From The Workshop: Stacked Bridle & Box Joint Jig Part I

This is my second home-built precision jig (the first being my cam-set groove jig). I made a simple 1/8" box joint jig for my drill press stand build, but it was a bit unwieldy and more like a temporary jig than a long term tool. I actually started this build during the summer, but more practical non-'shop matters took precedence. Now that I'm back in the 'shop full time, the half completed jig was getting in the way and I need a new box-joint jig for future projects anyway.

Inspiration

I draw a lot of inspiration for Matthias Wandel's work. He was the first woodworker I found online and after a few years of unrelated interests, he was the one woodworker I looked up when my "hobby cycle" came back to woodworking. Part of the draw is his engineering heavy approach, some of it is his humble, down-to-earth manner, and the rest is his reliance on home made, budget friendly, tools and materials. From what I can tell, he could probably afford all the fancy tools and wood that his youtube contemporaries show off (more-so if he shilled out to sponsors like so many have), but he seems to gain enjoyment from finding solutions to problems in his unique way, and all the better if he can do it for less money. Sometimes (quite often actually) I feel his approach to a problem is a bit on the over-engineered side, and I don't really have any interest in incorporating wooden gears into my own tools and jigs, but as a role model to aspire towards, I think I could do worse.
For this jig, I cribbed design elements from Matthias's linkage-based tenon jig. I thought about building a stepper motor controlled version of his box-joint jig years ago, but it never went beyond the drawing board. His linkage based tools and jigs have always been more interesting to me (like the pantorouter) for their ability to use scaled up templates for high-precision work. This preference is driven, at least in part, by my forgetfulness and inability to keep track of things like revolutions or steps: templates, while easy to screw up during setup,  save me from having to keep track of how many click of a lever or turns of a wheel I need to make the next cut. (Still won't stop me from putting the workpiece in upside down though...)

Linkage Design

One of the issue with his linkage design in the tenon jig is the way the primary pivot is attached to an articulating arm. In his approach, it makes sense, but it still introduces a lateral misalignment as the control arm swings through its range of motion (see video below). The lateral misalignment results in as-expected relative scaling between the carriage pivot and template pivot, but inconsistent absolute x-y postions of the same.

Matthias says it's not much, but I wanted to try and eliminate the effects completely. For his tenon jig, the dial indicator means he can set the absolute position of the carriage and ignore the misalignment. I'm planning to run templates exclusively, so I want the best scaling accuracy possible from my setup. So, instead of a linkage to semi-restrict the movement of the primary pivot, I went with a linear guide to restrict the primary pivot to only vertical movement. My thinking is that as long as the primary pivot stays in the same vertical (y) plane, the horizontal (x) absolute positions should remain unaffected by linkage misalignment. Of course, there will be vertical misalignment at the template pivot, but a large enough template should overcome the vertical movement.

Carriage Design - Doing Double Duty

Nearly all the existing box-joint jigs use a moving carriage that is indexed with a threaded rod or indexing groove-plate. Many tenon jigs also use some sort of carriage system. The difference is that that tenon jigs cut workpieces "in-line" with the blade, where-as box-joint jigs cut boards perpendicular to the blade. Tenons usually require a lot of vertical blade clearance, while box joints typically are not that deep; this is why most box-joint jigs aren't suitable for cutting tenons, even if the have a suitable clamping face that is in-line with the blade. Because I am an idiot, I prefer to jump in head first with the most complicated design I can think of, despite knowing better. As such, I decided to build a jig that would cut both tenons and box-joints. Using a tenon-jig as the basis, I ended up with this "outside-corner" carriage system:

As you can see, it has clamping faces for both tenons and box-joints. Since the rails for the carriage are fixed to the right of the blade, there is no danger to cutting through the guide-rails (8 mm shafts). There is still a maximum depth that the box joints can be cut to, but I designed it to specifically handle a standard 2x4 box joint. Likewise, I designed the tenon cutter around 2x4 stock.
A non-trivial restriction of the linkage system compared to an indexing system (like a box-joint jig) is reduction in the total x-axis movement possible: the scaling effect means that for a 4" carriage travel, the template pivot would need to move at least 8" for a 2:1 linkage. Anything more than a few inches of travel or a low scaling ratio would result in a jig that is so wide as to be unusable. With the rails and guide-blocks I bought, I ended up with 4.5" of lateral carriage travel. That basically meant I was wasting my time since many box-joints would be cut on panels greater than 4" wide. After some deep thought, I had the idea of a telescoping carriage system that would double the overall travel with a typewriter-like reset halfway through the travel.

 

In part II, I'll show the final product and see if it works. I tried to model the jig in sketchup, but with the linkages, I couldn't really get a feel for the range of motion. I played with some cardboard models then went YOLO and just started cutting and gluing.