finger joints in plywood

[shoggoth]

I'm (attemping) to build a 2x12 speaker cab, and I'm finger jointing the edges. It's a Kevin O'Connor "detuned" cab, so it's pretty deep (16 inches).

I've got a Keller dovetail jig (http://www.kellerdovetail.com/models/1500.html), and it's attached to a 2" inch thick piece of mahogany lumber I had laying around (it was milled flat already), and finger joint bit.

I had success clamping to smaller pieces of pine and routing those out. When I clamp to the 3/4" birch plywood, though, eventually the clamps !$@#$ start slipping and now all my fingers are lined up wrong. The clamps are these things with 2" wide rubber grips that press into the wood.

Any ideas on how to keep the jig from slipping? Maybe screw it into place, and fill the screw holes later? Or are my clamps just crappy?

[xtian]

I use something like this:

http://www.harborfreight.com/dovetail-m ... 34102.html

Clamps both pieces for you. I get pretty good results, and you can't beat the price...

[cbass]

Table saw make a jig clamp pieces together and run them through super easy once you get it set up.

[Cantplay]

Sandpaper under clamp instead of rubber.

Make sure your cutter is really sharp.

The movement is from vibration, and it could be from pushing the router too hard.

Try cutting with a narrower cutter first so the finish cutter has less work to do. If you can't do this cut shallower and work up to the depth.

I see they offer their own clamping setup. You could duplicate that cheaply with 2 5/8" carriage bolts and some washers. All their hardware does is add speed.

John

[Geeze]

I made a jig for a table saw - basically a big U shaped thing about 12" wide. I use a pair of metal pad Vicegrips to hold the piece being cut - hard enough to hold without marring the wood. Most rubber padded clamps don't have the adhesion for this.

Personally, unless you are going for what I call the 'Hollywood' look of a finger joint, I wouldn't bother as today's glues like the Titebond series make joints that are stronger than the wood they bond. If you are hard on the cab you can always reinforce the joint with a corner block.

Russ

[shoggoth]

I figure I'll re-route the existing fingers, they'll have massive gaps but will at least fit together. Then bond w/ epoxy I have left over from my boat-owning days, fill gaps with wood-flour-thickened epoxy, and epoxy in some wood blocks in the corners for the structural support. Just need to come up with a way to get the jig attached so that it won't leap all over the place while I route.

[cbass]

Does it have its own clamping system ? It really shouldn't take much pressure to route are you using good quality bits? Cheap router bits are no bargain.

[Phil_S]

I have the Keller. My only attempt with plywood was a total failure. It needs a backer board and that means you are pushing it with respect to the thickness limit. As for clamping, I put the work in an old Black and Decker Workmate along with the jig. It will slip sometimes, so you have to be very meticulous about the clamping tension being even. I think this is better than clamps.

Here's the new model. They got fancy. I think mine is at least 30 years old.
http://www.target.com/p/black-decker-wo ... Alp28P8HAQ

[shoggoth]

The bit is sharp (well was, after all the joints I've just cut it may be getting a bit duller).

I don't own a table saw, so that's out of the question - did try a buddy's with a jig he had, unfortunately his table saw didn't have a locking height so the depth of the cuts would keep going deeper when you turned it on

The problem I had with slipping is because the router kicks whenever it passes through the other side of the plywood. Anyhow, I just ended up screwing the jig to the plywood. No slipping after that.

The tearout is kind of annoying, but I'll fill it all in with thickened epoxy, and then tolex will hide the ugly. Pine was definitely an easier material to work with than plywood.