Facing Error

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So perhaps we need to pause for thought ...

... What exactly is Dave's measurement illustrating ?

My current guess is that the nascent faceplate was deflected by the force applied for cutting, and sprung back when that force was removed. ... A very light finishing cut, with a very sharp tool, would quickly confirm or refute.

MichaelG.

The faceplate is nearly 18mm thick, but I wonder if the casting is rocking in the chuck. It seems firm enough, but the chuck isn't gripping a perfect round section. Now I think of it, I wish I'd used my 4-jaw.

When domestic duties permit, I'll face off with a sharp HSS tool and report back.

Ta,

Dave

I think Michael G is on the right track. A light cut with a sharp tool is in order before conclusions can be drawn.

As for the original measurement with a dial indicator mounted on the cross slide; it's measuring nothing much. Certainly not the amount of dish in the faced surface as the dial indicator is following the original tool path, which could be anywhere, depending on wear, looseness etc.

BUT, repeating the test with a known flat (or near flat within Schlssinger limits) faceplate will give a meaningful measure of how far out of alignment the cross slide may be sitting to the lathe axis.

The test suggested by Perko way back when is again more accurate, if you use a ground parallel strip held in the four jaw. Set it so that each end gives teh same reading on a dial indicator when rotated 180 degrees end to end. Once set like this, it is dead square to the spindle axis. So then repeating the test of running the cross-slide mounted dial indicator along it gives a very accurate read of cross-slide to spindle axis alignment.

It this test shows that the cross slide is out of square with the spindle axis, the next step is to check the spindle axis is parallel with the bed ways by a simple turning test piece test, which will measure tapered if the spindle//bed alignment is out.

Can't say I've ever really worried about it, possibly due to insouciance.

However, I've just measured the smokebox door I've been working on recently. There's a flat ring on rhe outside, a gap, and then a central boss, all aligned. So not complete facing, but all done at one setting. A rule shows some light, but I can't get a 1.5 thou feeler gauge in anywhere under the rule. Diameter of the door is 210mm.

Andrew

Why is Blowlamp's comment being ignored?

Measured mine when we did the ML7 Wide-Guide conversion to check the new guide was holding the cross slide nicely square to the unworn new guide shear at the back. Here is a pic of the test described above., using a homemade, hand-scraped parallel bar in the four-jaw chuck. Each end of the bar was set to a 0 reading before taking a sweep along the full length.

Because the error is so great that even half of it is about ten times what it should be?

Or maybe it's because we all like to pee Blowlamp off by ignoring his comments so we can laugh at him behind his back and congratulate each other on being superior human beings? (Hmm, the last bit seems unlikely.)

Note that the parallel bar held in the four jaw as in the pic gives the actual error when sweeping along its full length though.

Edited By Hopper on 13/08/2018 11:03:43

Perhaps I have misunderstood (quite likely) but why hold the backplate in a chuck. When I did a new backplate for a collet chuck on my WM250 I simply bolted the backplate on and turned the face. Are you making the whole backplate from scratch? I agree with the comments that you cannot measure the flatness of the surface using a dial indicator held in the toolpost and traversing the cross-slide.

By the way the item seems to have started as a backplate and morphed into a faceplate!

John

Edited By Journeyman on 13/08/2018 11:15:36

Not sure if this is of any value,but I found a somewhat similar while facing workpieces and I found that the height of the tool had a lot to do with it. May not be the problem with Dave,s job,but worth looking at.

Another thing that influences facing is surface speed. It's high at the outer diameter, low in the centre area. So this can affect the way the tool cuts. Sometimes its good to stop and bump up to the next speed pulley halfway through (or tweak the VFD speed controller as you go if you live in the 21st Century.)

Well you don't really need to go to the trouble of the parallel bar once you've done the facing cut as you will have just generated the perfect reference surface to check against.

That is: A DTI, secured to the cross slide, and swept across the near side of the backplate will show the deflection/play during the cut, and when swept across the far side will show how large the error is. So no rule, feeler gauges or straight edge are required.

