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Thread: Rotary Adapter Problems

  1. #1
    Join Date
    Dec 2012
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    Brenham, Tx
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    109

    Rotary Adapter Problems

    I just finished making my vacuum system. I didn't buy anything other than fittings, bearings and the pump. I made my own Rotary Adapter out of maple.
    I have a Oil Bath Robinair 5 cfm pump. It pulls 29"hg.
    I hooked up all my hoses and valves and my spindle adapter. I closed the valve to the rotary adapter. It showed 29"hg. I opened the valve and put my thumb over the end of the lamp rod. I only could get 21"hg.
    I fooled with it and determined it was leaking through the bearings. I have two bearings with rubber seals on both sides. I put saran wrap over the bearing and it sucked it down and the pull went up to 26"hg or so.
    What can I do about this?
    It is not leaking around the bearing, it is coming through the bearing.
    RP

  2. #2
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    Quote Originally Posted by Robert Edington View Post
    I have two bearings with rubber seals on both sides.
    RP,

    I am certainly not a bearing expert, but when purchasing bearings I noticed they come in a variety of types of seals. I'm wondering if some types of seals are better for vacuum than others.

    I bought double sealed ball bearings from here:
    http://www.veneersupplies.com/produc...uum-Chuck.html
    In my experience, Joe is pretty good about sourcing quality parts.

    sold as part of this kit:
    https://www.veneersupplies.com/produ...ng-Add-On.html

    for this design:
    http://www.joewoodworker.com/veneeri...umchucking.htm

    I too would want as much vacuum pressure as possible. But I have no idea as to what is acceptable for a vacuum chuck. Just curious, is 21" enough to hold the pieces you plan to turn? In the last link above he describes how to calculate the holding forces: "For every inch of Hg vacuum, you get .49 lbs per square inch of pressure. Multiply .49 by the vacuum level that the pump can create. "

    JKJ

  3. #3
    Join Date
    Sep 2015
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    San Diego, Ca
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    I don't think that there isn't a lot of difference between 25 and 29 inches in terms of holding bowls and platter on a vacuum plate. Draw a vacuum and try to pry it off. If it is really difficult then you're okay.

  4. #4
    Join Date
    Mar 2005
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    Elkhart, IN
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    431
    Try wrapping white teflon plumbers tape around the spindle threads before screwing on your vacuum chuck. It helped me with leaks at that location.

    Ricc

  5. #5
    Excessive vacuum can dent or even collapse your work piece. It sounds like you're getting plenty. Like John Jordan said, you're getting 1/2lb pressure per square inch of the workpiece inside the vacuum chuck, per inch of mercury. That's a lot!

  6. #6
    Join Date
    Sep 2015
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    San Diego, Ca
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    By the way, some woods are so porous that you can't draw much of a vacuum anyway. I was sanding some Jacaranda and used an airhose to blow the dust out of the inside of a vessel. I could also see some dust flying off the outside.

    I liked John's explanation. Makes sense. IIRC the pressure on everything at sea level is around 14.7 pounds per square inch (PSI). So if you could draw a theoretical perfect vacuum, that is the PSI pressing on your work piece. So if your vacuum chuck rim is 5" in diameter you have nearly 20 square inches of surface area and you would have about 280 pounds of pressure against your work piece. If you could only draw 20 inches of vacuum, you'd have about 10 psi pressing against the workpiece. If you do the math, that would mean that you have a force of 200 pounds keeping the workpiece in place. That is still a whole lot of force !! I think that I recall people using bleeder valves to bleed off excess pressure of very thin or delicate pieces.

  7. #7
    Join Date
    Nov 2007
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    Quote Originally Posted by Brice Rogers View Post
    snip
    I think that I recall people using bleeder valves to bleed off excess pressure of very thin or delicate pieces.
    Exactly true.I have split pieces and had thin bottoms pop.
    Retired - when every day is Saturday (unless it's Sunday).

