Q. I recently purchased the loose tenon jig. It allows a furniture maker to create a mortise with a hand held drill and use precut tenon stock. It certainly seems to save a lot of time. But is such a mortise and tenon joint as strong as the normal mortise and tenon? Do the same rules of thumb apply to tenon size and depth? The jig allows for 3/8" and 1/4" tenons and virtually any depth desired.
A. Michael Dresdner: For the most part, yes, or rather, it is certainly strong enough. One could no doubt run lab tests and show minor differences, but generally speaking, a well-fit floating tenon is as good as a fixed one (and better than a poorly fit fixed tenon). Taking the discussion any further is tantamount to arguing how many angels can dance on the head of a pin.
A. Simon Watts: I would call these guides rather than rules. In general, I'd say keep the same proportions whether using fixed tenons or loose ones.
Q: At what width would you suggest a twin tenon becomes advisable?
A. Andy Rae: Twin (or multiple) tenons aid wood movement in that they localize the amount of movement in a rail, reducing stress to the joint. A good rule of thumb is that any piece of wood over four inches wide (in just about any species and cut of wood, such as quarter sawn, rift, or plain sawn) will have sufficient movement across the grain to affect joint integrity in a cross-grain construction, such as in a mortise-and-tenon joint. Therefore, a four-inch wide tenon on a five-inch wide rail or apron is fine because wood movement is minimal. But in rails six inches or wider, it's best to divide the tenons and house them in separate mortises.
A. Richard Jones: I think you mean a forked tenon, as you might see in the middle rail of an architectural frame and panel door. I consider forking the tenon where the rail is five inches wide or wider.
A. John Brock: I guess as a rule of thumb, I use a second tenon when the horizontal piece is as wide or wider than it is deep.
Q. I have read that stub tenons are not recommended for kitchen cabinet doors or heavy use doors • because of the short length of the tenon. Would gluing the panel add strength to this joint? I know that gluing a solid wood panel would interfere with wood movement, but what if I used 1/4" plywood panel and glued along the entire inside edge to reinforce the joint?
A. Michael Dresdner: Yes, gluing in a plywood panel would most assuredly add strength to the frame.
Q. When mating wide boards with mortise & tenon joints, what is a good rule of thumb to use for the maximum width of the tenon, or at least, the maximum width to which glue should be applied?
A. Michael Dresdner: Solid wood tenons should really not exceed two inches, or you may get problems with wood movement. Much more than that and you run the risk of blowing out the top of the mortise. This is not a hard and fast rule, because it depends in part on the type of wood you are using - some woods move more than others. You can get away with a slightly larger tenon if you're using more stable wood. For very wide boards, use two tenons."
A. Rob Johnstone: "Although it is a bit more work, splitting the tenons into multiples not only helps deal with the challenges of seasonal wood movement, but adds glue surface. So, when faced with a wide tenon ... over two inches ... I like to make it into a dual tenon. Not only is it a good technical choice, but it looks so cool, you feel like a better woodworker!