GB2055474A - Measuring device - Google Patents

Abstract

A self-contained hand-held comparator unit 1 which can make a direct comparison between a setting master of dimension (d+e) and a work-piece of dimension (d+e'), the unit including a displacement transducer and electrical means having a memory for storing e during measurement of the master and a digital display for displaying directly the value e' during measurement of the workpiece. The unit may be designed for external or internal measurements. <IMAGE>

Description

SPECIFICATION Measuring device The present invention relates to measuring devices, particularly to portabie, hand-held, electronic measuring devices.

In many areas of industry workpieces are required to be manufactured with specific dimensions to very small tolerances. Very accurately manufactured solid anvil gauges are at present generally employed to provide an indication during manufacture of whether or not the specific dimensions have been attained.

However such gauges do not give any sure indication of how much material needs to be removed before the workpiece achieves the correct dimensions, and serious inaccuracies are introduced as the operator will often have removed too much material, putting the workpiece outside the prescribed tolerance.

Regard must also be had to current methods of presenting tolerance data to machine operators.

The International Standards Organisation recommends that tolerances on engineering drawings be specified as a nominal dimension in millimetres and the tolerances as micrometres in relation to the stated nominal dimension. An example of the above system of specifying dimensions on, say, a shaft of nominally 30 mm is given below -0.010 29.990 30 not -0.018 29.982 An object of the present invention is to provide a measuring device, either an external or internal measuring device, that is designed to exploit the above system of dimensioning.

An external measuring system is known which can exploit that dimensioning system and it is referred to as the Sigma Vertical Mechanical Comparator. It comprises a rigid stand carrying two measuring elements to engage a workpiece, one an anvil supported by the base of the stand and the other a knife-edge carried by a measuring unit mounted so that it can move vertically on the stand. The knife-edge is movable relatively to the unit a ,d a transducer detects the position of the knife-edge relative to the unit and displays that position on an analogue meter. The meter carries a scale which can be set to zero, or some other value between +0.007 mm.

Assume a setting master is provided of external dimension 30.004 mm as determined by a standards room, i.e. the master has a normal dimension d = 30.000 mm with an error e of +0.004 mm. This master can be engaged between the measuring elements (anvil and knife edge) and the height of the measuring unit adjusted by a fine vertical adjustment provided on the stand until the meter scale reads the value e, i.e. +0.004 mm. With the measuring unit held in that position, any subsequent measurement on a workpiece will cause a display of the value e', where (d + e') is the actual dimension of the workpiece. Thus, a measure is given corresponding directly to the dimensioning system described above.

However, this is a very robust and bulky piece of equipment and is not suitable for the machine operator wishing continually to check a workpiece in his machine.

Thus, according to a first aspect of the invention, there is provided a comparator for measuring a dimension of a workpiece relative to a nominal dimension d, the comparator having two measuring elements which have an adjustable spacing for engaging the workpiece dimension, a transducer responsive to the spacing of said elements for providing data relating to said spacing, electrical means incorporating a display device for displaying information corresponding to said data, and means enabling said electrical means to be adjusted to display a known value e when the elements are engaging a calibrated setting master of known dimension (d + e), whereby, in subsequent use, the display device will display directly the error value e' of said workpiece dimension relative to the nominal dimension d, the measuring elements, the transducer and the electrical means being provided by a hand-held comparator unit in which the display device is a digital device, the comparator unit comprising battery supply means enabling the unit to operate as a self-contained unit, and the electrical means of the unit including an electronic memory for storing data relating to the values d and e, means for entering that data into the memory during calibration by the enabling means and means for utilising that data, in use, to cause the digital display means to display directly the error value e' of a workpiece.

In one embodiment, the means enabling the electrical means to be adjusted is a control unit releasably connectable to the comparator unit for calibration purposes. The control unit has data input means (e.g. a keyboard) by which the value e may be input during calibration.

An external or an internal measuring system may be constructed in accordance with the invention. The adjustably spaced measuring elements can be biased towards each other in the embodiment for measuring external dimensions, and can be biased away from each other in the embodiment for measuring internal dimensions.

