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Couple of vacuum related questions

Discussion in 'Woodturning Discussion Forum' started by Ric Williams, Sep 28, 2020.

  1. Ric Williams

    Ric Williams

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    I'm not much good at fluid dynamics, but I'm sure there are a lot of folks here who have enough practical experience to know the answers to these questions.

    I've been thinking and reading about vacuum chucking and understand that the degree of holding is directly related to the area of the mouth of the vacuum chuck. With that in mind, there must be a practical limit to how small the chuck diameter can be, and still have enough gripping power to be useful for turning at typical levels of vacuum. Anyone know what that limit is?

    Another thing that I wonder about is, with a dust collection device such as a shop vac or large dust collector, the suction power seems to drop off as the collection bin fills up. I have cyclonic separators on both my shop vac and dust collector, and have noticed an increase in suction after emptying the bins on both, without having done anything to the filters. What is it that causes the apparent change in suction with a change in available volume in the bins?
     
  2. Dwight R Rutherford

    Dwight R Rutherford

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    I use this chart to estimate holding power relative to chuck diameter and inches of vacuum.
    6D836A5F-B0BB-4471-8E60-8392417127CD.jpeg
     
  3. Ric Williams

    Ric Williams

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    Thanks for the chart! I figured that was the reason that no one markets anything smaller than 3". Judging by the large change when doubling a 4" chuck, a 2" one would be unusable for turning.
     
  4. Mike Peace

    Mike Peace

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    You can see why a flat plate chuck holds so well for bowls with a flat rim. A 12" bowl on a flat vacuum plate is like using a 12" chuck. I do know someone who uses a vacuum chuck for pendants. If all of the force is going straight into the HS it is more stable than cuts that are pushing the bowl sideways. Keep in mind lots of times you are only removing the nub on the original tenon with most of it removed while you still have TS support. And for sanding the very bottom.
     
  5. hockenbery

    hockenbery AAW Advisor Staff Member

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    I use a 2” vacuum for holding ornaments, pendants, small spheres....
    Holds these things fine for sanding and light turning.

    I use the 2” Rubber chucky vacuum seal on pvc pipe.

    You can see from the chart how easy it is to break things with larger chucks.
    Basically you have the whole of the atmosphere pushing you piece against the chuck.
     
    Tom Gall and Timothy White like this.
  6. Tom Gall

    Tom Gall

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    Yes, you can within reason. Like Al said....light turning & sanding. I used to make disc ornaments (like a shallow bowl) that were about 2" diameter and about a 1/16" thick in the center and less toward the outer edge. The 'sucking' portion of the chuck was probably about 1-1/2". Use the tailstock center until the last bit of turning. The vacuum was used to drive the disc and the TC was used to push it against the chuck. With a little thought and ingenuity you can hold almost anything on a vacuum chuck.
     
  7. JeffSmith

    JeffSmith

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    I use the 2” rubber chuckie on a pvc adapter as well - it holds smaller items, even small (under about 5”) plates just fine. As Mike pointed out, use tailstock support for all but the nub on the tenon.
     
  8. Ric Williams

    Ric Williams

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    I stand (actually, sit) corrected. I'm fairly new to this turning madness and if I didn't make mistakes every day, I wouldn't be learning anything. o_O

    I gotta say, I've never been involved in another activity where there's such a universal attitude of friendly openness and sharing of knowledge and skills (and tolerance for rookie goofballs like me). That's true of the online community and my local club, Detroit Area Woodworkers. Glad I stumbled into this.
     
    Last edited: Sep 29, 2020
    Richard Aldrich likes this.
  9. Dennis Weiner

    Dennis Weiner

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    Al, how deep is the 2" pvc chuck? I heard that as the volume of air increases, so does the vacuum holding power. Is that true?
     
  10. Clifton C

    Clifton C

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    Interesting question Dennis. I would think it would be the surface area of the piece, within reason. As in, a plate on a 4" chuck at 10mm of mercury would take x amount of force to pull it straight off, while a bowl, having more surface area, under the same conditions would require more.
    For sphere sanding a 2" chuck is great because you can reposition the piece by just breaking the seal and repositioning, no turning vac on and off or fiddling with the bleeder valve...
     
  11. Mark Jundanian

    Mark Jundanian

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    The force holding a piece on a vacuum chuck is a function of the surface area of the chuck opening and the amount of vacuum created. The volume of the space evacuated is not a factor. A larger space will take longer to equilibrate than a small one once the pump is turned off.
     
