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I want to glue two 3/4" (11/16" actual) sheets of Baltic Birch plywood together to make a 1.5" (1.375" actual) thick sheet.

I then want to add two "legs" coming out of the edge of the plywood, so that it looks like this:

 _________________________ 1.375" thick
/________________________/|
|                        ||
|                        ||   <- two sheets of BB plywood glued together
|                        ||
|________________________|/
   |_|/             |_|/

^
|



dowel rods (diameter tbd)
   each ~6" long
   3" glued into edge of 1.375" thick plywood, 3" exposed

Not pictured is a "base" with holes that receive these legs. The idea is that this piece will usually sit in this base but can be removed by hand. The holes in the base will be a touch larger than the dowel legs so it'll go in and out without any force.

  1. Is it feasible to drill into the edge of glued-together plywood like this?
  2. Is it feasible to glue a dowel into this edge? Should I use a fluted dowel?
  3. How big a dowel diameter can I safely use? I was hoping to use 3/4" so the leg is substantial, but that is over half the thickness of the 1.375" thick plywood sheet. Could 1/2" work?
  4. Is there a better option? I'd like to keep the legs centered on the edge (i.e. not attached to the front/back of the piece). I had thought of using three 1/2" thick sheets to build these pieces up (instead of two 3/4" sheets), using the middle layer to create the legs and holes, using taller pieces for the legs and leaving pieces out where the holes need to be. I prefer the drilling option if feasible, because it simplifies my materials list and plans.

EDIT: The sheet is a removable backrest of a sofa. It's two sheets of Baltic Birch glued together to make total dimensions of 79"L x 20"H x 1.375" thick. I had originally planned to use four 3/4" thick dowel rods for the posts, spaced 16" apart, leaving about 6" of space on either end. I had planned to wrap the upper exposed 1" of the leg in some felt, and the very bottom 1" of the hole in felt, so that the leg goes in/out of the hole in the base easy, but fits snug once fully inserted.

Max
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  • Welcome to Woodworking SE, also that name is taken ;) – Max Oct 25 '22 at 12:34
  • Whats the size and application of the plywood sheet? Is it just a small "rail" or more like a mounting point for a TV. If the sheet is a lot bigger than the dowels the main problem, I'm guessing, will be either the dowels breaking or tearing out the holes if the sheet "wobbles" forwards and back. – Max Oct 25 '22 at 12:49
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    Ha! Thanks for your comment Max I edited the question to add more info – Max Oct 25 '22 at 12:57
  • The drilling part shouldn't be a problem. I've drilled holes in ply in a similar setup that were more than half the thickness. But that was horizontal holes for small floating shelves and my "dowels" went almost the way through, so not enough torque to tear out anything. The recess for the mounts was even bigger, only about a 1/16" of the veneer remained on top or bottom, but there is no force acting on that part of the shelf so its ok. – Max Oct 25 '22 at 12:59
  • Whats the base made out of? And for clarification: do the dowels go into the base with the legs sitting on top or do the legs go in (in square holes I assume) and the main sheet sits flush on top of the base with the legs invisible? – Max Oct 25 '22 at 13:12
  • Welcome to StackExchange, and kudos on a well-asked Question (with extra credit for the ASCII art!). Just one thing, it's a a virtually irrelevant point but it's not a good idea to abbreviate Baltic birch to BB, because some plywood is letter-rated, with B being a common grade for one (and occasionally both) faces. So BB plywood of a given type is actually a thing :-) Edit I actually forgot to hit send on this when I wrote it about 40 minutes ago when I started writing my Answer... which now needs to be modified because of the added details. – Graphus Oct 25 '22 at 13:38
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    I agree with bowlturner's answer, but think the problem may be extend beyond attaching the back in that the dowels themselves may not be adequate to handle the loads the back will receive when someone sits on the seat and leans back. The problem is that the dowels must be vertically secured for some length at the base to transfer the rotational force. You may also want to consider leaning the back at an angle for better seating comfort. A better approach to the design may be to create a rigid frame from solid wood members and then place the plywood back against the angled frame back. – Ashlar Oct 25 '22 at 13:46

2 Answers2

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As I was reading this I was thinking there is nothing wrong with doing this, a 3/4" dowel should be able to be drilled and glued in with no problems. Then I read the part about being the back of a sofa.

The rotational torque on the board being the back of a sofa won't hold up for very long. The plywood will be twisted against it's weakest direction.