A self-contained hand-held digital comparator is known, this being the Micro 2000 manufactured by Moore and Wright. It can be caused to engage a standard and have its display then set to zero, so that in a subsequent measurement, the difference from that standard is displayed. Presumably an electronic memory is incorporated to store the display offset corresponding to the standard. However, there is no provision for compensating for any error e in the standard relative to a nominal dimension d.

The workman himself must mentally apply a correction to the display.

Thus according to a second aspect of the invention, there is provided a hand-held comparator unit for measuring a dimension of a workpiece relative to a nominal dimension, the comparator unit comprising two measuring elements which have an adjustable spacing for engaging the workpiece dimension, a transducer responsive to the spacing of said elements for producing data relating to said spacing, electrical means incorporating a digital display device for displaying information corresponding to said data, the electrical means having an electronic memory for storing data defining the dimension of a first component when engaged by said elements and also having means for utilising that data, in subsequent use on a second component, to cause the display device to display a value defining the difference in the dimensions of the first and second components, and battery supply means enabling the unit to operate as a self-contained unit, wherein the memory has input means enabling data to be supplied to the memory corresponding to the error e between a nominal dimension d and the dimension (d + e) of a calibrated setting master used as the first component, the utilising means being operable to cause the display device to display directly the value e' of a second component of dimension (d + e').

In other words, the comparator unit can be designed to be set to give a digital display of 000 to an accuracy of one micrometre when the distance between the measuring elements or anvils corresponds to some predetermined nominal dimension, i.e. it has the facility for being set to give a digital display of d plus or minus value that corresponds to the accurately determined error in the setting master. In one embodiment the comparator unit is connected to a suitable control unit but in use and for maximum convenience it functions as a fully operational hand-held measuring device that is not connected to any form of external supply.

When the instrument is being used by a machine operator, but not making a measurement, it preferably has provision for being located in a battery charging device.

Preferably the comparator unit has a range of accurate measurement that covers all known tolerances associated with any nominal dimension to which it has been set and in addition the size of a part before a workman adjusts his machine for the final machining operation. This later facility enables a machine operator to find and then make the amount of adjustment to his machine that will cause the part to be made to a size that is very near the mean of the tolerance. To bring this facility into most beneficial effect, the range of measurement is not equally spaced about the nominal size. For example, on external measurements the range of size below nominal must be a small amount below the largest minus tolerance associated with a particular shaft. On a 30 mm shaft this would be about 0.050 mm.The range of size above nominal must enable the machine operator to make an accurate measurement when the 30 mm shaft he is making is up to 1.500 mm above the required tolerance. It will be observed that the maximum spacing between the elements and therefore the maximum size of a workpiece which can be accommodated can be significantly greater than the maximum displacement which can be sensed by the transducer. For example, the ratio of maximum transducer response to maximum element spacing could be less than 1:5, perhaps less than 1 :10, or even less than 1:20.

It is also possible to provide for storing the tolerances required so that an 'OK' indicator activates when the part being measured is inside the predetermined tolerance and a 'plus' or a 'minus' indicator activates when the size of the part is outside the tolerance boundary.

As indicated above, the unit may be designed as an internal measuring device to cover the same wide range of accurate measurement as the external unit. In one embodiment, the measuring elements are a fixed anvil and a movable measuring probe coupled to the transducer, the fixed anvil being provided by a range of interchangeable and spherically ground measuring discs that are relieved by a predetermined amount diametrically opposite to the measuring probe.

The relief is spherical in form and produces with a workpiece two fixed location positions that have the same angular relation to the measuring probe on all sizes of measuring discs. This arrangement produces the following conditions.

1. An accurate two position location in the hole being measured that causes the measuring disc to be located on a diameter that passes through the measuring probe.

2. A common cosine factor that can be incorporated in the electronic circuit of the comparator unit to cause the display of measured information to refer to a true diameter.

3. It is possible to gently rock the comparator unit through a small angle that causes the line of measurement to pass through a position that is normal to the axis of the hole being measured.

During this operation the display of measured information will fall to a minimum when the line of measurement is normal to the axis of the hole being measured and then begin to increase. A facility is preferably provided that will retain this minimum reading which is the true diameter of the hole. The retained signal can be cancelled by a separate manual operation.

