Box Joint Jig Problem

[Toller]

After reading a thread about box joints last week I was inspired to make a jig for my router table. It is just a sled that runs in the miter slots (I have one on both sides of the table). I ran the sled through the router with a 1/2" bit in place, and mounted a 1/2" piece of wood 1/2" from the slot. One piece of wood is held against the wood and run through the bit; then the cut out part is put on the wood, and so on. The mate is first run through at the edge of the slot, and then continued like the first.

The pieces dry-fit beautifully, no slop anywhere. I am using almost no glue because the parts fit together so tightly that most of it is coming out. After the glue dries some of the places actually have spaces in them. It is almost like the wood has shrunk a bit. The joint is plenty strong, but it isn't right; but I can't figure out what is going on. If anything the wood should be expanding, not shrinking. Maybe my joint is too tight, and not leaving any room for glue? I dunno.

Any insight would be much appreciated; I can't make any sense out of it.

[Charlie Self]

toller asks:

Do you have to force fit the pieces or can they be push-fitted? If they're a push fit, there should be plenty of room for a modest amount of glue. If you have to really force them together, with a mallet or hard hits with the heel of your hand, then you may be getting some glue starvation, but that shouldn't affect the fit that much, or so it seems from your description.

How dry is the wood you're using? Are both pieces of wood from the same stock?

Is the bit a good one? Are its sides exactly vertical?

Charlie Self "Brevity is the soul of lingerie." Dorothy Parker

[Toller]

I can almost get them together by hand, but it takes a couple mallet wacks to get them together all the way.

Its some scrap oak that has been in my basement for a few months. The cabinet I made out of it is stable, so it must be pretty dry.

Its a brand new Freud. It is 2" long, which is much longer than I need for this, but seems to cut pretty consistently. Tomorrow I will cut a slot, and turn the piece around and see if it is true that way also. If it isn't, that would probablly be the problem, right?

[Charlie Self]

Toller notes:

Shouldn't box joints be the same all the way in? In other words, if they'll push fit part of the way, they should push fit all the way. That might be the problem.

It might be--see above. That 2" bit may be making a slightly off vertical or otherwise scroogy shaped slot, due to problems with router collet runout. Or the bit might be off, though I've never had a problem with Freud bits.

Charlie Self "Brevity is the soul of lingerie." Dorothy Parker

[roger]

Sounds like the joint is way too tight--by forcing the joint together you're compressing the wood fibers. Cut your joints about .002" wider, use normal glue-up, you'll find a gap-free joint. I'm talking table saw here. You might have to have your local machinist modify your bit to give you some clearance. Are you indexing correctly? Roger

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[David Chamberlain]

Another solution is to put the joint together fry and then use "Chair Doctor" glue. It is available from Lee Valley at:

chair doctor glue expands the wood permanently and locks the joint together so tightly that it is hard to believe. The advantage is that you can do all you set up and squaring and everything with no time pressure. Then when everything is perfectly square, you squirt the chair doctor glue onto the joints and the joints lock up tight. Give it a try.