How do I mark out a circle or an arc bigger than my compasses?

Curves come up more often than people expect — a shaped shelf, a rounded worktop corner, a garden bed, an arched opening, a curved bench seat. Compasses run out at about 200mm, and the three alternatives each suit a different kind of curve. Choosing the right one is mostly a question of whether the radius is actually specified or whether what matters is that the curve looks right.

What you'll need

• A straight batten
• Panel pin, nail or screw for the pivot
• A scrap block to pivot on, so you do not pin the workpiece (optional)
• Non-stretch line and pegs, for outdoor work (optional)
• A thin flexible batten, for a sprung curve (optional)

Step by step

1. Decide whether the radius is actually fixed.If the curve has to match something — a pipe, a hole, a matching piece, a specified radius on a drawing — you need a trammel. If what matters is that the curve looks fair and meets its two ends smoothly, a sprung batten is better and quicker.
2. Make a trammel for a defined radius.Take a straight batten longer than the radius. Drive a fine nail or a screw through it near one end as the pivot, then drill a pencil-sized hole at exactly the radius distance from the pivot. Push a pencil through and swing it.
3. Pivot on a scrap block, not the workpiece.Clamp a block of scrap where the centre falls, and drive the pivot into that. It keeps the nail hole out of your finished piece, and it lets the centre sit off the edge of the work — which it usually does for a shallow arc.
4. Keep the pencil upright and the batten flat.A leaning pencil shortens the effective radius on one side of the sweep, which produces an egg rather than a circle. Hold the batten flat on the surface throughout and let the pencil trail.
5. Use a pegged line for large outdoor curves.Drive a peg at the centre, tie a non-stretch braided line to it, and hold a marking spike or a bottle of sand at the radius. Keep the tension identical all the way round — string stretch is the main error, and ordinary garden twine stretches enough to matter.
6. Spring a batten for a fair curve.Drive a nail at each end of the required curve and a third at the top of the rise. Bend a thin, even, knot-free batten round the outside of all three and draw along it. The batten takes up a naturally fair curve, which the eye reads as smooth even though it is not a true arc.
7. Draw the mirror half from the same setup.For a symmetrical curve, mark the centre line, draw one half, then flip your template or reverse the batten to draw the other. Two independently drawn halves never quite match.
8. Make a template if you need it more than once.Cut the curve in hardboard or thin ply, fair it with a plane and sandpaper, and use it to mark every subsequent piece. A drawn curve is a one-off; a template is repeatable and can be corrected by eye before it is used.

Tips

• For an arc through three known points, the sprung batten is the only method that does not require you to calculate the radius. That is exactly the situation with a shaped shelf or an arched head.
• A trammel with two pencil holes marks the inner and outer edges of a curved piece in one sweep, which keeps the width constant.
• Sight along a curve before cutting it. The eye detects an unfair curve — a flat spot or a sudden change — far better than any measurement, and it is the eye that will judge the finished piece.
• Thin MDF strip springs more evenly than solid timber, which follows its grain and produces a lopsided curve.

Common mistakes

• Using stretchy string for a large radius — The radius grows as you pull harder, so the curve ends up an inconsistent shape and rarely closes back on itself. Use braided non-stretch line or a rigid batten.
• Putting the pivot nail through the workpiece — It leaves a hole exactly at the centre of the shape, which is often the most visible point. Pivot on a clamped scrap block instead.
• Springing a knotty batten — It bends less at the knot and more either side, producing a visible flat spot in an otherwise fair curve.

Questions people ask