Thinking About Jigs

Over the past couple of weeks I’ve been working on a set of four stools for a customer. One of the tricky areas involved in the making of these stools is cutting the tenons on the bent laminated stretchers, which are cut and glued together to form a triangular unit that secures the legs of the stools. When I made the first version of this stool several years back in NY, I used a router and a special jig to cut the tenons. Upon moving to Japan and making more of these stools here in my tiny Tokyo shop, I initially used the sliding tablesaw along with another jig to cut the tenons. But that method was slightly sloppy and it was hard to fine tune the accuracy of the tenons. So I began to think about my old setup again, and for this latest batch of stools I decided to make up another jig using mdf, and paired with my little Makita laminate trimmer and an edge guide attachment, cut some tenons.

Here’s how the jig looks with the curved stretcher assembly clamped in place.

To properly align the stretchers I simply used a set of layout lines drawn directly on the jig itself to position the stretcher assembly. A pair of clamps hold the stretcher assembly in place. The alignment and clamping method is somewhat provisional. I think I’ll take some time to create some adjustable stop blocks to make alignments easier and I’d also like to install a set of toggle clamps to simplify clamping. (You can see I installed a very basic stop block on the bottom side of the stretcher assembly above, which aided in positioning the stretchers from one direction at least.)

The jig works by aligning the stretchers such that the center of the tenon, gets aligned with the center line of the jig itself, and most specifically the centerline of the top surface which the router sits on. With the workpiece centered relative to the top plate, the edge guide of the router can reference off of both the right and left sides of the top plate for cutting each side of the tenon. You start by cutting one side of the tenon using the edge guide to refence off the corresponding side of the jig, then pick up and rotate the router to reference off of the opposite side of the jig to cut the other half of the tenon. One side at a time. The beauty of using the edge guide is that the cut is really safe since the edge guide stops the router at whatever distance you set it to, and assuming you set the guide to the right distance, the setup prevents overcutting into the tenon. In other words, the edge guide makes it’s a really easy to gradually sneak up on a very precise tenon thickness, safely and accurately

Here is a close up of the tenon after cutting. You can see the centerline drawn on the surface of the jig which I use to align the tenon.

Another view from the side, and you can just make out the tenon shoulder.

I’m pretty sure I first came across this kind of jig from Chris Hall’s blog (The Carpentry Way), and also from some articles and books by Pat Warner. From what I can tell it’s also basically the same concept behind the Leigh FMT tenon jig, minus the fancy templates and router base plate. I’ve never used the Leigh jig but I like that it’s made of machined aluminum which is far superior to the MDF I’m using here. That said, at $1200 for the Leigh jig, I think I’ll stick with MDF for the time being especially since it allows me to make modifications to the design for different projects.

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I’ve started thinking more about the design of the jig and drew up a quick model in Blender. Here it is with some improvements including some aluminum t-channels for an adjustable stop that can be used to align straight stock.

Not pictured are the necessary toggle clamps, but I figure those are pretty easy to attach just about anywhere. But it would be nice to figure out how to attach toggle clamps in a way that allowed their position to be easily adjustable… something to ponder.

Something I realized after using my jig for cutting the stool tenons above, is that it would be nice if the top plate could be adjustable side to side as well. So in the image below you can see how the vertical surface is dadoed into the top plate, and the top plate has an aluminum channel allowing it to be adjusted and clamped down. The thickness of the materials drawn in the rendering is somewhat arbitrary, and the top plate should really be thicker to accommodate the aluminum channel.

From the back you can see a couple more knobs for locking down the top plate.

My experience so far using jigs like this is that they can be really accurate and allow a lot of safety and control when cutting tenons, but the overall process is definitely a bit slower than other methods.

So far the best method I know of for cutting tenons is the Japanese tenon cutting saw that I’ve posted about before. Ultimately I’m hoping to get mind hands on one of those at some point, but for tricky situations, or when you don’t have a better option, I highly recommend this kind of jig for cutting tenons.