A further embodiment of the internal comparator unit has an alternative measuring plug that covers the necessary wide range of measurement, is self-centring but does not require the rocking action or cosine correction as is necessary with the spherical measuring discs.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a cross-sectional side view of one embodiment of a comparator unit according to the present invention; Figure 2 is a sectional view of a part of Figure 1; Figure 3 is a back elevational view of the comparator unit of Figure 1 positioned on a control unit; Figure 4 is a plan view of the comparator unit and control unit of Figure 3; Figures 5 and 6 illustrate stages during settting up of the comparator unit of Figure 1; Figure 7 is a cross-sectional side view of a second embodiment of a comparator unit according to the present invention; Figure 8 is a cross-section at right angles to that of Figure 7;; Figure 9 is a detail view of the unit of Figures 7 and 8 carrying a smaller measuring disc; Figure 10 is a view of the comparator unit of Figures 7 and 8 with a control unit; Figure 11 is a diagram illustrating use of a measuring disc; Figures 12 and 1 3 show a modification of Figures 7 and 8; and Figures 14 and 1 5 are block diagrams of the comparator and control units.

Figures 1 and 2 illustrate a hand-held comparator unit 1 operable to measure the difference e' between a nominal dimension d defined by the external dimension (d + e) of a calibrated setting master and the dimension (d + e') of a workpiece.

Figure 1 is a cross-section of the comparator unit 1 and shows a main body 3 slidably supporting an anvil 2 and a measuring rod 5 having opposed measuring faces 2a and 5a for engaging opposite sides of the object, setting master or workpiece, concerned. The anvil 2 also has a face 2b providing a support for the object.

The anvil 2 is clampable to the body 3, by tightening screws 4, in a range of positions to cater for workpieces in a range of sizes, e.g.

10 mm to 40 mm.

Rod 5 is coupled to a linear voltage differential transformer (LVDT) 6 and is urged towards the measuring face 2a by means of springs 29, which are shown in Figure 2, which is a cross-section taken at right angles to that of Figure 1 a. The linear voltage differential transformer 6 is operable to sense the position of the rod 5.

The unit 1 has a space 8 containing electronic circuitry and a space 9 for batteries. Batteries in the space 9 can be re-charged via a socket 10.

The unit 1 is switched on and off by means of a switch 11 which is illustrated as a sliding switch.

The electronic circuitry in space 8 is electrically connectable to external conductors via a multipoint socket 1 2. A digital display 13 is provided opposite to socket 12. This embodiment is particularly suitable for an operator who likes to apply the measuring unit to a workpiece from behind. The position of the digital display 13 and the socket 1 2 are reversed in another embodiment of the invention which is more convenient for an operator who prefers to apply the unit to a workpiece from in front. In both cases it will be noted that the visual display 13 is easily readable by an operator, which would not be the case if the display were positioned on the side of the unit as in the Micro 2000.

The linear voltage differential transformer 6 comprises a transformer 20 having a single primary winding energizable by alternating current, and two secondary windings. A magnetic core 24 couples the primary and the secondary windings, and is connected to the rod 5. The coupling between the primary and the secondary windings of the transformer 20 is determined by the vertical position of the core 24 which is indicative of the vertical position of the rod 5. The two secondary windings are so connected that when the core 24 is positioned centrally in the transformer 20, two equal and opposite voltages are obtained from the two secondary windings.

The linear voltage differential transformer 6 has a range of, say, + 1.000 mm to -1.OQO mm. This range is determined by the limitations on the movement of a guiding plate 25 in a guiding chamber 28. The guiding plate 25 is attached to the core 24 and abuts upper and lower walls 26 and 27 respectively of the guiding chamber 28 at the extremities of its movement.

The two secondary windings of the transformer 20 may be connected to a bridge circuit to which offset voltages may be applied. Any displacement of the core 24 from the central position results in the two secondary windings producing unequal output voltages, and the magnitude of the difference between the two output voltages can be used to indicate the magnitude of the displacement of the core 24 and hence of the rod 5. Alternatively, the phase difference between the two output voltages may be used.

