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hold fast vacuum chuck question

Discussion in 'Woodturning Discussion Forum' started by Dave Fritz, Jan 20, 2011.

  1. Dave Fritz

    Dave Fritz

    Jul 24, 2008
    Location (City & State):
    Montfort, Wisconsin
    When I use the chuck's supplied with the hold fast system they leave a black ring in the bottom of the wood. Any suggestions, thinking of cleaning but not sure what to use.


    Dave f.
  2. john lucas

    john lucas AAW Forum Expert

    Apr 26, 2004
    Location (City & State):
    Cookeville TN USA
    I'm not sure either. I had that trouble on one of my home made chucks and just replaced the material with something else.
    Here is something I know that will work. Get a piece of closed cell foam sheeting. I buy what's called Fun Foam at the local craft store. Cut a piece that is square just a little larger than your chuck. cut a 1" or so hole in the middle. Place this over the chuck, put the bowl in place. You can use one of the square corners to readjust the foam for better positioning if necessary. Then turn on the vacuum.
    This has 2 advantages. One is it will stop the black ring. The other is if your bowl is slightly out of round it will help fill the void and make a better vacuum seal. Any time I'm having trouble getting a good vacuum seal I pull out the fun foam and stick it on the chuck. It works really great. I suppose you could put a thin layer of clear silicone caulk on the chuck and just glue the foam to it.
  3. Ronald D. Black

    Ronald D. Black

    Dec 19, 2010
    Location (City & State):
    I now have a question. Since the larger the area the greater the pressure, if you only have a 1" hole, is your bowl stable? Would it not be better to have as much of the inside diameter of the chuck open as possible? Sorry that is two questions. :confused::confused:
  4. Vaughn


    Sep 8, 2006
    Location (City & State):
    Ronald, you get the same amount of vacuum on both sides of the sheet of foam, even with with small hole. The amount of vacuum (or pressure, if you want to look at it in reverse) doesn't change because the diameter of the chuck making contact with the bowl remains the same. If the chuck itself was only 1" in diameter, then yes, you'd see a reduction in holding power.

    I do something similar to keep from getting the black ring (or dents, in softer woods), except I use a piece of neoprene wetsuit material. Fun Foam would give similar results, though. (I need to try leaving it square. I like John's idea about using the corners to hold it into position while lining everything up.)
  5. Bill Boehme

    Bill Boehme Administrator Staff Member

    Jan 27, 2005
    Location (City & State):
    Dalworthington Gardens, TX
    Home Page:
    Ronald, as Vaughn said, the pressure differential is zero. If, instead, you were talking about a situation where the foam sheet were a diaphragm with a pinhole and if there were some leakage through the bowl then, yes, there would be a difference in pressure. Also, I think that you are confusing pressure and force. For instance, a 1" diameter vacuum chuck and a 6" diameter vacuum chuck would have the same pressure differential between the inside and outside of the part of the bowl in contact with the chuck. However, the force is considerably different. Suppose that the vacuum that you have applied is 10 PSI differential (vacuum is typically stated in terms of inches of Mercury -- in-Hg, but never mind that minor detail -- 10 PSI differential is approximately 20.3 in-Hg) below ambient (ambient is the current atmospheric pressure in your shop). For a chuck with a 1" interior diameter, the force would be about 8 pounds force. For the 6" interior diameter chuck, the force would be a bit more than 28 pounds force. Just for grins, suppose you had an 8 inch diameter chuck and applied the same amount of vacuum, the force would then be just over 50 pounds force. Bigger is not always better!

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