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Thread: Shop-built Plywood Question

  1. #1
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    Shop-built Plywood Question

    Building a bench top from a thick slab brings some challenges; slab availability, drying time, stability to name a couple. I have been thinking that building a slab up from thinner stock with the grain angled about 5 degrees might stabilize it. And premium wood could be used for the top, or an existing bench could be renewed and made thicker by adding a couple of 1/2" toppings with the first angled a few degrees for stability.

    Anyone try this type of lamination? Was it stable.

  2. #2
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    I follow several wooden boat building channels & it seems like that's a common way of fabricating bulkhead walls. The slats looked to be somewhere around 1/4" thick.

    This should be an interesting discussion.

  3. #3
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    Quote Originally Posted by Tom Bender View Post
    ....building a slab up from thinner stock with the grain angled about 5 degrees might stabilize it...
    Not quite sure what you are describing. Are you saying that you would alternate the grain direction with the layers?
    Brian

    "Any intelligent fool can make things bigger or more complicated...it takes a touch of genius and a lot of courage to move in the opposite direction." - E.F. Schumacher

  4. #4
    I would think any panels built that way would need to be balanced, that is alternating layers on each side of the core running the same direction. A five degree off set doesn't amount to much. To restrain movement overall the best bet would be to have ninety degrees between layers. I know there are thick laminated lumber panels available to the building trade but how suitable they would be for a workbench whose first requirement is to stay flat, I don't know. I think David Sochar has experimented with "shop made plywood" for door panels; perhaps he will chime in.

    I don't see why a solid slab would be a great choice for a bench except for appearance. A reasonably stable top can be made by ripping kiln dried flatsawn 8/4 hardwood into 3-4" strips and face laminating them to get a rift sawn slab.
    Last edited by Kevin Jenness; 09-14-2021 at 9:28 PM.

  5. #5
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    If you want plywood with alternate layers oriented only about 5 degrees from one another, try making it from elm, which grows that way. It's called interlocked grain, and it makes splitting very difficult. I don't see an advantage for a workbench.

  6. #6
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    90 degree plys restrict movement. I doesn't work if the plys are thick. 5 degree plys would allow movement so they could be any thickness. An existing 2" thick bench top could be topped with a 1" ply and should remain flat. Neither ply could crack. I think,

  7. #7
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    Fascinating question Tom. May I ask some clarification questions please?

    What are you trying to accomplish? How much trouble are you really willing to go to to reach that (unknown to me) goal?

    Briefly, I see most of the timber in Michigan is grown in the UP and I don't know where in Michigan you are. I do know you are in the lower 48 and have ready access to relatively plentiful and inexpensive wood in enormous quantities, relative to me.

    The vast majority of timber I can lay hands on is west coast Doug Fir, for lumber sized bits more Doug Fir and Spruce-Pine-Fir from Canada (SPF) distinct from SPFs, Spruce Pine Fire from the lower 48.

    If you are looking for laboratory grade flat on day one that is going to last for decades you might consider MDF layers for weight and perhaps quarter or half inch melamine for the top surface. I had never seen melamine more than a few mils thick before 2021 and I don't know if they make it half inch. I have some quarter inch thick pieces on the property now that boggle my little brain. Building up melamine and MDF for thickness and weight should be fairly straight forward.

    When I think of shop built plywood I think of boat builders with a significant capital investment in tooling, like these guys, see the third pic down from the top: https://epoxycraft.com/projects/caro...ic-with-epoxy/ , not straightforward process in a typical suburban garage.

    Anyroad, don't want to shoot my mouth off in ignorance. Still a fascinating question. Peace be with you.

  8. #8
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    Look at Cross laminated timber: https://www.apawood.org/cross-laminated-timber Solid wood at 90 degrees into monster panels. But I really don't see the need nor benefit for a workbench. The old way of edge glued solid lumber or several layers of plywood, MDF, or particle board, skinned with whatever you want, have been used for hundreds and dozens of years, respectively. My favorite bench top is an old edge glued lab bench that I flattened and then skinned with 1/8" Masonite, which I change when it gets all beaten up.

    John

  9. #9
    That's pretty much how I build these. With the waterfall miters, any faultline wouldn't be constrained by the adjoining piece. So, I build the panels in layers so no checking can go all the way through. the rabbet hides that its not a solid pane. I've had no issues so far.
    Attached Images Attached Images

  10. #10
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    Quote Originally Posted by johnny means View Post
    That's pretty much how I build these. With the waterfall miters, any faultline wouldn't be constrained by the adjoining piece. So, I build the panels in layers so no checking can go all the way through. the rabbet hides that its not a solid pane. I've had no issues so far.
    Very nice work

  11. #11
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    Ok one thing I'm trying to get to is thick veneer, maybe 1/4" or more. If applied cross grain it will crack due to moisture effects. But if it is approximately parallel to the substrate it may be more stable.

