Showing posts with label Forms. Show all posts
Showing posts with label Forms. Show all posts
Wednesday, March 9, 2011
Monday, October 25, 2010
More Ultralight.
We've started a new session of classes in my cedar-strip canoe building class. I have two new students joining the class this year along with previous students returning to complete boats that are in-progress. One thing I am pleased about is that my two new students are experienced woodworkers. One is a retired contractor and the other still works in the trade running his own business.
These two students are building the Wee Lassie II design to Mac McCarthy's plans. After our successful use of box-beam construction last year for the Prospector Ranger we opted to use this strongback style for the Wee Lassies as well. We've also removed material from the station molds to remove weight there as well.
One thing to remember is that a strongback and station molds need to be robust. Most builders do not move the forms once built so that the shape of the canoe doesn't change as it is being built. For our class, we need to move the boats on a weekly basis so the forms need to be both light and rigid. The box beam does this for us.
The box beam consists of a 1/2" thick plywood deck 1/4" thick Luan ply sides and 2x6" blocking. It is remarkably stiff once assembled and has the advantage of being stable with temperature and moisture changes.
We rip the 1/2" plywood deck to be 7-1/2" wide and the 2x6 blocking to match. Because the ply is only 8' long, we glue and screw the blocks to the underside of the ply with a block beneath the center joint.
The Luan is ripped to be the width of the 2x6 (actually 5-1/2") and the thickness of the ply (*close* to 1/2", but not exact), so it is nearly 6" wide. Again, because the sheet goods aren't long enough for a continuous piece the sides have a patch that is 5" wide and 2' long that gets glued to the side panels at the joint. We stagger the joints so that they are not in the same place, weakening the structure.
Once the sides have cured, the whole structure is glued and screwed (or in this case stapled - one of the students had a pneumatic stapler which made life very easy) to assemble the beam. When the cut edges are lined up, the structure is straight and flat. The large surface area that is glued up when the sides are assembled makes the structure very rigid.
We then mark the centerline and lay out the location of station molds. The station molds get screwed (but NOT glued) to the 1/2" plywood deck.
You will note the unsupported ends which have a taper cut on them. The taper is to clear the hull that we'll be building on the forms. We're also avoiding fiberglassing the hull to the strongback with this clearance cut. It is likely that we will support this with a small piece of 2x4 screwed into the end blocking and to the deck so it isn't flapping in the breeze. You will also note that the stem molds are not in place yet - we use them as forms to laminate the stems before they are installed on the strongback.
We have decided that there are a few flaws here, however. The old method, which used dimensional lumber, gave us a "ledge" which we could clamp to when screwing down the station molds. I'm concerned that we could crush the 1/4" sides if we clamped to them, so I need to either design in a ledge, or have a removable block to fit under the deck to clamp to when screwing down the molds. The other concern is how to support this at a reasonable work height. Small sawhorses seem to be the best solution for this at the moment. The Luan is also a bit splintery, but not bad - a little sanding at the top edge seems to solve that problem.
These two students are building the Wee Lassie II design to Mac McCarthy's plans. After our successful use of box-beam construction last year for the Prospector Ranger we opted to use this strongback style for the Wee Lassies as well. We've also removed material from the station molds to remove weight there as well.
One thing to remember is that a strongback and station molds need to be robust. Most builders do not move the forms once built so that the shape of the canoe doesn't change as it is being built. For our class, we need to move the boats on a weekly basis so the forms need to be both light and rigid. The box beam does this for us.
The box beam consists of a 1/2" thick plywood deck 1/4" thick Luan ply sides and 2x6" blocking. It is remarkably stiff once assembled and has the advantage of being stable with temperature and moisture changes.
We rip the 1/2" plywood deck to be 7-1/2" wide and the 2x6 blocking to match. Because the ply is only 8' long, we glue and screw the blocks to the underside of the ply with a block beneath the center joint.
The Luan is ripped to be the width of the 2x6 (actually 5-1/2") and the thickness of the ply (*close* to 1/2", but not exact), so it is nearly 6" wide. Again, because the sheet goods aren't long enough for a continuous piece the sides have a patch that is 5" wide and 2' long that gets glued to the side panels at the joint. We stagger the joints so that they are not in the same place, weakening the structure.
Once the sides have cured, the whole structure is glued and screwed (or in this case stapled - one of the students had a pneumatic stapler which made life very easy) to assemble the beam. When the cut edges are lined up, the structure is straight and flat. The large surface area that is glued up when the sides are assembled makes the structure very rigid.
We then mark the centerline and lay out the location of station molds. The station molds get screwed (but NOT glued) to the 1/2" plywood deck.
You will note the unsupported ends which have a taper cut on them. The taper is to clear the hull that we'll be building on the forms. We're also avoiding fiberglassing the hull to the strongback with this clearance cut. It is likely that we will support this with a small piece of 2x4 screwed into the end blocking and to the deck so it isn't flapping in the breeze. You will also note that the stem molds are not in place yet - we use them as forms to laminate the stems before they are installed on the strongback.
We have decided that there are a few flaws here, however. The old method, which used dimensional lumber, gave us a "ledge" which we could clamp to when screwing down the station molds. I'm concerned that we could crush the 1/4" sides if we clamped to them, so I need to either design in a ledge, or have a removable block to fit under the deck to clamp to when screwing down the molds. The other concern is how to support this at a reasonable work height. Small sawhorses seem to be the best solution for this at the moment. The Luan is also a bit splintery, but not bad - a little sanding at the top edge seems to solve that problem.
Tuesday, January 13, 2009
Tech-Tip Tuesday : Pattern Stock
When building a strip canoe, it's necessary to make patterns which are the cross-sectional shape of the canoe. Over time, I've seen a wide variety of materials used for this task. I've seen rigid foam (don't ask me how they did it...), Plywood (of various ilks...), MDF, glued up pine, and chip-board. They all have their ups and downs.
Usually, you are looking for a consistent material with no voids that is flat and thick enough for your purposes - 1/2" to 5/8" thick is usually fine. You want something inexpensive and easy to bevel. It should also resist warp and hold staples well. In the case of my students, I'm usually looking for an additional feature - light weight. (We move our forms in and out of the shop at every class!) While the pine isn't bad if you alternate the grain direction to avoid warping, I tend to prefere sheet goods for cost and ease of use.
Personally, I prefer plywood underlayment with patched voids. It's inexpensive and does the job well. If I wasn't lifting the boat and forms, I'd go for MDF as it's cheaper and very stable, although heavy. Chip-board is about the worst material I've used. It doesn't machine well, has random voids and it doesn't hold a staple well for some reason. One fellow I know prefers MDO for the nice paper face it has so that he can lay out patterns with ease. Unfortunately, it's a bit on the pricey side.
When choosing materials, I think it's very important to talk to the people at your local lumberyard. There are a wide variety of sheet goods out there - some are very similar in function, but can vary widely in price. Very often, they can point you in the direction of a less-expensive substitute if you tell them what you're doing with it.
Subscribe to:
Posts (Atom)
Posts
