Blind finger joint?

[Jamie Buxton]

A finger joint is a good carcass joint with lots of glue area. For instance, it could be used to join the top to the sides of a solid-lumber bureau, when the grain of the sides goes up/down. However, the usual finger joint leaves the joinery very obvious. I'd like to find a way to make a blind finger joint -- one that has all the structural integrity of the finger joint, but is invisible when assembled. Below is a sketch of what the two pieces would look like. Has anybody seen a jig design which would make this?

[Brad Schmid]

Wow, that's alot of little tiny mortise & tenons I can see why you'd want a jig Maybe CNC router with tiny cutter??? Can't wait to see if someone has a simple solution.

On the other hand, how 'bout a different joint, like a stopped sliding dovetail?

Good luck!
Cheers,
Brad

[Jim Becker]

I believe you may be able do this with the Leigh FMT and maybe a variation with the D4 and M2 template system. The tenons and mortises would be rounded, of course.

[Doug Shepard]

Hmmmm. I hope somebody chimes in with a jig idea as I've been contemplating something like a blind mitered finger joint that might be feasible with a jig similar to one that could make your joint. But looking at your sketch, the top piece looks to be the only real challenge. You could get the bottom (vertical) piece with a standard table saw box-joint jig then cut away the shoulders and cut the fingers shorter. Getting that to align correctly with the holes put in the top piece by a separate jig might be pretty tough though.

[Lee DeRaud]

But looking at your sketch, the top piece looks to be the only real challenge. You could get the bottom (vertical) piece with a standard table saw box-joint jig then cut away the shoulders and cut the fingers shorter. Getting that to align correctly with the holes put in the top piece by a separate jig might be pretty tough though.

Of course, the solution to that problem is to use the same jig for both pieces and a whole bunch of tiny loose tenons.

The question I have is, is this really going to be all that much stronger than using a couple of loose tenons oriented the "normal" way, or, for that matter, dowels?

[Keith Christopher]

I think what would look even cooler here is if they were thru tenons and sanded flush. I mean what a great symmetrical detail on the top surface. I would think making a jig Ok so here's my idea for a jug to cut the mortises with some "backer blocks" this could also be used to make the tenons.


A long "box" with a threaded screw rod running it length and about 3" past each end. Some cap screws that can be used to tighten the jig to the stock. The sliding frame can move along the surface supported by the "box" a self adhesive measure can run along the support box for accurate repeatable mortises. with an upright support frame and something to back the stock it could also be used to make the tenons. clear as mud ? No ? Well I hope this image helps a little.

Keith

[Jamie Buxton]

Doug -- you're right: the male piece is easy. It is the female piece that is difficult.

Lee -- yeah, dowels sorta work. That's what I do now when I want this kind of joint. I use a template intended for shelf pins and a plunge router. Using those ensures that the dowels line up. With the right template, I get 1/4" dowels on 1" centers. Individual dowels aren't real strong, but having one every inch seems okay. However, always in search of better ways, I like the much larger face grain glue area offered by the joint in my sketch. Of course, if I can't figure out how to build it, the benefit is only theoretical.

[Ken Garlock]

Not hard with an Incra router fence system. Make yourself a template for the fence and get with it.

It is just like making dovetails only different. Come to think about it, you might be able to use a dovetail template directly. Just change the bit to a straight bit. Put some stop blocks on the fence for the plunge cuts.

[Doug Shepard]

Doug -- you're right: the male piece is easy. It is the female piece that is difficult.

Don't know for sure if this would work well or not, but I had another thought that involves using the male piece as the 'jig' to make the female piece. Make the male piece with a std. TS box joint jig, and make the fingers extra long. Then place the male piece over the female and offset it. Use a bearing guided router bit with the bearing the same size as the cutter, and use the long fingers on the male piece to guide the bit, with an appropriately placed stop block to limit how close to the edge you get. Then trim off the x-long fingers and shoulders on the male piece. This still has plenty of problems - square up the ends of all those holes, or round over all those fingers. I think you'd also have to have finger/slot sizes big enough that you can fit a bearing guided bit between the fingers and the size of the TS cut fingers would have to exactly match the router bit diameter. Just another one of my hare-brained ideas FWIW.

[Steve Cox]

I guess my question would be why? If the tenons went through the piece to be seen on top that would be one thing but otherwise carcase pieces have been assembled for centuries with standard mortise and tenon joinery and I don't see a need for the increase in strength this would give.

[Mike Cutler]

Jamie. I have to ask, what is the purpose of this joint? It would be fun to do I agree, but are you looking for a specific function to be performed.
Depending on the orientation of the grain, I'd fake the joint. I'd cut a standard finger joint and offset the fingers to leave a rabbetted edge. I'd then glue a seperate piece that matched the grain to hide the joint.
The standard finger, or box joint is not inherently very strong by it self. It relys on the strength of the glue, or mechanical fastener for it's strength.
The supers, or boxes, that make up a beehive are normally glued and nailed box joints. A full deep super can weigh 75-90 lbs when full. Even when the joint is epoxied and nailed with ring nails the joints loosen up. Of course they see alot more stress than most rational folks would apply to any joint. 75,000 bees per hive,can really motivate you at times
Long story made short is, are you looking for greater mechanical strength from the joint you've laid out?

[Mike Parzych]

I'd have to wonder why also, unless you just enjoy that aspect of woodworking. Since modern glues are stronger than the wood they join even in the simplest joinery, I can't imagine an application that demanding of a joint.

[Jamie Buxton]

Ken -- Can you provide a more-detailed explanation about how to use the Incra jig to make this joint?

Steve -- maybe I wasn't clear. The side of this bureau would be planks, edge-glued to form a panel. The top, too, would be a glued-up panel. The grain in the top runs the long direction -- left/right if you're standing in front of it. The grain in the sides runs up/down. The appearance of the carcass is very clean -- if there's no joinery showing. The grain just flows across the top and down the side.

Doug -- using the male fingers as routing guides doesn't seem to me to do the right thing. You want to bore the holes where the fingers are, not where they aren't

Mike Cutler --- I'd say the finger joint is a very strong joint. It has lots of face-grain glue area, and modern glues stick wood together very well.

Mikes Cutler and Parzych -- The purpose of this joint would be to make the kind of carcass I described above.

[Chris Padilla]

Jamie,

The Incra jig gives you precision to 1/32" steps (0.001" repeatability) due to its interlocking design. I have one on my TS and one on my router table...you're welcome to have a go at it any time you like.

The "tenons" or "fingers" would be fairly straight forward although it would involve moving the plank through the cutter and if it is large (as I guess it is), you want a way to move the tool (handheld router) through the wood. Keith's idea of a jig might be helpful in that respect. The Incra stuff is only going to help you if you can move the wood through the tool and not the other way around. However, the Incra could help you make a very precise JIG to use a handheld router with. But, I think you will end up with nice square tenons.

Now the mortises in the other half of the joint could be done on the Incra jig on the router table however they would be rounded and not square.

I'm not sure which is more evil: rounded tenons or rounded mortises.

[Doug Shepard]

...
Doug -- using the male fingers as routing guides doesn't seem to me to do the right thing. You want to bore the holes where the fingers are, not where they aren't
...

That's why I said "Then place the male piece over the female and offset it." Still don't know that this is the best answer, but maybe my earlier description wasn't very clear.