Est. 1998 — 27 years of woodworking knowledge
Domino Sizing Questions
Doing an end-grain join in walnut that's 15/16" thick. I have 8mm and 12mm cutters.
Which size do you (the WC Domino experts) use in this situation?
The max depth on the 8mm is mol 1", but the 12mm can go deeper.
Suggestions welcome.
TIA
Re: Domino Sizing Questions
Depends on application and the strength needed
Structural Stress (PIC)
will be minimal. The dominoes will be as much for registration as for strength. Join is in the center. M&T to bed posts on each end with 2 struts at 12" on either side of the centerline.
Re: Structural Stress (PIC)
Mark
If they are primarily there for registration, primarily, I would go with 8mm.
Disclaimer - Domino user, not an expert.
Henry
Yes, 8mm
Thanks
8mm
Done!
Re: Yes, 8mm +1
Agree
The only time I use loose tenons that are thicker than one third of the rail thickness is when the rail is going into a table leg or some thicker stock. In your application, I'd want one or two longitudinal beams on the back of the headboard to counter deflection. The M&T joints aren't going to do it.
Ellis
Re: Agree
Two planned. Will be placed 12" on either side of the center point. 
Not seeing it
Mark, I'm talking about lengthwise beams of some sort, like from bedpost to bedpost or at least long enough to stiffen this assembly. If you're talking about 12" either side of center, that would be 24" apart. How big is this headboard? Will it conceal two stiffeners in those locations?
Ellis
Thisaway
The headboard is 76" between posts for a king size bed. The piece will "float" between the top cap and the bottom rail with 2 verticals much like this 
I could add a stiffener between the two verticals as it won't be seen.
Re: Thisaway
Ah, now I get it. Thanks. Stunning use of your crazy walnut, by the way. 
Admittedly, I do tend to over-engineer things, but I would definitely find a way to screw a couple horizontal straps across the back of the bookmatch. as a mechanical insurance policy on that joint. The vertical straps suspend the slab in the space, but they also act as fulcrums for forces against either the middle or the outboard ends, which could lead to some level of articulation of that center joint.
Ellis
Re: Thisaway
Ellis explained it very well. Just the force of a pillow and someone's head while reading a woodworking magazine could be enough to stress the joint held just by the tenons. Some horizontal reinforcement is in order.