Monday, March 3, 2014
Laying out curves
Fairing a template curve: Looking down the curve is a remarkably sensitive way to detect lumps and bumps
In nearly all of my woodworking projects, there are curves that can make or break the design. These are almost always curves without a constant radius, so I am relying on what looks "right" to my eye.
Acu Arc flexible curves are my favorite tools for laying out curves. They are available from Highland Hardware in 24", 48", and 72" lengths, and from Lee Valley in 18" and 36" lengths. French curves from an art store in regular sizes, and from Woodhaven in giant sizes, are also helpful for sections of curves.
I generally do not like flexible rules or laths because it is awkward or impossible to control the contour in mid-curve. The curve is regulated by the endpoints and maybe an intermediary point, but then is passively subject to the material properties of the layout device which may not produce the desired contour. I rarely use trammel points or a compass.
To design curves in a project, such as for table legs, I refine sketches into scale drawings. Then I make a full size mock-up by transferring the key dimensions onto scrap wood and use these to lay out a smooth curve. I saw the curve and refine it with rasps, spokeshaves, and planes. When it looks right and feels right, it is right! Then I go back and modify the scale drawings as needed.
For curves applied to multiple parts, and most of them are, such as with table legs, I next make a template by transferring the key dimensions of the final scale drawing onto quarter-inch MDF and use these to lay out the final curve. MDF works better than plywood, solid wood, or cardboard since it holds a clean edge and there is no grain to distract my eye or misdirect tools.
The Acu Arc has a natural tendency to produce a curve without bumps as you shape it. Hold it on the wood, trace only a nearby section of the curve with a pencil, hold it further along, trace more, and so forth. It is made of translucent colorless plastic but I would prefer opaque plastic that would show the curve better against the background of the wood.
Saw the template curve, then clean it up with rasps and sandpaper. Now you are on your own with your eyes and hands as you "fair the curve." Looking down the curve, as shown in the photo, is a remarkably sensitive way to see bumps and lumps that must be eliminated. Gently running your hand along the curve, like a sleigh ride over the hills, is also a very good way to sense irregularities. The edges of the template should be square if it will be used on both faces, as for legs with curves in three dimensions, or as a template for router work.
I position the finished template onto a squared-up blank of the project wood, clamp or double-stick tape it if needed, and draw the layout lines for each leg. With thinner stock, such as a table top or apron, the template may serve as a guide for pattern routing.
I label the templates and save this artistic capital which can be reused for future projects.
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