do tenons need to be in the middle?

[engineerone]

although i have read the books, i can find no actual comment about whether a tenon has to be in the middle of the piece.

i realise where the mortice is is not too important, but is there a strength issue about the tenon??

paul

[Scrit]

Neither mortises nor tenons need to be in the middle of a component, i.e. with the cheeks equidistant from either face, but I feel it is easier to mark-out and cut if they are. Certainly on shaped components such as arched doors or chair parts placing the tenon in the centre meand that one holding jig can often be used to work the left and right side tenons if using a spindle moulder or tenoner.

As to strength that may well depend on the function and loading of a piece. In a lightweight table offsetting the tenons towards the outsides or the rails may allow a longer length of tenon in the legs (and thus a greater/longer glue surface or pinning zone), whilst in a structural framework offsetting the tenons can have strength consequences with one side of the frame being more likely to fail than the other under load

Scrit

[andy king]

Hi Paul,

If you are working stock with rebates, its common practice to offset the mortice to sit so that one mortice wall is aligned with the rebate edge, so if for example, you have 45mm stock with a 20mm rebate, the mortice would be place 20mm from the face to align with the rebate edge.

Andy

[engineerone]

as it happens guys i am working with a small light table, and i need all the strength i can get.

the legs are 17.5 mm square, and three of the side panels will be about 12mm thick, set back about 3.5 mm from the outside edge, so i will have about 4mm thick tenons.

question therefore is since it will be by hand, mainly, which way to go.??

second question, at one end will be a drawer, and i am planing to put dovetailed top and bottom rails, but given the design, they will need to be horizontal, not vertical. will this cause any more problems??

paul

[mrgrimsdale]

4mm is a bit fragile for any sort of table. Why not go for the full 1/4inch? Mortice abt middle of legs and tenon set on the rails as and how you want.
Drawer rails - the bottom one has to be a stub tenon as you can't poke a dovetail into a mortice, but the top one can be dovetailed into the top of the leg. This is a very common traditional way of doing it. It's there in Joyce somewhere.

cheers
Jacob
PS mortice or tenon don't need to be exactly central and as Andy says if there are other details like rebates it's practical to line up with them. Tenons can be offset to have only one shoulder, if it fits the design usefully.
I'd worry more about marking up properly from the best faces or edges so that everything lines up, central or not.

[mrgrimsdale]

engineerone said:

although i have read the books, i can find no actual comment about whether a tenon has to be in the middle of the piece.

i realise where the mortice is is not too important, but is there a strength issue about the tenon??

paul

Another PS - it's the other way around - more important to get the mortice towards the centre so that it's not too thin walled. Tenon can be offset right to the edge with only one shoulder, if needs be.

[mrspanton]

mrgrimsdale said:

I'd worry more about marking up properly from the best faces or edges so that everything lines up, central or not.

As always jacob gets down to basic's and hits the spot. If mortices and tennons are narked up consistently from face side/face edge on every component, working off a rod, any slight discrepancie's in stock size wont affect the construction. On timber building frames the framers (especially american square rule ones) use mortice's and tennons that are definately "off set to one side". They do as standard a 1 1/2 inch mortice and tennon 1 1/2 inche's in from face side/edge, or a 2 inch mortice and tennon 2 inch'es in from face side/edge. No matter wether the beam is 7, 9, 12 inches or more wide or thick. Thats why a framing square has a 1 1/2 inch part and a 2 inch part, they dont use marking gauge's. The frame will then be all flush outside for the claddings