Machine Tool Metrology



The dimensions of a gauge, its surface finish and geometry are dependent upon the inherent quality and accuracy of the machine tool for its manufacture. Also in mass production the various components produced must be of such accuracy that they may be assembled on a non-selective basis. The accurate production of the component parts depends upon the accuracy of the machine tools.
The continuously increasing demand for highly accurate machined components has led to considerable research towards means by which the geometric accuracy of machines may be improved and maintained. It is very important, therefore, that under static conditions, the alignment accuracy of the machine tools is checked by some geometric tests. The tests which check the alignment accuracy of the various parts of a machine tool are called static rests.
A machine tool, at the same time must be able to display the desired behaviour and characteristics under normal operating conditions as operator is concerned with the actual working of machine tool. Therefore, some alignment tests are performed under dynamic loading conditions and these are referred to as dynamic tests. Thus it is obvious that the satisfactory working of a machine under static loading only is not sufficient but account should be taken of the vibration and deflection of various machine parts under actual dynamic loading also. In dynamic tests, the various members are actually loaded and alignment tests are carried out. Also various parts are excited at working frequency and behaviour of machine observed.
In other words it could be said that machine tools for the workshop must be able to produce work-pieces of given accuracy within prescribed limits consistently and without requiring artistic skill on the part of the operator and that the quality of workpieces depends upon :
(i) Stiffness and rigidity of the machine tool and its components parts.
(ii) The alignment of various machine parts in relation to one another. This is very important because the geometry of various shapes is based on the relative motion between various machine parts and hence on alignment of various parts.
(iii) The quality and accuracy of the control devices and the driving mechanism. Stiffness and rigidity are a matter for designer and once tests on a prototype of a certain design have given satisfactory results, there is no need to test the machine of same design over and over again. The accuracy of manufacture, the precision of control devices and driving mechanisms such as lead screw of lathe, all should have relative alignment as accurate as possible. This depends upon the quality of manufacture and may vary from machine to machine. Therefore, each machine is subjected to an acceptance test concerning items {ii) and (iii).
The accuracy of machine tools (which cut metal by removing chips or swarfs) is tested by means of geometrical checks and practical tests.
Geometrical checks include checking of dimensions of forms and positions of components as well as checking of their displacement relative to one another. These comprise all the operations which affect the components of the machine and concern only sizes, forms, positions and relative movements which may affect the accuracy of the machine. It may be appreciated that the geometrical definitions are abstract and relate only to imaginary lines and surfaces. In practice, therefore, metrological definitions are followed which are concrete and take account of real lines and surfaces accessible to measurement. Metrological definitions cover in a single result all macro and micro-geometrical errors and allow a result to be reached covering all causes of errors without distinguishing them from one another.
Practical tests include the machining of test pieces appropriate to the fundamental purposes for which the machine has been designed and having predetermined limits and tolerances.
The results of practical tests and geometrical checks may be compared only in so far as these two kinds of tests have the same object; in case both do not give the same result, the results obtained by carrying out practical tests are accepted as valid. In case where it is expensive or difficult to conduct both types of tests, the accuracy of a machine may be checked only by geometrical checks or only by practical tests.
The various geometrical checks generally made on machine tools are : (Before conducting these tests, it is essential that the machine is set up and principal horizontal and practical planes and axes are checked with spirit levels, etc.

(a) Straightness

(i) straightness of a line in two planes ;
(ii) straightness of slideways of machine tools ; (iii) straight line motion.

(b) Flatness

(c) Parallelism, equidistance and coincidence.

The parallelism includes (i) parallelism of lines and planes ; (ii) parallel motion.

(d) Rectilinear movements or squareness of straight lines and planes.

Quality of the guiding and bearing surfaces of beds, uprights and base plates are also tested.

(e) Rotation

This includes (i) out of round, (ii) eccentricity, (iii) radical throw of an axis at a given point, (iv) out-of-true running (run-out), (v) periodical axial slip, (vi) camming.
Main spindle is the fundamental element of the machine and is tested for concentricity, axial slip, accuracy of axis and position, relative to other axes and surfaces.

(f) Movement of all the working components.

We shall now discuss the definitions and various tests to perform these geometrical checks.


The definition of straightness of a fine in two planes is given in Chapter 7. The various methods to check the straightness are :
(i) Straight edge method. This method is described in Chapter 7 and is limited to lengths below 1600 mm.
(ii) Spirit level and optical methods. These are also described in Chapter 7. The spirit level method enables checking the straightness only in the vertical plane and may be used for lengths below as well as above 1600 mm. The optical methods are used for lengths above 1800 mm only.
(iii) Taut wire and microscope method. In this method a steel wire of diameter 0.1 mm is stretched and arranged to be approximately parallel to the line to be checked. A microscope
equipped with a horizontal micrometers displacement device is placed vertically on the surface of line to be tested and moved along the line to be tested. The deviation of the line to the taut wire in the horizontal plane of measurement can thus be noted down. This method should be avoided when the sag of the wire has to be taken into account as the sag is extremely difficult to determine with any great accuracy.
The conditions for the straightness of the slideways of machine tools are same as for a line. The methods indicated above are applicable for plane slideways : but, however for V-slideways the tests are performed on the surface of a cylinder or an intermediate piece (made to the shape of the slideways) placed on the V-slideways.
It may be noted that checking of straightness of slideways or beds is not as important as checking of straight line motion which takes into account all factors likely to effect motion. The expression “accuracy of straight line motion”, refers to the trajectory of a point on a component of the machine when affecting a working or setting motion. The various possible cases of straight line motion are :
(i) motion of an axis on itself, when this axis remains within the two right angled planes which contain it at rest;
(ii) motion of a plane surface in its own plane, when this surface remains in its own plane;
(iii) motion of a component parallel to a straight line or a surface, when any point on this component remains at an equal distance from the line or from the surface ;
(iv) motion of a component perpendicular to a given plane, when each point of the component describes a trajectory perpendicular to the given plane.
The straight line motion can be tested for smaller travels with a straight edge and dial gauge and for longer travels with microscope and taut wire. In the method of checking with a straight edge and dial gauge, the dial gauge is fixed to the moving component of the machine whose straight line motion is to be tested and the feeler slides along the straight edge representing the reference line. In the method of checking with microscope and taut wire, the reference line is represented by a lightly stretched thin steel wire and the dial gauge is replaced by a microscope, the deviations being read directly from the scale of eyepiece by sighting the wire through the microscope.
The straightness of lathe slide displacement for lengths smaller than 1600 mm may be checked with mandrel and feeler gauge method and for the lengths longer than 1600 mm with taut wire and microscope method. In the former method, a mandrel is mounted between the two centres of the lathe and adjusted such that the dial gauge indications are equal at two ends. A dial gauge is mounted on the saddle is place of tool and its feeler brought into contact with the surface of the mandrel. The first set of readings is taken by traversing the dial gauge along the surface of mandrel, the second by rotating the mandrel through 180°. The similar two sets of readings are again noted after turning the mandrel end to end. The mean of the four sets of readings gives the actual error in straight line motion and this eliminates any possible error due to errors in mandrel.
Under some circumstances, the deviation of straight line motion in relation to a^traight line can be permitted in one direction only. This should be clearly specified e.g., as “trajectory concave only in the horizontal plane”.
The tests for the remaining geometrical checks are covered in Chapter 7.
Dr. G. Schelesinger has for many years been concerned with the various alignment tests of machine tools for satisfactory working and much pioneering work has been done by him in devising and evaluating suitable tests. We will deal here latest techniques used in alignment testing of machines.

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