A two-point device uses probes 180° apart and often records only an average value, so a bore with a polygon error can look round. A three-point instrument centres itself and picks that error up. It is also easier to handle, because rod and swing systems need the operator to stabilise the instrument in two axes.

Polygonal deformation, which 3-jaw chucks or collets can cause during bore machining. Measuring in different positions gives the difference between the largest and smallest diameter.

In a cone system the probes touch the cone along a line, which causes local, non-linear wear and limits measuring range and accuracy. The Microtest spindle system has a large contact surface on the thread flanks, so wear is minimal and is compensated linearly during routine adjustment.

The bevel gear system keeps the measuring pressure constant, giving operator-independent repeatability of ±1–2 µm up to Ø400 mm.

The Microtest system compensates temperature-related expansion errors, so near-laboratory accuracy can be reached even away from the nominal 20 °C. All contact points are insulated to reduce heat transfer from the user’s hand.

They are used to check bore diameters against a tolerance in production and inspection work.