29 August 2012
Although this is a detour from the mainly hand tools direction I've been on for a while now, I needed this jig for small drawers around the workshop, as well as for box making. I like the aesthetics of an exposed finger joint on small boxes and providing the drawers are small and don't carry too much weight, the finger joint is very quick to make in multiples and strong enough too.
—It is crucial that the finger joint jig is guided in a straight line. In most designs I’ve seen, this is achieved through an existing mitre slot or a T-track.
—My router table doesn’t have these for two reasons, namely the slot or track weakens the table which must be co-planar, and it conflicts with the fence in terms of parallel thus affecting accuracy.
—Therefore two temporary parallel guides were clamped to each side of the table and the jig (which is a sled in essence) rode between the two.
—In order to ensure an almost piston fit, two paper-thick shims were placed between the guide and the sled before clamping up. The photo above shows the side fences (held in place with the orange clamp). The blue clamp is holding the stop block to prevent the jig from running too far over the router bit.
The setup above shows a ½" up spiral router bit with the same spacing as same size registration pin. As you know, the three must be exactly the same to ensure a tight fit. The ability to fine adjust the distance of the pin from the router bit is the difference between a tight, neat fit and a sloppy one. Enter the micro-adjuster below: The front sacrificial fence in a jig of this type must be allowed to move left or right to adjust the width of the fingers. This threaded bolt allows me to move the fence (once loosened from the permanent back fence), in small increments.
Below is a view from the back of the jig. You can see the two coach bolts with large washers which fix the front (movable and sacrificial) fence to the back (fixed) fence. These get loosened for the micro adjusting process.
You may also notice that my jig runs front to back on my router table as opposed to most that move from left to right. Direction of motion is not critical here as we are not routing lengths of wood where a climb cut will occur if you travel in the opposite direction. I simply don't have much room behind the router bit.
Here is my 3rd attempt on a piece of pine. The pins or fingers protrude slightly on purpose. They get sanded down at the end for a flush fit. This is controlled by the height of the bit. Remember to take the thickness of the jig/ sled into account when calculating this!