# Vacuum clamping--venturi vs electric pump



## azlogger (Dec 20, 2013)

I built a 24" x 24" MDF vacuum table for my CNC shark. It's made with two pieces-the bottom one has a 1" grid of 1/8" wide x 1/8" deep saw kerfs, the top is just machined off on both sides and the air will supposably flow through it. Not sure how it'll work; we'll see.

The question is: should I power it with a venturi or an electric vacuum pump? We have an industrial duty ~80 gallon compressor, but the CNC and the compressor are both on a somewhat spongy wood floor. If the compressor runs while CNC is running, I get terrible finish quality because of vibration. I'm sure my table will leak some, so I might need a couple CFM to be safe?
My budget: no more than about $150 for the pump itself.
Thanks!


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## stevepeterson (Dec 17, 2009)

It seems like you know what you want (an electric vacuum pump). There are several to choose from at joe woodworker, but the price will be well over $150. Even their cheapest venturi system will run around $150 and will force your compressor to run a lot. Is there any way to isolate the compressor?


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## wapakfred (Jul 29, 2011)

Actually Joe has one that's in your price range. I have one of these and they are excellent pumps.


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## bigblockyeti (Sep 9, 2013)

The electric pump will be much more efficient, not to mention the vibration issue. I see used ones between $50 and $150 regularly on CL, sometimes good ones, sometimes junk.


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## azlogger (Dec 20, 2013)

Thanks for the quick replies! I like the looks of the pump Fred mentioned. I do have a ~25 gallon air tank that is old and unsafe for compressed air. So I should be able to pull a vacuum on the tank and have some capacity, so the pump can cycle on/off right?


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## hydro (Aug 9, 2013)

You might want to re-think the flow through spoil board idea on a budget of $150.00 for a pump. To make that work you need to be able to pull a lot of air and at the same time make a high vacuum to effectively hold the parts. Commercial CNC flat table routers use huge (big $$$) pumps for that purpose.

If you want to try it with a smaller pump, first get an "LDF" low density board for the platen and surface off both the top and bottom to remove the most compacted material and improve flow. Next, when you place your work piece, lay paper or plastic to block air flow through everything except the work. That will allow the pump to pull only on the work.

Finally, if you use a groove and gasket to seat the work piece on a platen that is NOT porous, and drill a small hole to pass air for vacuum, you will most likely have better results. Program the router to cut the groove, place the gasket, then drill the hole. When done, plug the hole with a dowel and do the next piece. When the platen gets too cut up, simply surface it off and start again. This is the process window manufacturers use to cut those odd shaped components. (they use their scrap wood for the platen and just replace it when it gets too thin)


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## azlogger (Dec 20, 2013)

I am currently using stretch wrap to seal unused areas. The edges and bottom will receive a thick coat of paint to seal them. Also, there is no gasket. I pull a vacuum on the entire table and seal it off with the stretch wrap. (In case you were wondering, this is my third table, I'm just wanting to upgrade from the old vacuum pump: the shop-vac


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## woodchuckerNJ (Dec 4, 2013)

You may want to use a shop vac too initially to figure it all out.
You will need to bleed some air into the shop vac so that it can breathe, since most breathe through the intake, some do not, and if you have one that has a separate cooling impeller great.

I have a GAST medical grade pump, not sure how many cfm it is, but it is too small I think.
They are great because they can run continuously, a shop vac won't… But for your testing the shop vac is a good indicator of how it is going to perform and what the needs are. That pump referenced above at 3cfm seems like it might do the job if there are no open holes on your platform. It's a two stage pump so that is good too.


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## azlogger (Dec 20, 2013)

Thanks Jeff!
I've been using the shop vac on my first two, I'll go ahead and machine the surface of the latest model and see how it performs with a shop vac.
I didn't know I needed to bleed air for the vac and I don't think it has a cooling impeller…LOL! Hasn't failed yet!!


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## Loren (May 30, 2008)

It seems to me venturi will be annoying to be around due
to the cycling of the compressor. I've never used one
so I don't know what it's like. I do know my compressors
are super annoying to listen to for me.

I use a GAST pump I bought on ebay for like $50. It 
makes a sound but is not very annoying.

Using a Shopvac for pressing seems like a good way 
to burn up the Shopvac. Consider how long you're supposed
to keep the vacuum going for cold press glues.


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## wapakfred (Jul 29, 2011)

That 25 gallon tank will work as a reservoir (if it's air tight), though that's pretty large. Actually if you check Joe's site you'll find he has all the gizmo's needed to set that pump up to hold a vacuum and cycle as needed. I built my pump from his plans and he uses a much smaller reservoir (a couple of pieces of PVC pipe) but the tank will do.


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## bigblockyeti (Sep 9, 2013)

You could have problems using stretch wrap with a vacuum pump vs. a shop vac as most vacuum pumps will pull a significantly higher vacuum than any shop vac could come close too. The concept sounds good as your consumables aren't that expensive, but finding a much heavier stretch wrap may be necessary to prevent it from failing. I've had hundreds of shop vacs apart and while many home vacuums have through flow cooling, I've never seen a shop vac from any manufacturer that does. The life on universal motors that you'll find in a shop vac is certainly going to be less than an induction motor you might find on a vacuum pump, but if it's working for you and you can stand the noise, you won't be out more than $100 or so for a replacement if it does fail.


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## crank49 (Apr 7, 2010)

HF sells a nice vacuum pump in your price range.
Here is a link 
http://www.harborfreight.com/two-stage-3-cfm-air-vacuum-pump-66466.html
Very similar to one I have used for years for "lost wax" casting.
It can pull very high vacuum, enough to make water boil at room temperature, at about 3 CFM.

Venturi systems usually require about 10 to 20 times more CFM than they generate so a 2hp compressor will only generate less than 1 CFM of vacuum flow.

But, if you were using a shop vac before, you might be needing high volume instead of high vacuum.


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## azlogger (Dec 20, 2013)

Reasons to stop using the shop vac:
1:I'm afraid it'll burn up in the middle of an important project. (I'm not worried about losing the shop vac, as that'll be a nice excuse to buy a dust collector)
2:It's too noisy.
3:Not strong enough for clamping small objects. (Big parts hold fine)

The longest I ever ran the shop vac was a two hour program, shut it off long enough to set up the next…then ran another two hour program. Tough old boy eh?!

Our compressor is a 5hp 16cfm. So divide that by 20 and we have .8cfm…I doubt that's enough…

I like it that joe's pump that Fred mentioned is oil-less. Means it should be pretty well maintenance free right?
Thanks again.


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## crank49 (Apr 7, 2010)

That "oil-less" pump is a diaphragm type pump I believe.
They are generally pretty reliable in vacuum service.
Not able to get really high vacuum pressure like an oil flooded pump, but way higher than a shop vac.
Might be a good choice.

The oil flooded pumps are quite trouble free as well, but you do have to keep an eye on the oil level and top it up every once in a while. The exhaust tends to have an oil vapor in it that can be messy. I installed a better filter on the output and solved that problem. Otherwise I have been running the same 1/2 hp single stage oil flooded pump that generates 27+" vacuum at 3 CFM since 1985. Not too shabby.


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