25.8.
The suitability of CMM can be estabilished by specifying its length measurement uncertainty i.e. the range in which the measured value lies. For CMM, uncertainty estimation is essential part as CMM is used to inspect components which are to serve as standards. Quality in today’s era is an essential requirement and it can be only guaranteed through proper measuring techniques.
CMMs are normally calibrated using precise physical standards of known length. Step gauge can also be used for this purpose.
Uncertainty in length measurement is the uncertainty with which a CMM can determine the distance between two ponts located on parallel surfaces. It can be specified as
Uncertainty could be determined either (z) theoretically taking into account all the contributing factors, which can influence the correctness of measurement results, or (») graphically, i.e., by actually plotting the deviation of measured data on CMM from true value given by calibration certificate of step gauge. This way plots are made for several readings at a number of steps along entire measuring range.
Because of liberalisation of economy and globalisation of trade, it is essential that we adopt internationally agreed common method for evaluation and expression of uncertainty of measurement.
As per universal method, for calculation of measurement uncertainty, uncertainty of the result of a measurement consists of several components and these can be grouped into two categories, size Type A—evaluated by statistical methods and Type B—evaluated by other means.
Type A evaluation of standard uncertainty is calculated by estimating standard deviation of the mean of a series of independent observations.
Type ‘B’ evaluation of standard uncertainty is based on scientific judgement using all the relevant information available, like previous measurement data, data available in calibration/other reports, manufacturer’s specifications, uncertainties assigned to reference data taken from hand topic, experience with behaviour and property of relevant materials and instruments.
The reliability of the measurements depends not only on the metrological confirmation of CMM but also on the measurement process such as the method adopted, skill of personnel, environment condition, and all factors which have an influence on the measurement.
The possible factors which could contribute significantly to the uncertainty of measurement in CMM are
(i) Uncertainty due to deviation of mean calibration temperature from standard temperature of 20°C. This will affect the coefficient of linear expansion (s) of material of step gauge and CMM.
Uncertainty contribution on this account
Expanded uncertainty is defined as the measure of uncertainty which defines an interval about the measurement result within which the value of measurand can be said to lie confidently.
This expanded uncertainty U is determined by multiplying value of Uc with a coverage factor k, the value of which is chosen on the basis of desired level of confidence, for instance for normal probability distribution and for level of confidence of 95%, value of k = 2.
Q. 25.5. What are the functions of an electronic control for computer numerical control (CNC) ? What are the characteristics of drivers provided ?
Ans. Both straight line and continuous path controls are adopted. Necessary controls for drives, probe head, probe changing system, measuring systems, etc. are provided. Subcontroller is provided for material flow systems. A computer system for comprehensive computation based on data provided by pickups is incorporated to display measurement results on dimensions, form and position of the probed features. Drives (d.c. torque motors) can operate over extremely wide range of speeds (1 : 105) and with very high efficiency so that heat generation is minimal. Power transmission is based on tape drive so that guideway properties are not impaired. Spindle drives incorporate universal suspension joints.
Q. 25.6. The axis of rotation and angle measuring system of a rotary table must be of the utmost precision. What is the conventional angle measuring resolution of CMMs ?
The environmental requirements for vibrations and temperature during measuring conditions have to be met.
Q. 25.8. How CMM is monitored regularly ?
Ans. Each CMM is provided with one calibrated test standard made of invar, which comprises of nine spheres and a ring gauge (Refer Fig. 25.9). It permits virtually automatic monitoring of CMM in very little time.
Test standard is measured in two vertical setups displaced to each other. Thus 72 measurement lines, including the space diagonals get checked. The test standard can be checked by means of a test and calibration procedure incorporating measurement in 2 setups rotated through 90° and gauge block comparison.
Fig. 25.10. Invar test standard.
Q. 25.9. What are the broad functions performed by a computer provided with CMM?
Ans. Computer incorporates necessary software to display results of measurement on dimensions, form and position of probed features, collecting data and analysing them as obtained from readings of probes. It performs several important functions like
(i) Performing corrections
on each probing point (to reduce and eliminate all systematic influences) with regard to guideway deviations, squareness deviations, bend influences, thermal influences, probe tip radius and mid-point probe bend, brobe head deflection, etc.
(ii) Transforming the machine
coordinate data in volumetric coordinate system (described by six parameters—3 angles and spatial zero point defined by 3-coordinates) to workpiece coordinate system. The workpiece may have several coordinate systems.
(iii) Computing of substitute elements wherever feasible—
In general the probing points must be as large as desirable to cover the geometric element completely. However in some cases, shape may differ minutely from ideal substitute elements within permitted tolerances, then number of probing points can be reduced and properties of substituted geometry computed. Different best fit methods can be used by computer to obtain consistent results even with only a few probing points.
(iv) Evaluation and Control.
The dimensional specifications in the technical drawings often make it necessary for the measured elements to be linked mathematically so that further related elements are obtained. Very often measurement results have to be converted from one type of coordinate system to another.
Large data quantities need to be processed for complex shapes. Complex shapes are given numerical description with patches, and also using tangential bordering lines between all adjacent patches.
