vac hold down

[woodman08]

I would like to ask if anyone has experience with vac puck hold downs,i need to know the size of vac pump i should be looking at to run say 6-6x6 in vac pucks and double acting ---sucks the table and sucks on the work piece
A shop vac will not do the job ,the vac starts to heat up

[steve_g]

Stan...
A shop vac motor is cooled by the air going over it... by trying to create hold down vacuum you have stopped all its cooling air.

I don't think there is any way to get enough vacuum from a shop vac for hold down, but if you did you would have to leave enough leakage for motor cooling.

I was semi successful using my dust cyclone for hold down on one project by leaving an onionskin of material uncut and using down cut bits. I used a vacuum fixture of 24" X 36" or a very large "puck" my blanks were precut to fit in the fixture snugly and the only thing the vacuum was doing was preventing uplift.

SG

[Bob Eustace]

Stan I use a vacuum generator powered by compressed air. Very low cost if you already have the air.

[woodman08]

so you're using a venturi and what size is it ,as i did see one at 3.2 scfm and 28 in,will this be ok for 4-6 pucks and is it deamnding a hugh airflow

[bleeth]

Stan: I use a compressor run system for vacuum bagging. That uses a lot more compressor action when first engaging the unit as there is a much larger volume to evacuate. In the case of hold down pucks there is very little evacuation necessary and you shouldn't tax your compressor much at all. It is important to ensure hose and fitting leakage in your system is at least minimized or, at best, eliminated.

[billp]

If you don't want to run your compressor just to use a pod/puck system there is an easier alternative. Get a "laboratory pump" (shown in picture #1) . You can get them used on Ebay ( go oil less rather than oil filled) for between $75-$200. Gast is one of the better pump manufacturers, and you can buy replacement parts for all of their models if you ever need them. Since you are really evacuating cubic INCHES of air, rather than cubic FEET, pumps as small as 1/8 hp will work, however you want to be sure you are pulling at LEAST 18"of mercury , and preferably more to offset ANY system losses due to bad gasketing, poor connections, etc. I use a 1/4 hp Gast that pulls 25"of mercury on my pod/puck setup.
Picture two shows a 1/2 hp pump used by a Shopbotter to run HIS double sided pucks, but please be aware that many people can NOT run pucks/pods that are double sided because their table surface is either MDF or Trupan (light MDF). These surfaces are porous so you are going to get inefficient, or unusable results. You need a surface that will NOT bleed air, such as sealed plywood, plastic,sealed composite, solid PVC, etc., etc.
And you DO need gasketing ( All Star Adhesives .com) because having such a small surface area means you can NOT rely on just a surface to surface interface to avoid air/vacuum from escaping.
In picture#3 you'll see a pod with thick gasketing ( too thick really) and a brass air fitting. The brass fittings seem to hold vacuum much better than the aluminum fittings and don't cost a lot more. You can make pods with fittings on both ends, and then daisy chain them all over your table in whatever array you need them. Instead of using a double sided pod simply screw these pods to your current table, OR build a "pod sled"which is simply a piece of sealed plywood upon which you have pre fastened some pods. You then simply drop the sled on your existing table and start to cut.
And pods are SO powerful their effect is still usable beyond their own physical limits. In picture#4 you see a pod upon which I cantilevered a piece of wood. Even when carving out on the piece's perimeter it is still held rock solid by the pod. Pods can be ANY size. For years I used an 11"by 14" pod to do lithophanes.
And as Dave points out above, once you have some sort of a system in place you can get double duty by using the SAME pump for vacuum bagging, or thermo forming.
Pods can be made of plastic decking, ( "Trex" ) Pods can be made from PVC. Pods can be made from ANY sealed/solid material that does not leak air, so they are certainly cheap enough to experiment with.
I'll be bringing a vacuum pod/puck setup to the Summer Camp Shopbot at Colby College in Maine on July 13th.
There used to be an entire section of the Shopbot Wiki dedicated to setting up a pod system, but it seems to be "down"at the moment. I had listed suppliers, parts numbers, etc. for building a complete system. Maybe that will reappear in the near future.

[curtiss]

Great information Bill / 25 in of Hg wow...

I guess that is why you get to wear the cool hat....

[billp]

The Wiki is now back up and running. Vacuum pod info can be found here- http://shopbotwiki.com/index.php?search=vacuum&fulltext=Search

[jerry_stanek]

I have a harbor freight brake bleeder that I use for my puck. It is the one that uses the compressor model 92924. Tried it and it worked really good

[woodman08]

thanks for the info on the pucks,pictures were great.
I have ordered a gast pump 5.5 sfcm an 28in mercury,i should be set
good point about the cnc table and sacraficial table ,both mine are mdf but the cnc table is very dense the sacraficial one might be porous and i may have to remove it,but the sled sound good as i have lots of z axis room .
The idea of using the deck plastic is good too but i managed to locate some 1 inch plastic for pucks.
I really appreciate the article you wrote ,helps me stay on the right track

woodman ontario