Home made table saw box joint jig
The joints are always the weakest part of any piece of woodwork. Because of this, I'm always obsessing over making better joints. Dovetail joints are cool joints, but way too much work to cut by hand, or require a relatively expensive jig to make with a router, and even then, they don't end up that accurate and still take lots of time. However, accuracy is the key to having strong wood joints. So I'm quite sure an accurate box joint is much stronger than a typical dovetail joint, provided that both have been properly glued.
Cutting a box joint |
I use this in combination with my cheap dado blade substitute to cut slots. However, for this application, even without a proper dado, a proper set of dado shims is almost a must, as the width of the cut has to be adjustable to about 0.1 mm.
The device is essentially a board that I can push back and forth with a threaded rod. On the far end, the threaded rod, there is a block that holds the rod, and a crank for turning the threaded rod with. This block I clamp to the fence of my sliding table.
Crank block |
Ball bearing and crank |
To allow the threaded rod to turn freely, but fix it in the lateral direction, I run the 5/8" threaded rod through a 5/8" ball bearing. The ball bearing is clamped between two half thickness nuts on the rod. The ball bearing is the 'sealed' variety. Hoepefully, this will keep the dust out of it. It does have a small amount of lateral play however, despite being brand new.
The threaded rod then goes through a nut which is fixed to the end of the board that slides side to side. The nut on the threaded rod also has a tiny bit of play in it. Having a bit of play in the system is unavoidable. The solution is when adjusting the screw, to always approach the desired point from the same direction. The friction in the board sliding side to side will ensure that the play is always going the same way, so its net effect on the overall accuracy cancels out. The spring loaded roller guide ensures that there always is adequate friction so my jig won't wiggle around.
Roller guide |
To make sure the board is always stays firmly pressed against the fence, I added a spring
loaded roller to push it against the fence. When I bought the ball bearings, I wasn't yet
sure about what design I'd use, so I figured I might need two bearings for the rod. As
I didn't use it on the rod, it came in very handy for the roller. I stuck the bearing on the
end of a 5/8" dowel, put that through a block which is mounted on a hinge (black). The
hinge is a funny offset hinge, with the actual pin relatively high up. This gives the two
springs that tension the roller against the fence more mechanical advantage. I just
happeend to have a hinge like that lying around.
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To allow for a large amount of lateral adjustment, the screw needs to be able to go into the board quite a bit. This requires either a very deep hole, or a cutout as shown in the image above. The coutout essentially extends the hole without having a super long drill. Even if you do have a super long drill, if you drill to a depth of 20 cm, you may very well find your drill wandering off to the side, and the hole won't be properly aligned with the threaded rod. Worse yet, with enough error, the drill may break out of the side of the piece of wood.
Also note on the above picture the slots on the right side of the board. I took this picture after I had just cut one joint with it. I don't bother trying to align subsequent joints, so the bottom right of the board eventually mostly gets cut away. If I need a solid backing to my piece, I can always clamp an extra piece of scrap wood to this board for that purpose.
box jointed box corner |
When designing a joint, its best to design it so that the advance between each cut is an exact number of turns on the crank, so you don't need to remember which angle the crank is to face next time. Unfortunately, a 5/8" threaded rod is 11 threads per inch, an odd mreasure even by awkward imerial units. Because I do anything that needs precision in metric anyways, the units really really don't work out to anything evenly. I usually make joints that involve 4 turns per cut, or 6 turns per cut. With 4 turns per cut, I need two saw blades plus about 0.015" of shim. With 6 turns per cut, I need 3 saw blades and much more shims. Having a good metal venier or dial caliper is essiential when seting up for a joint. I set it up by guessing the shims I need, making a series of cuts, and then measuring the width of the cut slot and the width of the remaining fingers. Divide that difference in half, and you get the thickness of shims that must be added or removed. Then make a test joint to see how it goes together. I always aim for a joint that holds together by friction even before any glue is applied.
Chances are, whatever you have sliding in the T-slot of your table saw will also have a bit of play to it as well. To eliminate this source of error, its important to always be pushing the sliding table exactly the same way every time. Its best to apply a bit of a turning force to the sliding table while pushing it across, and maintain this turning force when pulling it back. I do this by pushing it forward by the left end (where I have the crank) and then, pulling it back, I grab it closer to the blade and pull it back while still maintaining abit of forward force on the left side. Spraying a bit of WD40 into the T-slot before starting makes it go a lot smoother.
Read More about my box joint jig.
I have since built a much more elaborate box joint jig that uses gearing to give more flexibility in terms of tooth spacing.
Computer controlled box joint jig
A good page on conventional box joint jigs
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