Results 1 to 7 of 7

Thread: Question about vacuum chucks

  1. #1
    Join Date
    Sep 2018
    Location
    Kensington, Maryland
    Posts
    274

    Question about vacuum chucks

    I know that people use vacuum chucks to attach a bowl to the lathe so that the bottom of the bowl can be finish turned (i.e., removing a tendon or decorating the bottom).

    Here’s my question: can you use a vacuum chuck to attach the bottom of a finished bowl to the lathe (assuming the bottom is nice and flat) so that you could — for example — freshen up a friction polish on the bowl or apply a fresh coat of hard wax to the bowl?

    Thanks,

    -Dan

  2. #2
    Join Date
    Sep 2015
    Location
    San Diego, Ca
    Posts
    1,647
    Probably, depending on how much force you would be applying perpendicular to the vacuum force. It also depends on the area that the vacuum is applied to. If the mouth of your vacuum chuck is small, the force is small and vice-versa. On my own set up and altitude, I get at least 10 pounds of force per square inch of area of the vacuum chuck. The force depends on your system (pump, leaks, seal, altitude, area of the chuck, and maybe a couple of other things.).

    What could definitely help is to also use the tail stock. I've seen people who will either add a tennis ball on the tail stock live center or make and use a cone center on the tailstock live center. Or you could put a pad onto a chunk of wood that your live center in the tailstock presses against. But you could probably do something similar w/o the vacuum system.

    Perhaps others will have additional ideas.

  3. If you want the piece to run true, it may be a challenge getting it centered on the vacuum chuck. When you use the vacuum to finish the bottom, you usually have the center point to assist in centering the piece on the chuck. A center point will not be present on the top surface.

  4. #4
    Join Date
    Sep 2013
    Location
    Wayland, MA
    Posts
    3,655
    Yes. I've re-sanded and refinished previously finished bowls when I've decided I'm not happy with them. Only had mine for about two months, but I really like it a lot! The force with which the bowl is held is a function of the area of the chuck, the vacuum level, and, related to the vacuum, the porosity of the wood. A high vacuum at seal level will give you about 14.5 PSI, the 25 inches that you get from the kinds of pumps most systems use will give you 12 PSI. A 6" chuck is about 28 sq in, so 12 x 28= ~340 pounds of pressure holding your bowl in place. No way you're moving that, but conditions are seldom so ideal. I've seen in practice so far that as long as I can pull about 20" vacuum bowls up to ~14" in diameter are held plenty securely for sanding or light turning. Smaller chucks of course have less holding power.

    Centering can take a while without a centering mark; fortunately for sanding and finishing a little wobble doesn't seem to hurt.

  5. #5
    Join Date
    Feb 2008
    Location
    E TN, near Knoxville
    Posts
    12,298
    If you have enough surface area to hold it securely and if you can get it to run true. I use a dial indicator to help true up pieces on vacuum or jam chucks:

    Dial_indicator_IMG_20141122.jpg

    But another idea for your consideration for the future. Over the last 6-8 years I've been leaving the recess in the bottoms of many bowls and platters. I initially got a lot of resistance from some other turners ("You can't do that, it's against the rules." But now, guess what, some of these same guys are now using the same method! This greatly simplifies holding and finishing a piece since I typically hold the piece by the top to turn, smooth and even apply finish to the bottom before reversing to hold by the recess and turn/finish the inside of the bowl or top of the platter. Then I'm done. In addition, the area around the recess can be detailed for visual interest and provide a nice "shadow box" effect for signing. And another "guess what", I've had nothing but positive comments from non-turners when they turn it over: "Ooo, I love the bottom!") Some examples.

    penta_maple_ellis_c_IMG_5435.jpg bottom_maple_IMG_7444.jpg bottom_IMG_4687.jpg bottom_cherry_IMG_7424.jpg bottom_IMG_4749.jpg

    bottom_PC012804_e.jpg platter_PC012780_e_comp_med.jpg

    On the last one the chuck recess is just the circular portions of the three small triangles. Less than 1/8" non-continuous recess was enough to hold the large sapele platter for turning and finishing the top.

    Another big advantage is I can do most of my smoothing/sanding/finishing off the lathe while the piece is still held in the chuck, held in a carving/finishing post from Best Wood Tools. Using hand scrapers and hand sanding off the lathe has improved my finishes tremendously.



    JKJ


    Quote Originally Posted by Dan Gaylin View Post
    I know that people use vacuum chucks to attach a bowl to the lathe so that the bottom of the bowl can be finish turned (i.e., removing a tendon or decorating the bottom).

    Here’s my question: can you use a vacuum chuck to attach the bottom of a finished bowl to the lathe (assuming the bottom is nice and flat) so that you could — for example — freshen up a friction polish on the bowl or apply a fresh coat of hard wax to the bowl?

    Thanks,

    -Dan

  6. #6
    Join Date
    Sep 2018
    Location
    Kensington, Maryland
    Posts
    274
    All — thanks these are very helpful replies. John those bowl bottoms look great!

  7. #7
    Join Date
    Aug 2007
    Location
    Lakewood, CO
    Posts
    761
    Quote Originally Posted by Dan Gaylin View Post
    Here’s my question: can you use a vacuum chuck to attach the bottom of a finished bowl to the lathe (assuming the bottom is nice and flat) so that you could — for example — freshen up a friction polish on the bowl or apply a fresh coat of hard wax to the bowl?
    Yes, I've done this a number of times. Even though I raise the grain on a bowl prior to finishing, sometimes after a coat of finish the surface isn't as smooth as I like. I'll put the bowl on the vacuum chuck and use a soft interface pad to sand the inside and outside again. Like others have said trying to center the bowl can be a challenge, but for what I'm doing I can get it close enough even if it's not perfect. I have minimum vacuum holding the bowl while I lightly tap it one way or the other, then when it's "good enough" I'll increase vacuum.

    You don't necessarily need a flat bottom bowl to do this, in fact that's the weakest way to hold the bowl. I have 3 different size vacuum chucks and I try and use a chuck where the bottom of the bowl fits down inside the chuck a bit. This way I have vacuum on the bottom of the bowl and part way up the side.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •