CNC Routed Salt Box #3: Redesigned Vacuum Clamp

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Blog entry by SPalm posted 11-20-2011 06:16 PM 8959 reads 2 times favorited 12 comments Add to Favorites Watch
« Part 2: New Vacuum Clamp & Ooops Part 3 of CNC Routed Salt Box series no next part

The vacuum clamp is now working like a charm. I also tried a new router collet and I was able to route a box with no problems.

After thinking about this (and getting suggestions), I changed the way it works. The new concept is to pull the work piece all the way to the flat of the HDPE clamp plate which will supply a flat, level and strong grip. I added dados that are just a bit shallower than the foam is thick. I also drilled a two inch ‘well’ for the vacuum to grab onto the wood. Yes, she sucks now. Thank you very much.

I still have to seal the bottom of the wooden work piece. I just used sanding sealer which dries pretty fast. With it I can pull 27 inches of Hg and my pump shuts off (I have a vacuum gauge). Without sealing the work piece the best I could pull was 18 inches, the pump never shut off, and I could twist the wood. So I will seal the wood first. But I can deal with this.

So to do a little math, 27 inches of Hg is the same as 13.25 psi of pressure. My small foam outline is 2.5” x 3” = 7.5 square inches. So 13.25 * 7.5 equals 100 pounds of pressure. I plan to add extra dados to be able to increase the total area when using larger work pieces. The foam is able to be reused and moved from dado to dado.

I remember now why I stopped using this CNC. It takes so much time to get everything right. I keep thinking I am close, but maybe this time….

-- -- I'm no rocket surgeon

12 comments so far

View blackcherry's profile


3338 posts in 3851 days

#1 posted 11-20-2011 06:28 PM

Set-up set-up set up when will it ever end, it time consuming to say the least Steve. The older I get the more impatient I get and it take more money to get to the end product. But it what make the whole thing worth while in this thing of ours / woodworking…lol best of luck hope this is the answer to your CNC set up delays…BC

View Ger21's profile


1075 posts in 3159 days

#2 posted 11-20-2011 06:51 PM

Your actual vacuum area used to calculate holding force would really just be the round pocket area, as there’s no vacuum where the wood makes contact with the plastic. At least in theory, there isn’t. ;-)

-- Gerry,

View Lee A. Jesberger's profile

Lee A. Jesberger

6859 posts in 4008 days

#3 posted 11-20-2011 07:05 PM

Hi Steve,

As always, very impressive. It’s amazing that simply sealing the block would increase the vacuum that much.

I had a few Lumberjocks at a picnic a few years back. I was demonstrating the vacuum clamping system I use on my work bench. I was using a piece of ultra lite M.D.F., and attempting to clamp it to the edge of the work bench. Something I do fairly often when edge banding a panel. The piece kept falling on the floor.

I couldn’t figure out what the problem was. A few days later it occured to me… it was the ultra lite M.D.F. permiting the vacuum to pull air right thru it. Just not dense enough. A hardwood, or even plywood panel would hold tight enough to drag the bench around.

Every little detail…

For a guy who isn’t a rocket surgeon, you do very well.


-- by Lee A. Jesberger

View Bob Kollman's profile

Bob Kollman

1798 posts in 3219 days

#4 posted 11-20-2011 07:33 PM


Looking good, I added to pictures of the vacum plate I use frequently. It is 13” diameter
the face grooves are about .08 deep when the plate is freshly resurfaced and regrooved.
I use plastic tubing as a seal, the tubing is held togeather with scotch tape (high tech).
The plastic hub in the center only acts as a centering pin and must be tight enough for
no vacum to leak, about .001 under size to the drilled hole in the center of the work media.
All the vacum pumps we use at work run constantly as the parts are held in place. I don’t
think I’d ever want mine to shut off when my work is spinning at 2000 rpm.

-- Bob Kenosha Wi.

View SPalm's profile


5320 posts in 3910 days

#5 posted 11-20-2011 07:51 PM

Thanks everyone.

Hey Gerry, that has been the question of the day around here. That nasty theory stuff. I too was calculating the clamping pressure by the size of the 2” hole, but I wasn’t sure. So I talked to my son who is a mechanical engineer and trained in all this stuff. He said very firmly that it is indeed the size of the foam box.

