diff. between a dead & live center ???
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  1. #1
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    Ok, I've been in the trade for over 11 years now, and I've never really thought about this one:
    What's the REAL reason for having both a live & dead center ??
    I know enough to use the dead center for big, heavy parts...and to make the parts more rigid (for milling, 4 axis work, etc.)......but when does the live center come into play?
    Doesn't the dead center do well enough for basically ANY type of turning job?
    Is the live center used for high-RPM turning?
    Enlighten me, Oh chip-making ones.....

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    Pazuzu: I,ll give this one a try. Back at the beginning of machine development, it seems that the first one to name a machine part usually had a good chance that the part name would stick...these names were also very descriptive.

    Originally the name "live center" applied to the center in the headstock spindle of a lathe and the "dead center" was the one used in the tailstock since it does not rotate. Since the development of the rolling element(ball or roller bearing )tailstock center for high speed turning, life has become easier for machinists. At the speeds at which modern turning centers operate, even carbide centers would fail due to excess friction and heat at the center/center hole interface. The "live" ball /roller bearing center eliminates the sliding friction by allowing more pressure to be applied to the part center hole,transferring the rotation to the rolling elements in the live part of the center.

    You are correct that for milling operations the solid or dead center is the better choice since the rotatinal speed of the part being worked is very slow, such as in helical milling and enough pressure can be used to provide better stiffness. Besides, the vibrations set up by a milling could cause denting or Brinelling of the bearing raceways of a stationary live center.

    I hope this helps, but these are just my observations. EAH



    [This message has been edited by EAH (edited 06-06-2003).]

  3. #3
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    Everything EAH said.

    Albert

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    Any type of turning work you normally use live center!
    Only when you go slow, like milling, and grinding between centers, the centers are stationary,-- more ridged!

    Konrad

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    You forget to mention...

    When a "Live" Center locks up it becomes a "DEAD" Center.

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    If you want to turn something to best roundness on a conventional lathe; an example being a large rotor on a shaft; you can often get better roundness by using two dead centers and driving with a dog. Obviously using moderate speed. Same goes for cylindrical grinding of shafts for extreme roundness. The two points define a line and you avoid the ball bearing noise and error of using live centers. Same principle as jewel bearings in instruments.

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    CASS hit the nail on the head. The live centers are only as good as the bearings inside of them. You use a dead center on a lathe only when roundness or concentricity is an issue. A good live center will usually work for .0003" - .0005". Use high pressure lube and a low RPM with the dead center for closer tollerance. The only time I have used a live center on a mill is with a 4th axis or rotory table.

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    im currently a apprentist and they're starting to make it very clear on such things as Dead centers and Live centers. they make you remember EVERYTHING! well i guess thats the way it is. just 8 days untill my level one trades test, wish me luck

  10. #9
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    Don't spell apprentice like that on a test PLEASE for your sake

  11. #10
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    On the Dead Center - Live Center issue.

    Live centers for lathe head stocks were usually left soft. When the center was installed in the lathe spindle, the center was trued up by taking a cut across the 60 degree center point.

    Assuming the lathe spindle doesn't have excessive radial play, this gets rid of any eccentricity in the head stock center.

    The Dead center in the tailstock will not run eccentric at all (obvious) where as a "live" or rotating tailstock center will have some eccentricity.

    A very simple lathe in reasonably good condition can turn out very accurate work with the traditional between centers set up.

    It is The Essence of Lathe Work.


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    Default Using DEAD and LIVE centers together

    Quote Originally Posted by Pazuzu71 View Post
    Ok, I've been in the trade for over 11 years now, and I've never really thought about this one:
    What's the REAL reason for having both a live & dead center ??
    I know enough to use the dead center for big, heavy parts...and to make the parts more rigid (for milling, 4 axis work, etc.)......but when does the live center come into play?
    Doesn't the dead center do well enough for basically ANY type of turning job?
    Is the live center used for high-RPM turning?
    Enlighten me, Oh chip-making ones.....

    The proper use or method to use both DEAD and LIVE Centers is when turning a part between centers. Remove the chuck from the lathe and insert a DEAD CENTER in the SPINDLE NOSE. Use the LIVE CENTER in the TAILSTOCK. Now, the spindle will rotate the dead center in the spindle nose along with workpiece while the live center in the tailstock will follow. This way there won't be any friction between the centers and the workpiece. They ALL spin together.

  13. #12
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    In Construction and Use of Universal Grinding Machines , B&S has a single sentence that explains it for me

    Under Head-Stock...."When it is desired to grind work on dead centers, the spindle is held from revolving by a pin, as shown in Fig. 9"

    To me this simply means a dead center never revolves with work piece

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    Quote Originally Posted by Rotate View Post
    Everything EAH said.

    Albert
    Does anyone know who patented the first live center?

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    Young one...I love it....do not fret...if your old man is a union boss,you will fly through never so much as touching any nasty smelly metal bits.......fifteen years ago...wonder if he made good?


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