Scarf anxiety

It’s cold outside (for Georgia) and the wind is blowing a half-gale. Most of this week I have been inside, thinking about scary things. Like how easily I could mess up $380 worth of marine plywood if I am unsuccessful at making two eight-foot-long scarf joints.

Scarffing comes into this project, because it will be necessary to make three 4 x 8 marine plywood sheets into one 8 x 12 sheet. The new top, which will be about 7 feet wide by 11 feet long, will be cut from this big sheet.

Here’s a definition of a scarf joint, taken from Wooden Boat magazine:

A scarf joint is made by joining two pieces of wood having tapered, beveled, or chamfered ends which over-lap together, as opposed to a butt joint where squared ends of the mating pieces simply butt together. Scarf joints are used to make longer members where single members of sufficient length are not available or are too costly. Both solid wood pieces and sheets of plywood can be scarf joined using epoxy. With the proper cutting and gluing methods, such joints will be amazingly strong, exceeding that of the joining wood members.

Duckworks, an online boat builder’s magazine, showed this side-view diagram of a scarf joint:

 

This looked simple, but the prospect of screwing up expensive plywood still haunted me. So, I fell back on my usual response to frightening woodworking projects: I ordered a new tool.

But today, the last refuge of the carpentry coward has fallen away. My new tool, the John Henry Planer-Scarffer Attachment, has arrived. Progress compels me to make some test scarfs.

So here we are, making scarfs:

 It is good that I did this. I learned a lot. The first thing I found out is that this thing can make a heck of a lot of sawdust real quick. Said another way, it really removes the wood.

Another lesson learned is that there is at least one very good reason to pay the money for marine plywood. Marine plywood has no voids.

My test pieces were not marine grade, just some 3/8" scraps I had in the shop. In just a few passes, the scarffer revealed numerous voids between the plies of the wood--places where there was neither wood nor glue. Filling these voids with epoxy could make a smooth joint. But when the epoxy glue cured, there would be hard spots where the voids were. These hard spots would seriously reduce the joint's flexibility and strength. It is unlikely such joints would bend enough to make our new top without breaking.

But probably my most important learning of the day came with actually cutting some scarfs. I found I could do it!  Behold:

Admittedly, the tapers I scarfed on these two short pieces are not beautiful. I had to touch them up with a block plane. Then, there are the voids....

But, I have familiarized myself with the new machine. I have made a small start toward conquering scarf anxiety.

I will work with these test pieces some more. In a few days, I will glue them up with epoxy and test them for strength. I'll see what it takes to break the joint.

Then I'll move on to the big sheets. Still scary work, but I think I can do it.