Frankly, Windows are a Pain by Jim Tolpin
Well now I know why windows are called panes...because they are truly a pain to make. Especially when you are challenging yourself as I am to do all the molding profiles, rabbets and joinery entirely with hand tools. (I’m not quite crazy enough to do all the stock sizing with rip saws and foreplanes).
Why I’m making the sash for my new stand-alone shop with hand tools is another story for another day---suffice it to say that I’m driven to extend and refine my self-education in woodworking (as opposed to wood machining).
Today I’m making the rail-to-stile joints for the big, 24-pane front window, which are franked (rather than haunched) through-wedged mortise and tenons. I was going to do my usual draw-bore pinned M&T, but a bit of research--especially in Charles H. Hayward’s book on joinery--convinced me otherwise. So I’m going down that road---and I have some theories about why this choice is typical of traditional sash joinery.
I’m pretty sure about the through tenon: With typical usage (sliding up and down in the case of double-hungs), strain on the pins would likely lead to enlarged holes and loose joints. Also, due to the sash’s exposure to the elements, the wood is going to undergo a lot of seasonal movement--again leading to loose pins. The through-tenons are much less subject to these symptoms of wood movement (no holes to enlarge for one thing). And unlike the pinned mortises---which eventually would have to be disassembled and re-bored--the wedged mortises could be tightened up by simply driving in (or possibly adding) the shims.
As far as franking goes, this is, according to Hayward, the joiner’s typical choice while cabinetmaker’s go with haunched. (see his illustration on the left). I think this is due mostly to the fact that cabinet frame joinery usually encases a panel, which means the stiles have a groove and the tenon is simply haunched to fit in it. In sash joinery--which is traditionally in the realm of the joiner--there is no groove worked into the rails and stiles. So its then a matter of chopping back the tenon to fit the ridge left on the stile stock when the molding profile is cut away. There does seem to be a clear advantage of franking in this application: The side of the tenon that will be angled by the wedges will be longer than it would be if there were a haunch. This allows more bearing surface for the wedged tenon, and therefore more strength and more resistance to loosening.