Saturday, January 28, 2017

Chirp Chirp, It's a Birdsmouth

I've been itchin to get started on the Apple Campion build ever since I purchased the plans but in addition to getting the shop in order, I have been without a truck since my son decided to ram it into an embankment (fortunately only his pride was hurt).  So getting lumber has been a bit of a challenge and I will need to get something soon if I am to transport the plywood and avoid delivery costs.

With that said, I found two really nice 16' Douglas fir boards for the main mast at the local specialty lumberyard (Goosebay Lumber) yesterday and immediately hatched a plan to get them home.  I won't go into details, but it involved my wife's Mazda, a 14' aluminum ladder, and a lot of rope.  I should have taken a picture because it was pretty comical looking and marginally legal, but I got the boards home without issue (other than being $135 poorer).

Birdsmouth reprinted
from Duckworks Magazine.
The plans call for either a solid or hollow spar; I am opting for a hollow spar using the 'birdsmouth' method.  I've never built one before, but I'm pretty sure it's in my wheelhouse and there is a lot of literature out on the interwebs detailing the procedure. One of the better ones is on the Duckworks Magazine site here.  In a nutshell, making the birdsmouth spar involves cutting a bunch of staves, and then cutting a notch out of the end each one, making the profile look like a birds mouth (hence the name). You can use anywhere from five to sixteen or more staves to make the hollow spar, but I opted for eight because it is the easiest to cut the notch (it uses a 45 degree notch).

To get started I used the birdsmouth calculators on the Duckworks Magazine site to figure out my stave size.  The plans call for a 90mm section tapering to 50mm at the top so I just plugged in the number of staves (8) the outer diameter (90) and an inner diameter.  I chose 60mm based on at least 20% wall thickness.  I know that 20% of 90 is 72mm, but it came down to me being lazy.  To get the 72mm would mean that I would have to plane the boards down from 18mm to about 12mm and since my planer blades are really dull, I decided to say I am overbuilding the spar, and may trim the final outer diameter a little less than 90 (solid spar calls for 84mm).

Anyway once I figured out my H(18mm) and L(35mm) for the staves I ripped the two boards up into eight staves.   Once they were cut, I set the table saw to 45 degrees and ran a few test pieces through to dial in the depth of cut and the fence alignment.  I should say that there are a number of ways to cut staves to get the birdsmouth cut, but I am using the table saw method (it seems faster and less messy than a router).

Once I was satisfied, I had my son come over to the shop and help hold the unwieldy staves as they came off the saw.  Basically, you run the stave through the saw for the first 45 degree cut and then turn the stave around and pass it through again leaving a satisfying little 16' triangular piece of wood leftover waste.  It took about 20 minutes to run each of the eight 16' staves through the saw twice and once completed I plunked all of them into a series of 90mm semicircle forms clamped to the bench and stepped back to see the magic.  Woohoo!  Tomorrow I will work on the taper to reduce the final diameter from 90mm to 50mm over the top 1.5 meters and then wait for my 3 gallon epoxy order to show up in the mail.

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