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  1. #1
    Join Date
    Mar 2012
    Posts
    26

    Vacuum pump combination

    I've tried searching for this, but I'm not sure how to word it...

    I found some rebuilt Gast 1/2hp rotary vane vacuum pumps locally for 120 bucks each. These pull high vacuum but only about 6cfm @10"hg. What I'm wondering is what would happen if I used one of these pumps in combination with something like one of the Ametek vacuum pump/blower motors? Gast pump pulls high vacuum and the other motor makes up for the flow. This makes sense in my head but I'I really have no background in any of this to know if it would actually work. I'm kindof assuming for some reason that it wouldn't work, or I would have found other people trying this by now.

  2. #2
    Join Date
    Jul 2005
    Posts
    12177
    It will not work either with the pumps in series or parallel. In parallel the lowest vacuum will be determined by the pump rated lowest for vacuum, in series the flow will be limited to the lowest flow pump. If you want high vacuum and high flow you have to go to large size multi-horsepower vane pumps.
    An open mind is a virtue...so long as all the common sense has not leaked out.

  3. #3
    Join Date
    Jan 2006
    Posts
    2985
    Think of it like combining a pressure washer hose and a garden hose into one pipe. It will act pretty much like just a garden hose with just a tiny boost. As Geof says, it will not work. Some commercial machines have vacuum pumps on the order of 50HP or more.

  4. #4
    Join Date
    Mar 2012
    Posts
    26
    Yeah, after thinking about it a while longer I think I see why it wouldn't work. I'm sure Geof is spot on. I think I'd rather go with the rotary vane vacuum pump and use a tank of some sort for an initial seal. I have a lot more reading to do.

  5. #5
    Join Date
    Jan 2006
    Posts
    2985
    A tank won't help you. There really is no substitute for a big old vacuum pump. Most folks are using MDF or similar as a spoilboard and it is basically a sponge, one huge leak. You just need lots of flow to overcome the "leak" that is your spoilboard.

  6. #6
    Join Date
    Sep 2012
    Posts
    1195
    I'm going to guess that this is for a hobby machine, so I'll base what I say on that a bit. From an industrial perspective, it seems that the norm is to have about 1/3 hp per square foot. If you have a 4x4 table, you need around 5hp minimum. 10hp for 4x8, etc.

    The real question is how big is your table? Depending on how big your router is, you could use a group of these pumps with each one set to perhaps a 18"x18" zone. If you have a 3'x3' table, you could use 4 of those pumps and probably get pretty good vacuum levels. Of course, it may not be that much of a savings over buying one larger pump at $120 each, but on the other hand there is some redundancy and you can split the power consumption over several electrical circuits. It's definitely not a totally crazy idea if your needs are minimal.

  7. #7
    Join Date
    Jan 2013
    Posts
    119
    The principle would be the sane as a two stage air compressor
    When I need to use vacuum, I use my air compressor as a vacuum pump and vacuum pods.
    I can pull about 23 hg, and I am at 2400' MSL.

  8. #8
    Join Date
    Mar 2012
    Posts
    26
    Well, this machine is a CRP 4x8 router. I don't know if you would consider it hobby or not, but it's being used in a production facility. It's our initial step into CNC machining some parts that we have been outsourcing. So for reasons I'd rather not discuss, I was given this project and I have been charged with the task of making it work on a "hobby" budget so to speak. I'm cutting full 4x8 sheets of 1/2" treated plywood into lots of large curved parts. I'm basically planning on making designated spoilboards with the cut paths sealed. Biggest hurdle right now is deciding on a vacuum source.

    If air compressors work so well when converted to vacuum pumps, why are they not more commonly sold that way? I get that they are essentially the same concept, but vacuum pumps seem to cost so much more than air compressors.

  9. #9
    Join Date
    Dec 2010
    Posts
    1230
    That is a damn good question jet8300. I've wondered the same thing.

    Decent multi horse pumps can run quite a few grand, but I would be happy to pick up another 3.5hp compressor to convert if it works as well.

    Brian
    WOT Designs

  10. #10
    Join Date
    Mar 2003
    Posts
    35538
    Check out these motors and the links on the Shopbot forum for systems using them.
    ShopBot Vacuum Motors

    I would expect that you need 4 of them for your application, which is probably going to cost you about $1000 or more to get up and running.