Figures 3 and 4 show the unit of Figures 1 and 2 in conjunction with a control unit 1 5. This control unit 1 5 is used to lock the dimensions of a calibrated setting master into the unit 1, and to enter any error e in the dimensions of the calibrated setting master into the comparator unit so that such an error is taken into account in subsequent measurement of a workpiece.

As shown in Figure 4, the control unit comprises a compartment or slot 1 6 for receiving the comparator unit 1, a mains supply switch 17, a setting circuit switch 18, a keyboard 19, and tolerance limit switches 53. It is connectable to the comparator unit 1 by means of a multi-point plug 30 (shown in Figure 1) which can be plugged into the multi-point socket 12 on the measuring device 1 and is connected to the control unit 1 5 by a lead 30a attached to the control unit.

The control unit 1 5 has a plug at the bottom of the slot 1 6 for engaging the socket 10 at the bottom of comparator unit 1 so as to provide means by which the batteries in the unit 1 may be charged.

The method of operation of the comparator unit of Figures 1 and 2 will now be described with reference to Figures 3 to 6. Like reference numerals are used throughout to denote like elements.

First, the measuring device 1 is provided with information defining the size of the nominal dimension d with which an operator has been provided (with tolerances). A setting master is used which has been carefully made to the required dimensions and which has been carefully measured so that its error e from nominal (d) is known. This setting master 7 is placed between the anvil 2 and the rod 5 of the unit 1 and a support plate 14 (Figure 5) of U-shape, to allow rod 5 to engage the setting master 7, is placed underneath the setting master 7. The anvil 2 is then adjusted to make firm contact on each of its faces 2a and 2b with the setting master 7 and clamped in this position.The use of the support plate 14 during this operation ensures that there will be sufficient space between the anvil and the portion of the unit 1 containing the rod 5 subsequently to insert a workpiece (which will normally be larger than the standard setting master during machining).

The setting master 7 and the support plate 14 are then removed, the unit 1 is switched "ON" by means of switch 11, and then the unit 1 is placed in the slot 1 6 of the control unit 1 5, as shown in Figures 3 and 4. The unit 1 is electrically coupled to the control unit 1 5 via multi-point plug 30 (shown in Figure 1) plugged into socket 12 and attached by lead 30a to the control unit 1 5.

The setting master 7 is then again placed in position between the anvil 2 and the rod 5 of the measuring device, and a wedged-shaped support plate 44 is wedged under the setting master 7 to hold the setting master 7 in contact with the faces 2a and 2b of the anvil 2. The control unit mains supply switch 17 is then turned "ON" and the setting circuit switch 18 is put to "SET". The keyboard 19 of the control unit 15 may then be used to enter data defining the units to be used (mm or inches) and any error e in the dimension (d + e) of the setting master. For example, if it is desired to manufacture a workpiece to have a dimension of 30 mm%0:1180, an operator will obtain a setting master produced by a standards department and of known dimensions, e.g.

0.004 mm too large. In this situation such an operator would use the keyboard 19 to enter data in the following sequence: C, mm, +, 0,., 0, 0, 4.

The digital display 13 on the measuring device will then display: "+0.004". If, however the setting master is known to be of exactly the correct dimensions then the sequence: C, mm, O,., 0, 0,0, would be entered and the digital display 13 would display: "0.000". Once the error has been entered, the setting circuit switch 1 8 is switched to "LOCK".

The entered data "+0.004 mm" is stored in the electronic circuitry of the unit 1. This data indicates to the unit 1 that the position of the rod 5 at the time the data was "locked" was 0.004 mm below the position which it would occupy if the unit had been measuring a dimension of exactly 30 mm, i.e. the unit 1 has been calibrated: the zero has been set.

If illuminated plus, minus and OK signs are used on the hand comparator unit to signify the high and low boundaries of the tolerance proceed as follows.

Put the 'plus' tolerance switch 53 to 'set'.

Depress the 'plus' or 'minus' key to indicate that the digital display of size is above or below datum size.

Depress the numbered switches as described above but to a value that causes the digital display to equal the number of micrometres that corresponds to the high limit of the tolerance band in relation to the datum size of 30 mm.