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  12. hockenbery

    hockenbery AAW Advisor Staff Member

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    Ditto on @Clifton C answer.

    Mine is about 4” deep using some pvc couplings that give a 2” ( 1.75 with the rubber chucky thickness) opening on 4” base from rubber chucky.

    what volume under vacuum does is give you more time if the power goes off to the pump.
    Lot of folks I know put a small tank in line with the vacuum. So when the power goes off they have a a minute or so to get the piece safely off the vacuum.

    I thought that was a good idea so have been hanging onto an 8 gallon tank for about 10 years
    Maybe I’ll plumb it into the system some day after I loose a piece.

    holding power is the surface area of the piece cut by the chuck contact.

    I am a big fan of Rubber chucky. Full disclosure. Don Doyle the owner used to be in our club. I got to be his tester and sometimes consultant so ended up with lots of rubber chucky stuff. The
     
  13. Timothy Allen

    Timothy Allen

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    The standard atmosphere is about 14.7 pounds of force per square inch (relative to a good vacuum) -- so a 2" diameter chuck at full vacuum could generate about 46 pounds of holding force....
     
  14. hockenbery

    hockenbery AAW Advisor Staff Member

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    That is correct for a disc - plenty of holding power for polite cuts.

    A 2” diameter box 3” deep has a whole lot more surface area - more holding power when you add the surface area of the cylinder.

    A fun experiment - put a five gallon bucket (a 300 pound guy can stand on one) on a disc used for a vacuum chuck with a 1” diameter hole. Close the bleed valve for ax vacuum. Stand back. When the bucket implodes it gets exciting
     
  15. Tom Gall

    Tom Gall

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    While all that has been said above is true to some extent - IMO Tim's response is the most correct. Atmospheric pressure is what holds a piece on the vacuum chuck. The surface area (sq. in.) at the vac chuck's opening is what determines the holding power (atmospheric pressure against the piece) and not the volume of the chuck. But because wood is porous (most of the time) the volume of the chuck helps with any possible recovery loss. Hope that makes sense. Every now & then I like to test my system - from pump through to the chuck - by using a non-porous surface like a plastic plate or piece of plexiglass instead of wood.
     
  16. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Not true, not to mention that you contradicted yourself when you earlier stated correctly that the volume inside the chuck doesn't affect the holding force ... which in essence is saying that the surface area inside the chuck doesn't affect the holding force.

    The force holding a piece on a vacuum chuck is the force that is in the direction of the spin axis (and the area would be the the area of the circle that lies on the plane of contact between chuck and turning). If we look at your cylindrical box example the force exerted on the cylinder wall is perpendicular to the spin axis and balanced by force in the opposite direction all the way around the cylinder. The only force that is holding the cylindrical box against the vacuum chuck is the force acting in the direction of the spin axis which in your example of 2 inches diameter is an area of π square inches.

    The five gallon bucket experiment is an attention getting demonstration of atmospheric pressure, but is misleading in that the 300 pound person is only exerting force on the end of the bucket and not against the sidewalls. If our 300 pound friend were nimble enough to stand on the bucket laying on its side he would surely cause the side to buckle.

    Unlike our 300 pound friend whose force only acts downward the atmosphere exerts force against all sides of the bucket. The force holding a vacuum chuck against the one inch hole would be π/4 square inches X 14.7 pounds per square inch = 11.5 pounds regardless of the volume of the bucket.
     
  17. hockenbery

    hockenbery AAW Advisor Staff Member

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    pretty sure you use the area of the bucket opening. Not quite 12” diameter so
    36 x pi x 14.7 Which is ball park 1500 lbs.
     
  18. Dennis J Gooding

    Dennis J Gooding

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    I agree with Bill, the area of the chuck and the inches of vacuum determine the the holding force along the spin axis. All other factors are irrelevant. This assumes only that the contact is circular and and perpendicular to the spin axis.
     
  19. Bill Boehme

    Bill Boehme Administrator Staff Member

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    We were both incorrect. To be clear what we are looking for is the force between the chuck and bucket (piece of wood or whatnot). The correct area for calculating the force would be determined by the inside diameter of the chuck. If the chuck had an inside diameter of 3 inches then we would have:

    equation1.jpg
    where
    F = resulting force from the product of area and pressure
    d = inner diameter of chuck
    P = sea level atmospheric pressure

    equation.jpg

    F = 103.9 pounds

    While it's true that the atmosphere would be exerting a force of approximately 1500 pounds against the top of the evacuated bucket (not to mention the crushing force against the side of the bucket), that IS NOT the force of the vacuum chuck against the top of the bucket. The force of the chuck against the bucket is about 104 pounds.

