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Thread: MDF or Baltic Birch torsion box top?

  1. #31
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    Quote Originally Posted by Don Stephan View Post
    Perhaps I'm too old and uneducated, but for me an assembly table does not get holes drilled in it, which could get glue along their edges. When gluing I use clamping cauls and bar clamps, and check for square by measuring the diagonals. Planing faces and edges is done on a Klaus style workbench.

    My assembly table is a torsion box, with a 1/4" x 4' x 8' melamine foil sheet lightly screwed to the top of the torsion box. I did wonder if the foil surface would be durable enough, but after about 6 or 8 years it is still in good condition.
    No idea Don, i'm a newb with a track saw, hoping to make an accurate repetitive cut mft type table to make a bunch of cabinet carcass and learn to make segmented bowls, that triples as an assembly table and work table.

    I'm hoping to make a large portion of the table have no holes in it though, but if need be, i'll probably throw down a few sheets of rosin paper.

  2. #32
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    Quote Originally Posted by Kevin Jenness View Post
    The key to getting a flat surface is to start with one and clamp to it. You can make a carefully levelled base with freshly milled 2x4's or the like, but it is best to clamp the new assembly to the base no matter how you assemble it. Just laying one skin on the base and pocket screwing the core elements to it won't ensure that everything is in plane. A set of curved cauls is very useful for gluing up.
    I've seen several done in this way, milled 2x4s with shimmed saw horses, hot glued to the floor and measured for level at a ton of different ways....

    So do you recommend clamping the ends of the MDF to the ends of the 2x4s, and many clamps along the long edge to the 2x4 under its edge?

  3. #33
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    Quote Originally Posted by Jon Steffen View Post
    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?
    I bought a 49x97 sheet of mdf today, its exactly 49x97 and corner to corner was spot on. SO....hopefully MDF 6-10 years from now will still be 49x97 and perfectly square.

  4. #34
    "So do you recommend clamping the ends of the MDF to the ends of the 2x4s, and many clamps along the long edge to the 2x4 under its edge? "

    I would recommend clamping curved cauls under the base and over the torsion box. Lacking sufficient clamps and cauls for that, the assembly should be at least clamped down around the perimeter of the base and checked for contact in the center before gluing.

    If you are drilling system holes around the edges at least 3/4" should be left between the hole edges and the outside of the rim.

    If you are going to screw the top to the core, grid members should be at least 1/2" thick, and the screws will need to be laid out accurately to get a reliable purchase. The screws should be located to miss the holes and the holes, if you plan to insert Festool style f clamps, should be clear of the interior grid. If you want to get the full structural advantage of a torsion box both skins should be glued, or at least fastened very thoroughly. Be aware the screws in the top are always a risk for cutters set too deep.
    Last edited by Kevin Jenness; 08-03-2020 at 12:28 AM.

  5. #35
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    I built mine with 3/4" ply. The lumber store called it "Russian Birch"-pretty much Baltic Birch as far as I can tell. I used the Perf Guide system, and drilled a full grid of the tiny pilot holes then drilled every other pilot hole with the the full 20mm holes. The other, every other grid, was the grid for the torsion box ribs. Then, I used the pilot holes to screw the whole thing together while the glue dried, then I took the screws out. It worked really well. The resultant cells, which are 192mm center to center, are the perfect size to fit the Festool clamps and still be able to rotate 360 degrees. It is my assembly bench. I also have a Roubo style woodworking bench, which I like, and use a lot, but I use this torsion aft for 85% of my work.

    To answer the original question, I considered both mdf and ply, and I am very happy I chose ply. Its finished with Waterlox, which is what I had on hand and durability of the ply is a non-issue with my work. I think the top weighs about 180-190 lbs, and it floats on the base, which means I can move it around for flexibility.
    torsionmft.jpg
    Last edited by scott lipscomb; 08-03-2020 at 1:14 AM.

  6. #36
    I have a 4' X 8' assembly table on a metal base. My torsion box top is MDF on top and Particle board on the bottom. A bit less weight and $$$. It is 5" deep and extremely heavy. I has supported woodwork, metalwork, motorcycles etc. Great shop addition. Good luck with yours. Oh yeah, I vacuum bagged mine no fasteners in the faces and I just throw a 1/8" HDF sheet on top when things get ugly.

  7. #37
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    Quote Originally Posted by Kevin Jenness View Post
    "So do you recommend clamping the ends of the MDF to the ends of the 2x4s, and many clamps along the long edge to the 2x4 under its edge? "

    I would recommend clamping curved cauls under the base and over the torsion box. Lacking sufficient clamps and cauls for that, the assembly should be at least clamped down around the perimeter of the base and checked for contact in the center before gluing.

