Fingerjoint Splices in Plywood

[gbinga]

There was a thread in the general section -

http://www.neilbank.com/phpBB2/viewtopic.php?t=6019

- that linked to a video where they were making wooden kayaks in a factory, and instead of scarf joints or butt splices they were using finger joints. There was some discussion about that being an interesting way of joining two pieces of plywood. I think it was Mick from Australia who opined that trying to do that with hand tools was more trouble than it was worth - impossible or impractical to get the kind of precision that you would need. I tend to agree with Mick about that.

But...

What if you knew someone with a CNC router, and you got them to make a pair of templates for you out of, say, phenolic sheet, or perhaps even aluminum. Then you use the templates as a router guide, with a pattern bit. The kind of bit with a bearing above the cutter. The bearing rides the pattern, and the cutter cuts the stock.

Once you had the tool (the pair of patterns, that is) you could finger joint plywood again and again, without the hassle of trying to make an actual scarf joint.

I know of a business with a CNC router near where I live. They do specialty work on a job-by-job basis. A lot of work for sign companies, some for countertops, just about anything that a customer could bring in a CAD drawing on. They used to cut out a lot of sign material from 1/4" aluminum sheet, and could cut just about anything else in the way of sheet goods.

I wonder if this would be a practical idea?

gbinga

 

[Jimmy W]

I also think that it is more trouble than it is worth. To me it seems like a solution to a problem that doesn't exist. A simple butt splice covered with fiberglass or plywood seems to be plenty strong enough. I've never heard of a butt splice failing while in use. On plans like from JEM. the panels would have to be redrawn to take into account the amount of space used by the joint.

Jimmy

 

[dawallace45]

Some time back [ about 6 years ago ] I used my router and dovetail jig to make similar finger joints and it worked fine but seeing as how I don't scarf any way and only butt joint with glass cloth or ply plates I've never followed up on it , but it did work for my sample piece , figured it would be too difficult to stuff around with and get the joint angle just right in real life

David

 

[JEM]

The finger joint splice is strong and does a good job. But unless you have the panels predesigned to use them, it's hard to work out on your own.

I've never heard of a butt-block or fiberglass splice failing (that's not saying it can't happen).

Also, on a natural wood finish, the joint looks funky to me.

 

[keith]

i'm with the boys above. will it work, yes, a butt joint will work as well with less work and less wood. it would take more time but you can make most any pattern you want ( mouse ears or fleur de lis or arrow head) draw in on one side and trace it on the other. it can be a good fit or not and epoxy the heck out of it. you dont need a fancy machine, they just make it faster and easier. and that is why people use a butt joint. later keith

 

[hairymick]

In my experiment, I used a dovetail jig and router with a pattern bit as described.

The broblems I had were several. Panel alignment was the hardest.

The other one was supporting the ply underneath the dovetail jig to ensure a neat and perfect cut.

It all looks good in theory but there are so many variables to overcome to achieve the desired result that unless one were a seriously hard core perfectionist in their wood work and set up, the end result would be inferior to the standard panel splic.

finger jointing ply is not an easy alternative to a panel splice.

 

[gbinga]

....To me it seems like a solution to a problem that doesn't exist. A simple butt splice covered with fiberglass or plywood seems to be plenty strong enough....

That seems to sum it up. I've used butt joints on each of my three boats and didn't have any problems, either. I feel quite properly swatted down. :?

I will say that Keith's thought about turning the fingerjoint into a decorative detail is interesting - seems like there ought to be some possibilities there... of course that would make the whole idea of a fixture for cutting the fingerjoints moot, unless you had a design that you wanted to use on every boat, like a personal trademark.

gbinga

 

[keith]

when i'm making a round bottom pirogue ( for going in the mud when the tide is out) , i will notch the round ribs and inlay a keelson with a arrowhead or fleur de lis on the ends. later keith

 

[ancient kayaker]

I think the idea of the fingerjoint is a good one for kits, it pretty well ensures the plank parts will be glued together in alignment. It isn't worth the trouble otherwise and doesn't look great either. The best looking joints I ever did were scarfs modified so the outer veneer was butted for a fraction of its thickness. Bit fiddly to setup the jig but worth the effort, almost invisible, and a lot less trouble than finger joints.