Vacuum chuck size

[Ted Evans]

Can someone tell me what the smallest practical size is for a vacuum chuck? I know the less surface for the vacuum equals less holding power. I was wondering if a 1 - 1 ¼ chuck into a vase or HF would hold well enough to clean up the bottom. The chuck’s OD would match the vessel’s opening size. Thanks for any info. Ted

[Bruce Stangeby]

I have used a 1 1/2" with sucess. I was carefull to take light cuts. This was on a 8" tall vase and I finished the bottom with no problem.

[John Keeton]

Are you asking about placing an extended chuck against the floor of a vase?

I haven't studied this, and have only had my vacuum system for a few weeks, but seems that the holding power would be a factor of the surface area on which the suction was acting. So, if the chuck was on the mouth of a vase, the entire interior of the form is involved.

This could be an incorrect perception, but seems logical to me.

[David E Keller]

I would think a deeper chuck that sealed near the maximum diameter of the form along it's outside would be better... Seems like it would shorten the portion of the form being supported and allow for a kind of jam/vacuum fit. Take that with a grain of salt as I don't currently own a vacuum setup.:mad:

[Leo Van Der Loo]

Can someone tell me what the smallest practical size is for a vacuum chuck? I know the less surface for the vacuum equals less holding power. I was wondering if a 1 - 1 ¼ chuck into a vase or HF would hold well enough to clean up the bottom. The chuck’s OD would match the vessel’s opening size. Thanks for any info. Ted

The maximum holding on a 1¼" area would be about 10 to 11 pounds of pressure holding the vase maximum, atmospheric pressure is only about 15 pounds per square inch.

That is not enough to hold a vase and finish the foot IMO.

I would make a chuck, like a dowel kind of, that goes into the vase and then with the help of the live center in the tailstock to hold it, you could then finish turn the bottom.


The length of the vase doesn't make for any more holding pressure, it is only the area that butts onto the chuck that counts, a big round globe with a area of half an inch square, that sits against the chuck, has a maximum of 7½ pounds holding it, a half mile long hose that has a 1" of area against the chuck has a max of 15 pounds of pressure holding it, I suppose you all know this, but I just added this for clarity.

[Larry Marley]

I have used a 8" long chuck that was 3" in dia against the interior bottom with success.
Please be VERY CAREFUL about using a vacuum chuck around the opening and drawing vacuum using much of the vessel. Every inch of exposed vessel will add to the total pressure and you can EASILY collapse a vessel.

[Ted Evans]

Thanks to all for your comments, suggestions and/or information.

Bruce - sounds like you had success with your attempt and if I understand you correctly, your approach is what I was thinking of doing. The chuck would be sized to fit snuggly in the opening and reach to the bottom of the vessel where suction would be applied.

John - Yes, that is my thinking. I agree that placing the chuck in the mouth of the vessel would produce more holding power, my concern with that would be the possibility of collapsing the vessel.

David - Perhaps I do not understand correctly your suggestion, how would you get a chuck that would enter a 1 ½” opening to seal, say a 3” diameter mid section? Have I totally missed your point?

Larry - I have experienced no problem using a 3” chuck as you suggest when the opening would permit access, I was hoping to find out if something in the 1 - 1 ½” size would be feasible without the effort of making the chuck to try it.

Leo - thanks for the info, my plans were to use the tailstock with the chuck up to removing the nib, then removing the tailstock to clean up.

Again, thanks to all for your comments. Ted

[John Keeton]

Ted, I haven't crushed one yet, but it sure is a possibility!! I think much of that risk would have to do with the shape of the form and whether it is long grain (end grain turning) or flat sawn - and, the size (surface area) of the form. Seems an endgrain turning would be more resistant.

I just finished the bottom of a small 5" form that is 1/8" thick with 25 Hg. It survived!

[Ted Evans]

Thanks John, if you had no problem at .125, then I should be ok. By design,:) I try to stay about .125 - .187, unfortunately, there have been times when they were less than intended, if you get my drift.:o

[John Keeton]

Ted, not sure we are talking about the same thing. I was pulling close to 25" Hg on this form, which was plenty enough to hold it to turn off the waste block(except for that little catch once on the epoxy seam!!;)) My concern was whether it would, in fact, crush the form. That was why I commented that it survived.

I have very little experience with the vacuum system, and most of the time I get about 25" Hg. I have used it as low as 15", but not below, though it says the minimum is 12".

[David E Keller]

David - Perhaps I do not understand correctly your suggestion, how would you get a chuck that would enter a 1 ½” opening to seal, say a 3” diameter mid section? Have I totally missed your point?


I don't think I was entirely clear. I would consider using a chuck that is slightly smaller in diameter than the widest portion of the vase. I would then insert the vase into the chuck as deep as it would go(vase into chuck, not chuck into vase). That would give you a larger surface area for chucking(at least in my mind), and it would also potentially shorten the portion of the vase that extends past the end of the chuck.

Another way to explain it... If you created a hollow cylinder that you were going to jam chuck the vase into for turning the foot, the vacuum would be the same with the addition of vacuum allowing you to remove the tailstock for the final passes.

Again, I have a sum total of zero experience with vacuum chucks, so my idea is likely flawed in a number of ways.

[Thom Sturgill]

David is right on. My vacuum chucks are sections of pipe with foam padding on the end. Use a pipe that is long enough that the vessel does not bottom out and which will support the outside if the vessel.

I would not use a chuck inside unless i made a jamb chuck that just fit the opening and would go all the way to the bottom of the vessel, supporting both the bottom and stabilized by the opening. Then carefully clean the bottom with cuts that provide some pressure to hold the piece on the jamb chuck.

[Ted Evans]

Thanks David, now I understand your approach and it sounds plausible to me, it is one that I will experiment with.

Thom - what kind of padding do you use for your chucks? I think I recall someone saying that they get sheets of foam from Wal-mart for this but I could not find any there today. :( I picked up a mouse pad that looks as if it should work but have not had a chance to try it yet. Ted

[Thom Sturgill]

I use the foam sheets from Walmart or Michaels. They are in the craft area at walmart, 5x8 and 8x12 sheets, self adhesive. Several people report using a mouse pad. I llike the foam sheets becasue I can clean them off and re-apply or build them up if needed to fit a contour.

[John Giem]

TEd,
This is a new post to an old question, but I have possibly a better solution to the small area of the vacuum chuck. In the June 2012 issue of the American Woodturner, I have an article on a Compliant Vacuum Chucking System that helps solve a lot of tough holding issues. I think that the techniques described would work quite well for your case and for many small items that cannot be safely held in a conventional vacuum chuck.
John Giem

[Faust M. Ruggiero]

Ted,
It's terrific to own a vacuum system but it is not always the best way to hold all forms. Leo described a jam chuck used by many of us that do a lot hollow forms. It does not use the vacuum. Remember, if you hold a long vessel with a vacuum, the smallest movement at the chuck turns into a bigger movement several inches away. Also, the mechanical advantage you assert that far from the chuck can easily unseat the vessel from the chuck and break the vacuum causing the piece to fly. Google "Rubber Chucky" and you will see what he was describing as well as the system David has spoken of several times. The pressure of holding the piece is solidly on the bottom of the vessel only. Good luck.
faust