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Dividers and compasses can both transfer the shape of a curved timber profile, but they do different jobs. Dividers carry a distance or step along a curve; a compass draws an arc from a fixed centre. For copying an existing moulding, bevel, or irregular edge, dividers are usually the more useful choice. For laying out a known radius, a compass is simpler and often cheaper.
The basic difference
A pair of dividers has two pointed legs. Set the points to a distance, then use them to walk that distance along a line, compare two gaps, or pick up a short section of a profile and transfer it elsewhere. The points mark locations; they do not make a continuous curve by themselves.
A compass has one point that stays at the centre and a pencil or marking point that swings around it. It draws a circular arc at a fixed radius. That makes it useful when the timber profile is genuinely circular, or when a drawing specifies a radius. It is a poor copier for a curve whose radius changes along its length.
Which tool fits the job?
| Job | Better choice | Why | Main limitation |
|---|---|---|---|
| Transfer a known radius to a board | Compass | Set the radius once and sweep a clean arc | Only matches a circular curve |
| Carry a width or offset along a curved edge | Dividers | Step the same distance around the curve | Small errors accumulate with each step |
| Copy an irregular moulding or curved template | Dividers, used with a rule or story stick | Can pick up and compare local distances | Needs several measurements and careful reference points |
| Mark a broad, shallow curve by eye | Neither alone | A flexible batten or template gives a smoother line | Requires a suitable batten and enough room to bend it |
Transferring a curved profile with dividers
For a practical copy, first establish a baseline: a straight edge, centreline, or two end points that can be reproduced on the new work. Mark several stations along the original profile, especially where it changes direction. On a small bracket, stations 25 to 50 mm apart may be enough; use closer spacing around a tight transition. Set the divider points to measure from the baseline to the profile at each station, then transfer those distances to matching stations on the new timber.
Join the transferred marks with a flexible batten, a thin strip of wood, or a fairing curve. Dividers preserve the measured offsets, but they do not decide how to blend the marks. If you use too few stations, the line can flatten a tight curve or develop a kink. If the profile is a continuous moulding, take measurements from the same reference face each time; switching faces introduces a systematic error.
Dividers also work well for stepping an offset around a curved edge. Set the span to the required distance and walk the points along the curve, keeping one point on the edge at each step. This is useful for estimating length or marking repeated divisions. It is not an exact way to make a parallel curve: the chord between steps differs from the true curved path, and the error grows with wide spacing. On a tight curve, reduce the step to roughly 10–20 mm; on a gentle curve, larger steps are usually workable.
Using a compass
Use a compass when the profile is an arc with a known or recoverable centre. Mark the centre, set the radius, and sweep the line without changing the opening. For a large radius, a beam compass or a simple trammel made from a batten is more stable than stretching a small compass to its limit. A short compass can flex, slip at the pivot, or leave a wavering line when opened too far.
If you do not know the radius, you can sometimes find a workable centre from the arc. Draw chords between points on the curve, then construct perpendicular bisectors; their intersection gives the circle centre. In a workshop, this is often more effort than tracing the original curve with a template. It also fails if the profile is not truly circular, so check the fit at several points before cutting.
Accuracy and common mistakes
Sharp, equal points matter. Blunt divider tips skate across hard timber and can leave marks several millimetres from the intended position. A compass pencil that is loose or poorly sharpened changes the radius or line width. Make a test mark on scrap, and measure the setting against a steel rule before transferring a critical dimension.
Do not force either tool around a curve. A divider step that is too large can jump across the edge; a compass pivot that shifts changes the whole arc. Clamp the work where possible, and keep the reference face clean of dust. On finished or valuable stock, use light marks and transfer to a paper or thin-card template first.
What to buy
For general bench work, a basic steel pair of dividers is a sensible first purchase if you regularly compare distances, step divisions, or copy offsets. The inexpensive versions are adequate for rough layout and occasional use. Their drawbacks are usually a loose joint, legs that do not hold their setting, or tips that need sharpening. Check that the legs move smoothly and stay put before relying on a measurement.
If your work mainly involves circles and rounded corners, a carpenter’s compass is the better fit. For arcs wider than its comfortable reach, look for a beam compass or make a trammel from a straight strip with a fixed pivot and pencil point. A home-made trammel is often more accurate than an overextended small compass, provided the pivot holes are tight and the beam does not bow.
For copying a flowing, non-circular profile, spend first on a decent flexible curve batten or make one from a thin, straight-grained strip. Dividers can supply the measured points, while the batten helps fair them into a clean line. A cheap compass is fine for occasional arcs; it is not a substitute for a flexible template when the curve keeps changing.
A reliable workflow
Identify whether the curve is circular or irregular before reaching for a tool. For a known arc, use a compass or trammel. For an irregular profile, establish a baseline, take divider measurements at close enough intervals, and fair the transferred points with a batten. Make a paper or scrap template before cutting the final timber, then check it against the original at the ends and at the tightest part of the curve. That extra check catches the common failures: a shifted datum, accumulated stepping error, and a curve that looks smooth on the bench but does not fit the work.