Facing parallel between centres.

Indeed it could, with the high point front or back it could correct nod issues r placed at the side tram, though with a swivel head I'm not sure if it's worth worrying about as you would tram head to table anyway.

Well I have read and re-read this entire thread. What machine are you turning the riser on? Is it a new and calibrated tool room lathe? Or a Chinese hobby lathe or maybe a Myford?

Before all the discussion about concave faces, turning between centres etc etc it would be reasonable to ask if the face plate to which the riser was bolted has been clocked to determine if it has no run out. Then of course the inevitable question has been missed.........just how accurate do you expect it to be and what tolerences are you working to?

I am amazed that as "engineers" nobody has even attempted to get to the bottom of the component tolerancing and the machine and even the machinists capability.

Many thanks for replies.

Looking at Hopper's diagram I see where I went wrong in my thinking - my mental model was that in an ideal world where the cross slide is perfectly perpendicular to the lathe axis it would have to cut a plane perpendicular to the lathe axis. Obviously (now) not true across the diameter - I did say I have problems with 3D visualisation! It's embarrassing as I know full well that the lathe faces slightly concave under the best conditions and understand the how and why of that. I knew there was something suspicious about my reasoning, just couldn't decide what and missed the wood for the trees. I blame the guy who taught me about conic sections!

I finished the riser by bolting to the face plate with M8 studding though the spindle and tailstock support:

I'm happy, in the light of comments, that I got as good as half a thou. On a Far Eastern  lathe!

Robin

Edited By Robin Graham on 14/09/2020 00:31:51

Edited By Robin Graham on 14/09/2020 00:34:42

A clue was the opening line "I've posted recently" a quick look at Robin's posts will see it started with this then this to this where lathe and accuracy of faceplate was established early on and the suggestion of doing it between ctrs was first raised, now it seems most are suggesting this way won't be accurate

Edited By JasonB on 14/09/2020 08:21:08

.

But, in his home workshop, Robin has already done better than that ... and is now exploring the limits.

Bravo !!

MichaelG.

Yes exactly, he got to about half a thou over six inch diameter four inches from the chuck, plenty good enough in my opinion.

Best thing to come out of this discussion was the suggestion to use a jubilee clip as a driver where a conventional driver cannot be used. To turn a part to obtain good parallism use a substantial cast iron sub plate bolted to the faceplate and then lightly skimmed flat, I use a scrap rotary table casting then bolt the work to this subplate , for brake discs they are bolted to the subplate and then skimmed on both sides at one setting, face plates are a useful tool though they will distort if clamps are over tightened, or the work mounted to the plate is not flat, Face plates in practice cannot be made too thick or rigid as they would reduce the lathe gap clearance when used on general work.

So am I correct in thinking that you started this thread after you had actually completed the job

Martin.

I have had my squared paper out and exaggerated the centres being offset in both directions, if the saddle is set to an exaggerated look in it faces concave with both offsets, turning end for end gives me parallel faces that are faced slightly concave, I don’t see how if the saddle is looking in how the centre position can effect it cutting anything but concave.

Mike

"Facing parallel between centres" is the title of this post. Failure to get a good result is hardly a claim that "it does not work". Especially if the methodology of this process is not fully understood.

I do wonder how much of the of the aversion to this "between centres process", is based on disturbing the tailstock setting. It has always been a big No, No, with some individuals. The same is true when it comes to moving the topslide, when anyone advocates the set over top-slide method of Screwcutting.

Those who know their machine tool history, will know, " Dead Centres" were the basis of all turned work at one time. It was not until later on that the "Live Mandrel" was developed and the faceplate became available. Dead Centres are still used in cylindrical grinding, for most work. The lathe Mandrel on the lathe in the picture above was no doubt finished this way.

Regards

Gray,

parallel faces that are faced slightly concave

I object to the description of parallel being applied to anything other than lines or flat planes. Parallel planes have rotational and translational symmetry.

I'm perfectly happy to describe the periphery of the two ends of the cylinder as being parallel when concave (or convex) by not the face.

regards Martin

How about 'evenly spaced' or just 'offset'?

Martin.

.

[ my emboldening ]

But surely that is what distinguishes them as planes [a special case of the spherical surface]

MichaelG.

A while back in one of the magazines was a discussion about producing some form of solid square where both faced and had to be parallel. The method was to bolt down on the miller.....  Not the lathe. 
machine one end flat then invert and machine the other next check the dimensions for parallel and if there’s any error pack up the low side with a shim and re-machine. Repeat until u get the correct result.

this workpiece would need slots or holes in each end to clamp it down and the work would need to be stood on strips clamped to the table . There would need to be stops to locate the x and y positions and after checking for parallel before repositioning, this should be achieved without any significant rotation of the billet. The clamping holes in this case will not be any problem

Edited By Zan on 14/09/2020 12:38:32

Yes, exactly. For example I would not consider two concentric circle as having parallel circumferences or stacked cones either. I just think it's important in discussons to use accurate terminology so we all talk about the same thing.

Evenly spaced or offset as suggested works better.

regards Martin

Yes, if you read it Robin was seeking clarification about the advice previously given, he said nothing about wanting to remachine it between ctrs.

.

Interesting [‘though not worth debating]:

You say ”Yes, exactly” and then describe how we disagree.

... It obviously worries you, and I respect that.

MichaelG.

Hi Michael

I'm agreeing with you that it's the translational and rotational symmetry that distinguishes planes which is what you wrote.

regards Martin

.

Sorry, Martin ... I must have read it in haste

[feeble excuse] : I had just been summoned to go out for the afternoon.

Happy to know that we agree

MichaelG.

Well said Graham. Moving-jaw chucks were an accessory at one time. Lathes would be supplied with centres and drive dogs.

One should be able to produce exceptionally good parallelism working between centres.