  12. #12
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    Quote Originally Posted by Tom Bender View Post
    Ok one thing I'm trying to get to is thick veneer, maybe 1/4" or more. If applied cross grain it will crack due to moisture effects. But if it is approximately parallel to the substrate it may be more stable.
    The laminations in cross laminated timber are thicker than 1/4". I think I've seen reference to some of them with plies 3/4" thick.

    Some folks are building exterior door panels in much the same way, using a thin plywood core, then a thick piece of solid lumber on each side, then two cross veneer layers over that.

    What I take out of the exterior door panels in particular is that only the surface layers need to be thin to prevent splitting. The interior layers are not subjected to the same unbalanced stresses so they can be much thicker. So if you want to make a laminated bench top I would make it with cross laminated lumber and two layers of 1/8" cross banded veneer for the top/bottom surfaces. Personally, however, I'd just use two or three layers of plywood and skin it with Masonite.

    John

  13. #13

    Overkill?

    I think you are trying to build a stable bench top by building it up from laminations running at 5 degrees to each other.

    I think you are doing what I often do - overthinking it. A bench top needs to be sturdy, flat and reliable. A shop made plywood bench top can b e very stable - the key, as Kevin J said - is balance. Think of the top and bottom layers as "control" layers, both running the length of your top. Inner lamination should be at 90 degrees to each other. You will end up with an uneven number of layers (5,7,9, etc) and a top like commercial plywood.

    I have run various thicknesses at 90 degrees to each other for several years now in exterior work - demanding conditions that will bring out any weakness. After about 30 years of experimenting with this, I can't get a panel to crack. The usual configuration now is a core of 1/4" or 1/2" exterior ply, with solid wood applied either side, both plies with grain the same directions, then a 1/16" " cross band at 90 degrees to the previous layers, and finally a face veneer, 1/16" thick, going the same direction as the second layer. The solid wood on=e either side of the core is always the same both sides, and can be from 1/16" thick up to an inch. Balance is the key.

    Yes, this appears to break all the rules, but it performs perfectly. Think of lumbercore ply. Or the old Singer sewing Machine tops - that is where I first saw the rule flagrantly violated, without problems. I even have samples I have made and thrown out in the rain gutter to see how - and if - they come apart. They try, and they try, but they only fail after a few years of abuse in the gutter.

    As for your bench, I'd ask why the extreme? And more importantly, how will you keep everything flat at assembly? I am old school on this: make the top out of nice, 3" wide stock - 5/4 or 6/4 being my first choice - glue up into 12" wide planks, face and plane, and then assemble those "boards" together for your top. Solid, flat and renewable.

    My primary bench is made from 5/4 Red and White Oaks, and has worked extremely well since 1985. 2-3/4" x 42" x 90" - it could be bigger, but then I'd need more shop. It has produced over $10M in work, so it is the most important tool in the shop. I check for flat about once a year, an it is very flat, essential in my work.

    2nd Best Bench.jpg
    I want to stay as close to the edge as I can without going over. Out on the edge you see all kinds of things you can't see from the center.
    - Kurt Vonnegut

  14. #14
    David, I see the photo is labeled "second best bench.jpg." What is the first best bench?

    Nice to see a shop with signs of use. Some posted here look like a surgical theater. I like the doors in the background too.

    How's the book coming along?

  15. #15
    Kevin, well it is the best bench. The photo was labeled "Second Best" in the line up for the book. It is a jpg file, and publishers want tiff files. So, only tiff photos are 'First' rated photos. I love the bench dearly, and only the second bench in the shop is my second favorite.

    Funny you should mention it, but the book is done, and is being printed this week. It should be available next week at all the online outlets. Underline 'should' since nothing in the publishing has happened the way we thought it would happen. It has taken about 6 months just for editing.

    "Small Shop Production of Custom Wood Doors" will be a bit expensive, but will easily be worth if for anyone interested in the subject. About 30 drawings and 25 photos will help explain what in the heck I'm talking about. As we have all lamented before, there just is no good published info. Even the Internet, gawd bless it, is hit and miss on reliable info. Thanks for the opportunity to plug the book, but I was hoping for an interview from Oprah.
    I want to stay as close to the edge as I can without going over. Out on the edge you see all kinds of things you can't see from the center.
    - Kurt Vonnegut

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