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How to Set a Beam Compass for a Large Timber Radius
A beam compass is the practical way to mark large arcs on beams, posts, rafters, and curved furniture parts. It is simply a long straight beam fitted with a fixed pivot point and a marking point. The beam can be timber, plywood, aluminium extrusion, or a purpose-made bar. Unlike a short compass, it does not need to span the entire radius with a bulky hinge, but it still needs to be straight, stiff, and accurately marked.
The important measurement is the radius, not the diameter. If the finished curve is 1,200 mm across, the compass must be set to 600 mm from the pivot point to the pencil or marking knife. A common mistake is measuring from the end of the beam instead of from the actual pivot tip.
Choose a Straight, Stable Beam
For radii up to about 600 mm, a straight length of good-quality timber can work well. For larger radii, use a stable strip of plywood, a straight aluminium bar, or a commercial large-radius beam compass. Avoid a twisted or bowed offcut. Any bend in the beam changes the radius as you swing the compass, producing an arc that is not truly circular.
| Radius | Recommended beam | Useful marking method | Main concern |
|---|---|---|---|
| Up to 400 mm | 18–25 mm straight hardwood | Pencil or knife | Pivot and point must stay tight |
| 400–1,000 mm | 25–40 mm plywood or hardwood | Fine pencil or marking knife | Beam flex and inaccurate graduations |
| 1,000–2,000 mm | Stiff plywood, aluminium, or commercial bar | Knife, pencil, or router guide | Movement at the pivot |
| Over 2,000 mm | Very stiff purpose-made beam | Sharp knife or scribe | Deflection and layout errors |
A 1-metre strip of 12 mm plywood may appear straight but can flex when you press the marking point into timber. That flex changes the effective radius and can leave a shallow, uneven curve. For a long compass, stiffness matters more than having a very narrow beam.
Make a Simple Beam Compass
For occasional work, a homemade compass is often the sensible choice. Cut a straight beam longer than the required radius by at least 150 mm. Drill a clean hole for the pivot near one end. At the other end, fit a pencil holder, awl, marking knife, or fine nail. A screw through a small metal plate works better than a loose nail because it is less likely to shift sideways.
Use a woodworking marking knife when the line must guide a saw, chisel, or plane. Use a pencil for rough cutting or when the surface is liable to be damaged. A knife point gives a narrow line, but it can follow the grain and wander on softwood. A sharp pencil is more forgiving.
For repeatable settings, add a sliding block with a bolt or wing nut. Mark the beam in millimetres from the pivot centre, not from the beam end. If the beam has a 6 mm pivot hole, measure from the centre of that hole. Make a small reference mark at 500, 750, 1,000, and 1,500 mm, then verify them with a tape measure. Factory graduations are convenient, but they are not automatically accurate.
Set the Radius Correctly
Place the beam on the timber and identify the centre of the required arc. Drill a small pilot hole or press an awl point into that location. The pivot should be sharp enough to locate positively but not so large that it leaves an oversized hole. A 2 mm to 3 mm pilot hole is usually sufficient for layout work.
Measure from the pivot point to the exact tip of the pencil or knife. Set this distance to the required radius. If the radius is 900 mm, the point must be 900 mm from the pivot centre. Tighten the sliding block firmly, then check the measurement at both edges of the beam. A block that sits at an angle can put the marking point several millimetres out.
Before marking the workpiece, test the compass on scrap. Swing it through the full arc while keeping the pivot vertical. If the beam catches, flexes, or rocks, correct that problem first. Do not compensate for a loose pivot by pressing harder; pressure only bends the beam and makes the curve smaller.
Mark the Arc Without Losing Accuracy
Support the timber securely on trestles or a bench. The work must not move as the compass travels. Keep the pivot point planted and sweep the marking end in a smooth motion. On a large arc, it is often easier to make several light passes than one heavy pass.
For a knife line, make the first pass lightly, then deepen it on the second pass. Keep the knife tilted in the same direction throughout. If using a pencil, use a sharp point and avoid leaning the beam sideways. A sideways load moves the pivot and creates a kink at the start or end of the arc.
When a curve must meet straight edges, extend the arc beyond the visible joint. This gives you a clean intersection to check with a square or straightedge. For a template or repeated parts, cut the first piece slightly outside the line, then refine it with a hand plane or spokeshave. Do not rely on a rough bandsaw cut to confirm the compass setting.
Check the Common Failure Modes
The most frequent error is confusing radius and diameter. Another is measuring from the end of the beam rather than the pivot centre. Loose hardware is also troublesome: a sliding block can creep during the sweep, especially when the marking point catches in end grain.
Check the timber itself before blaming the compass. A curved or cupped board can make a correct arc look wrong when viewed from above. Mark from a flat reference face, and use a straightedge to establish the centre and end points. On a 2-metre radius, a 3 mm setting error is normally acceptable for rough carpentry, but it may matter on a visible joinery curve.
A purpose-made compass is worth buying when you need several accurate radii, quick adjustment, or a built-in pivot and pencil holder. For one porch brace or garden bench, a straight strip of plywood, a screw, and a sharp pencil are usually enough. The cheaper option is fine when the beam is stiff, the measurement is checked, and the work is marked lightly before cutting.
If you want repeatable layout across a site, keep the beam with your carpenter’s measuring and marking kit. Label the verified radius marks and protect the pivot tip so the setup is ready for the next large timber job.