Measurement of Limit Gauges (Metrology)

10.3.
Before making any dimensional measurement of limit gauges, preliminary inspection is made about in general and the non-gauging dimensions of the gauges checked. In addition, the flatness, hardness and straightness of the gauging surfaces are also checked. The checking of the general dimensions is very useful to find the reasons of wear of the gauge etc.
The hardness of the gauging surface may be usually measured on a hardness testing machine, preferably on the diamond pyramid machine. In certain types of gauges like plain and screw ring gauges, hardness is checked by means of a scratch test. A scratch made on gauging surface with the help of a suitable scriber is compared with the scratch put with same scriber on a mild steel piece. The comparison of the two gives the desired result. Same pressure should be applied in both the cases.
The flatness and straightness of the gauging surface may be checked accurately and quickly by applying a knife edge straight edge to the surface and viewing any gap between the two against an illuminated background. Under these conditions it is possible to see a gap of light as small as 0.008 mm. Substantial error of flatness may produce a gap of light exceeding 0.008 mm in magnitude and estimation of gap of such a size is not possible with any degree of accuracy by the above method. In this case slip gauges of different size should be introduced between the gauging surface and straight edge so as to reduce the gap of light to a value which can be accurately estimated. If this method is not convenient, the gauging surfaces should be set up horizontally on a surface plate, the flatness or the straightness being checked by means of a sensitive dial gauge mounted on a stand which is placed on the surface plate.
Having completed the preliminary inspection of gauges, the gauging dimensions are measured. This is described below for different gauges.
10.3.1.


Parallel Gap Gauges.

These may be measured directly by means of slip gauges and length bars depending upon whether the gap is smaller or larger than about 10 cm. The slip gauge or the length bar combination should be laid on its side on a surface plate with the end faces vertical. The gap gauge should be supported lightly at both ends and lowered over the slip gauges combination so that only its own weight allows is to move downwards, and the slip gauge combination should be gradually increased until the gap gauge just nips the slip gauge combination and yet moves freely over a combination which is 0.002 mm smaller in size (Refer Fig. 10.1).
The size of gap gauge is determined like this for both “GO” and “NOT GO” dimensions.
In order to detect errors in parallelism of the gauging surface of the gauge, it is important to present both sides of gauge to slip gauge combination, every time keeping one face in exact contact with slip gauges and noting the difference in other face of gauge surface.
Small gap gauges may be measured to an accuracy of ± 0.001 mm by above method but the accuracy of measurement, of larger gap gauges is not so high, due to probable springing open of jaws when inserting the slip gauge combination and any other reason such as temperature difference between the gap gauge and the slip gauges. Other methods of testing parallelism of surfaces have been described in chapter 7.
Checking dimensions of gap gauge.
Fig. 10.1. Checking dimensions of gap gauge.

Plain plug gauge
Fig. 10.2. Plain plug gauge.
10.3.2.

Plain Plug Gauges.

These may be measured by hand micrometer to an accuracy of ± 0.002 mm. For such measurements the micrometer should be used as a comparator, i.e. by taking successive readings on a standard known size and on the gauge and then adding or subtracting the difference between the readings in relation to size of the standard. The standard size should be quite near to the size of the plug gauge so that the travel of micrometer
spindle during the measurement is small enough to minimise the effect of any progressive pitch error of the micrometer screw. The standard should preferably be a plain gauge and not a slip gauge combination to lessen the effect of any possible errors of parallelism of micrometer anvil and the spindle faces and any compression effects caused by measuring force on the gauge and standard. If the tolerance on the plug gauge is appreciably less than 0.025 mm. it is necessary to use more accurate method of measurement. In that case, bench micrometer, diameter measuring machine or high magnification dial comparator should be employed. The previous way of using the hand micrometer also applies to bench micrometer or the diameter measuring machine. High magnification dial comparators can only measure over a limited range of size at any one setting and the size of the standard has to be very approximately close to that of the gauge ; slip gauge combinations are, therefore, used for setting standards of comparators. The measuring forces of a dial comparator are very small and the errors caused by different compression effects on the flat surface of the slip gauges and gauge surface may be neglected. An advantage of the use of slip gauge standards for dial comparators is that size of the standard may be equal to one of the limits of the size of the plug gauge and hence the effect of any errors of the comparator scale graduations is minimised.
When measuring the plug gauge, sufficient readings should be taken to ascertain the presence of any ovality and taper of gauging surface. It may be necessary to check the gauge for “lobing” by rotating it in a suitable angle block, beneath dial gauge of high sensitivity or by some other suitable method.
A reading on standard should be taken after the plug gauge has been measured, to make sure that the instrument setting has not altered during measurements. This rule should always be followed for all types of measuring instruments.
10.3.3.

