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Thread: MDF or Baltic Birch torsion box top?

  1. #16
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    Just did this a couple of months ago for my new "Paulk style" workbench top. I used 3/4" birch ply for the top, screwed on for when I want to replace it. I also used the Parf system for drilling my holes in the top, which worked out great. I left the bottom (glued on) as a solid surface for an assembly table. One thing which I realized about 1/3 of the way through drilling all the holes in the top is the horrible blow-out when the drill went through the last layer of the ply -- I started using a clamped-on backer board, but still got mediocre results.

    On my next top, I'm definitely going to drill the holes before attaching it, and use a proper backing board. MDF is probably more forgiving, since the ply I used just looks terrible when I have the thing flipped over in assembly table mode (even though I am the only one seeing it, and you have to look inside at an angle, it still bothers me.) With one sheet each of 3/4" birch ply for the top & bottom (and 1/2" for the webbing), the table is still light enough for me to pick up, maneuver around, and flip without too much difficulty. I'm not sure if the added weight of MDF would make it unwieldy. The top is flat enough for my purposes (I can't see a light underneath a 4' level, but it's not surface table flat.)

    Hope you have much success!

  2. #17
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    Quote Originally Posted by John M Wilson View Post
    Just did this a couple of months ago for my new "Paulk style" workbench top. I used 3/4" birch ply for the top, screwed on for when I want to replace it. I also used the Parf system for drilling my holes in the top, which worked out great. I left the bottom (glued on) as a solid surface for an assembly table. One thing which I realized about 1/3 of the way through drilling all the holes in the top is the horrible blow-out when the drill went through the last layer of the ply -- I started using a clamped-on backer board, but still got mediocre results.

    On my next top, I'm definitely going to drill the holes before attaching it, and use a proper backing board. MDF is probably more forgiving, since the ply I used just looks terrible when I have the thing flipped over in assembly table mode (even though I am the only one seeing it, and you have to look inside at an angle, it still bothers me.) With one sheet each of 3/4" birch ply for the top & bottom (and 1/2" for the webbing), the table is still light enough for me to pick up, maneuver around, and flip without too much difficulty. I'm not sure if the added weight of MDF would make it unwieldy. The top is flat enough for my purposes (I can't see a light underneath a 4' level, but it's not surface table flat.)

    Hope you have much success!
    I'm glad to hear I'm not the only one that gets annoyed by minor mistakes that only I know about.

  3. #18
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    Have you ever heard of Xtira MDF? .
    Looks like its made with an exterior moisture resistant glue. Would this make it resistant to wood glue? I'm calling them tomorrow.
    https://miratecextira.com/extira-exterior-panels/
    Extira-board-outlined-1-1.jpg

    Its $60 a sheet, but for my torsion box top, might be sweet.

  4. #19
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    $60 for a sheet of Extera is a pretty good price, Jon. (.75") And yes, the benchtop inserts on my guitar bench as well as the top for my auxiliary bench are Extera. Yes, it's water resistant. No, it' not glue resistant. You sill will want to put a finish on it to help with that. Guitar bench has oil. Auxiliary bench has "left over oil based poly".
    --

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

  5. #20
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    Quote Originally Posted by Jim Becker View Post
    $60 for a sheet of Extera is a pretty good price, Jon. (.75") And yes, the benchtop inserts on my guitar bench as well as the top for my auxiliary bench are Extera. Yes, it's water resistant. No, it' not glue resistant. You sill will want to put a finish on it to help with that. Guitar bench has oil. Auxiliary bench has "left over oil based poly".
    Bummer, perhaps the malamine mdf cored top with a finish on it will be best for glue resistance then?

  6. #21
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    Melamine is inherently glue resistant, at least for PVA type glues, but it's still best to wipe it off at first opportunity.
    --

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

  7. #22
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    Also, when I'm doing a glue up, where I know I'm going to get drips that I can't get to, I'll put a layer of parchment paper down. That keeps the mess off my work tops. This is especially true for cutting boards where you have a lot of glue lines, but really want to reference the flatness of the assembly table.

  8. #23
    With melamine and laminate work surfaces I find it easiest to keep them waxed and pop glue drips off with a scraper once cured.

  9. #24
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    some great input here... these projects require a lot of planning and foresight. i did the same, and it was no easy task... but maybe I did not think things through as well as you are. Starting here was a great idea.

    To make the torsion 4x8 FLAT, how will u do this? long straight edge references? Straight on one plane is NOT FLAT over the entire surface. You can just imagine all the scenarios of square and straight, but NOT flat. Square is easy, Flat is not... I am big on FLAT surfaces. MDF will "give" or conform to the flatness of your torsion box. I would love to read a good article on how to build a large FLAT torsion box dead FLAT. Not sure what reference to use that is larger than the 4x8 to assure the torsion box is FLAT on top. a 5x10' Flat Granite surface would be NICE!

    When you build your torsion box, remember, to fine tune it to flat, you might be hand planing areas a few thou here and there...MDF does not plane... BB does with a sharp blade.

    I went with 1/2" MDF top x 2, the top sheet being sacrificial. Only the Festool right angle clamps seem to require 3/4" max thickness top to insert, which I dont use, all other components, I have NO issues. So many great 20mm clamp accessories available today, many from UJK in the UK, which I order from. To replace the top sacrificial panel, I use the panel below as the new guide to drill holes for perfect alignment, so no need to return to the Parf II system (PITA) which I used to drill the original holes, i.e. both 1/2" sheets secured together to act as a 1" sheet. I nail the edges down mostly at the corners of the MDF to the torsion box, so I dont hit nails while using the Festool saw. it works pretty good... not much pull up. If you dont plan to use the top to cut with track saws, this is a non issue and I agree, secure it everywhere.

