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Thread: Live center help!

  1. #31
    Quote Originally Posted by Ted Reischl View Post
    My guess is that he has a draggy bearing in the live center. As he puts the tool to the work he slows the work, a draggy bearing in the live center would cause it to ever so slightly try to loosen up.
    Can't happen. As long as the lathe direction of rotation is correct for spindle turning, no way the cone loosens up because of a "draggy bearing." Even if the live center freezes/locks up solid, the lathe rotation will be such that the cone is tightened on the live center - not loosened. There must be something missing from the problem statement. - John

  2. #32
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    Quote Originally Posted by Bill Blasic View Post
    Ricc I was talking about the point in the live center that is there before the cone is attached not the point of the cone itself.
    It would appear that you do not have that live center, there is no point on the large cone, it is wide open on both ends and can be installed either way on the centers threaded nose.

    Oneway live center set.jpg large cone fits either way.jpg


    Have fun and take care

  3. #33
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    Rick, what you are telling us, is something that actually is impossible to happen, taking the way things do fit together, however in a case like this, maybe you would be better off to build a tapered cone on your live center.

    Take a good piece of hardwood and drill an opening the right size to tap tight fitting threads in it, fitting the centers threaded nose.

    Stick a pin through the hole blocking the rotation of the center and install it in the headstock spindle, turn a taper on the wood that will fit you workpieces.

    Install the center in your tailstock again and start turning, that wood should stay on and not loosen up IMO.


    Have fun and take care

  4. #34
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    Quote Originally Posted by Leo Van Der Loo View Post
    Rick, what you are telling us, is something that actually is impossible to happen, taking the way things do fit together,
    It happens. It's not impossible. I have the same center on mt Jet and it happens to me. This is extremely interesting to me too.

  5. #35
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    Quote Originally Posted by Kyle Iwamoto View Post
    It happens. It's not impossible. I have the same center on mt Jet and it happens to me. This is extremely interesting to me too.
    No Kyle, only if the live center is turning faster than the workpiece it is supporting, would this be possible, and how would that happen ??, as I say, in the way these pieces are put together and the direction the pieces are threaded and the direction the lathe turns it is not possible, or could not happen.

    The live center is not going to start turning faster just by itself, so in order to happen something else is going on, elasticity of the wood that after it was under tool pressure goes back and forth rotationally, and backs the cone off of the center ???, I don’t know, but a tighter fitting cone should make this less likely to happen I believe.

    Not much to loose but some wood and time turning a new cone.


    Have fun and take care

  6. #36
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    Mr. King, you are definitely right, my thinking was going backwards.

    Here is a thought, and someone else sort of wrote about this. The only way it can come unthreaded is for the center to be turning faster than the workpiece. This might happen when the part gets thinner, pressure is relieved reducing friction between the center and the piece. Then when the tool is applying force the center has enough momentum to start creeping. I would not bet the farm on this idea.

    All that said, I would not use a cone on a thin walled workpiece to start with. The cone is like a wedge trying to drive the piece apart. Instead, I would turn up a boss with a shoulder so the force is applied in line with the spindle when tightened. That is why I bought a live center with a threaded end, no cones. I have made up my own cones out of wood but rarely use them. Oh, and edit. . . I turned my cones with a 120 included angle so they apply more force in line with the spindle than outwards.

    This is definitely a nasty problem. I am thinking that the aluminum cones are being applied to situations that they were not intended to be used for.

  7. #37
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    From a mechanical view there is a possibility there is a crack in the threaded section. If a cracked nut is tightened onto a bolt the crack will spread and continue to turn on top of the threads until the threads are aligned again and then snap back into the thread grooves. At this very moment the nut will be loose again. Inspect the threaded section of the cone for cracks. Logistics say it can't possibly come loose in it's normal operational mode but stranger things have happened.

  8. #38
    It is possible with a real technical reason. There were telescopes with a hypercyclonic drive reduction. Incredible reductions in the order of 1000 to 1 or more. That reduction would put amazing pressure on the mug.

    Just a very slight misalignment could cause this. As an example of the basic principle... spin a coin on a surface and watch as it slows down. The coin will rotate slower and slower the flatter the coin is to the surface.

    The reduction ratio can be calculated by comparing the circumference of the coin's edge to the the circumference of the circle the edge of the coin takes on the surface. Very high reductions for the last few rotations of the coin.

    How's that for a strange explanation... So... The cone is obviously not co-planar with the lathe spindle or chuck.

    Solution... Make the tailstock rotate in the same plane as the spindle. Perhaps an accurate faceplate on the spindle and a faceplate on the tailstock. If there is any misalignment at all fix it. The smaller the misalignment the greater the reduction ratio.

