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Thread: Vacuum holder cup choice

  1. #1
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    Default Vacuum holder cup choice

    In order to be able to process the edges of a large piece in one go, I want to build a DIY vacuum holder for my milling machine. I can get a suitably powerful vacuum pump here, shouldn't be a problem. But all suction cups that I find online seem to have rubber orings, which seems to me would cause chatter during milling. I would probably need something that either has very hard rubber seals, or no rubber at all and some kind of oiled seal. Can anybody suggest suitable suction cups? Initially I will be working with acrylic sheets, but it would be nice to know what options are available for working with mild steel.

    So here's the workflow I am thinking about: Two steel rods with a groove. Each rod receives two suctions cups which slide in and are fixed at a desired position. Then the two rods are fixed onto the milling machine table at a desired position. The hoses from all four cups are combined and go to the vacuum pump. I place the acrylic on top of the cups, turn on the vacuum pump and the acrylic is held in place while I mill all four sides of it.
    Last edited by taydin; 02-01-2019 at 11:47 AM.

  2. #2
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    Default

    Helder Ferreira
    Setúbal, Portugal

  3. #3
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    MDF. It's porous enough that you don't need grooves.

  4. #4
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    I suspect you still need the seals. Just that if they are sized correctly in the grooves that they'll crush down and the material will still sit against the solid support.

    MDF. It's porous enough that you don't need grooves.
    I've heard of that too. But I suspect that the platten used to hold the parts would still need to have a seal around the edge and be at least painted with some sort of sealing enamel over all the surfaces other than the suction bed so only the bed forms the vacuum. And possibly a thin smear of silicon rubber sealant around the edge of the bed for a better edge seal of the part being machined?

    And it may take a while for the air to seep through those pores. So a "starburst" extending from the air port of at least deep scratches would likely help speed things up.

    The nice thing about a platen like this is that it can be stepped and set up with dowel pins to align the part before the vacuum is turned on then the alignment pins removed. And the acrylic won't try to suck inwards like it would with regular suction cups. And finally the greater surface area will give more retention force.
    Last edited by BCRider; 02-01-2019 at 05:36 PM.

  5. #5
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    I have made quite a few Vacuum tables. The sizes vary from 2 inch square to 24 x 48 inches.. For a seal I have used a 1/8 inch diameter foam cord. My source is MSC The last I purchased was $20.00/100 feet. If the panel being held is thin/flexible I place pieces of the cord throughout the "chamber" to support the material.

    The 3 pin option allows you place pre-cut material in the same place relative to X and Y Zero. I try to draw my part with the 3 pin corner as X0.00,Y0.00. If you wish to insure accuracy drill the pin holes with the fixture on the machine.

    The greater the surface area the greater holding force. Or the smaller the surface area the greater in/hg necessary.

    This is going on the assumption that you do not want to cut all the way through the material.
    Last edited by Stepside; 02-01-2019 at 08:11 PM. Reason: More Info

  6. #6
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    At a prior employer, we had a CNC router. The machine took complete sheets of plywood, OSB, or MDF, and cut out parts completely.

    There was a sacrificial table cover of regular particle board on the table. It worked fine with anything that was at least about 0.5 sq foot. Ubder that size, there could be problems with the part shifting. And that was with a wood router. The forces were considerably smaller than they may be with metal working. The vacuum system was a multiple stage centrifugal pump, which pulled a good vacuum even with a decent amount of airflow. There were no special seals necessary, the table just needed to be covered, so if half sheets, or metriic sheets were used, the extra space had a piece of plastic thrown over it.

    For metalwork, I would suspect the issue of cutting forces would be more significant. For the acrylic, it might be in between, and presumably the parts might be similar in size to the wood pieces we had with the CNC router.

    However, acrylic is smooth and slippery. You probably want to have a fairly high friction surface, like rubber bonded to some base material, to assist the vacuum in preventing sliding around of the hard, slippery acrylic parts.

    With a vacuum table, the amount of friction is limited with any given material, because the maximum pressure is only about 14.7 PSI (or metric equivalent), so the resistance to movement is limited. With a rubber or other high friction surface, the vacuum is less critical, and smaller parts can be held.

    One good thing is that the acrylic is smooth and would seal well against a rubber gasket. The gasket does not need to be thick, it can be quite thin, so part stability is not compromised.
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  7. #7
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    Quote Originally Posted by elf View Post
    MDF. It's porous enough that you don't need grooves.
    Can you please elaborate a little? So I placed the acrylic on the MDF. How do I suck the air so that the acrylic sticks on the MDF with sufficient force?

  8. #8
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    I guess a vacuum table where each of the high points between the grooves also has a high friction surface (some kind of hard rubber or something similar) would really help increase the holding force. Without the high friction surface, the acrylic will be pressing against the aluminum (or whatever the vacuum table is made out of) and it could slip given enough cutting forces.

  9. #9
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    One more question guys ... Let's say I buy a 300 mm by 300 mm vacuum table. When I want to process the edges of a 200 mm by 200 mm acrylic sheet on it, is there a way to have the sheet be a few millimeters higher than the table so that the cutter won't plunge into the table itself when cutting the sides? For example placing a number of standoffs underneath the sheet ... But will the seal still be good enough when it isn't pressed all the way down?

  10. #10
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    Quote Originally Posted by taydin View Post
    Can you please elaborate a little? So I placed the acrylic on the MDF. How do I suck the air so that the acrylic sticks on the MDF with sufficient force?
    You have to have a vacuum system that pulls air through the particle board. MDF is not a good choiice. The particle board is a cheap material that is like MDF but less dense and more porous, the MDF is denser, has a lot more glue content, and will not pass as much air, so holding force is not as good.

    You put something like the particle board down and then you accept the fact that you must cut through the acrylic and into the particle board, but not all the way through the particle board. If you are making the same parts many times, the cuts are in the same places, so you do not need to change the particle board, just keep using it.

    Standoffs would not be a good idea, sealing would be an issue. It could be done, but probably is more trouble than just putting down some rigid, but porous material like particle board.
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