Universal Measuring Machine (Metrology)

17.4.
These machines can be utilised to measure length and diameters of both plain and threaded work, tapers and the pitch of the screw threads to a high degree of accuracy. Some machines employ two and three co-ordinate measuring^systems based on the use of line standards. The scales are generally enclosed within the machines and thus not liable to damage
by physical contact. The basis of the accuracy of measurement of these machines is an accurately divided scale viewed through a microscope. The microscope is used to determine the movements of the measuring anvils of the machine. In order to satisfy the principle of alignment, the scale is positioned in line with the line of measurement.
17.4.1.

The Societe Genevoise Universal Measuring Machine.

It has two carriages which move at right angles on hand scraped flat and vee-shaped guiding surfaces. The upper carriage is capable of being lowered or raised and carries a tool holder at the front end. The tool holder is fitted with a sleeve of hardened steel which is finely ground to ensure the accurate centering of the various tools such as locating microscope, feller microscope, goniometer microscope, spotting tool and locating indicator. Both carriages carry a standard scale in box and the scale is viewed by a measuring microscope. An auxiliary scale is also attached to both carriages to register their approximate positions. There is arrangement to carry the cast iron table, glass table centre supports, rotary table and other accessories on the scraped bearing surfaces of the machine. It is possible to make measurements in all the three co-ordinates and also polar co-ordinates. Thus the curved surfaces such as aerofoils, turbine blades etc., can also be optically tested by use of a short-focus microscope carried in the special tool holder. The goniometer microscope comprises a viewing microscope fitted with two super imposed graticules each carrying diametral dotted lines. One graticule is fixed, and the other can be rotated and carries a circular glass scale divided in degrees. This enables the measurement of thread angles and flake angles etc.
17.4.2.


The Matrix Internal diameter Measuring Machine.

This machine is capable of measuring plain and threaded work, and also parallel or taper work. It can work in the range of 6 to 150 mm diameter and to a depth of 50 mm. The special feature of this machine is that it relies on end standard. The measurement of diameters is made by comparison with a box standard built up from matched slip gauges. This is also capable of measuring work which differs in size from the standard by several millimetres. These differences are measured by means of a micrometer used in conjunction with a fiducial indicator. The principle of its operation is rather unusual in the sense that it measures diameter by measuring two radii at exact 180° spacing. The gauge whose diameter is to be measured is supported on a circular table, which can be precisely rotated about a vertical axis. There is an arrangement by which the stylus can be exactly located at diameter end. For taking the measurements, the gauge is first approximately centered on the table with the help of concentric ring engraved on it. Further with the measuring stylus in contact, the whole table is traversed on a precision slide until a maximum reading is obtained. The table is then rotated by exactly 180° to a stop, and another reading is taken again after traversing to obtain the maximum reading. The readings on the gauge are compared with those obtained on a ‘box’ made up of two equal piles of slips on to which parallel end pieces are wrung and held with screw clamps.
For getting exact 180° rotation of the table, two limit stops are provided in the machine. For location of maximum diameter of workpiece, the table is provided the traverse motion by a knob and screw which transmits cross-wise movement to the table.

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