    See... math to the rescue.

    clint
    Last edited by Clint Bach; 05-14-2018 at 12:38 PM. Reason: Typo

  9. #39
    It is caused by a misalignment between the two rotating axis. As the cone rotates the slight clearance in the threads allows the cone to slowly walk as the misalignment applies an uneven loading to the cone as it rotates.. In this case it just happens to walk towards the headstock. If you run the lathe backwards, it will likely cause the cone to screw onto the center.

    A possible solution would be to turn a plug with a shoulder that fits snug into the open end of the cup, and use the lathe center to hold the plug in the cup and provide support for the turning. ( no cone needed)

    Another possible workaround would be to put the cone in the open end of the cup and apply a slight bit of pressure with the hand wheel. Then with the lathe running about 150-200 rpm, reduce the clamping pressure that holds the tailstock to the bed until you can crank the hand wheel and move the tailstock backwards about 1/4 inch with the lathe spinning. Sometimes this will allow the sideways misalignment to correct itself when the tailstock is moved. If you have a sliding headstock, try the same method to slide the headstock. I have done metal spinning demos where the misalignment was enough to cause a piece of metal to walk even though the clamping force between head and tail stocks was very strong. Allowing both head and tail stocks to 'self align' this way corrected the problem.

    If the piece was put between a center in the headstock and the cone in the opening, it is doubtful you would see the cone unscrew. The misalignment would be absorbed at the ends of the drive center and there would be very little force left to cause the cone to unscrew.

  10. #40
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    Alignment

    As mentioned earlier, I'm an advocate of using a wooden support piece (with a shoulder) to hold such a piece.

    However, you bring up a good point about the alignment - even if it's not the problem here the centers on a lathe should be aligned. This is usually a simple process and only takes a moment. For those who haven't checked the alignment, simply put centers with central points into both the headstock and tailstock and slide the tailstock close to the headstock. The points should be touching. Typically, if out of alignment one point will be closer to you than the other. The fix is to adjust one leg of the lathe by cranking up on the leg leveler, or by adding a shim if there is no leveler. I usually raise one of the front legs since they are easier to reach - if the tailstock point is closer towards me I raise the front right leg until the points align. (It might seem odd but cast iron lathe beds are flexible and will easily twist.)

    I check the alignment EVERY time I move a lathe, even if moved only a few inches since even then it can be forced out of alignment unless the floor is dead flat. When Mark StLeger came to the Knoxville turning club last year the club Jet 1642 was terribly out of alignment and caused a problem for him. When we took a break I checked the alignment and fixed it with a slight turn on one leg leveler.

    If the misalignment is in the vertical plane, that's a different problem. I've never seen that.

    JKJ


    Quote Originally Posted by Dale Miner View Post
    It is caused by a misalignment between the two rotating axis. As the cone rotates the slight clearance in the threads allows the cone to slowly walk as the misalignment applies an uneven loading to the cone as it rotates.. In this case it just happens to walk towards the headstock. If you run the lathe backwards, it will likely cause the cone to screw onto the center.
    ...

  11. #41
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    As the OP says, the centers are aligned and there is no slop in the thread of the cone and center, then the previous reasons are not logical.

    But as I mentioned before chatter could cause this, where the wood is flexing and rotational going back and forth, grabbing the cone in the forward direction just so slightly, thereby rotating the cone and loosening it off.

    A tight wooden cone could help in preventing this I do expect.


    Have fun and take care

  12. #42
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    Ok, time for my stupid question.
    If you are using carbide they MAY cut so... are you turning in reverse?
    Told you it was stupid but no one else asked.
    "I became insane, with long intervals of horrible sanity." - Edgar Allan Poe

  13. #43
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    Quote Originally Posted by Michael Mills View Post
    Ok, time for my stupid question.
    If you are using carbide they MAY cut so... are you turning in reverse?
    Told you it was stupid but no one else asked.
    LOL, I was going to ask the same question but was afraid I would offend the OP. I even wondered if he was standing on the wrong side of the lathe.
    I suppose the live center could come loose but for the life of me I just can't see how.

  14. #44
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    Well I thought of it because I have done it a couple of times when I forgot to switch back after sanding. The first time I even re-sharpened my gouge because all I could get was little chips.
    But I did remember that here (or a different form) recently two folks posting that their lathes came with the switch reversed... or if it read forward it was really reverse and...
    "I became insane, with long intervals of horrible sanity." - Edgar Allan Poe

  15. #45
    Aligning the tips point to point does not definitely mean the axes ( plural of axis?) are aligned. If the axes of rotation are not aligned they are out of alignment even if the tips of the centers are exactly touching. An extreme example is if the tailstock and the headstock were at ninety degrees to each other the tips could touch perfectly. Would they then be aligned? Absolutely not! Assuming the axes could be even slightly misaligned on their axes of rotation (not just point to point) the above reasons can hold true. I don't know how to test for that...

    Another solution to the cones unscrewing could be to put less pressure from the tailstock therefore allowing a slight bit of wiggle. Using just enough pressure to make the spindle run true may solve this problem.

    if you try that report back.

    Clint

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