  8. #8
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    Quote Originally Posted by Brice Rogers View Post
    ... I think that I recall people using bleeder valves to bleed off excess pressure of very thin or delicate pieces.
    Every vacuum chucking system should have a bleeder valve. This picture, again, is from the excellent Joe Woodturner vacuum chucking page:

    vacuum_manifold.jpg

    The vacuum pump is connected at the bottom. The elbow at the top connects to the rotary adapter on the lathe. The valve has a filter on the left side to keep sawdust and the like out of the vacuum pump.

    For those new to vacuum chucking, this is a typical way to use the bleeder valve:
    - the valve is opened to the atmosphere so almost no vacuum is on the chuck,
    - the vacuum pump is started,
    - the piece positioned on the chuck in the approximate center (with help from the tailstock, if possible),
    - the bleeder valve is closed enough to hold the piece lightly on the chuck,
    - the work piece is adjusted on the chuck until it is exactly centered when the lathe is rotated by hand (I like to use a dial indicator on a magnetic base for this),
    - the bleeder valve is closed enough to hold the piece firmly and (hopefully) without cracking.
    - the lathe speed is turned up slowly.

    Joe says this about the amount of vacuum needed: "You'll find that most projects will have no trouble with the pressure from the vacuum. It is generally accepted that 15" of Hg is the minimum safe vacuum for most small to medium sized projects. As always, exercise caution and good judgment. Wear appropriate protection and don't crank the lathe up to 8 million RPM. And for goodness sake, make sure the project is centered on the jig before you get the lathe up to speed."
    http://www.joewoodworker.com/veneeri...umchucking.htm

    JKJ

  9. #9
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    Jul 2008
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    Bob as you indicated, you feel that the bearings are leaking, and more than you expected, and yes there is differences between different makes of bearings, however I would expect that the seals will seat better after some use of them.

    You can always change the bearings, but try them some first, they might do well as is and get better after some use, I have a vacuum system but hardly ever use it, but found that with most regular size bowls, even 10 Inches of mercury is enough to hold it.

    It is something that can happen with the best of systems, as leaks can be enough to have the vacuum not get any better than that by porous wood or else.
    Have fun and take care

  10. #10
    Join Date
    Dec 2012
    Location
    Brenham, Tx
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    109
    Thanks for all your replies. I am still installing it. I just got my Beall Tap today. I will soon be making my own chucks. I also machined a fitting from PVC so I can port the exhaust out of the shop. Those oil bath vacuum pump really put out the fog/mist.
    I am anxious to be able to use this new equipment.
    RP

  11. #11
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    Quote Originally Posted by Leo Van Der Loo View Post
    ... leaking... It is something that can happen with the best of systems, as leaks can be enough to...
    Completely off topic, but this reminded me of my player piano that gradually became so leaky it was "almost" impossible to play - it took a herculean effort by the two strong legs of an energetic young person to drive the mechanism. (Player pianos run completely on low-pressure foot powered vacuum - quite sophisticated) It took me a month to rebuild the bellows (nearly 100) and mechanisms and stop most of the vacuum leaks. I could then make it work with just the power of one thumb!

    But it worked fine for years with a number of leaks. I agree to try using the vacuum chuck as is - it may work fine. I don't use a vacuum chuck often but when needed, it is a wonderful thing.

    JKJ

  12. #12
    Join Date
    Jan 2006
    Location
    Ottawa, Canada
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    21
    It looks like you have enough vacuum to securely hold the workpiece, but I wouldn't rely on a sealed bearing to hold pressure (vacuum). The seal is there to keep dirt out of the bearing raceway, it was never intended to hold pressure or vacuum. The seals are just lightly pressed in place and could pop out if enough pressure is applied.
    To hold pressure safely you need a lip seal. I put a lip seal in my rotary adapter, and can pull 29" at the chuck with the bleeder valve closed. Just be sure you install the lip seal in the correct orientation - they only work in one direction! However, a lip seal wouldn't work on a wooden shaft - they need a smooth metal shaft to get a good seal.
    Ed.

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