So, this is the gizmo I've been working on. It's in a mostly-done state right now, still needing to be burnished and waxed.
What you're looking at is a stoutly made, fully enclosed vise, made with a veneer press screw. The entire top surface is reference-flat, and at 90 degrees to the inside surfaces of the vise. The purpose of this device is to assist in cutting co-planar tenon shoulders, regardless of the angle that's required. The basic theory is this: clamp the part to be cut into the vise, and align the shoulder scribe lines with the top edge of the gauge stick. The saw will then be used to cut the shoulder, sliding back and forth across the top surface of the vise. This will ensure that all shoulder surfaces are 100% in the same plane. In the photo above, the part being cut is at a simple angle. I will also make a pair of pivoting pads, so that parts may be set up for compound angles.
Why did I bother with this?
The tenon shoulder is the most critical, visible part of a mortise and tenon joint. If one of them is poorly cut, there will be a gap, or the specified dimension will be changed. On a table stretcher, for example, the distance between legs is defined by the distance between tenon shoulders. If you're just bracing table legs, maybe this isn't such a big deal. But if you're making another part that will form a drawer enclosure, that dimension is more critical. If you're making a dresser, and you have many such parts, you really want them all to be the same. When you're fitting the joint, mortise walls can be pared back, tenons can be trimmed and shimmed, but cutting the shoulder is a one-shot operation. It has to be done right the first time.
Cutting square tenon shoulders is a snap with a table saw, and it's not much harder with a hand saw. Cutting simple angled tenon shoulders is pretty easy, too, though grain direction on porous wood can throw the saw off track if you're not careful.
Cutting tenon shoulders at a compound angle is possible on a table saw, but it's a hassle, and it took some interesting jiggery to make it work when I was learning to make chippendale style chairs in school. Cutting a compound angle joint with a hand saw is probably easier with a hand saw in that regard, but even still, there's a lot of care required. I suppose in a production environment, and given reference faces, I should just use a Domino machine and have done with it. But in a home shop environment, how do you set up for compound angle joinery and make it easy? Well, the gizmo at the top of this post is the answer.
This isn't my design. This is something that was shown to me when I was in school, that was brought over by a french chair maker. There's a companion jig that provides a way to clamp a piece and plane a reference flat surface, which will then be mortised into. That'll possibly come next, but for now, this seemed like it would provide more capability.
