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Scarfing half inch plywood

Posted by Sandy 
Scarfing half inch plywood
November 04, 2016 04:32PM
I'm trying to get back in the shop. Haven't done much for half a year because of two eye surgeries, retina and cataract. Still can't see shit but new glasses should fix that in two weeks.

I'm building a 66" inch wide 17' foot long decked white water boat. The bottom and 8" up the side will be 3/4" inch Plascore/fiberglass. Everything else will be 3/8" inch and 1/2" inch Meranti Hydrotech NOT covered in fiberglass. This will be bare oiled plywood attached to an all fiberglass bottom and chine. Should be an interesting boat.

However. That means I have to be able to scarf 3/8" plywood for the sides and 1/2" plywood for the deck. And my 7-1/4" worm drive saw won't cut the mustard. I used to have an 8-1/4" inch Skil circular saw I used for scarfing, for most of a career--but it died a while back and I couldn't get parts. That was a 30 year old saw they don't make anymore.

So I bought an 8-1/4" Skil worm drive this morning. It will cut all the way through 3/8" plywood at a 12:1 scarf slant, and almost all the way through 1/2" inch plywood. $209 dollars for the saw and $43 dollars for the blade. Those are expensive scarf cuts. I suppose I could have cut through the plywood from both sides with a 7-1/4" inch saw and then cleaned up with a sharp hand plane. But fuck that. Man needs a saw.

Makita makes a not worm drive 8-1/4" saw that theoretically makes smoother cuts because it spins faster (it's not a worm drive). But it gets bad reviews. The foot on the worm drive pivots fore and aft on two swivels so it's rock solid. The Makita foot is attached to the saw in the front only with a plastic sleeve angle adjustment. And the online reviewers say the saw bends away from the foot attachment and torques on the blade too easily. So I bought the worm drive.

Re: Scarfing half inch plywood
November 04, 2016 05:13PM
Here's the old 7-1/4" saw attached to the same home made scarfing jig.

Re: Scarfing half inch plywood
November 05, 2016 03:41AM

Re: Scarfing half inch plywood
November 06, 2016 04:51PM

Here is a diagram and a photo of some cuts I made yesterday in 3/8" inch thick Meranti Hydrotech, with an 8-14" inch saw and the home made jig pictured above.

Even the 8-1/4" inch saw does not cut all the way through at an 8:1 scarf (run to rise like a roof rafter, where 8 is the run and 1 is the rise or thickness).

But the little nubbin left after the cut is easily cleaned up with a hand plane.

When scarfing 1/2" plywood the nubbin is fatter, approximately 3/16" inch high. Still easily cleaned up with a block plane. Scarfing like this can be done with a more common 7-1/4" inch saw, still at 8:1 and still cleaned up with a block plane. But it is a bit more work. Reducing the angle so the scarf face is more like 2" inches long rather than 3" is possible. Then the 7-1/4" saw cuts all the way through. But then you get a weaker scarf too.

It's best to stick with a full 8:1 slant on the scarf, and then to clean up the resulting bump on the scarf cut with a block plane. Then you get a full 3" scarf face for gluing (with 3/8", more like 4" inches with 1/2" inch plywood).

At some point I'll post a diagram complete with dimensions, for making the scarfing jig itself.
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