    The one inch hole is a red herring. The hole size could vary, but the resulting force is still the same.
     
  20. hockenbery

    hockenbery AAW Advisor Staff Member

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    I think we agree except on what diameter to use for a disc chuck.

    When using a disk as a chuck, I believe the effective diameter of the disc Chuck is the inside diameter of the rim Of the object held in the disk. Is this not correct?

    My 4” and 6” cup chucks have 1” holes just like the disc. We already agree that the volume of the chuck has no effect.

    so the force on a bucket with 12 “ diameter inside opening at its rim is (12/2)x(12/2)x pi x 14.7
     
  21. Lou Jacobs

    Lou Jacobs

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    I’m not a physicist, but Hockenbery’s take makes perfect sense to me. Once a good seal is made, the system does not “know” where the orifice of the chuck is. The smallest opening is in fact the 3/8” tube that feeads it. The relevant dimension is the widest part of the bowl. I’d even think that a closed form, with a small orifice but a large belly, so to speak, would be subject to more vacuum pressure measured from the wider diameter. (Although you’d want to be cognizant of the leverage effect of having a wider piece supported by a small mating surface). Just my $.02, which is exactly what it’s worth!
     
  22. Bill Boehme

    Bill Boehme Administrator Staff Member

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    No disagreement. In rereading your post it wasn't obvious to me that you were putting a 12" disk chuk against an open top bucket. My bucket has a lid with a hole in it.
     
  23. Dave Landers

    Dave Landers

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    I stick a piece on a chuck, turn on the pump, close the bleed valve, and get on with turning.
    I do know that a larger chuck will hold more than a smaller one. That seems to be enough for me, and involves a lot less math.

    Heck, my cheap vacuum gauge is sorta busted - the needle zeroes out at around 3 whatevers. So I don't even have good data to plug into any formula. But I know that if it points around 5, I can move the piece around, and if it's up over about 20, then I got a decent seal.

    I get that it's possibly "interesting" (and maybe even "fun") to play with the math, but I've never really felt the need to calculate force or whatever with any of my chucks or holding methods (vacuum or not).

    Just thought I'd throw that out there in case we scare someone away from purchasing a vacuum system thinking they're going to have to do math. :p
     
  24. hockenbery

    hockenbery AAW Advisor Staff Member

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    Al Stirt who I have known for along time was coming to do a class in our shop a few years ago. My vacuum gage had been broken for a long time and we just watch the gap that forms at the tailstock since every piece needs slightly different vacuum. I got a new gage But had to settle for a dual pressure/vacuum gage, the only one I could find in Lakeland. While Al was setting up I mentioned my quest for the gage and that he might not like the new dual gage. He laughed and said I shouldn’t have bothered. his gage had been broken for years and he just watches the tail center gap....
     
  25. Dennis J Gooding

    Dennis J Gooding

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    Let me suggest a way of visualizing the situation. Suppose that a rigid spherical object (possibly hollow) is being held in a round chuck of inner diameter D and imagine that the sphere consists of two parts: (1) The part that lies within a cylinder of diameter D that extends axially outward from the chuck and (2) all of the rest of the sphere. We are interested in the total force directed toward the chuck in response to a vacuum in the chuck. For the portion of the sphere outside this cylinder, the net force in any direction clearly is zero, because the force on any bit of surface of the sphere is exactly cancelled by the force on a diametrically opposite bit of surface. Therefore, the only force that is directed toward the chuck is that arising from air pressure on the domed outer surface of the portion of the sphere that lies within the cylinder. That force is clearly directed inward since any sideways force on any bit of surface is exactly balanced by the opposing force on a diametrically opposite bit of surface. The effective area of this dome of diameter D is the same as that of a disk of the same diameter, although it would require a bit of calculus to prove it.
     
  26. Tom Gall

    Tom Gall

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    Yeah ...... what Dave said. Everything is relative (re: gauge, etc.). All that math would make my head hurt. :confused:
     
  27. Bill Boehme

    Bill Boehme Administrator Staff Member

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    My gauge is also the dual pressure/vacuum type and the vacuum par of the scale is pretty small. I've had that gauge for a dozen years or so. I keep telling myself that I am going to get a nice big vacuum gauge sometime soon, but not right now.
     
  28. Bill Boehme

    Bill Boehme Administrator Staff Member

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    I thought that it was all about the math and the woodturning part was just incidental. ;)
     
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