Vacuum chuck pump questions

Hi all,
I want to build a vacuum chuck. I'd like to build one that can hold a piece
up to 10" square with the ability to block off some holes and use it for
smaller pieces. I'm trying to figure out what pump is appropriate to try to
find used. In looking at what's available from MSC and others, it appears as
though a pump that pulls at least 26" is about right. How important is the
CFM rating in this application? I'm assuming that a pump's max CFM will
influence how quickly the chuck builds vacuum, and depending on how much
leakage gets past the item I'm machining and its contact with the chuck, how
easily it'll maintain its hold. Any suggestions about minimum specs I
should be considering.
Thoughts and suggestions welcome.
Peter
Reply to
Peter Grey
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We have used the really inexpensive venturi type vacuum pumps when the leakage was kept to a minimum. These pumps work on shop compressed air and have no moving parts. CFM rating will depend on how much leakage there is in the setup. If coolant is used in machining a catch tank in the line will be necessary to keep it from getting to the pump.
Reply to
ff
Those venturi vac pumps are real air hogs. You can use up to 10x the energy to run a compressor to get the same vac as a tiny vac pump would give you. Fine for occasional use but miserable on a continuous basis like you would have for a vac chuck.
ff wrote:
Reply to
RoyJ
26 inches of vac works out to about 85% of your potential 29.92 inches, not a bad number to shoot for. But given the potential for leakage and open holes, you should be a lot more worried about CFM.
Peter Grey wrote:
Reply to
RoyJ
"Peter Grey" wrote: (clip) Thoughts and suggestions welcome. ^^^^^^^^^^^ I am not a machinist--I am a woodturner. Vacuum chucks are becoming very popular on wood lathes, and I use one a lot, so I will tell you what I can. First, the amount of vacuum you need obviously depends on the area holding. If the contact area is several inches in diameter, you can probably get by with less than 26" Hg. However, I believe the tool forces in metal turning are much higher than wood, so I think you will have to experiment. It is tricky to get a good vacuum seal, which means you will be fighting leaks. If your pump has limited volume, small leaks will reduce the vacuum, and drive you nuts. My pump is a 3/4 HP Gast vane pump--I can be fairly casual about leaks, 'cause the pump is that large.
In wood turning, we use foam gaskets to form the seal to the vacuum chuck, but I am afraid that if you tried that on a metal lathe, the deflections due to compression of the gasket would destroy your accuracy. If you can use a machined fit between the vacuum chuck and the work piece, possibly with a little wax or grease to help with the seal, I think you will be good to go. Another possibility would be to use hot glue to seal leaks.
It is good to have a close-fitting ridge to hold the workpiece against radial slippage--the vacuum force is good for holding the piece ON, but it isn't too good for holding it CENTERED.
Reply to
Leo Lichtman
I should have mentioned that this for use on my mill. I want to hold some fairly thin pieces for finish work. Double sided tape (in conjunction with clamps) works for some of the pieces, but when using the tape, the thinner ones are susceptible to bending when removing them from the table; hence the desire for something less "permanent".
Peter
Reply to
Peter Grey
Peter, I was lucky to find a nice vac chuck at auction. It uses a Gast rotary vane type pump that pulls about 27" under load. I have sen these in surplus catalogs for well under $100. For light finishing cuts it works just fine. The bigger the piece the better it hold. I just use duct tape to mask the unused areas of the chuck plate. Another easier solution is to use fixturing tape. I think MSC or J&L Supply sell it. It is a very thin tape that releases under low heat, like from a heat gun. It holds well and won't distort the piece or leave a residue.
Glen G.
Reply to
glen
I built a waffle plate, a plate with a grid pattern of slots. They are .125" wide and .058' deep. I found some .063" diameter O-ring material. With a hole in the middle you can put the O-ring material in a pattern to match the part. You can overlap the O-ring for one grid space. No need to cut it short either. Finer grid, better match to the parts.
Next part, new pattern. This also works great for parts with a center cutout, you can outline "exclude areas" with the O-ring.
Pete
Reply to
Pete Logghe

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