    Re-reading your post, though, if your cutting treated plywood, you might not have much luck with any system. We have two 10HP Becker pumps on a 5x12 table (pulling through an MDF spoilboard), and plywood is the toughest thing to hold down. The reason, is that it's rarely flat. I'd guess that treated plywood is even less flat. I've frequently had plywood that was warped too bad to hold down as soon as we cut the bands off the bundle.
    Gerry

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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  11. #11
    Join Date
    Jan 2013
    Posts
    119
    I did not say it works as well as a regular vacuum pump.
    " When I need to use vacuum, I use my air compressor as a vacuum pump and vacuum pods.
    I can pull about 23 hg, and I am at 2400' MSL."
    23 inches hg is only about 84% of potential vacuum at the altitude I am at.
    All I do when I use my compressor for vacuum is the remove air filter, insert hose and
    connect a open hose fitting.
    As far as piston vacuum pumps, look here.

    Indo Air Compressors Pvt. Ltd. manufacturers reciprocating air and water cooled versions compressors. It has a most modern factory, which spans across an area of 6300 sq.m. and is fully equipped with modern manufacturing and testing facilities. These

    I would guess that the piston vacuum pump has a longer stroke, in relationship to its bore.

  12. #12
    Join Date
    Mar 2012
    Posts
    26
    Well, I rushed into buying a used Becker U2.165 because it was a great deal locally but after some experimentation it seems like it's not powerful enough. It's 5HP but from what I can tell it looks like it only flows about 100cfm. I'm having a hard time getting the plywood to seal to the spoil board even after sealing everything up in between really well. I have a square grid plenum and ran EPDM o-ring material around the outer perimeter. If I put a big sheet of plastic over the spoil board, I get almost full vacuum. With the plywood on it all I get is around 5 in/hg.

    Safe to assume I'm not moving enough air? I want to look at running 4 of those Ametek motors, but even that sounds like it probably wouldn't work based on your info ger21.

    So now I'm looking at just screwing the plywood down to the table, but even that isn't going to work because the plywood isn't flat. Really grasping at straws here and getting frustrated.

  13. #13
    Join Date
    Mar 2003
    Posts
    35538
    Whatever is under the spoilboard (plenum) needs to be 100% leak proof. Is the pump connected in one place only? Perhaps a Y fitting and two lines?
    Also, make sure the spoilboard edges are sealed. If the spoilboard is regular MDF, remove at least .02" off of each side.

    With nothing on the spoilboard, does the vacuum hold the spoilboard down?

    Plywood is the worst material to use with a vacuum table, because it's rarely flat. On our 5x12 table with two 10HP beckers, I can lay a single piece 15"x30" in the middle and cut through it at 1000ipm without it moving, but if a sheet of plywood has more than an inch of bow when lying on the table, it won't hold down. Make sure you put it on the table with the corners bowing down and the center up. It will hold the best that way. Sometimes we need to climb on top of the sheet to get the vacuum to grab.
    Gerry

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    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  14. #14
    Join Date
    Mar 2012
    Posts
    26
    I made a vacuum plenum out of two sheets of MDF glued together. I then painted it and coated the whole thing in urethane. If I put a sheet of plastic over the table i get very close to what the vacuum pump is rated for.

    I've attached a couple photos of the router and the plumbing, as well as a picture of one of the layouts i need to cut.

    Attachment 212850

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    Click image for larger version. 

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  15. #15
    Join Date
    Mar 2003
    Posts
    35538
    The simplest thing to do is to lay your plywood directly on the gasket (no spoilboard), and leave about 1/16", then cut the parts free with a laminate trimmer.
    Like I said, make sure the spoilboard edges are sealed.

    Will it pull a sheet of particle board down? If so, then the problem is the plywood.

    The small parts are going to give you trouble even with good vacuum. Make sure you cut them first. I'd onion skin them all first, then go back and cut them all the way through. But there's still a good chance that they'll want to move.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  16. #16
    Join Date
    Jan 2021
    Posts
    1

    Post Re: Vacuum pump combination

    I don't think it's working motherly. In parallel, the lowest vacuum was determined by the pump rated lowest for vacuum, If I put a big plastic cover over the spoil board, I get the almost full vacuum. many times the problem will happen even using a good quality vacuum pump.
    If ir air compressors work so well when converted to vacuum pumps, then it's the very complicated way? I get the result with very essentially the same concept but the vacuum pumps are more expensive as compare to any Reciprocating Air Compressors.

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