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When making a box joint how do you determine how wide to make each finger so they come out even?

I know there is an easy way to do this that I just am not getting, trial and error sucks:o

jcraigie wrote:When making a box joint how do you determine how wide to make each finger so they come out even?

I know there is an easy way to do this that I just am not getting, trial and error sucks:o

Divide the height of the box by the number of fingersx2.

i.e, 4" tall box, 8 'sets of fingers' x 2 = 16. 4" / 16 = 1/4" per finger.

2" tall box, 8 'sets of fingers' x 2 = 16. 2" / 16 = 1/8" per finger.

The 'end' fingers do not HAVE to be full width if the height is not a multiple of the finger width.

Are you using a jig to set the spacing?.

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Or... Just make the box pieces over height and after you cut the finger joints then saw the excess height off at an even tooth.

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jpg, that's what I was looking for thanks! I plan on building a jig to use, I had a plan for one but I like the one you showed better, thanks.

Dumb question #2

How do you determine the number of fingers, is it just based on how you want it to look or is it also by the thickness of the stock?

jcraigie wrote:jpg, that's what I was looking for thanks! I plan on building a jig to use, I had a plan for one but I like the one you showed better, thanks.

Dumb question #2

How do you determine the number of fingers, is it just based on how you want it to look or is it also by the thickness of the stock?

Yes! Whatever floats yer boat. Too many fingers makes for too many 'joints' and more difficult assembly. i.e. thicker material = bigger fingers = easier 'fitting'.

Thanks again, that's the info I was looking for exactly.

Box joints are really not as difficult as many might think.

Some suggestions:
a) Make sure the space between the dados are EXACTLY the same width as the dado itself. We can get away with a little variance if the joint is small, say ±5" or so. The longer the joint = the less tolerance for variance between finger and dado width.
b) cut the dados a little deeper than the width of the finger/dado. This way, you can sand off the "sticking out fingers" for a real good look.
c) do several dry fits of all box joints before gluing. This will help remove any small pieces of wood that will restrict perfect mating of the joint. Then use a little glue in each dado before assembly and put the joint together forcing the glue toward the outside of the joint. This method will only work for the first two corners of a box. Keep the glue toward the outside of the dados for the last two joints, because these will be assembled at the same time. Maybe the use of an assembly hammer will be needed.

jcraigie wrote:When making a box joint how do you determine how wide to make each finger so they come out even?

I know there is an easy way to do this that I just am not getting, trial and error sucks:o

I think the first task is to determine the size of the joint and how you are going to cut it. I prefer a router bit but that is just personal preference.

For discussion only, let's use 1/2" fingers and we'll use the dado blade. Cut a couple sample dados and measure them as accurately as you can. Whatever the dado measures, that is the width of the fingers.

Now the width of the stock to be used. If at all possible, make it an exact multiple of finger width. That way the end joint will be equally sized.

If the stock is not an exact multiple of the finger size (1/2"), do a layout of the joints starting in the center of the stock. I have a "center zero" ruler that I use for this. In this fashion, the end joints will be equally sized (whatever they turn out to be). Hopefully they are not splinter size.

What I actually do is cut the stock over sized, cut the dados and then trim the sides down to match.

VERY IMPORTANT NOTE: Don't commit to the finished size until it is finished. That way there are no design flaws, ever.

I was interested in box joints as well after watching this Saw Dust session http://www.shopsmithacademy.com/SS_Archives/SS102/SS102_Finger_Joints.htm. It seemed like a pretty good jig to make and I have made it but not actually used it yet. I used some leftover birch ply and a piece of walnut for the locating pin. I need to redo the walnut pin as I glued it in the wrong direction.......DUH! It seems to be an idiot proof jig as long as you get it precise when you build it which is not exactly easy for me. Specifically how do you get the pin exactly 1/2" x 1/2" for making 1/2" box joints? Also I have to play around with my dado blade thickness to see if I can get it exactly 1/2" wide. When I set it up the way it says to for 1/2" it is not 1/2" exactly. But like everything it is a learning process for me.

When watching the video again I remembered I could not figure out exactly how he was keeping the jig attached to the mitre guage? Is he just using the hold down to do that? Also his jig is slightly different than the plan shows as the through bolts are offset on his jig.

Dispatch from Rube Goldberg:

Here's another way to address box-joints:
http://www.shopsmith.net/forums/showpos ... ostcount=8