Mortise and loose tenon with rounded ends
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The general recommended procedure is to create the mortise on the router table then square off the ends to accept the rectangular tenon. Bob’s video is great on this.
https://www.finewoodworking.com/2018/09/24/ep-4-router-table-fundamentals-mortises
But then I thought, if you are using a loose tenon, why not round over the loose tenon to fit the mortise. In the photo I was using a 1/2 inch by 2 1/2 inch mortise. Then made foot of poplar with the same dimension less 1/64 inch each dimension. Then ran that piece through a 1/2 inch bull nose router bit.
Replies
For loose tenon joinery I always round the tenons, thought that was the standard solution.
I prefer to round the tenon. It's easier to sneak up on the fit than removing corners in the mortise.
The strength of a M&T joint (loose or not) is in the glued long grain sides and the shoulders. Round or square ends don't matter and for ease of assembly, don't even need to be tight.
However, it is important to know that the rounded areas do not contribute much to the functional glue surface, so reduce the size of the tenon by the rounded over area. For example, if you have a 1/2" thick tenon, rounding over the ends would reduce the width of the glue surface by 2x(1/4") or 1/2". So a 2" wide tenon would only have 1.5" width of glue surface. If that is not a problem, round the ends. If it is, square the ends of the mortise. Just depends on the situation.
What jharvey said. Most of the time it won't matter. But on a narrow mortise you'd be losing a significant percentage of the glue area. You're better off squaring the mortise in those cases.
I'm not rounding the ends, I'm rounding the sides. I would think the sides on a well fitted tenon (even rounded) would contribute to the glue surface. But it would reduce the overall glued surface by the rounding amount.
The rounded areas of the mortise are functionally end grain, and don't contribute significantly to the glue strength of the joint unless you use a gap filling glue like epoxy. So the functional glue surface with other adhesives is the straight sides of the mortise. This is also a weakness of many dowel joints (which, in old chairs, I repair with epoxy.)
This^^
A lot of you seem to think that you can rely solely with glue on the face cheeks to achieve a strong M&T joint.
A strong M&T joint should be snug in all directions and mechanically tight to resist racking, the glue is there for insurance and serves the same purpose as a dowel pin.
The bottom of the mortise doesn't matter at all. It doesnothing to help the joint. I've seen a lot of really old joints such as the one below. All of the bottoms of mortises look like the one below. They horrifying modern woodworkers.
The x-ray was taken because these mortise and tenons are rock solid after 230 years, and they wanted to see why. I'd prefer the ends of my mortises to be tighter, but apparently it doesn't matter much either.
I never mentioned the end or bottom.
I was pointing out that the edges are just as important as the faces. Both need to be fitted properly for a strong joint that resists racking, (which leads to failure). Some seem to want to rely too much on glue alone and not the inherent mechanical strength of a properly fitted joint.
There must be close to a dozen articles on this very subject in the FW archives.