greetings,
I’ve been on woodworking hiatus for a while and now I’m happy to say I’m back.
My project is a piano bench, with storage underneath the seat/top/lid. I don’t feel like looking for a design or plan, I just want to keep it simple and unadorned like the one I grew up with and vaguely remember. My thoughts:
– Bench top (lid) is 27-28″ long and 13-14″ wide.
– 4 legs (1.5″ square) that are 19″ high.
– Aprons that I was thinking would be 4″ wide (stock is just under 7/8″ thick)
The aprons would have grooves on the inside bottom to capture what will be the bottom of the storage cabinet. The top/lid is fixed to the apron with simple hinges at the back and rear apron.
I’ll be using mortise and tenon joints, my question concerns what type. Does a 4″ apron require double tenons, or is single sufficient? One the one hand it’s only 19″ high, on the other there might be some sideways forces depending on how fidgety/violent the piano player is.
Any feedback is appreciated. Also, I just pulled 4″ as an apron width out of the air. It might be disproportionately large for a 20″ high bench. If you think 3″ is fine, please say so (that might solve my M&T issue…assuming I can get away with a single).
Thanks.
w2
Replies
A 4 inch apron will end up with a tenon about 3 inches. A single tenon is fine. You'd need to be a ton wider before needing a double tenon.
Draw it out on graph paper first, and see how the tenon width looks. You could make it a little narrower if you think it's too wide.
Are you tapering the legs? I like a tapered look better myself.
That’s great, thanks John. I planned to taper the legs to take some of the blockiness out, and maybe a slight arch on apron. Graph paper is a great thought to check proportions.
A couple comments:
1) I would probably try to start with 1&5/8" square legs, as their size will be limiting the size of the mortise/tenon as well. Then use the taper to lighten them up, or a gentle curve that reduces the size a bit more quickly. Start the taper or curve below where the aprons come in. The two mortises coming in from adjacent sides can seriously weaken the leg pieces, especially if they come near the top of the leg. I would probably leave at least 3/4" solid at the top of the legs, and a 1/4" wide shoulder at the bottom. The shoulders add significant racking strength, which you will need. I would stick with the 4" wide aprons.
2) You will need to miter the ends of the tenons, so that they just clear each other when inserted in the mortises.
Like every other area of woodworking, M&T joinery requires that we make compromises and account for cross grain dimensional change. I would not use a double tenon where I glued both tenons, as that would promote a crack in the apron, or a broken joint from the stresses. I normally use a single tenon, typically 2&1/2" wide and as deep as the situation allows, and keep the rest of the tenon piece aligned with a haunch, which is basically a 1/4' (or less) stub tenon in a very shallow mortise. The shallow mortise is slightly longer than the haunch, to allow the haunch to expand into the extra space if needed. The haunch is left unglued. The haunch is always kept at least 1/4" from the edge of the apron to hide the extra mortise space. The mortise can be 1/16" deeper than the haunch, to make sure it doesn't bottom out before the joint closes. The haunch is just to keep a wider apron from twisting/moving out of alignment.
Every time we remove wood from a piece to make a mortise or tenon, we make that piece of wood weaker. The closer we make the mortises in the legs to the outside of the legs, the longer we can make the tenons without them hitting each other. However, that also weakens the outside walls of the mortises considerably. So we have to balance the two. I usually make the outside walls at least 1/4" thick. If the aprons are not to be flush with the legs, I would make the tenons 1/2" thick, and (with 3/4" thick aprons), I would have 1/8" shoulders on each side of the tenon. That would leave a 1/8" reveal between the apron and leg.
Depending on how you plan to cut the mortises, if you do it with a router, mortiser, or drill press, cut each mortise so there is a bit of wood left in the corner of the intersection of the two mortises, and clean that out with a sharp chisel. If you go too deep with the initial cutting, you will lose a lot of glue surface for one or both of the tenons.
You can buy a very good book on all sorts of woodworking joints on Amazon (and probably other places): Woodwork Joints by Charles H. Hayward.
More questions, just ask.
Thanks jharveyb. Your explanation is really clear. One thing I’m not getting about the haunch tenon. The aprons will be flush with top of legs (but inset 1/8-1/4 from “face” of legs). Do I understand it right that the mortise on the leg for the haunch extends all the way through the top of the leg? I can see how that wouldn’t matter if you’re fixing a top to it, but this will be exposed when the top/seat/lid is open and the gaps might be unsightly (especially with no glue). If the mortise for the haunch doesn’t extend through top of leg, I’m not sure I see the benefit.
I was mentioning the haunch concept for wider aprons, for the future, as you had mentioned double tenons. If you make your aprons 4" wide with 3" wide tenons, 1/4" shoulder at bottom and 3/4" shoulder at top, you won't need a haunch, as the apron won't twist/warp appreciably over that short a distance. And any time you are doing a haunch, you can stop it short of the edge of the apron so it doesn't show. Sorry if too much information caused confusion.
Harvey
I’d leave extra scrap length at the top of the legs to support the mortise cutting and prevent a blowout.
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