Box Joint Help

mrhart · Post by **mrhart** » Wed Nov 16, 2011 9:45 am

So I'm making my grandson a small cherrywood tool box and tools for Christmas. I wanted to experiment with box joints to make it a learning experience for myself as well. I made a box joint jig out of 3/4 ply, with a faceplate thats interchangable for different size cuts. In this case I used 1/2" and have a walnut spacer in place. I made some test cuts and found the right spacing by small quill adjustments. I am using the shopsmith dado set.
First I noticed the dado set cuts are not completely flat and even like I had expected, its almost like one blade is bigger around than the other.
Second I was getting a little tearout on the backside of a couple pieces which was not consistant on the rest and I didn't see it until I was done.

Q. Is it standard practice to sand all the ends for a really good fit?
Q. Can I fill those small tearouts with something and still have this worthy of a nice gift?
Q. Should I just sell my wifes jewelry and buy a manufactured box joint jig and a nicer dado set?
I really like the looks of these joints and want to get good at them.
The cherry was hard to come by and I don't want to think I wasted it.

algale · Post by **algale** » Wed Nov 16, 2011 11:16 am

I just "built" my first box joint jig a couple of weeks ago. Made out of nothing but 3/4 scrap ply clamped to my miter gauge fence with a 1/2 inch wide key cut from oak. Works very well with my stacked dado head.

Sounds to me like your main problem isn't your jig. If your dado doesn't cut even height dados, a new commercial box joint jig isn't going to improve your box joints, although a nice new stacked dado set will.

Incidentally, there was a recent thread about a wobble dado not cutting evenly. Turns out the poster took the set off the arbor and discovered the arbor had been gouged out and one part of the dado was slipping or sitting lower than the rest. You may want to check that out to see if you have the same issue. But a wobble dado, even working properly, isn't going to give as flat a cut as a nice stacked dado, which may be important if you want tight looking finger joints.

As for cutting the box joints, experiment with some scrap (I used ply) and cut some box joints and test fit them for width (controlled by position of the key relative to the dado set and length (controlled by depth of cut). Since it is a pain if the "fingers" are too short and below the surface, make the fingers 1/32nd-16th too long so you can sand the fingers down to the surface.

As for tear-out, again a commercial jig isn't necessary. Once you've got the set up just right, slip a backer board (can be 1/4 ply or hardboard) between the actual workpiece and the box joint fence to prevent tear-out.

A couple of other things I learned while experimenting. It is important to keep track of the "top" and "bottom" edges of the boards or your box joint fingers won't line up with the notches on the adjacent boards. Also, after cutting all the fingers/notches on the first work piece, you must turn the leading edge of the board around and rest that first notch or partial notch over the key; now put the leading edge of the next piece up tight against first piece and make your first cut/notch on the second piece. Now remove the first piece and cut all the remaining notches on the second piece. You will get a perfect fit.

Good luck,

Al

Gene Howe · Post by **Gene Howe** » Wed Nov 16, 2011 11:24 am

Tear outs shouldn't occur if your cut in the jig is precisely the same size as the cut in the work piece. In positioning the dado blade, you may have widened the cut.
It is standard practice to cut the joint so that the ends are slightly (1/32) proud, then sand them even, after the box is assembled.
The only thing I can recommend for the tear out is to plane the pieces down. In which case, the fingers will stand really proud.

Some folks mix fine dust with glue to mask small gaps in the joints. That works well, but I'm not sure it would be satisfactory for a larger area.
Good luck.

PS. You could cut off the fingers and make a smaller box. Of course, after insuring that the tear out won't occur again.

JPG · Post by **JPG** » Wed Nov 16, 2011 12:22 pm

Replace dado blade with router bit!;) A few other alterations as well(jig,setup). Think table vertical.

Power Pro/M7 or MVII would be advantageous.

billmeyer · Post by **billmeyer** » Wed Nov 16, 2011 5:06 pm

I recently had a bad experience with my dado stack. I was getting tear out, burning uneven bottoms and slow feed rate. When I took off the set and turned it around it worked perfectly!

I really felt dumb! I remember getting interrupted a few times while I was trying to set it up, but still I should have caught it.

Bill

mrhart · Post by **mrhart** » Wed Nov 16, 2011 6:17 pm

Here is some pics

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pennview · Post by **pennview** » Thu Nov 17, 2011 7:47 am

A couple of things I would suggest for future use of your box joint jig:

- add a second runner which will give you much better control over any possible flexing or wiggling of the jig while it is being moved back and forth.

- add a sacrificial face to the jig made of masonite/hard board or thin plywood after you make some test cuts to ensure the width and height of the dado blade is correctly and index pin is located properly. You want the notch in the jig to exactly match the dado slot being cut, otherwise you'll get some chipping.

- if you add a sacrificial face to the jig and also use a clamp to hold the boards being cut against the the jig, you'll likely eliminate the possibility of any chip out.

- use a sharp and clean dado set. Remove any pitch that may have accumulated.

nuhobby · Post by **nuhobby** » Thu Nov 17, 2011 7:56 am

mrhart wrote:
Q. Can I fill those small tearouts with something and still have this worthy of a nice gift?
The cherry was hard to come by and I don't want to think I wasted it.

For something like this, for a 'rescue', I have tried filling the gaps with 15-min-cure epoxy. It is a little darker-colored than the wood but it's not too terrible-looking (tan to brown colored) when finish-sanded.
Don't ask me how I know this....

dusty · Post by **dusty** » Thu Nov 17, 2011 8:18 am

If there was no wood filler, I would probably never get a really good looking joint.

Given what this is, the joints all look fine. Furthermore, I don't believe the joints will be inspected enough to know that you used wood filler.

You ask about sanding down the joints. On box joints and some dove tail joints, I deliberately cut them just a little long and sand them down.

Tear out - best avoided by using a sharp cutting edge and (in this case) a backer board. I find a backer almost essential when routing dovetails and box joints. With a saw blade or dado blade, tear out is very dependent on the grain pattern and type of wood.

I don't care how you do this - don't start without having some wood filler available.

holsgo · Post by **holsgo** » Thu Nov 17, 2011 8:47 am

Here's a trick I've used to fill "mistakes". Sawdust from the project itself, preferable from the bandsaw or belt sander. Mix sawdust and gorilla (polyurethane) glue, wet one side of the joint with water slightly, glue press fit and allow to cure. Sand flush. Don't disturb the foam out until it hardens and you can use a sharp chisel to clean up and overage.