Put the 'plus' tolerance switch to lock.

Repeat the above setting procedure for the low limit of the tolerance band. Disconnect the multipoint plug from the hand comparator.

Once the "zero" position of the rod has been memorised and the unit device 1 disconnected from the control unit 15, the setting master 7 and the support plate 44 are removed. The rod 5 will then be free to spring up towards the anvil 2, under the action of the springs 29 acting on guiding plate 25, until it is restrained by the fixed stop comprised by the upper wall 26 of the guiding chamber 28 being abutted by the guiding plate 25. The digital display will respond to this movement and indicate a value of approximately "-0.100". On placing the hand comparator on the part to be measured and applying firm but not heavy pressure against the two faces of the fixed anvil, the digital display will show a value representing the magnitude of the difference between the size of the part being measured and 30.000 mm as an indicated number of plus or minus micrometres.

For example, if the workpiece being measured is 30.156 mm, the display will show: "+0.156". If the part is to be made to a tolerance of 30Z00:0011 he needs to remove about 170 micrometres more from the workpiece he is machining to produce a dimension of about 29.986, which is the mean of the tolerance range.

An alternative available to the operator is to initially set the unit not to 0.004 but to 0.004 + 0.014 (i.e. the mean of the tolerance). In that case the display will show "+0.170" in the above example, telling the operator directly how much material should be removed relative to 29.986 mm.

Figures 7 to 11 illustrate a second embodiment of a comparator unit according to the present invention which is adapted for measuring internal dimensions of workpieces. Like reference numerals are used to designate like parts.

Comparator unit 31 shown in Figures 7 and 8 comprises similar components to the unit 1 of Figure 1. It has a space containing electronic circuitry on circuit boards 55 and a space 9 for batteries. Re-chargeable batteries may be charged through socket 10. An ON/OFF switch 11, multi-point socket 12 and digital display 1 3 are positioned on the outside walls of the unit 13.

A measuring stem 32 with a measuring disc 33 at its free end is clamped to the main body of the measuring device 31. Its angular position is controlled by a clamping pin 34 making accurate register with a 90 V-slot 56 and the pin 34 is clamped by a screw 57. The size of the stem 32 and the measuring disc 33 are chosen to be suitable for the dimension to be measured. In the illustrated embodiment two stems 32 are available: Stem 1 for 14 to 20 mm dimensions, Stem 2 for 20 to 100 mm dimensions.

Stem 2 is the one shown in Figure 7 with a measuring disc suitable for measuring internal diameters of 30 mm. Figure 9 is a detailed view which shows stem 1 being used.

Figure 11 is a view of a measuring disc 33 and of the measuring probe 35a carried by the arm 35.

The measuring disc has a spherically ground side surface 33a that is relieved over a given area diametrically opposite to the measuring probe.

The relief area is also spherically shaped and produces two points of contact with a workpiece which subtend an angle 20 with the measuring probe when the probe is in a position corresponding to the mean nominal dimension for which the disc is designed. All the discs are machined to provide the same angle 20. The unit is therefore directly measuring the two equal chord lengths from the probe to the points of contact. The circuitry of the unit provides an amplification factor related to Cos 8 to compensate for this factor.

The unit 31 also has a movable arm 35 which is coupled to a linear voltage differential transformer 6 which operates as hereinbefore described. The arm 35 is pivoted on precision ball races 43 and is biased away from the stem 32 by a suitable spring system 58, being limited by an adjustable stop 59.

The unit 31 is calibrated in the same way as the unit 1 using a control unit 1 5 as described above.

This time, a disc-shaped setting master 37 is used (Figure 10). No support plate is needed provided that a sufficiently small measuring disc has been chosen. A location plate 36 (Figure 10) may be used accurately to align the setting master 37.

This location plate is clamped to the setting master 37 to ensure accurate alignment between the axis of the setting master and the measuring disc. This is achieved by the face of the setting master being square to the axis of the hole and the face of the measuring disc being parallel to a true diametral section of its spherical surface.

Once calibrated, the unit 31 is removed from the control unit 1 5 and the setting master 37 detached from it.

