Blind mortice tenon dimensions

[Sir Percy]

I've got some blind m/t joins to make; the pieces are all 35mm thick oak. Pieces to be morticed are 75mm deep. Tenons to be cut on pieces 100mm deep.

Hand tools, apart from drill for mortice.

What dimensions for mortices/tenons would you recommend?

First time of doing this.

Attached a shot of the assembly, hopefully that helps.

 

[Jacob]

Those cross grain tenons not a good idea at all. They want to be straight grained straight through.
As a general rule the tenon about a third of the thickness of the morticed piece and about half the width of the tenoned piece, plus haunch.
Might be better as a bridle joints with a couple of dowels to hold them together - or blind, not all the way through.
I see you have the drawing from Mike Jordan's excellent book - he gave me a copy some years back. He used to post on here. Sorry Mike I wouldn't do M&Ts like that - straight grain tenon and angled mortice would be better IMHO.
PS If you are going to paint it I wouldn't use oak it doesn't hold paint too well. I'd use good quality redwood.

 

[Sgian Dubh]

Those cross grain tenons not a good idea at all. They want to be straight grained straight through.
As a general rule the tenon about a third of the thickness of the morticed piece and about half the width of the tenoned piece, plus haunch.
Might be better as a bridle joints with a couple of dowels to hold them together - or blind, not all the way through.
I see you have the drawing from Mike Jordan's excellent book - he gave me a copy some years back. He used to post on here. Sorry Mike I wouldn't do M&Ts like that - straight grain tenon and angled mortice would be better IMHO.
PS If you are going to paint it I wouldn't use oak it doesn't hold paint too well. I'd use good quality redwood.

Actually, if you're talking about the tenons shown at the bottom of the triangular frame, at left in the image, those tenons seem to be impossible anyway assuming that rail starts off at the finished width shown, because the tenons extend to the right of that rail thicker than the rail itself. Like you though, I'd say the tenons need to be parallel with the rail's long grain direction meaning the mortices need to be chopped at a matching angle. As to tenon thickness, as you say, about 10 or 12 mm should do the job. I'd say much the same advice applies to the top narrow bit of the triangular structure, i.e., the tenon parallel with the long grain of the small bit between the long parts. . Slainte.