Vacuum chucking, advice request

[Alan Zenreich]

Just to avoid some confusion, it's "Morse", not Morris taper.

[John Keeton]

This may be similar to what Thom is thinking of -

http://www.woodturnerscatalog.com/wo...-mandrel_l.jpg

[Paul Williams]

Thanks for the photo John. That looks like an easily made tool that will work well for me.

[Paul Merrill]

Jon, check your PM (personal messages), I've got a few questions for you on your equipment. Thanks!

[Jon Murphy]

Bob, I nominate you for the Murphy prize for clarity (Nobel and Pulitzer don't have that category). I am a bit embarrassed, as I'm writing a book on musical strings and one of the criteria is understanding the volume versus the cross section. (The vibrating mass is a cubic measure, the tensile strength a square one - the string material has density (cubic) and gauge (square), and the instrument has length (fixed for guitar, variable for harp). Not an exact comparison, but like the matter of holding force with a vacuum there is a counter intuitive aspect. A string of a given length (VL) has a breaking pitch, given the material. My harpist friends keep trying to tune higher with a thinner string (same length), doesn't work.

OK, got off on a tangent - but your description has me thinking of the math. I suggest that you are wrong on one thing. David Ellsworth, the guru of "spirit forms", says that he can only do them with vacuum chucking, and some of them are less than one inch diameter. That suggests to me that your math is correct, but that David has such a gentle touch that he can finish the base with 11.5 lbs force, or less.

I do not have that gentle touch, I shall continue as I'm doing - and save a few bucks.

Best, Jon

[Jon Murphy]

David, you say thanks for starting the thread - and I say I'm thankful for starting it. A bit difficult to keep up with the format, but I'll get the hang of it soon.

I gather that the "jam" chucks you are using are specific to the piece and that you actually "jam" the piece on it. I use my formed jam chucks with half inch foam padding to provide friction - but that also reduces grip. Since writing the opening of the thread I've been using "blunt" tailstock centers and been getting a good concentric reversal.

It would seem to me that the thin walls (I turn to about 1/16th) would not support the "jam" without the pressure of the tail stock. A soft "hold" pressure, admittedly, but still the pressure of the tail stock.

Best, Jon

[David E Keller]

Jon, I don't think I've done any as thin as 1/16"... If I did, it was unintentional!

I've reversed little forms a number of ways... Sometimes I hollow out a piece of scrap to insert the reversed piece into, and other times, I use a small dowel or something similar to insert through the opening and rest against the bottom of the form. Lately, I've been doing more of the 1st scenario because it gives me total access to the bottom. I do use the tailstock with a small wooden insert replacing the normal metallic pin, but I remove it at the end for final cuts and sanding. If you turn a very small cone in the bottom of the jam chuck, it will help center the piece when reversing... Doing so introduces a whole new bit of fiddling since the cone has to be just the right size to work and not prevent the piece from sticking in the jam chuck.

I'm surprised that Ellsworth suggested that he could 'only do them with a vacuum chuck'... A man of his talents and creativity could likely do it a number of ways. I'm no Ellsworth, so if I can do it, I'll bet he could do it blindfolded.

[John Giem]

Hi, Jon:
The key to vacuum chucking is surface area for the differential pressure to act on. Small pieces don't have very much surface area. The amount of differential pressure directly affects how much surface area will be needed to develop an adequate hold.

A dust collector is typically just a centrifugal blower with the piping hooked up to the intake side. They can move a lot of air but do it at relatively low speed and do not develop much differential pressure. Plugging the system by closing all the blast gates simply stops the air moving and the blower doesn't really care.

A shop vac moves a smaller volume of air but at a considerably higher velocity than the dust collector, and the shop vac is more "determined" to keep that air moving. It can develop a significantly higher differential pressure than a dust collector. That allows it to be used for vacuum chucking as long as the piece being chucked has enough surface area for the air pressure to act on. In practice I find that the smallest piece that will be held well enough to do any work on is about 6" in diameter.

A vacuum pump system will develop a much higher differential pressure than the shop vacuum even though it is moving a relatively small amount of air. On a perfectly sealed system the theoretical maximum differential pressure is about 14.7 lbs./sq. in. since that is the difference between atmospheric pressure (at sea level) and a perfect vacuum. That means that on this hypothetical perfectly sealed system a 10" diameter bowl mounted on a 10" diameter chuck would be held on by over 1100 lbs. of air pressure (pi x radius squared x 14.7 psi) Unfortunately, since the surface area is directly related to the square of the radius, the holding forces drop off rapidly as the diameter goes down. A 5" diameter bowl would only be held by a total force of 288 lbs. That is still plenty for doing useful work but look how much change there is by cutting the diameter in half. A 1" diameter piece would only be held by 11.5 lbs. of force under the absolute best ideal conditions. That is really not enough to do much useful work.

Take care
Bob

Bob,
You are correct that the force holding the object on the chuck is the product of the pressure differential times the area. This is a problem for small items and in a few other cases. Since we are limited in the vacuum we can create, to increase the holding force the area must be increased. Yes, this is possible. I have developed what I call a Compliant Vacuum Chuck that addresses these limitations. (article to be published in the American Woodturner in early 2012.) The Compliant Vacuum Chuck breaks the barriers of small size, odd shapes, leakage of the wood, perforations and does not require cutting or penetrating your blank inorder to mount it. An explaination of how I can do this 'magic' is too long for this posting so if you are interested go to the AAW Forums, www.aawforum.org, and look for the thread 'Update on Vacuum Chucking Systems'. There are a couple of other strings there on vacuum systems that address some of the questions in this and other threads. The AAW Forum is free and does not require registration to read the posts.
Let me know if you have questions. BTW, I wrote the article 'Understanding - and Improving - Vacuum Chucking Systems' in the February 2011 issue of the American Woodturner.

John Giem