Cylindrical Ring Gauges.

Those gauges which are less than 1 cm. in diameter are difficult to be measured accurately and it may be best to measure the approximate size of
such gauges by suitable plug and then to insert a tapered mandrel having mean size equal to the measured approximate size and having a taper of 0.0002 mm on the diameter per mm. of the length. The size of the ring gauge is then given by the size of the mandrel where it fits the ring gauge. The measurement of the ring gauge by tapered mandrels or plug gives the minimum size of the gauge and so the presence of any ovality is not checked. Care should be exercised when inserting a tapered mandrel in the gauge, to avoid any considerable wedging force on the gauge. Ovality can be checked by setting the gauge concentrical-
Ring Gauge
Fig. 10.3. Ring Gauge.
ly on the face plate of the dividing head and inspecting the bore of the gauge with a sensitive dial indicator as shown in Fig. 10.4.
The size of the ring gauge having diameter more than 1 cm may be measured by using two small rollers or balls of equal size and slip gauges. The balls are placed diametrically opposite in ring gauge (Fig. 10.5) and a slip gauge combination is found which will be just nipped by the rollers while a combination 0.002 mm smaller will enter between the rollers by virtue of its own weight. The diameter of the ring gauge is then calculated by adding the mean size of the slip gauge combinations to the sum of the diameters of the rollers. By using rollers, the size of the gauge may be estimated to an accuracy of ± 0.001 mm for gauge upto 4 cm. in diameter, an increased accuracy can be obtained by the use of ball although more skill is required for that.
Inspecting bore of a ring gauge.
Fig. 10.4. Inspecting bore of a ring gauge.
Checking size of ring gauge
Fig. 10.5. Checking size of ring gauge.
Cylindrical ring gauges may be most conveniently measured on various types of pneumatic comparators ; the ovality and taper of the gauges may be quickly investigated: and accuracy of the order of ± 0.001 mm is obtained by using such comparators.
10.3.4.

Form of Profile Gauges.

These are most conveniently inspected by optical projection at a standard magnification and comparing the projected shadow image with standard projection diagram. The standard magnification most generally used is 50. Another standard magnification used is 100 but due to reduced intensity of illumination at the projector screen and consequent loss of the definition of illumination accuracy of measurement at this magnification, it is generally not more than 50.
Profile Gauges
Fig. 10.6.Profile Gauges
The magnified diagram with which the form is compared should be drawn on a material which has a good drawing surface and also is dimensionally stable.
Form gauges may be measured on Tool Maker’s Microscope, especially when minute detailing of individual dimensions of the form makes the projection of enlarged projection diagram difficult. I
10.3.5.

Height and Depth Gauges.

These may be measured by means of depth micrometers, bench micrometers or comparators. The ‘gap of light’ method is recommended for this. This gauge is placed on a surface plate in front of light and gauges are built up until the top of the slip gauge combination and the gauging surface of the gauge are at the same height above the surface plate. This is possible when no gap of light is visible below a knife edged straight edge resting simultaneously on the two (Fig. 10.7).
Alternatively a sensitive dial gauge mounted on a stand on the surface plate may be employed to measure the difference of the height of the gauge and the slip gauge combination. This difference should not
 Cheching height of depth gauge
Fig. 10.7. Cheching height of depth gauge.
exceed 1/8 mm for greatest accuracy of measurement otherwise errors in dial gauge graduations may have some effect.
10.3.6.

Taper Plug Gauges.

These are inspected in the following way (Fig. 10.8). The gauge is mounted vertically on a surface plate with the small diameter end of gauge downwards, and measurements are made across two equal sized precision rollers, which are supported on equal slip piles which may be altered so that the rollers contact the taper in various planes normal to the axis of the taper. The process is facilitated by taper measuring machine which is having standardised equipment, and taking various measurements is also very easy. Ovality may be checked by taking diametral measurements around the circumference of the taper plug in various measuring planes, and the uniformity of the rate of taper is checked by taking measurements in three or more planes uniformly spaced and normal to the axis of the taper. The diameter of the small end of the gauge is then calculated from the formula :
Checking taper plug gauge
Fig. 10.8. Checking taper plug gauge.
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10.3.7.

Taper Ring Gauges.

