filing, if you have the patience

... [not unlike Mr Whitworth's method for making surface plates]

MichaelG.

Yes Michael, that is what I have done when I had them at home, not knowing that that is Mr. Whitworth's method, Niko.

HI Clive,

I've been facing 2 3/4" blanks to make wheels for a driving trolley. It doesn't matter that they won't sit flat (although it is irritating). However, I will have two oversize slices of steel left at the end, and I want one to become a bench block. It will need good flat and parallel faces if it is to sit still and be useful for temporary jigs etc.

Neil

Neil

Assuming you propose to drill a hole through the centre of the bench block then you could at the same time counterbore the base a few thou deep to leave a rim of 3/8 or so for the block to sit down upon. That's how my proprietary bench block's base was done, as is the base of my height gauge. So it's possibly common commercial practice, to avoid the risk of a convex base.

Michael

Michael,

That was part of my plan, although it doesn't give me a flat top, but I've been offered a chance to get it surface ground

Neil

OK so after all that, a friend of mine has a big english lathe a proper one (DSG from memory) with a clutch and gearbox an everything (like power cross feed). This machine is facing off concave to the extent that it is buggering up some two inch washers he is trying to make.

He is not a machinist - where does he start pulling this into some sort of reasonable alignment?

Regards

Steve

First thing for him to check is that the saddle is not moving during the cut, and of course that the gibs are set for low play.

If it is just a few items to get done rather than a longer term solution then it should be possible to swing the topslide round to do the facing cut, though will take a bit of fine adjustment to get dead true.

Correcting wear is much more difficult.

Is that quote from ETW?

Very interested in this Badgers Arse spec,this might go some way towards explaining my table problems.!

I do have a badger come to visit from time to time,I,ve never seen him,but he leaves his mark. N o need to dig holes when they are around,nothing stops them. I dont fancy trying to measure his arse since I suspect he might object to it,and when they get annoyed,it,s time to hit the trail,fast.

Well as no real experts have suggested anything yet, I'd try to find out what gives under pressure by pushing and rocking the various bits and pieces. Nothing should move! In no particular order:

• Tool bending or not clamped firmly
• Loose cross-slide gibs
• Worn cross-slide
• worn headstock bearings
• headstock bearings need adjustment

Dave

Or possibly, given that he is not a machinist, he is facing a washer with a topslie/compound slide which is not quite zeroed, and thus not cutting perpendicular to the axis of the lathe?