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IndustryArena Forum > WoodWorking > WoodWorking Topics > Vacuum hold down for jigged parts without the weather stripping stuff?? Possible???
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  1. #1
    Join Date
    Mar 2006
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    168

    Vacuum hold down for jigged parts without the weather stripping stuff?? Possible???

    I'm about to start making parts. I'm going to be using jigged hold down boards (made of MDF most likely but not married to that) to locate the material blanks I'm cutting all the way through. I've got a decent little vacuum pump (maybe 3 - 4 times the size of the biggest one HF sells) and a vacuum grid made of High Density Plastic that's fixed to my tables T-slots.

    So, on top of this I'd like to have the MDF and cut ports in this to allow vacuum pass through to hold down each of the different parts cut. The MDF will be referenced to the vacuum grid that's fixed on my table and with holes that I'll position to work with each piece that's cut. when I switch to a different part, I'll just change the MDF that i have jigged to my table's vacuum grid and now I have a new fixture for the next part.

    But I'd like to avoid using the weather stripping tape. Is there a better way to create the seal?

  2. #2
    Join Date
    Mar 2003
    Posts
    35538
    I don't think so.

    You also might need something to keep the parts from sliding. http://www.allstaradhesives.com/ sells a "friction" tape called Z Grabber for this application. they also have many other vacuum hold down products.
    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)

  3. #3
    Join Date
    Jul 2005
    Posts
    12177
    Your pump is a bit small to do it by brute force, i.e. pumping fast enough to maintain enough vaccum despite leaks but maybe it is possible. You would need to seal the MDF used as the jig so that there was no leakage through it. When the jig piece is on the table seal around the edges with tape so no air can infiltrate between it and the table. Make sure the fit between the jig and the part is very good so the part cannot move sideways. Get everything setup and see what vacuum your pump can maintain; if it is 20 inches or better that will probably be enough.
    An open mind is a virtue...so long as all the common sense has not leaked out.

  4. #4
    Join Date
    Jan 2006
    Posts
    387
    So a 5 horsepower shop vac can't provide enough vacuum to hold down a piece of quarter-inch ply strongyl enough to let a half-inch bit cut through it.
    Wow.
    How big a vacuum pump do I need?

  5. #5
    Join Date
    Mar 2003
    Posts
    35538
    First, a 5HP shop vac isn't 5HP.

    How big a vacuum pump do I need?
    Depends on the application. If you try to pull vacuum through a 4x8 MDF spoilboard, you need a lot. If you have a dedicated fixture with a very good seal, a 1/4HP vacuum pump can work very well.

    Also, sometimes thin flexible panels can need more vacuum. Because they can flex, if the bit lifts a corner, the whole piece can quickly come loose.
    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)

  6. #6
    Join Date
    Jan 2006
    Posts
    387
    My shopvac is rated at 6 hp so I figure I have 3 hp on a regular basis. The table I'm trying to power with it is 14 inches by 18 inches. the vac fitting is on a short side, and there are 52 holes in the top for holding down stuff. but a quarter-inch-depth of cut with a .5 endmill at 200 ipm through pine will pull the piece off the setting every time.

    So I guess I recaulk the seams, gasketize the top, and use double-stick tape as well? I switched to GibbsCam from VCarve Pro and it doesn't have the capability of adding tabs to the workpiece easily. I may end up going back to VCarve Pro for everything that isn't a surface, if I can.
    Hm.......

  7. #7
    Join Date
    Jul 2005
    Posts
    12177
    A shopvac is not a vacuum pump. The best shopvac you will ever get is going to give you a vacuum of about half an atmosphere, yours probably does less than third of that which works out to maybe 5 inches of mercury. To hold things effectively you need more like 25 inches of mercury vacuum.
    An open mind is a virtue...so long as all the common sense has not leaked out.

  8. #8
    Join Date
    Jan 2006
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    387
    I will be using this setup with wood almost exclusively, what do you suggest I drive the table with?

  9. #9
    Join Date
    Mar 2003
    Posts
    35538
    I'd get a real vacuum pump and make dedicated fixtures, or pods. Vacuum Clamping (Vacuum Hold-Down Jig)

    But they can make it hard to cut all the way through.
    The best way is to pull vacuum through an MDF spoilboard. Shopbot guys are using 2-4 of these motors. You might be able to do it with 1 on a small table.
    ShopBot Vacuum Motors

    You also have to keep in mind that holding power is proportional to part size. Small parts will always be harder to hold down than small parts.

    On the big router we have at work, even with two 25HP pumps, any parts smaller then about 250 sq in will move if not onion skin cut.
    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)

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