Desk Legs: Tenon size on rails

[Andy Kev.]

Good Morning All,

my intended project for the festive season is to build a smallish - about 3' or 3' 6" by about 2' desk for my computer. The legs will be fairly sturdy at about 1 7/8" x 1 7/8". The rails are likely to be 4" - 5 " wide and 1" thíck.

All the recent discussion about tenons got me thinking about the rail-leg joint. Convention dicatates the two mortices meeting in the leg with the tenons being given mitre ends to accommodate each other, the idea being to get maximum length of tenon in order to boost joint strength. There must come a point where an increase in tenon length brings no extra benefit e.g. if, for the sake of exaggeration, the rails were going into 6" by 6" legs, would there be any need to have the tenons longer than, say, 2" i.e. thus avoiding the mortices meeting? If that idea is correct, would it apply to the above dimensions e.g. would 1" or 1 1/2" long tenons be adequate?

The second thing is the thickness of the tenons, the usual rule seeming to be a third of the thickness of the stock but I presume that is more to do with the mortice when the two pieces of wood to be joined are of the same thickness. Am I right in thinking that when the member with the mortice is much thicker than that with the tenon, the thickness of the tenon can be upped as one wishes e.g. I could go up to 1/2" thick tenons on the rails of my desk?

Thanks in advance.

 

[custard]

Somewhere I've got an academic treatise on how the strength of M&T's is computed, what it boils down to though is that within reason it's total gluing surface area that really counts and for maximum strength get a snug fitting joint and make sure you glue both the mortice and the tenon.

If the two mortices meet inside the leg then it becomes more difficult to glue up in sub assemblies (i.e. glue up the two ends first then connect the two ends in a separate glue up), the reason is that modern glues don't stick very well to themselves, so you don't want squeeze out from one joint getting into the mortice next door. Sometimes you don't have a choice, but there's no need for it in this case, 1" long tenons are perfectly adequate

Draw out your joinery full size or twice size and you'll soon figure out the optimum layout.

Your table is well over engineered so don't sweat the joinery decisions.

Personally I'd take the legs down to 1 1/2" x 1 1/2" and I'd taper them, otherwise it can look like Fred Flintstone furniture, but you're the one that has to live with it so entirely your call.

Five inches is very wide for apron rails on such a small table, remember you need room to get your legs under and the top is pretty much a fixed height (especially on a computer desk where you don't want the keyboard too high), 3" is perfectly adequate, 4" if you want to run a shallow curve on the bottom edges of all the aprons. I'd make the aprons 3/4" thick rather than 1", for such a little table 1" is adding more weight than strength.

Good luck!

 

[Andy Kev.]

Thanks Custard, that was the sort of stuff I was hoping for. I like the idea of drawing the joints out full sized as that really will help with visualising it.

I might have some slightly thinner pieces nearer to 1 1/2" x 1 1/2" so I could use them and save the 1 7/8" stuff for something bigger. The reason for the rails being big was that I want to put a couple of drawers in the front but come to think of it they won't have to hold masses of paper etc. what with it being a computer desk, so I could drop to 4" (with a gentle curve) and settle for 2" deep drawers. I was intending to put a gentle taper on the insides of the legs in any event, so I reckon 1 1/2" going down to 1" would probably do it.

Thanks again.

 

[David C]

"the truth about tennons" page 59 in my first book"

Joyce is pretty good too.

Best wishes
David