    As for the finish, love that water resistant MDF shown above, never knew about that before, damnit, I would have used it. I went with a black matte dye mixed with water based poly.... many many coats, sand, re apply etc. Matte finish... the look is superb, however, water still seems to penetrate to the MDF...so next time, I would use the water resistant MDF and do the same.

    Also.... The Parf II system is quite the novel invention. I had to return a few parts as they were not SPOT on which will throw off square. I finally ended up with what appeared to spot on sticks. I was meticulous in using the Parf system, took me forever... while the holes are damn good (as for square) considering the relatively low price of the Parf system, dont expect perfection, specially over 4x8'. Errors magnify over distance. Again, this assumes squareness of the holes matters to you... if only clamping, not an issue. But if you plan to use it as a fast cutting station, well then it does matter. After you are done, use the large Woodpeckers squres to check for square, if you are good at right angle math, you can easily determine how out of square you are... or use the 5 cut method, many videos on this, even by Peter Parf himself.

    Another thing I learned after the fact was... a few companies make "lifters", prob. not the proper name... which allows all the tracks to lift up 5mm IIRC, to place a 5mm sacrificial cut sheet under your stock and cut lines, this prevents cutting through your Parf top and avoid drilling all those holes again and again.... I would re consider this next time for sure. I bought some of them to experiment from a company that 3d prints all these 20mm components.

    The other lesson I learned is... 3/4" MDF is the material on the Festool tables... but remember, these are for field use, light weight fold-able, etc. But for fixed shop use, if you go with 3/4" use something super strong, such as ultra dense MDF or as u suggest BB Ply.... When mounting some bolted clamps in, i find a 1" top much more robust, it resist buckling. Of course, how tight your torsion box is, and how close the fasteners are that secure the top mattes here. AGain, if you plan to cut through the top, u must be careful with fastener locations, put them where you wont cut them, so less secure to the torsion box.

    My other lesson learned... on my front apron, which I use waaay more than I thought for holding long boards for edge planning... USE 1" heavy duty BB ply for sure! 3/4" can get sloppy with all the holes you drill to have the ideal hole in the right place for fast and perfect board placement. I under estimated the value of the apron, I use it as much as the top.

    Lots of improvements if I ever build my second table. Quite the project...

  10. #25
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    Just another plug for water resistant MDF. The brand I use is Medex. Superior to the standard stuff in every way. For the little bit of MDF I use, I only buy Medex anymore.

  11. #26
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    thx Frank
    where do u buy this Medex?
    Does it machine like MDF or more difficult I assume?
    Does water bead off it?
    Do u still finish it ?

  12. #27
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    Quote Originally Posted by Will Blick View Post
    thx Frank
    where do u buy this Medex?
    Does it machine like MDF or more difficult I assume?
    Does water bead off it?
    Do u still finish it ?
    PJ White Hardwood or Windsor Plywood in Calgary. Water mostly beads up but with extended exposure, the surface will wet. The stuff isn't intended for prolonged submersion. I left a piece underwater for a week and there was some swelling at the very edges. For exterior use, they recommend sealing cut edges with something like epoxy before painting.

    Glue bonds very well with it. It machines just like regular MDF. But edge delamination is not nearly as much of an issue. It's also a fair bit stronger.

  13. #28
    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.

  14. #29
    Ian Kirby wrote a how-to article on torsion boxes back in 1982 in FWW # 32, well worth looking up.

    I worked for 25 years on a 4' x 8' table with a core of basswood 5/16" x 3 1/2" strips about 4" on center and skins of 1/4" luan plywood with hpl surfaces. It sits on supports within 16" of the ends and 8" from the edges. If your support is smaller you may need to add strongbacks or make the box a little thicker. Mine is a little shabby from errant sawcuts and mallet blows, but still serviceable and flat overall, and can be moved by one person easily, probably 70 lbs or so. I think about 4" thick is about right for a really stout work surface that will remain flat no matter what it is set on. If you are going to drill holes through it you should lay them out to avoid the core grid. Any spots that need more solidity for fastening can get a filler block.


    I have found it a little easier to assemble a core of halflapped continuous strips than stapling numerous individual pieces. The hardest part is making the first table flat. Mine was second generation, clamped with cauls to a torsion box that was assembled on a 3x6 cast iron machinist's setup table. After that it is easy to breed more with the "beast with two backs" technique. If you want to screw to your worktop you will want thicker skins, in which case you can go with larger spacing in the core grid. The core merely transfers shear stresses to the skins, so tight joinery in the core is not important, just sufficient glue surface to keep the skins together.

    I have a lighter worktop that I pull out of the plywood rack for occasional use about 42" x 90" x 2 1/4" thick with 1/4" lauan skins. It's not perfectly rigid torsionally but if the horses under it are level it is good for veneering or cabinet assembly.

    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.

  15. #30
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    Quote Originally Posted by Kevin Jenness View Post
    With melamine and laminate work surfaces I find it easiest to keep them waxed and pop glue drips off with a scraper once cured.
    Do you mean that if something does drip onto it, you just leave it and then scrape it off later, rather than wipe it up right away?

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