runout after facing on the lathe, how can this be possible??
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  1. #1
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    Default runout after facing on the lathe, how can this be possible??

    Dear all,

    please assist on this, I am really puzzled.

    I have started on making a backplate, to fit a 4-jaw I recently got my hands on, on my old italian PAV lathe, threaded spindle. I will admit that I overshoot the backplate register ID, so it is not a tight fit on the spindles register, BUT, I still have the spindle's shoulder to register. So far so good.

    So, I mounted the backplate on the spindle and started facing. Of course, all I need to be true is the surface the mates with the chuck, but anyhow, I faces the spiggot as well. Not sure why, I setup to measure run out on the chuck mating surface on the backplate but first I measured runout on the spiggot face. Got around 0.03 mm or runout (more than 0.001''). So I faced the spiggot again and remeasured. Same result. Did some extra runs, increasing depth of cut, trying manual cross-feed, nothing, same runout.

    Here is a video on this:


    So, ok, it has to be the spindle bearing. Tightened the plain taper bushing a bit so that I can't measure more than 0.01 mm of radial movement while trying very hard to pull the spindle up, tried everything again, still getting the same runout.

    Checked it also on the granite surface plate. While left-hand side face of the backplate is nice and flat, opposite face, the prpoblematic one, still has the same runout as the one I am measuring on the lathe. Blue test on the surface plate was not good at all...:



    The thing is that I cannot understand how a lathe could have runout on a faced surface. I can understand taper on the face if cross slide is not perpendicular to the spindle. I can understand gaps or whatever if crosslide is very worn and has inconsistent travel. However I cannot understand runout. A lathe should face anything, no matter how crazy it turned to begin with, to a taperred surface with, ideally, 0 taper angle....And I think it also can't have something to do with runout on the spindle shoulder.


    Any ideas guys? I can't even say 'get a new lathe' since I can't identify the root cause for this...

    Thanks in advance for any help.

    BR,
    Thanos

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    Adding a video on the spindle shoulder runout, though I think that is should not have something to do with my issue...


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    Check the register dia' is running true

    If it is, lightly skim that spindle shoulder,(just enough to clean it up) fit backplate . skim that and try again

    Chances are the spindle shoulder has been damaged, either by being hit or having swarf trapped and biting in to the face.

    Failing that the spindle could be bent, but with the sound of that machine running, I think that's a real longshot.

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    My first instinct.. Your spindle is screwed up.

    I would try putting the indicator on the center of rotation on the lathe and spin it up,
    see what happens. Then maybe get a prybar in there somewhere and check for slop in and
    out (along the Z).

    That is an odd problem. At least not one I've seen.

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    Hi guys thanks for the reply,

    I would like to point out that, even if the spindle shoulder is bad, or even if the spindle is bent, one shoud STILL face true....(or, as I said above, at least face to a shallow taper).
    What is the difference between a bent spindle and a bent shaft chucked on the lathe? In any case one should be able to face true, at least that's how I am thinking right now....am I missing something?

    In other words, facing depends only on the shape of the cross-slide travel and I can't think of something that would make the cross slide move in such a way to cause a runout on the face....

    Damage on the shoulder could be the root of my problems if there was some kind of slack and movement. That is, in a specific angle, backplate has no support from the spindle shoulder so the cut is lighter and you end up with runout. The thing is that I screwed the thing in tightly and the cuts were very light....

    Any ideas more than welcome!

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    Quote Originally Posted by thanvg View Post
    .... the cuts were very light....
    Very light cuts? "Attempted cuts" those can be.

    Your blued pattern does show it worse towards lower-RPM center of the work than at outer periphery.

    Show us the cutting tool and its mount, please. This need not be a show-stopper.

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    Quote Originally Posted by Monarchist View Post
    Very light cuts? "Attempted cuts" those can be.

    Your blued pattern does show it worse towards lower-RPM center of the work than at outer periphery.

    Show us the cutting tool and its mount, please. This need not be a show-stopper.
    Hi Monarchist,

    thanks for the reply.

    Well, the material is cast iron, so I did see (hear) it cutting all the way. I was indeed surprised by the blueing pattern. I think I had deburred the outer edge with a file, lightly touched the whole face with some 400 grit emery cloth and cleaned before taking it to the surface plate. As I said before I have no (theoretical) problem with a small taper, what you mentioned as "...show it worse towards lower-RPM center of the work than at outer periphery". It is the runout I cannot grasp!

    Sorry for not being able to provide a better picture of the setup, have a look here:


    CCMT insert on 10 mm shank, AXA QCTP, minus the indicator of course...

    BR,
    Thanos

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    High and low spot always in the same location? Approach the face with a sharp tool, ever so slowly. Does it touch just the high spots as you creep in? Cray thought but if the material is harder on the high area it will put more force on your setup.

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    So far. all assume zero run out or END PLAY of spindle. Is this the actual case? Has END PLAY been measured?

    It was stated this is an OLD lathe

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    Quote Originally Posted by thanvg View Post
    Hi Monarchist,

    thanks for the reply.

    Well, the material is cast iron, so I did see (hear) it cutting all the way. I was indeed surprised by the blueing pattern. I think I had deburred the outer edge with a file, lightly touched the whole face with some 400 grit emery cloth and cleaned before taking it to the surface plate. As I said before I have no (theoretical) problem with a small taper, what you mentioned as "...show it worse towards lower-RPM center of the work than at outer periphery". It is the runout I cannot grasp!

    Sorry for not being able to provide a better picture of the setup, have a look here:


    CCMT insert on 10 mm shank, AXA QCTP, minus the indicator of course...

