Universal Measuring Machine (Metrology)

17.10.
With the introduction of concept of interchangeable manufacture parts, measurement of size and location of holes, contours and surfaces has attained more importance. Today the tolerances are specified in terms of fractions of a micron. Universal measuring machine, once thought to be a part of metrology laboratory, is becoming a part of the manufacturing process. The four essential components measured by vuiiversal measuring machine are : length, geometry, division of circle, and roundness.

17.10.1. Comparison of CMM and UMM.

Coordinate Measuring Machine (CMM) is useful for rapid inspection of production parts. It does not inspect the geometric features of the part, while universal measuring machine (UMM) does it. CMM is not as accurate as UMM. If a part fails to qualify the inspection criteria, it is revealed by CMM but it can’t reveal the
reasons for same but UMM is useful in this regard. Measurement on CMM is very fast but it takes time on UMM. CMM is, therefore, very useful for quality control. For calibration of hole location gauges, UMM is virtually a primary standard. A hole-location gauge is used to inspect a CMM and is calibrated on UMM. UMM is capable of resolving the exact nature of the error of all part features, regardless of complexity. UMM discloses measurement information which in turn permits the improvement of the machines or methods by which the part is actually brought to finished form.


17.10.2. Inspection on UMM.

It is very essential that the workpiece be mounted carefully on UMM. It must be flat and supported on accurate parallels. The flatness should be checked by traversing the machine axes and registering the indicator against the surface of workpiece. The clamping pressure should be directly over its supports.
For measuring the location of hole on a part, first the part needs to be clamped properly. For this purpose, the indicator probe is registered on the side of the workpiece and the long axis is traversed. The workpiece is held on one end and its side tapped at the opposite end to bring it into alignment with the axis. The part is then clamped and this process is repeated. The long axis is then moved until the spindle by eye appears to be centered over the workpiece longitudinally. This can be accomplished by extending the indicator linkage and noting if the indicator probe appears to be directly over each end when swung through a 180° arc. A “zero” is registered on one end of the workpiece at the lowest reading of the arc. Without moving the long axis, the lowest reading of the indicator is noted against the other end when swung through an arc. Leaving the indicator set against the latter end, the table is moved by fine-setting knob exactly half-way between this reading and zero on the meter. The above process is repeated to centre the spindle over the workpiece in the direction of short axis. Then the diameter can be measured using fine scale on the meter.
The awkward shaped workpieces can be located by using a rotary table to align reference holes or surfaces with the measuring axes. When the dimensions are specified w.r.t. centre hole in the part, then initial line-up is greatly facilitated by using the reference hole in the centre of the rotary table.
Microscope is used for measuring the location of small holes and for parts which might be damaged by physical contact. Parts like macro-circuit components, which have no reference surfaces against which an indicator probe-tip may register, can be measured by using microscope.

17.10.3. Mechanical Accuracy.

The attainment of a “cubic concept” of accuracy in the Universal Measuring Machine requires the individual mastery of four mechanical arts-geometry, length, dividing the circle and roundness. These four mechanical arts are truly the “Foundations of Mechanical Accuracy”.

Geometry—

The foundation of all geometric accuracy and indeed of all dimensional measurements is the flat plane. Meehante cast iron (close grained) is a prime material component of most of the elements of geometry because it is readily available, can be easily cast to shape, relatively uninfluenced by relative-humidity and is inherently stable, provided certain precautions are taken. Proper composition of the iron improves machinability, wear qualities and the quality of the scraped surface. Stability of the iron depends mostly on slow, uniform cooling in the mould after casting. For maximum accuracy during machining, irregularities in the supporting surface are balanced out through the use of shims and most of the heat and stress of machining is removed in a first rough-cut. After that the workpiece is undamped and permitted to relax. It is then supported evenly, clamped less tightly and finished with a light skin cut. Elastic deflection can be minimised through design and support. It is important to understand that geometric relationships are affected by the value of ambient
temperature, provided it is steady. However, temperature differentials (which could occur due to proximity to radiators, doors, windows or drafts ; heat from handling ; changes in room temperature ; stratification, i.e., non-uniform temperature from floor to ceiling) affect the accuracy. Localised heating/cooling of pieces subjects them to unpredictable distortion.

Standards of length.

All the lengths measured should be traceable to an ultimate internationally acceptable standard. The accuracy of derived length-standards such as gauge blocks, precision scales, step gauges, lead screws, and laser interferometers depend in large measure on how truthfully they represent the ultimate length standard.
Dividing the circle. Accurate circle division is a requirement of all laboratories and machine shops. The authority for “angles” of angle gauges, rotary tables, or the angular relationships in mechanical parts and assemblies is derived from the self-proving principle of dividing the circle.
Roundness. Precision machine tools must achieve roundness of many mechanical parts and especially with their spindles. The authority for roundness is a precision spindle. Location, effective size and the fit between mating parts are all dependent on the ability to measure and define the conditions of roundness.

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