He said if it was the size of the hole, then you don’t need the foam. But if there was is no foam, it doesn’t work. The smooth plastic to wood bond will leak until it hits the foam. Even if you get a perfect bond between the wood and plastic, the instant you try to displace the workpiece, the vacuum will leak to the size of the foam box and grab it back down. And according to him, vacuum travels pretty fast.

But what do I know? I plan to add a finger tracks to the pocket for the vacuum to travel around easier.


-- -- I'm no rocket surgeon

View Pete Jansen's profile

Pete Jansen

250 posts in 2950 days

#6 posted 11-20-2011 08:26 PM

CNC is all about the setup. A good setup makes for good parts. I sometimes setup
a job on a HAAS machine at work with 20 tools. Takes hours to setup everything
to maybe machine for a couple minutes to make a one off part. But it’s a really
nice one off part. I like the concept, but for me I’d rather clamp the bottom in a vise at about .300” deep and sacrifice a little of the wood by machining the
back off. That’s how it’s done in metal, that’s how I do it in wood. Fast and easy baby!

-- Lovin' sawdust in beautiful Fort Collins, Colorado

View Joe Lyddon's profile

Joe Lyddon

10123 posts in 4081 days

#7 posted 11-20-2011 08:42 PM

Hey Steve,

I’ve been sitting here thinking a little about what you’re doing… then, I thought of something! REALLY! LOL

Sometime ago, my wife almost had an accident while leaning too far forward on a stool when the front legs slid backwards… and she almost went to the floor…

I got some liquid rubber to dip the tips of the front legs in… to stop it from slipping…

Well, I’m wondering if it would work on your table to increase the vacuum pressure!

I see the table flat (no dados… just the Well)... with of this stuff poured into the middle & evenly spread over all of it… I don’t know how many coats it would take… maybe 3 or so…

What do you think?

-- Have Fun! Joe Lyddon - Alta Loma, CA USA - Home: ... My Small Gallery:"

View Jim Jakosh's profile (online now)

Jim Jakosh

20600 posts in 3134 days

#8 posted 11-20-2011 10:05 PM

Hi Steve. I was looking at all that setup and wondering if you could use steel hold downs like we used in a milling machine vise. I have a pair I made that are just like the Starret brand and the hold good. They are abou1/4” thick x 3/4” x 6” long they have an angle along the back and then a slight angle on the front edge that is tapered real thin. When you clamp them in a vice they put a tremendous amount of pressure to pull the part down to the parallels in the vice. I thought that might work on the CNC table if you had a pressure devise from the side. I would use mine in the shaper on a 6×6 block of steel and shaving off 1/4” cuts.

That vacuum table looks like it will work pretty good and not mark the part at all!

-- Jim Jakosh.....Practical Wood Products...........Learn something new every day!! Variety is the Spice of Life!!

View JL7's profile


8667 posts in 2993 days

#9 posted 11-20-2011 10:19 PM

Hey Steve – This is really quite clever, and the sanding sealer trick is interesting – I will have to remember that little tip….

My crude little vac clamp “sucks”, but you’ve actually figured out exactly how much yours sucks….!

Looking forward to seeing the box.


-- Jeff .... Minnesota, USA

View Karson's profile


35126 posts in 4429 days

#10 posted 11-21-2011 03:40 AM

Steve: A great modification. Nice job.

-- I've been blessed with a father who liked to tinker in wood, and a wife who lets me tinker in wood. Southern Delaware soon moving to Virginia †

View lanwater's profile


3111 posts in 2962 days

#11 posted 11-21-2011 07:58 AM

It looks like a professional setup Steve.

-- Abbas, Castro Valley, CA

View mafe's profile


11732 posts in 3118 days

#12 posted 11-21-2011 06:00 PM

Steve you might not be a I’m no rocket surgeon, but a vacum surgeon instead…
That is so cool, I would love to play with a tool like that, I cant even stop to think of the new ways I could work.
Best thoughts,

-- MAD F, the fanatical rhykenologist and vintage architect. Democraticwoodworking.

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