I’m looking for some help. First, some background.
My wife asked me to design an A-frame shelf for us, to hold kitchen stuff including some appliances. We both like the look of live-edge plywood, so my idea is to use my OX to route it out of baltic birch plywood. We also like exposed features. So I designed this tab-and-slot unit. I figured I might as well make it possible to break down while I was at it, so it will pack down very compact if we disassemble it, yet should be very sturdy assembled.
But I’m missing one part. I could make the part on lathe and mill, but figured it’s probably actually a standard part somehow and I just don’t know what to search for. I want a peg that fits into this hole in the tab. I need the peg body to be snug against the side of the shelf, but have some sort of asymmetric head, such that it will not fall through.
I can adjust the hole size easily enough if there’s a part that is a the right shape but a different size.
I’ve thought of putting stock on the lathe and cutting out a T profile, and then putting it on the mill and cutting off one side of the T. It wouldn’t take that long to cut 16 of those. But everything else in this design could be made with a CNC router and standard parts, and it would be great if I could share the design and someone else with a large enough CNC router and necessary bits could make one.
So does there exist any sort of peg with an offset head that would fit in a hole like this?
Worst case for someone without a mill they could file off the edge of a peg with a normal head. I was just expecting that there’s a standard part and if I knew what it was called I could find it on McMaster.
I must be missing something…
I would just use a hardwood dowel that is a bit oversized for the hole with the end split partway up.
I assume you want a jam fit.
That is more elegant than my solution. I’ll probably play with different wedge shapes in cad, maybe using truncated wedges.
Turns out that the way I modeled the shelf makes it hard to pull it apart in CAD to toolpath it for the CNC router, so I need to start over on the modeling anyway. I can account for this change in the process.
This is the way to do it as it will also tension everything and makes sure the sides are pressed solidly against the shelves. Remember when you screw things and there’s a bit of play? Everything wobbles. This is the same.
I’m still trying to decide whether to dogbone the shelves or sand the edges on the sides to fit the radius of the tool I use for the finishing pass on the shelves. Thinking about ⅛" carbide to finish if I can put one long enough into my router, which would make light sanding job for a snug fit.
With the peg, it will be harder to get the shelf to draw tight to the side frame where there is only a small area of contact. Then as the load is applied and the wood is compressed the joint will likely loosen. The hole and peg position, size, and materials will be critical, not things wood, which is malleable, is good at. I have had this kind of problem using a dowel offset from its hole to draw in a joint.
Whereas:
The tusk tenon typically has a larger contact area against the side frame and the inner surface of the mortice. By definition, the wedge has the ability to draw the shelf and frame together over a larger range of tolerances. A plus is that you can tighten the joint as the wood moves over time.
One challenge with any kind of loose tenon is that with larger dynamic loads the wedge can work its way upward and loosen the joint. The angle of the wedge and the mortice-wedge contact area matter. The angle on both must match. Typically the wedge is made of hardwood.
There will be a lot of stress at the corners of the wedge’s mortice. I would endeavor to make the area around the mortice as large as practical and aesthetically possible. Especially pay attention to the corner of the mortice if there is inadequate material at that point the mortice can fail. Less likely with plywood.
And for maximum “draw,” the outer face of the wedge and wedge mortise should be beveled to a shallow angle of about 5°. The inner face of the wedge mortise extends slightly into the main mortise. This ensures that the joint will close up when the wedge is driven home. The pressure exerted by the wedge on the outer face of the wedge mortise is how the work gets done. This means that you want the angle of the wedge and the mortise to be a perfect match. And to avoid splitting, be sure to leave adequate material beyond the wedge mortise at the end of the tenon — a minimum of 1 ⁄ 2 .
Some practical design hints.
All of the above depends on the type of wood and the racking load expected. Certainly, plywood is more stable and does not have grain direction concerns. I wonder if there is enough surface contact area between the mortice and the wedge using 3/4" plywood.
Perhaps a shelf unit does not see much dynamic load and that small of a surface area will hold.
Yeah, since I’ll use the same plywood for the wedge and CNC route it like the rest of the shelf, it’s easy for me to choose the angle for the draw. I plan to use baltic birch which is relatively dimensionally stable, which should support a shallow angle.
Also, now I know the term “knock-down joinery” which I’m glad to have learned!
I would not use plywood for the wedge as it needs to be harder than the mortice and have a stable interface with the mortice. The edge of plywood is not very stable. When force is applied to its edge it compresses and splinters.
Are you sure this would be true of Baltic birch? It’s birch all the way through, not like the veneer over junk wood you can buy at the box stores. I’m sitting now in a chair that uses live edge Baltic birch plywood as its feet, and over 20+ years there has been zero tendency to splay or splinter.
What the wedge will be pushing against is another live edge of the same plywood, of exactly equal area. Therefore, if the wedge would splay or splinter, the shelf would, equally. That would be an argument for, if anything, cutting the wedge from a softer wood, so that the wedge is a sacrificial part.
The slot into which the shelf fits is intended to be a tight fit (I expect to seat it with a mallet and/or compression straps) with about 10 times the surface area of the wedge, so the bulk of torsion force will be taken between the side and the shelf in those slots, rather than on the wedge. My initial design intent was that the peg merely held the sides together and that the slots in the side absorb the torsion. I think that will be true also with the wedge, such that the wedge doesn’t need to be locked in place firmly to keep the shelves secure.
I do not expect to have heavy torsion loads on the shelf.
After having joints come loose over time I started using the recommended hardwood wedges, pins and pegs successfully. That is the experiential wisdom of years of woodworkers that have followed before me. I tend to value experience over theory :).
I suspect this is because the goal in this type of joint is to have maximum contact area when the wedge is inserted and minimal compliance after insertion. Otherwise the joint loosens over.
Never used, nor would I, use plywood like this so my experience may not apply to your project.
When it comes to experiential wisdom, baltic birch plywood has been a staple of Scandinavian furniture design over the past 70 years or so, and there’s plenty of woodworking experience out there with it. In particular, it is not prone to spalling and often does not require bushings where many other wood products, particularly softer woods, common plywoods, and engineered wood products like MDF, would.
There are a lot of sources of information on why baltic birch is different from typical plywood; picked completely from a random web search is this manufacturer’s summary which covers a lot of it:
It’s more like working with a very dimensionally-stable piece of solid hardwood than it is like working with typical plywood.
Incidentally, it can be a pain to find baltic birch, and it’s sold primarily in 1525mm x 1525mm (nominal 5’ x 5’) sheets. That’s an inconvenient size for shipping and even for fitting in cars/vans for hobbyists. I’m lucky enough to be able to pick up 4x8’ baltic birch marine plywood at a lumberyard 20 minutes from home as well as being able to put the 1525x1525mm sheets on the roof of my van, but many folks aren’t that lucky. One vendor explicitly cuts for reasonable shipping costs, though I personally haven’t had to use them due to my lucky location:
