Jiggy
This was my first finger jointing adventure. Go big or go home right?
I thought about building a jig that was adjustable and re-useable like you have. I'm glad it didn't do that. I was going to use brass or keystock as a guide, and ended up not doing it because i don't have a mill to knock it down to the right thickness.
So, I used some scrap 1/2" oak ply from home cheapo that was sitting around. I think the parts were about 8x48, 2 of them. I ripped a rabbit about 1/2 the depth about 3" in from the edge. Across the back, I ripped in rabbits all the way to the line from the back edge every 8". I glued one board vertical in the rabbit and then glued in 4 or 6" squares in ever slot to make a very rigid structure.
I don't have many good pictures, but hopefully from this description and the one photo you can see it.
SAFETY NOTE: I did not build any handles, and the blade comes back into the work area, so you need to be REALLY CAREFUL where you put your hands. I didn't cover the blade, because I wanted to see it!
Originally I was pushing it with a miter gauge similar to how you are. That had too much slop, so I added a wood piece to slide in the miter slot in the table saw. That still had too much slop, so I added a second one in the next slot, and spread them slightly. Lots of drag but no slop at all. And you can still push the jig.
The hardest part was getting the spacer blocks right. A deviation of only a few thousands of an inch here can cause an epic fail. I cut about 10 of them really close in thickness (trying to match the dado head at 1/2"). I picked the one that was closest, and failed miserably. Then I figured out to actually measure the thickness of the dado stack with a caliper, and found that the little guidebook included with my relatively nice dado set is not all at accurate. It was off by at least .004. After measuring that, I was able to measure the gap between fence and blade stack and cut much more accurate spacers.
Then I went and set up the first cut slot and set up the first spacer. I brad-nailed the spacer to the surface. I did about 6 slots and made another mating piece. It fit aswesomely. So I made a pair of 24" scraps and jointed them. They failed miserably. They first 3 slots or so were dead on and then they got steadily worse until there was almost 3/16 to 1/4 misalignment on a 1/2" tab. ******?
This is when I got the calipers out and discovered that the slot spacer was 0.004 thinner than the dado stack. Multiply .004 by each and every tab across 24" and you get something in the area o 1/4"
I figured out 2 problems:
I was only holding the work against the jig, so it ended up being pushed all the way against one side, making the slop all hit one side of the joint and not average out. After getting the right size spacer, I clamped every move, so I could sort of make sure that the slop felt even. I was down to about .001 now, and try to make that "feel even" on a table saw!
I also added 3 more spacers so that the work engaged 4 instead of 1. It sped up alignment significantly. After everything was done, I had sanded all of the joints with 120 grit to ease the transitions in and made sure that everything fit really smoothly.
I don't have any special skills, I'm just dead stubborn on doing things the way I want, and I'm willing to fail. I have built a lot of furniture and cabinetry, and houses and whatnot... so I don't scare easily. I would recommend anyone with a tablesaw, a dado set, and a caliper to try this.
Enjoy, it was very satisfying. And having a chuck box that fits perfectly in my jeep and holds my stuff perfectly is well worth the long winter nights in the shop... Especially, because winter in Fresno is freekin awesome!