Numerical Controlled (NC) Co-ordinate Measuring Machines (CMM) (Metrology)

17.11.
CMMs originally designed for high-precision measurement on relatively few workpieces in the metrology laboratory, are now being increasingly used both in the workshop and on the production floor because of the demand for accurate checking of dimensions, form and position of geometric elements of workpieces after machining. CMM (portable or bridge-type) with probe system (three dimensional probes suitable for dynamic measurements), probe changers, rotary table and workpiece handing devices becomes a multi function measuring system designed to maintain a high level of accuracy. With such an arrangement, a single measuring set up and hence a single workpiece clamping suffices to provide data on deviations in dimensions, form and position against three different set ups and three workpiece clampings in conventional methods.
Software, suitable for automatic calibration of the probe, determination of the workpiece coordinate system, calculation of geometric elements, automatic evaluation and recording of complete measurement is incorporated. Automatic compensation of workpiece misalignment is a common feature on many machines. Software for statistical evaluation of measured data, curve measurement, gear measurement and probing curved surfaces is also available. Fast feedback of measuring data significantly reduces idle times of the production equipment. After changing the probes and loading the workpieces, the automatic measuring sequence can be initiated by simply pressing a button.
The form or profile of cams, cutting dies, etc., is measured on coordinate measuring machine by a scanning technique which takes about fifty measurements and processes them every second.
In CMMs fitted with microprocessor based controls, adequate number of soft keys, i.e., keys with variable functions, which hadle all the commands needed for man-machine dialogue are provided. The soft keys change their functions throughout the measuring sequence. At each stage their function is clearly defined on the display immediately adjacent to the keys. Various commands provided are ; full 3 D alignment, choice of three working planes, multipoint measurement of circle centres and diameters, probe diameter or radius compensation, datum translation or transfer, sub-datum facility to match drawing dimensioning, axis rotation. A repeat function simplifies inspection of similar features.