Why does my vac table work?


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    Member Rink's Avatar
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    Default Why does my vac table work?

    Good afternoon. This is my first post here, although I’ve been lurking for some time now. I have a small hobbyist CNC (Next Wave Automation Piranha) purchased used from our son when he upgraded to a 4’X8’ CNC. The Piranha is a good starter/learning machine, and a larger CNC may be in my future, possibly an Open Builds Lead 1010 or similar. I use Vectric VCarve Destop and Fusion 360, depending on the job. My geographic location is close to sea-level.

    I spent many long hours reading this and other forums about CNC vacuum tables. I read about Pascals, inches of HG vs water, Microns, PSI, etc. I know about leakage, and the difference between hold down force and lateral force. Gasketed pallets, venturi pumps, rotary vane pumps, diaphragm pumps, two-stage pumps, zoned systems, etc. It seems there is an infinite array of approaches, with costs ranging from $50 to hundreds or even thousands…and results across a similar spectrum. I studied until my brain was mush. So after a friend with a CNC Shark build a simple vac table and got good results, I did the same and my table works incredibly well for the jobs I do. My question is how/why it works because something doesn’t seem logical to me and I’d like to understand it before possibly making it the basis for my next, larger, set-up.

    Specs: The spoilboard is 1/2" MDF, 12”x18”, with 199 quarter-inch holes. Therefore, each hole is 0.049 sq in, for total hole area of 9.75 sq in. The holes are spaced 1” apart on the x and y axes and there are a few extra holes around the side of the board for free air flow. Directly under the spoilboard is a gridboard of 3/4” MDF, with quarter-inch grooves in the X and Y directions intersecting under the spoilboard’s quarter-inch holes. On the bottom side of gridboard is a manifold to accept a 1” vac hose. The vac hose inserts into a plenum that is connected to the gridboard grooves. The two pieces of MDF are stacked, screwed together, and the edges are taped with blue tape for sealing. Otherwise, no sealing of the MDF surfaces. The whole contraption is permanently affixed to the Piranha’s aluminum t-slot table.

    The $35, 3-gallon, 3 hp Shopvac I’m using for suction has a 1” hose. Therefore about 0.7854 sq in of area. I would guess about 1.5 PSI, but that's not important for my question today. My last one lasted about a year, and I just replaced it. I figure $35/year is acceptable for my hobby. Pros: cheap, effective. Cons: loud, loud, loud.

    I’ve calculated that about 16 of my quarter-inch spoilboard holes will provide about the same area as the shopvac hose (0.784 sq in). Therefore, if I cover 183 holes, leaving 16 holes open…that should allow the shopvac to pull as much air through it as if the hose were completely unobstructed. So I do that…I place my workpiece on the table and cover all but 16 of the remaining open holes with various sized pieces of plexiglass. The suction force is great, well more than enough to hold the workpiece in place while milling. And the vac sound never changes…in other words, it never produces that sharp whine that you get when you start restricting the air flow.

    So…if the vac is getting air equivalent to a completely unobstructed hose…how can it still provide strong suction to the other holes? That doesn’t seem logical to me…but it works! And if it's getting air equivalent to an unobstructed hose, shouldn't it last a long time without "burning up"?

    Thx, rink.

    P.S. I’m thinking about whether a zoned table with a cheap vacuum pump would be a better solution for my next set-up. I’ll have to ask some questions about that in another thread.

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  2. #2
    Member ger21's Avatar
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    Default Re: Why does my vac table work?

    The vacuum is pulling through the entire sheet of MDF, not just the holes.

    Gerry

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    Default Re: Why does my vac table work?

    Hi Rink - is not about the air "flow". It's about the difference in pressure from the atmospheric side to the suction side (force = pressure x area). There will be porosity between the job and the spoilboard and as Gerry says the mdf is porous. So if you buy an absolute pressure gauge you can measure the atm pressure (say 1bar or 15psi) this is pushing down on your job surface (and your skin). The air pump is creating a lower pressure on the back side of the job so the pressure delta is what is holding the job down. The horizontal force is friction which is the friction co-efficient x the load on the job. So if its plastic it could be 0.1x this load if rough metal it could be 0.5x the load, if rubber it will be 1.0x the load. Rubber has a very high friction co-efficient.

    Now a proper vacuum pump can create near 1atm of pressure. So for small jobs needing high forces your air pump/motor won't do. I'm metric so 1atm is 10 tonne per sq metre which is a lot of force on a large object. If you use a proper vac pump you need to use a gasket and aim at zero leaks. This will give you maximum holding force. If the parts are sheet products and large, a relatively low delta pressure will work fine as you have found. That's why a vacuum cleaner or shop vac works, its static pressure capability is quite small... I just looked up a blower fan and it makes -7kPa pressure so thats 7% vacuum calling 100kPa 100% vacuum and my vac pump pulls 99.99% vacuum. cheers Peter



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    Default Re: Why does my vac table work?

    ger21 and peteng:

    Hi guys, and thanks for the info. I’ve read some of your posts on other threads also, and I’ve learned a lot, appreciate you sharing what you know.

    At some point in my planning for the next set up, I’ll post some questions in another thread about vac pumps, zones, reservoirs, etc.

    Thx, rink.



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    Default Re: Why does my vac table work?

    Hi Rink - you can't store vacuum. So vac reservoirs do not work., They only make the pump work harder to clear the bigger volume. The only case where they work is if you use a small capacity pump to pull down a chamber that is used to pull something down fast. But that's also limited if you want to do that often. Usually means the pump is undersize for the job or you should use a blower for the pull down and vac pump for the holding..... cheers Peter



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    Default Re: Why does my vac table work?

    You mustn't make the mistake of using a high vacuum/low flow type of pump.You need a high flow pump because of all the leakage in a typical system.The plumbing for a zoned system can be fairly extensive or you can just lay a piece of plastic sheet over the area that isn't being used.You need to be aware that some of the high flow systems have relief valves so that they don't pull too great a vacuum,additionally a shop vac needs some leakage to admit enough air to keep the motor from overheating.



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    Default Re: Why does my vac table work?

    You mustn't make the mistake of using a high vacuum/low flow type of pump.
    Unless you want to make dedicated fixtures, in which case, they can actually work better.

    You need to be aware that some of the high flow systems have relief valves so that they don't pull too great a vacuum
    What situation would you want less vacuum?

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

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

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


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    Member routalot's Avatar
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    Default Re: Why does my vac table work?

    A high vacuum/low flow system won't work well if it has to pull a vacuum through an MDF spoilboard.I've been there and done that and all that happened was that the leakage in all directions through the MDF gave insufficient clamping force.If you can make an impervious fixture and use it without using anything as porous as MDF at any point in the system it can hold very well.In fact it can hold too well unless the fixture is really well designed.I used a gasket type seal to hold a piece of tooling block and the gasket groove was a bit shallow which resulted in the tooling block bowing due to the forces generated.After gluing a slice on the back and re-cutting the gasket groove a successful outcome was achieved.In most situations I would prefer flow rate to ultimate vacuum.

    As for the use of vacuum relief valves-take a look at what Busch have to say on the subject https://www.buschvacuum.com/document...0870132194.pdf .If they think its necessary for their pumps I'm happy to believe them.



  9. #9
    Member ger21's Avatar
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    Default Re: Why does my vac table work?

    If the relief valve is that important, why is it an optional piece of equipment.
    The Beckers I've used don't appear to require one?

    Maybe for a different application where you are actually pulling max vacuum??

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

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

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


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Why does my vac table work?

Why does my vac table work?