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How much Vacuum is enough?

D

devonpalmer

Joined
Sep 24, 2004
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Location
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Website
www.devonpalmer.com
Greetings all,

I've recently assembed my first vacuum chucking system w/ an MDF faceplace, 8 and 4 inch PVC tubes, used gast pump etc...

My question is, what range of vacuum does everyone pull on items of difference size? Any guidelines/suggestions?

Also, how much vacuum should I get out of my Pump (3/4 horse). It'll pull around 23-24 inches alone, or around 20 attached to the lathe holding a piece (note: I haven't sealed my MDF faceplace yet. The PVC pipe sections fit tightly) How bad does it have to be before taking the pump in for rebuild?

- Devon
 
Thanks!

Jmarx,

What a fantastic reference! I'm sure it'll help delay the inevitable crack->split->separate->fling->duck! that happens with Vacuum chucking. The difference in strength between similar woods is surprising.

Please extend my thanks to your father as well...

- Devo
 
Nice article Devon. Do you mind if I post that in the Tips and Techniques subforum?

Also a side comment....there never seems to be sufficient vacuum in the house for some reason. I don't think is has to do with woodturning though.
 
More Data

A more extensive list of wood shear strengths is listed the USFS Handbook in Chapter 4, available online at

http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.htm

You will have to do a bit of conversion of the pressure units (listed in kilo Pascals, kPa), but you'll also find listings for green (wet) compared with 12% MC.

If we remember that vacuum chucking is only intended as a method to aid when we finish-turn the bottom of a piece where it was "fixed" to the lathe, most people are probably using way more vacuum than they need. If you had a piece spinning horizontally (as on a potter's wheel) how much weight would you think you need to hold it in place while you gently turn off the tenon and a little nubby? Consider that 20" of Hg roughly translates to 10 pounds of atmospheric pressure for every square inch on your drum chuck's area, and a 4" chuck has an area of about 12.56 square inches. Thus if you only pull 20" of Hg using a 4" chuck at sea level you're putting more than 125 pounds of pressure on the bottom of your bowl. Go to a 6" chuck and you more than double that to a bit over 280 pounds; at 8" it's over 500 pounds. I've held platters with an 8" drum chuck at less that 10" of Hg. without a mishap.
 
Small change/add

Jeff,

I left out that I've held 18-20" platters with that 10" of Hg. in my 8" chuck; makes more sense with that added.

Mark
 
1) Thanks Mark - I've made that change.

2) Also I mistakenly wrote "Mark Kauder" in a couple of places instead of "Mark Mandell". I apologize Mark Mandell ! That has been fixed.

3) Jim - thanks for the PDF version of the document, I've included it as well.

4) The "How Much Vacuum" section has been expanded to include a discussion from Mark Mandell from a separate posting. The changes are the 3 paragraphs starting with "A note of caution...".

5) I had to change the posting limits for the whole forum. Previously they were 10,000 characters per posting. The article is now 12,000 characters. The new limit for the entire forum is 15,000 characters.

6) -e- thanks for the vacuum gauge information, I'll add it to the article in the next 5 minutes. Since formatting options are limited in this forum, I'll make a link to your website. (Added to end of section "Minimum Setup (for Homebuilt System 2)". Very nice detailed pictures!
 
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Jeff,

Outstanding job; very good article. This will be a real resource to point new questioners to read.

Thanks for the effort.

Mark
 
Thank you very much Mark. I think this should help a lot of people.

I'll wait until Friday a.m., and if there are no more changes I'll post the extracted article in the Tips area.
 
Moving the article to the Tips section 5/13, and putting a redirect in the original posting in this thread. Thanks to everyone for the comments and the help.
 
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