Judging by the number of questions I received about making the box joint jig, it is evident that I didn't cover off with enough detail exactly now the "indexing" or "finger alignment" worked, so this video is to make up that shortfall. When you are making Lynn Sabin box joint jig, one of the key components is the Readi-Rod (also known as threaded rod, Redi Rod, Ready Rod and Thready Rod). The rod basically has three components, the length of the rod, the diameter of the rod and the number of threads per inch that are carved into the rod. Of Course the length is important as it needs to be the full length of your jig, the diameter is not so important but I found 3/8" to be a good weight to work with, and the final Very important component, the threads per inch ... should be 16 threads per inch. There are many other options of threads per inch and you can decide if another is best for you, but for most woodworkers 16 will be the best and here's how it works.
If you place a nut somewhere along the length of the threaded rod and hold that nut so that it doesn't turn, then turn the threaded rod 16 complete turns, that nut will have moved exactly one inch along the lenght of the rod. If you were turn that threaded rod only 8 turns, that nut will move exactly half and inch and of course if you only turn the rod 4 turns, the nut will only move 1/4 of an inch. All of this is important if you are going to be making box joints that are either 1/4", 3/8" or 1/2" which are the most common sizes for box joints. If you need to make another size box joint a different threaded rod may be a better choice.
Another point to know is that the threaded rod is attached to one side of the main body of the jig so it cannont move, this can be done by locking 2 nuts on either side or by using a single locking not or, as I did a combination of both. The threaded rod needs to be snug to the frame, but loose enough that it will turn easily.
As the threaded rod is what drives the carraige back and forth, there also needs to ve a threaded nut that attaches to the carraige so that when the threaded rod is turned it actually moves the carraige back and forth. For this I used a couple of "T" nuts on either side of the carriage. Attaching them is a bit of a chore, but I removed the tangs off the T nuts then used some very small screws to attachen the outer flange to the carriage. (in future weeks I am going to have a small steel, greaded plate made that will attach to the carraige more firmly).
The final not is about the crank, I did not go into too much detail on this because it is a pretty easy thing to make, or at least if you have a lathe it is, otherwise you may need to buy some dowling, or conversely, purchase something that will work equally well.
Now the way the box joint works once you get it all together and aligned on your table saw, is like this. If we assume you are using a 1/4" blade to cut your box joints (either a dado blade or some other 1/4" blade), all you have to do is align the carriage with the edge of your saw blade, and make your first cut. You will see that when you turn your handle or crank 4 complete turns it moves the carriage 1/4 inch, BUT that will place your wood where the next finger will be, so you need to advance the carraige another 4 turns and make your next cut there. So, every time you make a cut, you need to move the carriage 8 turns to move the carriage to the next place where the next cut will be made.
After a bit of practice, you will be surprised how easy and painless it is to make excellent box joints. And if you still have questions, please use the Forums on this site, or the comments to ask questions so they can be answered for all to see in the future
Copyright - Colin Knecht
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