Re: Tenon length?
Sgian Dubh
>Derek, let's take a realistic scenario. Draw this out on a bit of paper as if you're viewing a section through your two rails and leg as viewed from above.
1. Draw a rectangle 40 mm square.
2. Pick two intersecting faces as the outside faces.
3. From the inside faces of your leg draw a pair of lines 19 mm apart to represent the rails, but importantly in this example,
4. Inset the outside face of your rails 3 mm in from the outside face of your leg. This creates a 3 mm reveal. Reveals are optional so this is just for illustration.
5. Draw two lines to represent the thickness of a 6 mm tenon (which is the same as the mortise) centred on the end of each rail. The two lines will be parallel to the outside face of the legs.
6. Where these pairs of mortice lines intersect nearest the outside corner of the leg will determine the length of the mortise, and of the tenon.
7. If you've drawn it correctly you'll see that the depth of your mortices is 30.5 mm overall.
8. Draw a line to create a mitre at the corner of the mortice that goes from the inside corner to the outside corner of the leg.
9. The length of the tenon on the inside face of the rail is in theory 24.5 mm.
10. In practice you would cut the tenon about 28- 29 mm and mitre each one so that they almost meet in the corner.
Incidentally, I teach the rule of thirds to my learners too just as I was taught many moons ago. The wrinkle is that you pick a chisel one size up. In your case 19/3 = 6.33 mm and I'd use an 8 mm chisel.
This rule perhaps matters more where frame members are of equal thickness, such as a door frame. The idea is to remove 1/2 the material from each member to weaken each by the same amount. In a mortice and tenon of this type you generally add a bit of thickness to the tenon to try and even up the mass of material in each part, and this usually considered to be the best compromise. You do have to be careful not to make the mortice cheeks too fragile.
In your scenario because you have a greater mass of leg than you do rail there'd be nothing to stop you making a 10 or 12 mm thick barefaced mortice and tenon, i.e., one with a shoulder on only the outside face. Or you could put a 3 mm shoulder on the inside face, use a 10 mm tenon and 6 mm shoulder on the outer face. If you offset the rail as you drew by 3 mm you'd still have a 9 mm outer mortice cheek on the leg, which would likely be plenty strong enough.
You're looking to maximise long grain to long grain gluing surface area, and the dimensions you were planning on using seem a bit small to me.
Also don't forget to make the mortice length just over 2/3 of the rails width so that there is plenty of meat at the top of the leg to stop a plug of wood popping out if the joint should rack a bit, or the rail width grow. This also means making the tenon 0.5- 1 mm less in its width than the mortice is long to allow for the rail expansion and contraction-- this assumes a rail width of about 100- 120 mm.
If the rail is narrower you can reduce this sloppiness a hair. If it's wider you're starting to get into the territory where a forked M&T might be a good option where only the upper fork is glued. The forks don't always have to be to be equal in size. You could have a wide fork at the top and a narrower fork at the bottom.
Lastly, if there is a rule of thumb for a stopped tenon length/ mortice depth I'd probably say go for about 4 times tenon thickness, but that the depth can be adjusted a bit one way or another to account for circumstances. Slainte.
