Facing Error

Good idea. Pretty sure I don't have any bar-stock big enough in my junk box. I feel the urge to go shopping coming on...

Dave

As an inexperienced lathe user I've followed this thread with interest, but I'm struggling to understand the concept of the workpiece rocking in the chuck such that it produces the convex face. I can't imagine how rocking could induce a consistant tapering effect on a rotating face

The only ways I can think of producing a convex face are either the spindle axis or the cross-slide being misaligned ( which should result in consistency at least ), or flexing of the workpiece, tool, toolpost, topslide or cross-slide, which might be inconsistent.

Or I may have completely misunderstood the problem

Mike.

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Fair point, and well-made, Mike

I don't think we have actually seen solid evidence that the dishing is consistent, but I presume it is.

... The 'mechanism' would be broadly similar to the bending of the plate, which I hypothesised earlier.

[at least that's my story]

MichaelG.

So the topslide locking has got it down from 0.23mm to 0.08mm over 55mm. If you make this the side that goes against the flange which is not much more than 100mm dia then you are only looking at maybe 0.03mm over 25mm and a concave surface will pull up tighter I wonder if it is going to cause any problems.

I believe it won't matter in this application, and it hasn't mattered on anything else I've made either.

I wouldn't worry about the error if it wasn't so big and I'm intrigued as to the cause. Not made much progress but I'm about to fit the 4-jaw and face a bar. If it's good, I've probably had a chucking problem. Like Farmboy I can't visualise how the chuck could cause it, but thinking about it makes my head hurt. The symptoms imply the tool path isn't parallel with the headstock and the DTI suggests the tool tapers in towards the headstock in a straight line.

No proof of anything yet.

Dave

Did you face off from the bore out or from the OD in?

If from the bore out, Is it possible that you didn't take the backlash out of your topslide before taking your first facing cut?

Steve.

Both Steve, but mostly outside - in. Pretty sure it's not backlash because the bore allows plenty of room for overshooting and taking up backlash on return.

I did something stupid earlier: as Ian suggested I faced-off inside to outside with HSS. Just as I finished the door-bell rang and a delivery distracted me. When I got back I thoughtlessly removed the backplate for milling without first running the DTI over it. Arrgh!

Unlikely to get back to it tonight.

Dave

While you have the part out chuck up some round bar ( biggest dia you have ) and give the OD a skim over 3 or 4 inches of length just to confirm the lathe is turning parralell .

What are the saddle gib adjustments like - particularily the rear gib ?

Ah, sorry Steve, I misunderstood - with you now. Yes, that was part of the problem and locking the top-slide halved the error.

Might be able to have a little play before bedtime. As XD says a quick check the lathe is still cutting parallel is worth doing. It was OK last time I checked but ...

Cheers,

Dave

If the part was slowly working its way out of the chuck due to poor grip and the draft taper you would get a concave face.

It is not cutting convex, DTI getting pushed in as it goes towards the edge is concave.

Edited By JasonB on 13/08/2018 19:43:12

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Oops I missed Mike's typo [if that's what it was] when I replied.

... I usually do better than that.

Thanks for highlighting the concave, Jason.

MichaelG.

Oops! Yes, a typo . . . nearly got away with it though

If the part was working its way out, causing the concavity as Jason suggests, then Dave's latest facing cut from centre out should result in convexity, or at least flatness, and prove the case . . . but I'll be surprised if it does.

Mike.

It seems like the best procedure would be to follow the usual one for machining a chuck backplate (or faceplate) and now that the main face has been roughed out, machine the thread and register surface so it can be screwed on to the lathe spindle for final machining of the face and OD. No possibility of movement in the chuck etc then. (Although you would think that if the job had moved enough in the chuck to cause a major curve in the face, it would have fallen out altogether?)

Edited By Hopper on 14/08/2018 01:55:59

Hi, as it happens, I faced up a piece of cast iron cut from an old bearing plummer block for an 80 mm 4 jaw. I found no significant out of flatness at all. First checked it with a straight edge.

Then with a DTI and the error was not much more than the width of the pointer. This was done on my Boxford lathe.

I'm wondering if SOD's back plate has become progressively harder from the inside to the outside during its manufacture for some reason, which could attribute to the concaving, just a thought.

Regards Nick.

Again, we need to be sure that we are clear that mounting a dial indicator on the toolpost or cross slide and then sweeping the pointer across the freshly faced surface does not give any indication whatsoever of flatness. The dial indicator is following the same path the cutting tool just took. So if the tool cut a surface with a 5mm dish in it, the dial indicator will follow the same path and show no deflection of the needle. (Providing gibs are tight, topslide is locked and no other random slop or movement is allowed.)

The straight edge test is good.

I have one of those insert holders like the one SOD used to face the item. I only use it as a chamfering tool because the tip moves when trying to turn with it. My tool holder and tips are Sandvik so not a cheap set up of poor quality.

.

I stand to be corrected, but: I think that would be 'a little tricky'

Dave appears to have one of these lathes: **LINK** ... which are listed as having a flange fitting chuck.

https://www.warco.co.uk/metal-lathes-metalworking-lathe-machine/302996-wm280v-lathe.html

I am not aware [may have missed the disclosure] of the purpose of the job in hand, but I don't think it's just another regular backplate.

MichaelG.

Not cheap, but surely poor quality if the insert moves?

I don't have this problem with mine, which cost less than Sandvik.

Neil

As I've pointed out a few times, this is factually incorrect.

Martin.