    BR,
    Thanos
    "10 mm"? That's what, about .45 calibre? Bit of a noodle, no?

    IF I had the problem, I'd have a 3/4" square HSS-Cobalt or Stellite cutter shimmed across the top of the compound, wedge-shimmed to height, held with a top-bridge and clamp-bolt either side "borrowed" from one of the sets for the mills. No more overhang than could be eliminated.

    That's why I have 3/4" cutters for a mere nominal 10" lathe.

    OTOH, with a 4-Way TP and ignorant 1/2" +/- or so Rex 95, and over half a dozen faceplates as well as several CI backplates, I do NOT have the problem, so...

    To be fair, if you follow earlier advice as to insuring the spindle register is de-burred and true, and the backplate-to-be has no freedom of movement, AKA "wobble", you may not really have a problem, either!

    Where is the critical fit between that backplate and the chuck?

    If it is toward the outer edge, I'd be inclined to just relieve the center, apply pressure with a live centre from the TS to insure consistent loading of the spindle bearings, take my outer-periphery cut.

    Then triple-check the fit where it actually matters, abrasive finishing permitted, fit the chuck, and move on.

    Typical backing plate for a chuck is generally more useful if bored to not less than spindle through-bore, anyway.

    Not left solid, however flat and true.

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    Quote Originally Posted by thanvg View Post
    lathe should face anything, no matter how crazy it turned to begin with, to a taperred surface with, ideally, 0 taper angle....
    Doesn't help your situation, but the lathe cross slide is not supposed to be square to the spindle axis. when you scrape them (or make them) its suppose to turn a very slight concave face, and 1/2 a thou per foot irrc. The reason is there is no 'perfect' so at some level of magnification there is a taper....and since "convex" would be quite problematic, its made to cut a slightly concave

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    Quote Originally Posted by johnoder View Post
    So far. all assume zero run out or END PLAY of spindle. Is this the actual case? Has END PLAY been measured?

    It was stated this is an OLD lathe
    Hi johnoder,

    there is runout on the spindle shoulder, around 0.01 mm, shown on video as well. But, should this matter when we'are discussing facing...?

    Quote Originally Posted by Scruffy887 View Post
    High and low spot always in the same location? Approach the face with a sharp tool, ever so slowly. Does it touch just the high spots as you creep in? Cray thought but if the material is harder on the high area it will put more force on your setup.
    Well, Scruffy887, thanks for the idea. I'll try facing another material, e.g. a piece of aluminum. I may have hit a hard spot in the cast iron, does not seem crazy! Actually, makes more sense than anything else till now!

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    Quote Originally Posted by thanvg View Post
    there is runout on the spindle shoulder, around 0.01 mm, shown on video as well. But, should this matter when we'are discussing facing...?
    !
    If its a flaw on the shoulder then I agree no, but if was the whole spindle moving in and out, then yes. If the location is the same as the run out, the coincidence seems too great for it to not to be connected with the problem.

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    Quote Originally Posted by Mcgyver View Post
    If its a flaw on the shoulder then I agree no, but if was the whole spindle moving in and out, then yes. If the location is the same as the run out, the coincidence seems too great for it to not to be connected with the problem.
    Hi Mcgyver,

    I think I am dealing with some runout on the spindle shoulder, as seen in the video, BUT you are right: I have to measure runout on the face of some other shoulder on the spindle as well and check my thrust bearings also. Yes, this makes perfect sense as well, provided that my intermediate pulley has some wobbling. So, if my trust bearings are not ok, there could be some axial movement of the spindle caused by the belt! (not sure if the pulley wobble would be synchronized to the spindle 1:1, but it is a parameter I had not checked!)

    BR,
    Thanos

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    If you have hard spots in the casting, you should hear a change in the sound the cutter makes. I doubt that is the case as you would also see a different shade of grey.

    I'm just as stumped as you are on this one.

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    Quote Originally Posted by thanvg View Post
    Hi Mcgyver,

    I think I am dealing with some runout on the spindle shoulder, as seen in the video, BUT you are right: I have to measure runout on the face of some other shoulder on the spindle as well and check my thrust bearings also. Yes, this makes perfect sense as well, provided that my intermediate pulley has some wobbling. So, if my trust bearings are not ok, there could be some axial movement of the spindle caused by the belt! (not sure if the pulley wobble would be synchronized to the spindle 1:1, but it is a parameter I had not checked!)

    BR,
    Thanos
    No need to be 'stumped'. Stop guessing. Put pressure on it from the TS. Your chuck does not need the material at the centre. Same again with an aluminium test plate.

    Make it work for you to complete the detective work to see what's going on with your spindle, bearings, and that wobbly pulley.

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    Quote Originally Posted by Monarchist View Post
    No need to be 'stumped'. Stop guessing. Put pressure on it from the TS. Your chuck does not need the material at the centre. Same again with an aluminium test plate.

    Make it work for you to complete the detective work to see what's going on with your spindle, bearings, and that wobbly pulley.
    You are right, this is the detective work...if pressure from the TS solves it, my thurst bearings need some attention.

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    Quote Originally Posted by Rigor View Post
    If you have hard spots in the casting, you should hear a change in the sound the cutter makes. I doubt that is the case as you would also see a different shade of grey.

    I'm just as stumped as you are on this one.
    As a matter of fact I did not notice any audible abnormalities. But this applies both to a possible hard spot and axial play...I should be able to hear something in both cases

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    Is your saddle locked?
    Joe

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    I've never actually run a lathe with a bent spindle, but it seems logical to me that you cannot face a flat plane with a bent spindle. This is because of precession of the axis. To face a plane, there must be a fixed axis for that plane to rotate on.


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