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lathe chuck jaws

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  • lathe chuck jaws

    What is the rule of thumb for max diameter held in let's say an eight inch chuck? Now as you open the jaws the scroll is unscrew the jaws. Before they fall out one thread is holding them and this would allow a nine inch diameter to be held in place. But is that beyond max? Not safe? Not accurate due to one thread holding?? Does any company make jaw extensions? My Jaws are two piece.
    Last edited by ahidley; 11-12-2015, 08:44 PM.

  • #2
    I don't think there is a rule, as that would suggest that two, maybe three threads in engagement would be safe, but how can that be said- it depends on how tight you crank the jaws and how fast you turn the spindle. I've been in that situation many times, often with just one thread on one or two of the jaws engaged with the scroll. That usually means I'm using a slower speed to begin with since the workpiece is larger, so I don't worry about that although I do have it in mind. A lower clamping force means I lighten up on the cuts, and often means I supplement the holding with some pressure from the tailstock using a live center. This sometimes means I use a bit of a push rod so the tailstock doesn't have to come in as close, in case the carriage or cross slide needs the room.

    Reversible jaws are sometimes called for of course. It might make sense to check to see how far open you can go and still have at least two threads on every jaw engaged by the scroll. Note this number, and also note the point at which every jaw has at least three threads engaged. Just my opinion, but I'd say that at three threads minimum you shouldn't have to worry about it.

    You have two piece jaws- make yourself some pieces that will allow for better thread engagement with larger diameter work pieces.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


    • #3
      My experience has been that a 6" chuck will hold a 6" part. 8" an 8" part. 10" a 10" part. If you need to hold a larger part than your chuck will allow, then you get a bigger chuck. It's that simple.