    If you are drilling system holes around the edges at least 3/4" should be left between the hole edges and the outside of the rim.

    If you are going to screw the top to the core, grid members should be at least 1/2" thick, and the screws will need to be laid out accurately to get a reliable purchase. The screws should be located to miss the holes and the holes, if you plan to insert Festool style f clamps, should be clear of the interior grid. If you want to get the full structural advantage of a torsion box both skins should be glued, or at least fastened very thoroughly. Be aware the screws in the top are always a risk for cutters set too deep.
    Thanks Kevin, i'll have to research what curved cauls are.

  8. #38
    I do not use MDF for hardly anything. I hope it works out for you. It's dust is NASTY. It doesn't hold screws well. It swells from moisture very badly - even falling apart. It has very little strength - much less than plywood. So it sags at the slightest opportunity. Can you tell I do not like it?

    The top of my outfeed/accessory/work table is 3/4 plywood with 20mm holes on 4 inch centers. Works very well. I usually use a sheet of foam board on top when cutting up sheet goods with the track saw. But I do crosscuts on the table using dogs. So I have one kerf from that that can be filled in if it starts to bug me. It is built paulk style with a second top 8 inches below and ribs with big holes separating the two tops. Frequently used tools go on the lower top (or underneith) for easy accessability. I just added a leg vise at one end.

    I think you're worrying too much about replacing the top - but maybe not if you use MDF. I am pretty confident my table will never have to have the top replaced. But I don't worry about minor nicks and stains. It may not look pretty 10 or 20 years from now but I am confident it will still be structurally fine and able to serve it's purpose. If not, I guess I will build another. It's probably close to 10 years old already.

  9. #39
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    Quote Originally Posted by Jim Dwight View Post
    I do not use MDF for hardly anything. I hope it works out for you. It's dust is NASTY. It doesn't hold screws well. It swells from moisture very badly - even falling apart. It has very little strength - much less than plywood. So it sags at the slightest opportunity. Can you tell I do not like it?

    The top of my outfeed/accessory/work table is 3/4 plywood with 20mm holes on 4 inch centers. Works very well. I usually use a sheet of foam board on top when cutting up sheet goods with the track saw. But I do crosscuts on the table using dogs. So I have one kerf from that that can be filled in if it starts to bug me. It is built paulk style with a second top 8 inches below and ribs with big holes separating the two tops. Frequently used tools go on the lower top (or underneith) for easy accessability. I just added a leg vise at one end.

    I think you're worrying too much about replacing the top - but maybe not if you use MDF. I am pretty confident my table will never have to have the top replaced. But I don't worry about minor nicks and stains. It may not look pretty 10 or 20 years from now but I am confident it will still be structurally fine and able to serve it's purpose. If not, I guess I will build another. It's probably close to 10 years old already.
    You could be right. I'll see how much time is invested in drilling the parf grid before I decide if i'm gluing the top and bottom or just the bottom.

  10. #40
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    Quote Originally Posted by Kevin Jenness View Post
    "So do you recommend clamping the ends of the MDF to the ends of the 2x4s, and many clamps along the long edge to the 2x4 under its edge? "

    I would recommend clamping curved cauls under the base and over the torsion box. Lacking sufficient clamps and cauls for that, the assembly should be at least clamped down around the perimeter of the base and checked for contact in the center before gluing.

    If you are drilling system holes around the edges at least 3/4" should be left between the hole edges and the outside of the rim.

    If you are going to screw the top to the core, grid members should be at least 1/2" thick, and the screws will need to be laid out accurately to get a reliable purchase. The screws should be located to miss the holes and the holes, if you plan to insert Festool style f clamps, should be clear of the interior grid. If you want to get the full structural advantage of a torsion box both skins should be glued, or at least fastened very thoroughly. Be aware the screws in the top are always a risk for cutters set too deep.
    okay, i've read up on curved cauls a bit. When you do this, I can envision if you put too much clamp pressure on the ends of the curved caul, it could make the base actually bend and form to the curve.

    Do you fine tune the clamping pressure to try and make the skin have consistent contact with the gridwork, so it doesn't bend?

  11. #41
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    I haven't done a torsion top, but I always think of the old David Marks video when he made one. IIRC you set up sawhorses perfectly level to each other to avoid twist in the surface. Most torsion top videos have that as one of the initial steps.
    NOW you tell me...

  12. #42
    Curved cauls are used in pairs- when clamped together they should meet in plane.

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