Those gauges which are above 20 mm in diameter may be measured in a similar way as taper plug gauges (Fig. 10.9). The ring gauge is mounted on a surface plate with the axis of the ring vertical and the small diameter end uppermost. Measurements are made by means of slip gauge combinations between two precision balls of equal size supported on equal slip piles placed on the surface plate. The measurements are made in planes at different heights from the base of the gauge, the same criterion of fit being uses as for cylindrical ring gauges. The diameter of the large end of the gauge is then calculated from the formula
Checking taper ring gauge
Fig. 10.9. Checking taper ring gauge.
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Taper ring gauges of sizes below 20 mm diameter are best measured by inserting precision balls of various sizes in the gauge and measuring the distance between the top of each ball and one of the end faces of the gauge (Fig. 10.10). The diameter in the plane of one end faces is then calculated from the formula
tmp3F-12_thumb
10.3.8.

Taper Plate Gauges.

There are measured by the methods described for taper plug and ring gauges, depending whether the taper is external or internal. In the latter case it is more convenient to use precision rollers in place of balls.
10.3.9.

Parallel Screw Plug Gauges.

Various elements of a screw plug gauge are measured in the following way :
(i) The major diameter is measured on a diameter measuring machine which is standardised in a similar way as a bench micrometer for this purpose.
(ii) The minor diameter is obtained by making measurement on a diameter measuring machine between the flat ends of Vee-pieces placed in the thread groove on opposite sides of the gauge. These Vee-pieces have a radius at the apex which is smaller than the root radius of the thread so that during the minor diameter measurement they contact the root of the thread. The diameter measuring machine has arrangements for holding either gauge or a standard cylindrical plug. Two V-pieces hang freely and they can be placed across gauge in same plane and readings taken across them. First readings are taken on the cylindrical plug standard of known size with the Vee-pieces in position on both sides of the standard. The standard is then replaced by the gauge and a series of readings is taken on the minor diameter (Fig. 10.11). The minor diameter K of the gauge will be equal to the known diameter of the standard plus or minus the difference between the reading taken on the gauge and the standard respectively. The sizes K and S should be nearly equal for accurate results.
Checking small taper ring gauge
Fig. 10.10. Checking small taper ring gauge.
(iii) The effective diameter is obtained by noting the dimension measurements T under a pair of thread measuring cylinders placed in the thread groove on opposite sides of the gauge (Fig. 10.12). A diameter measuring machine is used for this purpose. Next readings are taken on a standard cylindrical plug of nearly same size as T with the thread measuring cylinders in position on both sides of the standard. The effective diameter is then calculated from the formula
Measurement of minor diameter of screw plug gauge
Fig. 10.11. Measurement of minor diameter of screw plug gauge.
E =T + P + Compression correction—C ; T = S ± Difference between the readings taken over the gauge and the standard.
Where T is the measured diameter under the cylinders, P is a constant depending on the pitch and angle of the thread, and the diameter of the cylinders; C iaa correction for the rake of the thread and the compression correction is for the compression of the
Measurement of the effective diameter of screw plug gauge
Fig. 10.12. Measurement of the effective diameter of screw plug gauge.
cylinders caused by measuring force of the machine. The rake and compression corrections for standard series of screw threads are usually very small and tend to cancel out, but to the small tolerances on screw gauges, they must not be neglected. (Also refer chapter 13).
(iv) The pitch is measured on a pitch measuring machine and the thread form, including crest, root radius and flank angles are examined by optical projection method.
10.3.10.

Parallel Screw Ring Gauges.

These are best inspected by plug check gauges. The minimum number of check gauges required for this purpose are :
(i) A ‘GO’ full form screw check gauge made to low limit of the screw ring gauge.
(ii) A ‘NOT GO’ effective diameter screw plug check gauge having threads cleared at the crest and roots made to the upper limit of screw ring gauge.
(iii) and (iv) ‘GO’ and ‘NOT GO’ minor diameter plain plug check gauges made respectively to the lower and upper minor diameter limits of the screw ring gauge.
In addition to this inspection, it is usual to measure the pitch of the ring gauges on a pitch measuring machine, and to inspect the thread form and measure the flank angles by ‘optical projection method’ by taking a cast of the inside of threads made from plaster of paris or a mixture of sulphur and graphite. (For small ring gauges a cast of dental wax may be used.)
The inspection of screw ring gauges by plug check gauges verifies that the ring gauges lie between their limits of size. If the actual effective diameters of the screw ring gauges are required, a suitable horizontal comparator is necessary, employing two styluses which seat in the thread groove on opposite sides of a gauge.
The major diameters of screw ring gauges are usually not measured but inspection of the thread form provides an indirect verification by checking of the depth of the thread.
10.3.11.

Position Gauges.

The design of position gauges varies so much that it is difficult to lay down any procedure for their measurement.

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