I am making droplights (the opening window panels) that go in the doors of an 1890s wooden railway carriage, under restoration.
They are ca. 28″ tall by 20″ wide, and fabricated from ca. 2.5″ x 11/16″ mahogany, and of conventional mortise and haunched tenon construction for what is in effect a panel with the rebate containing glass as opposed to e.g. ply.
The tenons, on the horizontal rails, as measured on the surviving originals are 1/4″ thick and full penetration of the verticals. The tenons are about 1″ high, 3/8″ haunch at outer edges as is conventional. Yes, thin and long tenons!
Implicit in the design is that they can be taken apart to replace broken glass without destroying the droplight. To facilitate this, the surviving original shows wedges applied to the outside of the tenons. Joints are NOT glued.
Rather surprisingly, the entire tenon is tapered, and so is the mortise – to the extent that the wedges are 1/4″ showing at outside edges.
I have to say the joints come apart pretty easily, Yes they are 130 years old, so I replicated a new one with snug fit tenons, but tapering BOTH M&T seems to allow then to be pretty much self-dismantling! Remember no glue is used.
My question is surrounding the wedges – the original style seems to (a) weaken an already frail tenon and (b) very easily knocked apart. My inate preference is not to taper the tenon but just the mortise – probably installing ca. 1/8″ wedges at the outer ends, and tapering only 2/3 – 3/4 of the mortise.
(Not relevant to the question but we may choose to glue one vertical and make the other removable if required – thus maintaining a bit more integrity. There is a right angle brass strip attached across the bottom of the droplight – which helps protect against the weight of glass trying to break out the bottom rail if the droplight is allowed to fall rapidly).
I agree with the idea of tapering the mortice and not the tenonsI have a workbench held together in this manner and it is very stable. Another alternative is to glue in the wedge with hide glue so it can be removed with heat if required
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