Inside & Outside Callipers

Inside or outside, and which one you need

The difference is in the legs. Outside callipers curve inwards, so the tips close around a shaft, a boss or a turned component to take an external diameter. Inside callipers curve outwards, so the tips open into a bore, a groove or a hollow to take an internal measurement. Most workshops keep both, because each one measures what the other cannot reach.

Dividers and trammels for marking out

Precision dividers hold two straight, sharp points for scribing arcs and stepping off repeated distances along a workpiece. When the radius runs beyond a divider’s reach, the 1680 500 trammels take over: a millimetre scale, interchangeable tips and fine tip adjustment to 0.02 mm, which is what makes them worth reaching for on layout work that has to be right the first time.

Choosing your size

The 150 mm callipers suit small turned parts and bench work. The 200 and 250 mm sizes cover most general engineering. The 300 mm is the one to reach for on larger bores and castings, where the extra leg length is the difference between taking a measurement and guessing at one.

Where callipers still beat a digital gauge

A digital caliper is faster and reads to a finer resolution, but it has limits. Callipers reach into grooves and hollow forms a straight jaw cannot enter, they do not mind heat, swarf or coolant, and they hold a setting so you can compare part after part without watching a display. On a lathe or a woodturning bench they are often the quicker tool, not the slower one.

Getting a reading off a calliper

A calliper transfers a dimension rather than displaying it. Set the legs to the work by feel, looking for light drag and no force, then lay them against a rule or a vernier to read the figure. If you want a direct reading instead, the 1679 universal gauge with contact roll reads to 0.1 mm on its own scale.