The measuring arm will have moved under spring pressure to the fixed stop restricting the amount of travel in the plus direction. The digital display will respond to this movement and indicate a value of approximately "+0.050". The hand comparator unit may now be used to measure the size of holes that are nominally 30 mm diameter in the following manner. Place the measuring disc 33 into the hole to be measured. With a hand grip that depresses a spring loaded switch 60 shown in Figure 7, apply a small amount of pressure to ensure that the side of the measuring disc opposite the measuring probe on arm 35 makes positive contact with the surface of the hole being measured. Measure;r,,,,: is made by a slow rocking movement in the plane of Figure 7.The position where the digital display falls to a minimum value is the line of measurement that bisects the circular hole being measured and is square to the axis of the hole. The electronic circuit is designed to retain the minimum value during the period when the spring loaded switch shown in Figure 7 is depressed. This retained digital display shows the magnitude of the difference between the size of the part being measured and 30.000 mm as an indicated number of plus or minus millimetres. The retained digital display of the minimum value registered during the above operation of measurement is automatically cancelled when the spring loaded switch is allowed to return to its normal position.

Figures 12 and 1 3 illustrate a modified version of the internal unit avoiding the use of a cosine factor.

In this case the measurement stems carry a measurement plug having four pistons 61 in respective bores of the plug, spring loaded by a tube 62 containing a spring 63 and slidable in a fifth bore in the plug.

This provides a four point location system that centralises the unit over a wide range of measurements.

The operation of the electronic circuitry of the comparator units 1 and 31 will now be described with reference to Figure 14.

The linear voltage differential transformer 6 comprises transformer 20 which has a primary 21 and two secondaries 22 and 23 together with a movable core 24. The primary 21 of transformer 20 is energized by alternating current and for this purpose an alternating current generator 62 is provided in the unit coupled to the battery supply 63.

The two secondaries 22 and 23 are connected in anti-series so that when the core 24 is in a central position the two equal output voltages generated in the secondaries 22 and 23 cancel each other out and give a net zero output voltage from the transformer 20. The net output voltage from the two secondaries 22 and 23 is fed to a synchronous detector 45 which produces a direct current output proportional to the displacement of the core 24, and hence of the rod 5 or the arm 35, from its null position. This direct current is fed to an analogue-to-digital converter 46 which is connected to a microprocessor chip 47.

Microprocessor chip 47 comprises ROM and RAM memory 47a and 47b, which may be partially external to the chip or wholly contained by it. ROM memory 47a will oe used to store conversion factors enabling different units of measurement to be employed (e.g. mm and inches), and to store a fixed offset voltage to compensate for errors in the transformer. RAM 47b will store the input error in the setting master when the unit is being calibrated. Control signals are entered through input 52 which will be connected via muiti-point socket 1 2 and plug 30 to control unit 1 5 during a calibration operation.

The microprocessor chip 47 calculates the required difference data and feeds it to the digital display 13.

An analogue processor chip 47' may replace elements 46 and 47.

Figure 1 5 illustrates diagrammatically the circuitry of the control unit 1 5. The keys 1 9 and switches 1 7, 18 and 53 feed a further microprocessor 64 programmed to interpret the data input and provide a value to feed microprocessor 47 with the error data e to be stored in its memory.

Preferably the circuitry is designed to provide a universal control unit applicable to all comparator units.

A complete system could thus comprise: - 1. A single universal control unit.

2. At least one simple battery charging unit.

3. At least one external measuring comparator unit (metric and inches).

4. At least one internal measuring comparator unit (metric and inches), with spherical measuring discs and having cosine correction and a retained minimum signal facility.

5. At leat one internal measuring comparator unit (metric and inches), with cylindrical measuring plugs.

The system would be kept in the standards room and a standards room engineer would set each comparator unit with appropriate setting masters as described above. Each machine operator would be given only a battery charging unit and the appropriate comparator unit which he would not be able to reset.

