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Large timber layout calls for a different compass than ordinary cabinet work. A small drafting compass is too short, too delicate, and usually too loose at the joint. For circles, arcs, and stepped-out dimensions on rafters, beams, slabs, and work platforms, you need a long, rigid beam with a point that stays planted while the marking end travels accurately.
The best setup depends on the diameter you need to reach, how often you will use it, and whether you are marking with a pencil, knife, or scribe. For occasional work, a straight timber with a couple of trammel points can be perfectly adequate. For repeated layout, a purpose-built beam compass is faster and more reliable.
What a beam compass does
A beam compass is essentially a long rule or bar carrying two adjustable heads. One head holds a sharp pivot point; the other carries a pencil, knife, or marking point. Set the distance between them to the required radius, lock both heads, and swing the compass around the pivot.
That radius is the important measurement. A 24-inch radius produces a 48-inch diameter circle, while a 36-inch radius produces a 6-foot diameter. Check the tool’s useful range rather than its advertised overall length. Some of the bar is consumed by the heads and end fittings, so a “48-inch” beam may not produce a true 48-inch radius.
For timber framing, the compass is useful for marking curved braces, wagon-wheel-style arches, large shoulders, scribed curves, stair or roof geometry, and layout lines that would be awkward to strike with a tape measure and pencil.
The main types to consider
| Type | Best use | Strengths | Weaknesses |
|---|---|---|---|
| Purpose-built beam compass | Regular large-radius layout | Rigid, quick to adjust, repeatable | Costs more and takes storage space |
| Trammel points with a wood beam | Occasional site work and custom lengths | Flexible, inexpensive, easy to replace the beam | Accuracy depends on the beam and clamping |
| Folding or telescoping compass | Transport and moderate-size work | Compact and convenient | More joints can mean flex and lost settings |
| Large divider or drafting compass | Small arcs and indoor layout | Fine adjustment and compact size | Usually too short for beams and large timbers |
For most carpenters, the practical choice is either a rigid large-radius beam compass or a set of trammel points. Trammel points are often the better value if you already have straight stock for beams and need different lengths for different projects.
Features that matter on timber
Rigid beam or bar: A narrow steel or aluminum bar is convenient, but it can bow when you press the marking point into rough wood. For radii over 30 inches, even a small amount of flex can make the curve visibly inaccurate. A broad, straight wooden beam—such as clear maple, ash, or straight-grained plywood—can be more stable than a thin metal strip.
Positive locks: Each head should clamp firmly with a wing nut, thumb screw, or lever. If the head creeps 1/16 inch while you swing the compass, the circle will not close correctly. Test the lock by setting a radius, applying moderate sideways pressure, and measuring again.
Separate pivot and marking tips: The pivot should be sharp enough to hold in end grain but not so thin that it bends. The marking head should accept a pencil for visible layout and, ideally, a knife or scribe for permanent lines. A pencil is safer on finished surfaces; a knife line is better where the layout must survive handling.
Fine adjustment: A rack, screw adjustment, or threaded fine-adjuster is useful when you need to hit a precise radius. It is less important for rough timber framing, where a strong clamp and a tape measure are often sufficient. Fine adjustment should supplement—not replace—a reliable scale.
Readable graduations: Marks every 1/16 inch are useful, but verify them against a trusted rule. Some inexpensive tools have scales that are adequate for approximate arcs but not reliable enough for joinery.
Choosing length and accuracy
Measure the largest radius you expect to mark, then add at least 6 inches of working margin. If you commonly mark 30-inch radii, a 36-inch usable beam is a sensible minimum. For 4-foot diameter circles, choose a tool with at least a 24-inch radius; for 8-foot circles, you need a 48-inch radius, which may be easier to achieve with custom trammel points and a long timber beam.
Do not assume a long compass is automatically accurate. On a rough 8-by-8 timber, the pivot can rock between growth rings, and the pencil can follow surface irregularities. Plane or belt-sand a narrow layout strip first when the curve matters. On a wide board, use a straightedge to establish a baseline and confirm the pivot is square to the surface.
For repeated parts, make a full-size plywood template after striking the first curve. A compass gives you the geometry; a template makes it repeatable. This avoids resetting the tool several times and exposes errors before expensive timber is cut.
When the cheaper option is enough
A basic adjustable woodworking trammel set is sufficient for occasional arcs, garden structures, rough braces, and non-critical decorative work. You can also make a serviceable beam compass from a straight 1-by-2, a nail, and a pencil holder. Drill or mark the pivot hole carefully, measure from the exact pivot point to the pencil tip, and check the setting at both ends before cutting.
The homemade approach fails when the beam is bowed, the pencil is not held square, or the pivot hole enlarges after repeated use. It is also slower to adjust. For one project, that may not matter. For a timber framer laying out dozens of matching curves, the time saved by proper locking heads quickly outweighs the purchase price.
How to use a beam compass accurately
Mark the center clearly and set the pivot point on it. Measure the radius from the actual tip of the pivot to the actual pencil or knife point—not from the edge of either head. Lock the heads, then test the setting by making a short arc on scrap.
Keep the beam flat against the timber and use light, steady pressure. Heavy pressure makes the bar flex and can pull the pivot out of end grain. Rotate from the shoulder or elbow rather than twisting your wrist. If the arc must be cut later, make a light first pass, inspect it, and deepen the line only after confirming the curve.
Before buying, compare the usable radius, head-lock quality, tip options, and replacement parts. A simple rigid compass with strong clamps is generally a better site tool than a sophisticated but flexible folding design. For very large work, buy the trammel heads and make beams to suit the job.