Probing for a CMM Machine

Published by CMMXYZ on May 31, 2018


Today there is an extremely wide range of probes on the market. There are two basic types, touch probes and scanning probes.

The most popular touch probe on the market today is the Renishaw TP 20, a modular probe that consists of two parts: the probe body and the stylus module. A kinematic coupling that allows for quick connection or disconnection, either by hand or by a stylus changer, joins the two parts. Stylus modules are available for all lengths of potential styli, permitting the probe to give optimum precision. There are three manufacturers of modular probes: Renishaw, Hexagon, and Zeiss, each of which has similar accuracy and repeatability specifications and is generally found on machines of the individual OEMs manufacture.

Scanning probes are generally two sensors in one, enabling the user to scan while allowing touch trigger technology for geometry. One of the most popular scanning probes is the Renishaw SP 25. Most coordinate measuring machines can be fitted with scanning probes when form deviation is required on geometric parts or when scanning contours for reverse engineering. Hexagon, Renishaw, and Zeiss all produce versions of scanning probes. This technology allows the ability to carry extra long styli, occasionally up to 200 mm in length.

Most coordinate measuring machines utilise motorised probe heads. These allow rapid and repeatable inspection of complex components with the minimum amount of intervention from the operator. The average probe head has two axes, permitting 7 ½° increments with 720 spherical positions. Other brands offer 5° and 2 ½° rotational increments, notably the Hexagon probe head. Again, Renishaw, Hexagon, and Zeiss all produce motorised probe heads that allow quick connection and disconnection of modular probes.

Manual and fixed probe head systems. A fixed probe head requires the building of our Christmas tree structure utilising styli in order to accommodate probing at different angles.  This is cumbersome at the best of times and requires a lot of preplanning prior to measurement. Manual probe heads must be indexed to position and are not suitable for a fully automatic operation of your CMM machine. Many manual probe heads also contain the probe; this greatly lowers the price of touch probing.

Never underestimate the role that styli play in the accuracy and repeatability of a coordinate measuring machine. The stylus is the part of the measuring system that makes contact with the part being measured. The part being inspected dictates the type and size of stylus used. It goes without saying that the rigidity of the stylus and precise sphericity are essential ingredients.

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