Thursday, July 01, 2010

Let's talk about Finger Joints

At last month's Sin City Woodworker's meeting, I gave a short demo on cutting finger joints. I have a portable Dewalt Tablesaw that I keep outfitted with a stacked dado blade set, and showed the group how to set up the jig for cutting the joint.

My buddy John blogged about the demo here. John teased me a bit in a recent e-mail, as when I did the demo, I had to make a slight adjustment for the fit of the joint. The best adjustment tool I have is a small hammer, and with one whack, I was able to get a perfect fit for those corners. Or as John says - I lead a charmed life in the woodshop.

Cutting all four corners is a breeze when you've got the jig properly adjusted. You can make a box in less than three minutes. Sweet.

But this post isn't about cutting finger joints - you can find info about how to do that in many different places. Tage Frid's book on Joinery is one of the best investments you can make, if you're looking to add new joints to your woodworking arsenal.

This post is about something a little more obscure - how to machine the dado around the inside of the box, to hold the bottom in place. I shared my secret weapon with the group, and I've already heard from several members who have gone on to purchase it.

When making a box with mitered corners, you can simply run your dados from end to end on the box sides. The dado will never show, so it's simple. But with finger joints, you have to machine stopped dados, and since you're working with short end grain, it's quite easy to blow out the ends of the grain. That's where this slot cutting bit comes in - from Lee Valley.

It's available in a few different configurations, but I just use the simple 1/4" slotting bit.

It's simple - just cut your corner joints and assemble the box. The run the router along the inside of the box, where you want the slot to be cut. There are only a few things to think about - you have to be careful when exiting the cut. Pull the bit away from the wood before you lift it up, or you'll end up cutting out an area of wood on the bottom that you shouldn't cut.

John said he did his machining on a router table, but I usually rout mine with a handheld router.

The slot that you wind up with has a rounded corner, matching the radius of the bit. You can either square off the slot with a chisel, or round the corners of the bottom panel. I prefer the latter.

See how the bottom has square corners, but the dado is rounded?

I usually just trace the radius of the bit onto the corners of the bottom, and sand them on the stationary sander.

A perfect fit!

Once everything is sanded, it's time for the gluing process. It usually takes a lot of clamps, since you want your joint pulled tightly together in each direction.

I applied the glue with a small paintbrush, and felt like I was putting a very light coat on all the fingers, but you can see, there is still quite a bit of oozing out. I'd rather see that, than a dry joint.

Eight clamps on a small box - can you ever own TOO many clamps?

And finally, after the glue dried, I sanded down the fingers that were proud of the surface.

Is there anything easier to cut and more attractive than this joint?

With all the gluing surface, this corner joint is exceptionally strong. People can argue all they want in favor of dovetail joints, but I find the set-up time for machining them to be too labor intensive. I can set up my saw and cut finger joints in a matter of minutes, and they're every bit as strong and attractive.

Anyone disagree with that? Let me know.

No comments: