My misconception about lathe vibration, and how to deal with it.....

Discussion in 'Newbie' started by odie, Dec 30, 2012.

  1. Bart Leetch

    Bart Leetch

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    If that's the case then why can't you have vibration at the end of a tool rest with the winter wood hitting your tool at hundreds of times a second?

    Blond answer: Well silly that is because it's winter wood & it's to cold to vibrate.:D

    Sorry I couldn't resist I've been following this thread & it's become to dry & serious.:eek:
     
    Last edited: Jan 8, 2013
  2. Bill Boehme

    Bill Boehme Administrator Staff Member

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    John, I'll stifle myself (as Archie Bunker used to say) and not make any comments about the WF beam and your noggin. I think that the Mystery Man's valid point has to do with other folks misapplication of things (even though I truly believe that he is mistaken about shrimp passing gas). Sound is transmitted through water and solids and other media besides air and generally more efficiently. The example of sensors on bridge columns are nothing more than an example of sound being transmitted and the sensors being capable or detecting those small inputs. You could call it sound or you could call it vibration because that is what our ears detect, but your observation wasn't the sort of data that the structural engineers were interested in gathering.

    The MM made a valid point regarding vibration that most others failed to appreciate (somewhat his fault for not clearly enough stating what he meant, IMNSHO). Things can't really be considered in isolation when trying to ascertain the source of vibration because it is a close loop system that involves a human in the loop. However, it is safe enough to say that the tool (unless a chatter tool or something left to float at the fulcrum) is not the source, but just a rigid conduit for all reasonable purposes between the two compliant components in the closed loop system -- the human and the thin bowl. From an engineering standpoint, the term compliant refers to something that can't be considered rigid when a force is applied to it. Everything has some amount of compliance, but something that causes a significant enough time lag between stimulus and response to lead to sustained instability .... or to use another word, "vibration" (but, no relationship to the tuning fork vibration caused by twanging a bowl gouge with a hammer) would be the primary causes. There are at least two inputs -- the mechanical drive which is fairly rigid and the human poking the bowl with a gouge. Since the human and bowl both have high compliance the poking force is not constant.

    Like I said, the shrimp "wind" thing may be fabrication, but I do know that our sonar could tell precisely where the Ruskie subs were no matter where they were in the oceans because of their noisy screws.
     
  3. MichaelMouse

    MichaelMouse

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    Why are there no chatter marks when the tool's being used on 3/4 thick, or as Hippy mentions, the outside of a piece? All's the same, save the thickness of the wood.

    Why is it that with the same extension of the same tool the chatter pattern varies with the velocity of the thin wood? Does a string change its pitch (vibration) when it's plucked more often?

    Does choosing a larger diameter tool reduce the chatter?

    When someone's innocent enough to believe almost anything Joe Turner with the aura tells him, he needs good information. The solution to chatter has to do with the wood, so let's stay it, dampen it, be gentle with it and solve the problem, not mislead.
     
  4. Matt Lewis

    Matt Lewis

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    And the person poking/turning the wood as Bill mentioned and as was stated way back in the beginning.

    Although this article is specifically talking about machining metal, I think folks will find it useful to this discussion. Particularly the info discussing dynamic stiffness, forced input and controlling chatter.

    http://www.ctemag.com/dynamic.articles.php?id=236
     
  5. Dale Miner

    Dale Miner

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    Matt,

    That is a good read.

    While it does assume machining metal, the principles apply to any material.
     
  6. Matt Lewis

    Matt Lewis

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    Dale,

    I concur.

    Best regards,
     
    Last edited: Jan 9, 2013
  7. Bill Boehme

    Bill Boehme Administrator Staff Member

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    It is a good article that does a good job of explaining things to a non-technical audience. I noticed a few errors, but nothing that would matter except to another engineer proofing the article.
     
  8. robo hippy

    robo hippy

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    I can get a 3/4 inch thick bowl to chatter/vibrate easily by 'rubbing' the bevel too hard.

    Again and again, "the bevel should rub the wood, but the wood should not know it."

    robo hippy
     
  9. Matt Lewis

    Matt Lewis

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    Hmmm, controlling the forced input.
     

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