Coordinate measuring machines (CMM) are mechanical structures that move a measuring probe to predetermined coordinate points on a work sample. The machine has three important elements, the mechanical machine structure, the measuring probe and the computer (hardware and software).
The three most important parameters of CMM are the measuring length of the X,Y, and Z axes; the measuring resolution, and the maximum weight of the item being measured. The measuring length is the total travel that the probe can move in each, the X,Y, and Z direction.
Three mechanical structures are common for holding the measuring probe. The articulated arm is used for portable (tripod) systems. The bridge and gantry system holds the probe vertically on a horizontal beam which is supported on both ends by posts in a bridge fashion. The bridge style is the most common design for metrology labs and manufacturing. Gantry systems are very big bridge shapes where the horizontal arm moves in a gantry form on top of the uprights. The horizontal or cantilevered probe is held by a single vertical movable support. The gantry and the horizontal measuring methods are used for large products.
This section covers bridge style CMMs only. The size of the items to be measured can range from a few centimeters to over a meter. In all cases, the volume space under the bridge has to be larger than the measuring range specified for any given model. Most important is the height available under the probe tip (Z axis) and
the width on the table (Y axis). The work surface of most models is made of granite. The X and Y axes are moved with linear electric motors or ball screws and ride on air bearings. The Z axis is driven by ball screws or pneumatically.
The software allows the programming of coordinate points to be measured or shapes and contours that the probe will follow. The degree of difficulty to
program the measurements will depend on the software as well as the type of measuring probe. The machine manufacturers state machine performance based on their own software and recommended probes.
Ambient temperature is an important factor when considering the accuracy of the system. 20 C +/- 2 C is the standard temperature range. Location of the system is therefore an important
consideration regardless of which model is chosen.