Facing Error

[blowlampParticipant @blowlamp]

Why is Blowlamp's comment being ignored?

[HopperParticipant @hopper]

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.

[HopperParticipant @hopper]

Posted by blowlamp on 13/08/2018 10:53:47:

Why is Blowlamp's comment being ignored?

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

[JourneymanParticipant @journeyman]

Posted by SillyOldDuffer on 13/08/2018 10:35:39:

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

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

[larry phelan 1Participant @larryphelan1]

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.

[HopperParticipant @hopper]

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.)

[blowlampParticipant @blowlamp]

Posted by Hopper on 13/08/2018 11:01:44:

Posted by blowlamp on 13/08/2018 10:53:47:

Why is Blowlamp's comment being ignored?

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

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.

[Ian S CParticipant @iansc]

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

[JasonBModerator @jasonb]

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

[SillyOldDufferModerator @sillyoldduffer]

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

[blowlampParticipant @blowlamp]

Posted by SillyOldDuffer on 13/08/2018 13:21:07:

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

[richardandtracyParticipant @richardandtracy]

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.

[SillyOldDufferModerator @sillyoldduffer]

Posted by JasonB on 13/08/2018 13:13:57:

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

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 13/08/2018 13:21:07:

… 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. …

.

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.

[SillyOldDufferModerator @sillyoldduffer]

Posted by blowlamp on 13/08/2018 13:34:09:

Posted by SillyOldDuffer on 13/08/2018 13:21:07:

…

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

…

Martin.

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

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 13/08/2018 13:42:05:

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.

.

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.

.

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

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

[SillyOldDufferModerator @sillyoldduffer]

Posted by Michael Gilligan on 13/08/2018 14:08:00:

Posted by SillyOldDuffer on 13/08/2018 13:42:05:

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.

.

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.

.

**LINK**

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

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

[FarmboyParticipant @farmboy]

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.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Farmboy on 13/08/2018 15:09:22:

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

.

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.

[JasonBModerator @jasonb]

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.

[SillyOldDufferModerator @sillyoldduffer]

Posted by JasonB on 13/08/2018 15:43:26:

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

[fishy-steveParticipant @fishy-steve]

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.

[SillyOldDufferModerator @sillyoldduffer]

Posted by fishy-steve on 13/08/2018 18:29:35:

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

[fishy-steveParticipant @fishy-steve]

Posted by SillyOldDuffer on 13/08/2018 19:01:06:

Posted by fishy-steve on 13/08/2018 18:29:35:

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

Dave,

I'm talking about backlash in the topslide not the cross slide. The tool can push away with backlash in the topslide. On saying that you would only get a concave face if you were feeding from the bore out. Convex if facing from the outside in.

Steve.

[I.M. OUTAHEREParticipant @i-m-outahere]

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 ?

[Author]

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