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Tuesday, February 19, 2008

Creating the Station Molds

A big chunk of work towards finishing the canoe is almost done. Over the last four weeks or so I have traced the station molds from the full-size plans ordered from Bear Mountain Boats onto plywood, then rough cut them with a jigsaw and sanded them down to the final shape.

I picked up a couple sheets of 1/2" plywood from Home Depot and had the guys in the lumber yard cut them into 15 pieces to fit each of the molds. It took them about 5 mins and saved me an hour or two.

The lines for the molds come on a single sheet of paper as half stations and need to be mirrored about the centerline to produce a complete mold. First, a centerline is drawn on the plywood perpendicular to the straight factory cut bottom edge. Then the plans are taped onto the plywood sheet lining up the centerlines. Two sheets of carbon tracing paper are placed back-to-back and slid between the plans and the plywood so that when the mold lines are traced with a ballpoint pen the line shows up on the plywood as well as on the back of the plans. Then the plans are flipped over to trace the other half of each station mold from the line on the back of the plans. I did a couple of these a night and it was a slow process. Altogether there are 13 station molds and 2 stem molds. Since the canoe is symmetric front to back the station molds come as six pairs plus the centre mold and the two stems are also identical.

As I started out tracing the patterns, I realised that the plywood sheets had a small amount of warp in them that could be magnified on the boat. My first thought was to scrap the plywood and go back and get some MDF (medium density fibreboard) that I have read is easy to work with. In the end though, I checked the forum at and found some other builders have had this same problem, which was easily fixed by screwing on a stiffening rib to each mold. Since the warp is quite small I have decided to go this route and otherwise these boards are behaving quite well.

With each of the station molds traced out, I set about cutting them out with my new Bosch jigsaw, cutting a millimetre or two off of the line. This went a lot quicker than I had thought it would and soon I was on to the sanding. I knew that using the random orbit sander to smooth the edges of the molds and bring them down to the line was not ideal, but I thought I could do a reasonable job with it. I quickly found out that it would not give me very good results. It was almost impossible to get a square edge and I was ending up with wavy edges as well. I have read that the best way to sand down the edges is to put a sanding disk on a table saw and place the molds flat on the table to get a perfectly square edge. Since I wasn't about to go buy a new table saw, I devised a little jig to hold my random orbit sander vertically offset from my work bench by about half an inch. The sander is designed to be held in the hand, not mounted on anything so it was a bit tricky, but an hour of cutting out plywood brackets and screwing them together produced a really solid mount that held the sander vertically without any wobble. Then the molds could be placed flat on the bench and run against the sander. In the end I think this has saved a few hours of headache and it has produced really good results. With a 60 grit sanding disk each station mold can be brought down to the tracing line in a few minutes. Keeping the edge of the mold moving against the sander produces a nice fair curve. The stem molds were made in same fashion and also have 14 1-1/2" holes spaced on 2" centres around the outer edge to provide a place for clamping the steam bent stems. Luckily, I just met our neighbour, David, from across the street who has a garage full of power tools including a drill press. He is working on building some custom made speakers for his house and he was kind enough to let me use the press.

This whole process produced a huge amount of sawdust. I have hung some plastic sheeting down the middle of the garage I am using as a workshop to try to keep most of it off of my neighbour's stuff.

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