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Thread: MDF or Baltic Birch torsion box top?

  1. #1
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    MDF or Baltic Birch torsion box top?

    I'm building a 4x8 torsion box top for my assembly table. I'm going to be drilling 20mm holes with a Parf Mark 2 system.

    The inside grid will be baltic birch.



    Should I make the top and bottom out of mdf for super flatness, or should I go with Baltic Birch that's lighter, less noxious dust?

  2. #2
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    I forgot to mention I'll glue and pocket hole screw is together.

  3. #3
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    What do you think of just screwing the top down, and no glue so the top can be replaced?

    The rest would still be glued and screwed.

  4. #4
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    My choice is the Birch. I actually used Birch and oak to build up a thicker top then put an 1/8Ē sheet of hardboard on top. I waxed it and Iím very happy with it. Iíve done that for my work table and my table saw extension table which also supports my router table.
    I can see either but Iím not a fan of working with the MDF and it is heavy if weight is an issue. Iíve used it before and it does have some benefits. So either isnít a bad choice.

  5. #5
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    If you already have stock, either material will do.

    If not, Plywood is more resistant to abuse and can be assembled with only glue and pin nails.

    Plywood will also readily accept the occasional screw for clamping fixtures, etc.

    https://www.canusawood.com/premcore

  6. #6
    For maximum strength and torsional rigidity, glue both sides to the core. You can add a sacrificial top layer.

    If the plywood is reasonably flat to start with and held down to a flat surface when glued up the torsion box should come out as flat as if made with mdf.

  7. #7
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    If you plan on using clamps that go through the bench dogs holes, your top is pretty much limited to 3/4" or the clamps will not fit through, so be careful with the idea of a sacrificial top as an add on layer.

    When I redo my bench, I will make the top a torsion box with the lower flat glued to the inner skeleton, but will screw the top on as it will be sacrificial with bench dog holes.

    Using either plywood or MDF, at 4' x 8', with either material, that torsion box will be heavy. I might be inclined to use a sheet of melamine for the top as it's easy to clean glue and other spills off.

    With a removable/replaceable top, if you use one material and don't like it, it relatively cheap to replace it.

  8. #8
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    MDF for me.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  9. #9
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    I'm planning to do the same. Its 3/4 in mdf for me. Planning on biscuits for outside for alignment and glue and 18 gauge brads. Thinned down poly for the top. My main goal is flat so mdf. If I wanted mobility I'd go with 1/2 ply.

  10. #10
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    Quote Originally Posted by ChrisA Edwards View Post
    If you plan on using clamps that go through the bench dogs holes, your top is pretty much limited to 3/4" or the clamps will not fit through, so be careful with the idea of a sacrificial top as an add on layer.

    When I redo my bench, I will make the top a torsion box with the lower flat glued to the inner skeleton, but will screw the top on as it will be sacrificial with bench dog holes.

    Using either plywood or MDF, at 4' x 8', with either material, that torsion box will be heavy. I might be inclined to use a sheet of melamine for the top as it's easy to clean glue and other spills off.

    With a removable/replaceable top, if you use one material and don't like it, it relatively cheap to replace it.
    life is a compromise, so I think i'm going to go the same route and do a glued bottom skin/inner skeleton setup, but the top will only be screwed down in MDF. if the skeleton frame isn't flat, glue and screws wont make the top flat.

    If the skeleton frame is flat then an mdf top screwed down should be pretty darn flat, AND i can replace it.

    Onto my next question. I'm going to wrap the box in maple I think. I want to drill 20mm dog holes around this perimeter.

    1.How thick should I make the perimeter pieces so I don't worry about them splitting if some force is applied to the dog holes for clamping?

    I'm thinking if the outside board is clamped inward towards the table, it doesn't need to be that thick, but if I ever applied pressure outward against a dog there, its possible it could split.

    2 more questions.

    I will be using through clamps on the table, like the Festool ones, but on the boarder I won't. I've read some people drill a hole all the way through the boarder board, this seems unessesary, but would help so wood dust/chips don't fill those holes.

    2. Do you guys think I should drill all the way through the boarder board?
    3. Does mdf come square from the factory, or do you still need to cut the edges to make sure its square?
    Last edited by Jon Steffen; 08-01-2020 at 12:24 PM.

  11. #11
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    1/2" mdf on mine with plenty of glue. 1/8" hardboard sacrificial top that is 8 years ish old and still good as new

  12. #12
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    I would drill all the way through on both the top and the side boards.

    I would use 3/4" Maple. You could use thicker, but I would hollow out the back to allow the Festool clamps to be used on the side as well.

    Just plan where the bench dog holes need to go and draw out your skeleton placement to give you the maximum use of the through clamps.

  13. #13
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    Quote Originally Posted by ChrisA Edwards View Post
    I would drill all the way through on both the top and the side boards.

    I would use 3/4" Maple. You could use thicker, but I would hollow out the back to allow the Festool clamps to be used on the side as well.

    Just plan where the bench dog holes need to go and draw out your skeleton placement to give you the maximum use of the through clamps.
    you think 3/4" maple with a 20mm hole drilled through it would be thick enough? I was thinking 1.5" of maple with the 20mm hole in the middle of it.

    I like your idea on the hollow out back side idea.

  14. #14
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    Quote Originally Posted by Dan Chouinard View Post
    1/2" mdf on mine with plenty of glue. 1/8" hardboard sacrificial top that is 8 years ish old and still good as new
    Ha, so in all likelihood I don't need a sacrificial top, because if I get 8 years out of one side, I can just flip it over have a fresh top. If I design it right that is.

  15. #15
    Double up the top with 3/4 BB. Add a 1/4Ē mdf overtop that if you need something replaceable.

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