Use a Compass

The most obvious way to draw a curve is to use a compass. When I was in school the teacher had a huge wooden compass with a rubber foot on one leg and a holder for chalk on the other. This size would be ideal for drawing large and small circles or arcs, but I haven’t seen one of those since I left school. If you are working on smaller projects, several woodworking supply houses carry larger sets for drawing circles.

Trammel Points Are Handy

At some point a compass becomes impractical. A far more common way to draw larger curves is with a set of trammel points, sometimes referred to as a ‘beam compass’. These function much like a compass, but the two ends are mounted along a beam made of metal or wood. One end is the pivot and the other end is for marking. The distance between the two determines the size of the arc. To use one of these, the anchor point is located on a centre line drawn perpendicular to the curve. To flatten out the arc, move the anchor point further down the centreline. Only the length of the beam limits the size of the circle or arc.

Make Your Own Drawing Batten

To draw the arc used on the blanket chest, I used some clamps, a few scraps of wood and a drawing batten. Before you begin, it is important to determine the length of the curve and its depth.

No fancy math is required – just keep playing with different combinations until you find one that is pleasing to the eye. In the example of the chest, I wanted the curve to begin 1 3⁄4″ out from the legs. By starting the curve away from the leg, the width of the lower rail is consistent on both ends. If the curve were to extend right to the very edge, even minor errors with the band saw or sander would have made it look lopsided.

Clamp a stop block on each end of the rail so that the space between the two defines the outer ends of the arc. Divide the space in half and clamp another support block in place at the center. To define the curve, insert a ‘drawing batten’ between these blocks. Drawing battens are very easy to make and it is a good idea to have several different sizes available. Select a piece of wood that is easily bent. All of mine are made of quarter-sawn white oak strips that were off-cuts from various projects. White oak is one of the premier bending woods and I can easily bend my 1⁄8″ x 7⁄8″ x 48″ batten into a ‘U’ shape. A batten made from maple, cedar or most other common woods would snap long before that point.

When selecting wood for the batten, choose only clear, straight-grained wood. If the grain runs out the edge of the batten at an angle it will most certainly snap at that point when bent. Flat-sawn boards used as drawing battens will tend to have flat spots and angular transitions as a result of the irregular grain and the resulting curve will not be fair. A compass or set of trammel points will always result in a regular, mathematically precise arc, but there are situations where you may choose to use an irregular, though fair, curve. These can easily be plotted with a drawing batten simply by adding extra stop blocks to hold the batten in the desired location.

Or Buy One

Lee Valley sells an excellent drawing batten. Their plastic drawing curve (item 07K01.01) is composed of several rows of a clear flexible material that are interlocked using splines. This curve can be shaped to conform to a series of plotted points. It can also be used to transition from curved sections to straight sections in a smooth and even manner.

Drawing Large Radius Arcs

An easy way of drawing perfect large radius arcs is with a jig, made from two long narrow pieces of wood and a couple of nails. Tack one nail at each end of the arc and find the halfway point between them. Next, mark off the height of the arc at the centerline. Place the two pieces of plywood against the nails at either end, and overlap them where they cross at the highest point of the arc. Screw them together in this position to form a ‘V’. Place a pencil at the apex of the ‘V’ and slide the jig along the two nails to draw the arc.

Drawing Complex Curves

To draw curves that are beyond the capabilities of shop made jigs, look to one of the readily available commercial products. For tight and irregular curves use a set of French curves. These are available from woodworking and art supply houses in a variety of materials and are useful when transitioning from a straight line to a curve. Simply select one with the right size curve, place it on your work and use a pencil to trace along the edge. There are times when you have to copy a curve from an existing piece. A flexible bending curve is a great time saver for this. These are made of two pieces of spring steel with a core of lead between them. This is then wrapped in a plastic or vinyl coating to prevent it from marking your work. Its design allows it to be placed alongside an existing curve and formed to its shape. When removed it will maintain the shape of the original allowing you to transfer that shape to a new piece of material.

Bends Ahead – Use Caution!

You should always wear eye protection when working with bending battens particularly shop made battens. When drawing tight curves, one side of the batten is in compression while the other is under tension. If there is a weakness in the batten caused by the grain in the wood, under extreme conditions it could snap.

In nature, curved lines are the norm and straight lines the exception, the result being that the eye is naturally drawn to curves. The curve on the blanket chest (this issue), was done without any exact measurements. It was simply what looked right for the piece. Once the chest was finished we were curious as to the final proportions and pulled out a ruler to measure it. At its height, the arc is 21⁄8″ tall and the rail at that point is 33⁄8″ wide. This was almost identical to the measurements of the golden rectangle, a ratio that is generally accepted to be the most pleasing to the human eye, a subject we’ll take a closer look at an upcoming issue.