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Thread: Is it ever OK to put screws in your bleeder board?

  1. #1
    Join Date
    Mar 2008
    Location
    Radford VA
    Posts
    586

    Default Is it ever OK to put screws in your bleeder board?

    I am helping a friend of mine setup a vacuum system on a Shopbot that he is in charge of at a local college. We are going to make the plenum out of standard MDF with 6 zones. Then glue down a 3/4" thick piece of Trupan for the bleeder board.

    I've had my own Shopbot many years without a vacuum table, so I am new to vacuum systems and I am accustomed to screwing everything down. Is it heresy to use screws to hold down small irregular pieces that don't go all the way through the bleeder board?

    I know you can use a sacrificial piece of plywood and screw to that and hold with the vacuum. But, I just wondered if your could screw into your bleeder board without ill effects in order to save turning on the vacuum for small jobs.
    PRT Alpha with 7.2 upgrade, indexer, and PC router

  2. #2
    Join Date
    Feb 2013
    Location
    River Fall WI
    Posts
    765

    Default

    Yes, that is fine we do it when small parts just will not hold.
    Kyle Stapleton
    River Falls Renaissance Academy
    Math/Technology Education Teacher


    PRS Alpha 96x60 2.2 hp spindle, Double Air drills, 6" indexer, Fein 5 zone vac table
    Desktop w/spindle
    Potter Pen
    Aspire 8.5, Creo 3.0

  3. #3
    Join Date
    May 2014
    Location
    MA
    Posts
    434

    Default

    You can also put a "carrier" board on top of your spoilboard and vacuum it down. Then you can fire screws into that carrier board. I've done that before to avoid putting screws in my bleeder board, especially when the bleeder board gets real thin and there's not much meat left for screws.

    I've even routed a little channel into the underside of a carrier board to direct a lot of suction at a smaller part.

  4. #4
    Join Date
    Nov 2012
    Location
    Lancashire, UK
    Posts
    64

    Default No screws or glue!

    Dear Kyle

    We have a series of thin laminated boards that we place next to the 'part' to (a) help with the vacuum and (b) help hold the 'part' from moving see attached picture.

    StairString.jpg

    Also we use a 12mm thick MDF sacrificial board. We don't glue it down as we change it every two or three months. There are some location blocks around the edges then use the vacuum to hold it 'flat' while it is surfaced also while it is being surfaced we place masking tape round the edges to stop it moving.

    Any way that's my two penn'orth.

    Sincerely and in good faith
    Martin

  5. #5
    Join Date
    Nov 2012
    Location
    Lancashire, UK
    Posts
    64

    Default Seal the plenum

    Dear Kyle

    I forgot.

    To maximise the 'vacuum' don't bleed air through the plenum. When you have machined the plenum zones, seal it with lots of coats of sealer and when you think you have finished sealing it - give it another coat. When we bought our first machine (second hand) the guy said his best decision was a vacuum bed and gave us the 'more sealer' advice. Of course we didn't follow his advice and gave it two coats of sealer, it looked fine. For months we couldn't understand why everyone raved about 'vacuum' we were putting screws in all over the place. Then while changing the sacrificial bed I decided to give the plenum another couple of coats of sealer. We couldn't believe the difference. We gave it more coats the next time we changed the bed.

    Remember; vacuums don't 'suck' down, they evacuate the air from underneath and atmospheric pressure pushes down. If you could evacuate all the air then the maximum hold down pressure (at sea level) would be 14.7 pounds per square inch. As soon as the air bleeds out from the vacuum you start to reduce the hold down. I've even taped over behind the kerf to help prevent bleeding the vacuum.

    Now I've had more than my two penn'orth.

    Again sincerely and in good faith.
    Martin

  6. #6
    Join Date
    Feb 2013
    Location
    River Fall WI
    Posts
    765

    Default

    Martin,
    Thanks, great tips for others.
    Your picture does not show a small part and the forces of a 3d cut are normally not the same as a 2d profile.
    Kyle Stapleton
    River Falls Renaissance Academy
    Math/Technology Education Teacher


    PRS Alpha 96x60 2.2 hp spindle, Double Air drills, 6" indexer, Fein 5 zone vac table
    Desktop w/spindle
    Potter Pen
    Aspire 8.5, Creo 3.0

  7. #7
    Join Date
    Dec 2007
    Posts
    2,300

    Default

    I split my spoil board into 4 sections with a 6mm piece of sintra between each. My plenum is 1 inch thick sintra also. The reason is I found out that when I was only using 1 zone I had a piece of material all the way over to the other side and at the other end of my table and I went to push it it was held down. I now get better vac hold down

  8. #8
    Join Date
    Nov 2012
    Location
    Lancashire, UK
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    Default

    Dear Kyle

    It is the only photo I have showing how we can 'zone out' each part of the table with laminated boards. Laminated because they don't bleed air. I don't have photos but we use this technique with the 19mm roto tip cutter when cutting vision panels in 54mm fire doors. That really tests the hold-down.

    Again sincerely and in good faith.
    Martin

  9. #9
    Join Date
    Mar 2008
    Location
    Radford VA
    Posts
    586

    Default

    Martin - Can you give specifics on what you used as your plenum "sealer"?
    PRT Alpha with 7.2 upgrade, indexer, and PC router

  10. #10
    Join Date
    Dec 2008
    Location
    Diamond Lake, WA
    Posts
    1,604

    Default

    Here's what I do. 1" MDO with T-tracks and clamps. Vacuum system holds the MDO firmly in place. Eliminates having to put screws into your spoil board. I've got two of these to provide 4' x 8' T-track hold down.

    Attached Images Attached Images
    Don
    Diamond Lake Custom Woodworks, LLC
    www.dlwoodworks.com
    ***********************************
    Life is not a journey to the grave with the intention of arriving safely in one pretty and well preserved piece; But to skid in broadside, thoroughly used up, worn out, bank accounts empty, credit cards maxed out, defiantly shouting "Geronimo"!

    If you make something idiot proof, all they do is create a better idiot.

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