Claims (14)

1. A comparator for measuring a dimension of a workpiece relative to a nominal dimension d, the comparator having two measuring elements which have an adjustable spacing for engaging the workpiece dimension, a transducer responsive to the spacing of said elements for providing data relating to said spacing, electrical means incorporating a display device for displaying information corresponding to said data, and means enabling said electrical means to be adjusted to display a known value e when the elements are engaging a calibrated setting master of known dimension (d + e), whereby, in subsequent use, the display device will display directly the error value e' of said workpiece dimension relative to the nominal dimension d; the measuring elements, the transducer and the electrical means being provided by a hand-held comparator unit in which the display device is a digital device, the comparator unit comprising battery supply means enabling the unit to operate as a self-contained unit, and the electrical means of the unit including an electronic memory for storing data relating to the values d and e, means for entering that data into the memory during calibration by the enabling means and means for utilising that data, in use, to cause the digital display means to display directly the error value e' of a workpiece.

2. A comparator according to claim 1, wherein the means enabling the electrical means to be adjusted is a control unit releasably connectable to the comparator unit during calibration in conjunction with the setting master, the control unit having a data entry facility for entering the value e of the setting master and means for presenting to the unit, for adjustment of its memory content, data corresponding to the entered value e.

3. A hand-held comparator unit for measuring a dimension of a workpiece relative to a nominal dimension, the comparator unit comprising two measuring elements which have an adjustable spacing for engaging the workpiece dimension, a transducer responsive to the spacing of said elements for producing data relating to said spacing, electrical means incorporating a digital display device for displaying information corresponding to said data, the electrical means having an electronic memory for storing data defining the dimension of a first component when engaged by said elements and also having means for utilising that data, in subsequent use on a second component, to cause the display device to display a value defining the difference in the dimensions of the first and second components, and battery supply means enabling the unit to operate as a self-contained unit, wherein the memory has input means enabling data to be supplied to the memory corresponding to the error e between a nominal dimension d and the dimension (d + e) of a calibrated setting master used as the first component, the utilising means being operable to cause the display device to display directly the value e' of a second component of dimension (d + e').

4. A comparator according to claim 1 or 2 or a unit according to claim 3, and having a battery charging connector, so that the battery supply means can be charged between measurements.

5. A comparator according to any one of claims 1,2 and 4 or a unit according to any one of claims 3 to 5, and comprising means for storing tolerance values and indicator means for indicating whether or not a measured workpiece is within the tolerance values relative to the nominal value d.

6. A comparator according to claim 1,2,4 or 5 or a unit according to any one of claims 3 to 5, wherein the comparator unit is designed for measuring external dimensions.

7. A comparator or unit according to claim 6, wherein a first of the measuring elements is coupled to the transducer and the second element is movably mounted and has securing means enabling it to be secured in any one of a plurality of positions to enable the unit to embrace a range of dimensions.

8. A comparator or unit according to claim 7, wherein one measuring element is biased towards the other, one having a workpiece engaging portion provided by two surfaces at an obtuse angle to one another, one of said surfaces being parallel to a measuring surface of the other measuring element.

9. A comparator according to claim 1, 2, 4 or 5 or a unit according to any one of claims 3 to 5, wherein the comparator unit is designed for measuring internal dimensions.

10. A comparator or unit according to claim 9, wherein one measuring element is a disc the side surface of which is spherically ground and having a relieved portion diametrically opposite to the other measuring element so as to provide for two points of engagement between the disc and workpiece.

11. A comparator or unit according to claim 10, wherein the electrical means has provision for recording the minimum value sensed by the unit whilst the elements are rocked within the workpiece being measured.

12. A comparator or unit according to claim 9, wherein one measuring element provides four distinct areas of contact with a workpiece additional to one djametrically opposite the other measuring element.

13. A comparator or unit according to claim 1 2, wherein the four areas are provided by respective spring biased plungers carried by the associated measuring element so as to locate the unit relative to a workpiece.

14. A comparator system substantially as hereinbefore described with reference to Figures 1 to 6 and 14 and 15 or Figures 7 to 11 and 14 and 15 or Figures 7 to 11 and 14 and 15 as modified by Figures 12 and 13 of the accompanying drawings.

1 5. A hand-held comparator unit substantially as hereinbefore described with reference to Figures 1,2 and 14, or Figures 7 to 9 and 14, or Figures 7 to 9 and 14 as modified by Figures 12 and 13 of the accompanying drawings.