After the first cut, try taking another at the same setting, preferably from the centre out, you most likely will get a little sliver off out toward the periphery.

Ian S C

Looks like Dave is using a raw sand cast backplate so can't bolt it straight on. To me the wrong face is being delt with first.

If I were him I would now turn the casting the other way round and clean up the back and machine off the boss which is not needed and bore the hole. It is not worth wasting time trying to get a flat cut on a rough casting held on a tapered (draft angle) boss which could be moving..

Once you have it bored and the mounting stud holes drilled and tapped fix it back onto the spindle and take your truing cut with a new insert and then see how flat the surface is. Only then start to worry.

Edited By JasonB on 13/08/2018 13:14:26

Firstly, I'm not ignoring anyone - as if I would!

Three things slowing me down: I've avoided moving the backplate yet because the job needed me to keep register until I finished machining; having to keep cleaning up; and the age it takes to finish a facing cut because the banjo is set for 0.03mm per revolution. Particularly long winded with HSS because the rpm is a third of what carbide can do. (Though HSS produced a better finish)

Anyway, I faced off as suggested by Michael with HSS and got exactly the same result. I'll try a second outgoing cut as Ian suggests after lunch.

In the last face-off I put my DTI on the side of the slide to see how much it moved during cutting. Not enough to explain the facing issue - less than 0.03mm over 150mm.

Thanks for suggestions and observations - I'm reading them all.

Beats working for a living!

Dave

You need the DTI magnetised to the cross slide - not the lathe bed!

Once on the cross slide, run the DTI on the operator side of the backplate - this will show deflection/play when you faced up the part. Then run the DTI on the other side (far side, same surface) of the backplate - this will show twice the actual error you are getting.

Martin.

Edited By blowlamp on 13/08/2018 13:35:21

SoD,

I really don't think that last measurement of yours is meaningful. As the cross slide goes past the DTI, all you have discovered is the flatness of an unimportant face on your cross slide. Imagine if there were corrugations on that rear face you've just measured, how would it affect the machining of the tool tip? Frankly, not one iota. It does show, though, that actually you have a nicely made machine. It is of interest, but of no consequence to the possible squareness or not of the cut.

Regards,

Richard.

That's exactly what I'm doing, and I think you're right about turning the wrong face first! Trouble is I had to start somewhere and it seemed easiest to hold the boss in a 3-jaw, face off, bore the register and turn the outer edge round. How best to hold a raw casting isn't obvious to me. As far as I can tell the 3-jaw is holding the boss firmly, but it's certainly not best practice.

Part two is to transfer the backplate to my milling machine and drill & tap the backplate mounting studs, also drilling three clear holes to take the chuck studs.

Part three is to bolt the clean face of the backplate to the lathe's spindle, face off the dirty side,and finish by cutting a register for the chuck.

Even though the casting rocking in the chuck might explain the symptoms, I can't change course as Jason suggests because the register is already bored to size. When I move the backplate to my mill I'll be able to hold a test bar in my 4-jaw and test facing again as advised.

Dave

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That's useful, Dave

It now suggests [to me] that the error may well be caused by the chucking, rather than the machine itself.

... keep it coming !!

MichaelG.

A misunderstanding Martin. When I measured the facing error the DTI was positioned on the saddle. This set-up was intended to show if the slide drifts to one side as it's moved along the saddle's dovetails. It appears not, though Richard makes a good point that the test is dubious because the face is cosmetic.

Keep it coming, I feel we're getting closer as possibilities are eliminated and my dodgy measuring technique is improved thanks to criticism.

Dave

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So ... Turn a 'stub mandrel' to hold it by the register, and re-face the plate.

a very, very, slight taper on the mandrel would probably hold O.K.

... alternatively incorporate an expanding collet arrangement.

MichaelG.

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http://www.homews.co.uk/page551.html

Edited By Michael Gilligan on 13/08/2018 14:10:41