It might handle it for a bit longer if you make the 3/4" dowel almost all 20" of the height and have 1 about every 10" along the length.

The better option would be to put in some triangles on the 'back' side to help support the back, I'd go with 3 or 4 of them. They can be narrow and tall maybe 18" tall by 6" wide at the bottom.

Otherwise I think you are going to find things breaking.

bowlturner
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  • I was in the same boat, this seemed fine until the added details LOL – Graphus Oct 25 '22 at 14:05
  • Thank you for your answer! I think the triangles are a great idea. For my design, I had hoped to use the removable backrest in another context where the triangles would have gotten in the way, but based on the feedback here I've decided to just permanently attach the backrest. – Max Oct 31 '22 at 02:05
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  1. Is it feasible to drill into the edge of glued-together plywood like this?

Yes.

Just a note for future readers: this is not a universal yes for all plywood, because of the limitations of some plywoods1. But Baltic birch is ideal for face glueing because of the uniform thickness of the surface and interior plies.

2a. Is it feasible to glue a dowel into this edge?

Yes, if you glue the plywood together well (see tips at bottom).

2b. Should I use a fluted dowel?

You may not be able to easily source fluted dowel in the length you're planning to use, making this almost irrelevant. And while you can quite easily make your own, see How can I flute my own dowel or create dowel with similar properties it's often not necessary — dowels that are custom in some way2 are almost invariably a plain, smooth dowel, without even a relief groove/channel down the edge.

It's really only if dowels are a very tight fit in their drilled holes that some kind of relief is needed.

  1. How big a dowel diameter can I safely use? I was hoping to use 3/4"

3/4" should be fine.

Edit for new details provided. Now that we know what this laminated sheet is for you're going to want to up the number of feet from two!! I suggest you use 5/8" in addition to increasing the number, and if were relying on the dowels alone and the back was thick enough to accommodate it I'd go with 1".

My gut tells me that you're going to want to use six at least, and note that this will make drilling the corresponding holes in the base a lot more challenging (and will make installation and removal of the back much more difficult, even if dowel alignment is basically perfect). Chamfer the leading edges of the dowels well!

  1. Is there a better option?

I think so yes.

Given the strength requirements if I were making a back of the same style as this I think I'd create a stopped groove in the base for the edge of the plywood to sit into (which would allow the number of 'feet' to go down to two or four). The plywood should be a tight fit in the groove so there's no play and should sit in 3/4" or 1"; again, chamfer the leading edge well to help ease insertion.

Here are the tips on glueing the ply.

  • First and foremost you have to lightly sand the mating surfaces to ensure the best glue bond, because only freshly worked wood glues well (here's why).

  • Don't skimp on glue. If you've never watched this being done, it's likely this will require quite a bit more glue than you might imagine! And it is important to apply some excess (which will then be squeezed out during clamping, so be prepared for the drips).

  • You might want to make a notched spreader (from an old credit card or store card) to ensure even glue distribution, or use a roller (hard rubber rollers such as brayers/ink rollers are excellent for this).

  • If you intend to use PVA to glue the two pieces together you must ensure good pressure to be assured of a strong bond; PVA glue is only strong in a thin (very thin) layer and this is only achievable with high or very high clamping pressure. This is easy along the edges where you can place clamps directly, but more difficult to ensure in the interior — use clamping cauls/clamping pressure pads for that, or very heavy weight stacked in the middle.

  • Don't underestimate the number of clamps you're going to need. This is really a subset of the previous point but deserves to be listed separately to make the point properly. Along the edges you might want to space the clamps no more than 6-8" (150-208mm) apart, so just a smallish panel could easily require every clamp in many workshops. I know it did when I glued up my workbench tops!

1 Some modern plywood has ridiculously thin surface veneer and this is not a good candidate for bonding face to face because these veneers are inherently weak and typically not firmly attached. In short this type of plywood, if used as-is, could quite easily separate along the user's glue line.

2 Diameter, length or species, or any combination.

Graphus
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    Thank you for all these tips, particularly with the glue-ups! You must have sensed I have not done this before LOL. I have decided to hold off on the removeability of the backrest for now based on the feedback received here and I'm just going to permanently attach it. – Max Oct 31 '22 at 02:07
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    You're most welcome Max. You'd actually be surprised how many woodworkers (including many pros) don't know that little detail that you need to refresh old wood surfaces prior to glue application IF a particularly strong joint is needed (as it would be here of course). – Graphus Oct 31 '22 at 16:36