GB2079460A - Caliper gauges - Google Patents

Abstract

A caliper gauge comprises a pair of spaced probes 9 each rigidly supported at one end within cylindrical members 7 & holders 1, 2 slidable on bars 3,4 and each adapted to flex about a portion of its length. Strain gauges 13 carried by the probes in the vicinity of the flexing lengthy portions are operable to transmit electrical signals representative of the compressive and tensile stresses induced by flexing of the probes to electronic circuitry which provides an output representative of the true distance between the surfaces under measurement. <IMAGE>

Description

SPECIFICATION Measuring apparatus This invention relates to measuring apparatus and especially, but not exclusively, to apparatus for measuring internal diameters.

Accurate measurement of internal diameters in, for example, the range 1 mm to 25mm is difficult to achieve. Known instruments, such as plug gauges, air gauges and dial gauge adapters all suffer from various disadvantages. Thus, a plug gauge can be employed only in the measurement of one diameter and is relatively expensive; in addition, it does not actually measure the diameter or cross-section of a hole but merely indicates that the hole is smaller or larger than a particular dimension. Air gauges and dial gauge adapters suffer from a similar disadvantage in that the range of a particular measuring head is limited around a particular dimension (commonly the range of a head is of the order of 0.1 mm). Thus, to cover a range of, say, 25mm requires a number of expensive heads.

According to the present invention in one aspect, apparatus for measuring the distance between two surfaces comprises a pair of spaced probes each rigidly supported at one end and each adapted to flex about a portion of its length, strain gauges carried by the probes in the vicinity of the aforesaid length portions and operable to transmit electrical signals representative of compressive and tensile stresses induced through flexing of the probes to electronic circuitry operable to provide an output representative of the true distance between the aforesaid two surfaces.

In one arrangement the probes are supported by relatively massive cylindrical holders mounted for sliding movement on a pair of parallel spacer bars.

Each probe is located within a recess formed in one end of a cylinder carried by a respective holder, the strain gauges being mounted adjacent flat flexible central portions of the cylinders.

The invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a plan view of measuring apparatus in accordance with the invention; Figure 2 is a side view of the apparatus illustrated in Figure 1; Figure 3 is a section taken along line Ill-Ill of Figure 2; Figure 4 is a plan view of the apparatus illustrated in Figure 1 when used in concert with micrometers; and Figure 5 is a circuit diagram of the illustrated measuring apparatus.

The measuring apparatus illustrated in the drawings comprises two cylindrical mild steel holders (1,2) mounted on two parallel ground silver steel spacer bars (3,4) so that the separation between the holders (1,2) can readily be adjusted by sliding one or both of them along the bars. Once adjusted, the holders can be locked onto the spacer bars by locking screws (5) giving a rigid measuring framework. The holders are preferably bored and reamered as a pair in order to produce a pair of parallel equally spaced holes to enable the holders to slide and lock easily on the spacer bars.

As shown in Figure 3, the upper end of each holder (1,2) is recessed at (6) to accommodate a smaller cylinder (7) whose centre portion has been machined down to a thin flexible flat (8). One end of each cylinder (7) is bored out to carry a thin measuring probe (9) whilst its other end is rigidly fixed in the bottom of the recess (6). The probes (9) are cranked so that when the holders (1,2) are nearly touching at their closest adjustment on the spacer bars (3,4), the probe tips are brought close together.

The inwardly facing sides of the probes are flattened to obtain the least combined diameter when the probes are close together and outwardly facing protruberances (12) are located at the probe tip ends; these protruberances define the points of contact with the inside of the bore being measured.

Strain gauges (13) are mounted one on each side of each flat (8). As will be seen from Figure 5, the four gauges (13) are connected to form a Wheatstone bridge (14), all four gauges being active. The bridge (14) is arranged to give diagonally opposite pairs of tension and compression gauges when the probe carrying cylinders (7) are bent towards or away from one another. The bridge is supplied with current from a stable constant voltage source (15) including an amplifier (16) and output transistor (17), and the output from the bridge is amplified by signal amplifier (18) which drives an indicating meter (19).

An F.E.T. input amplifier (21) is employed as an integrate and hold zeroing device and provides automatic zeroing for the system. The circuitry is energised by rechargeable nickel-cadnium batteries.

The operation of the circuitry will now be described in more detail. Initially after energising the circuitry, some error will exist causing deflection of the indicating meter (19). To eliminate this error, a zeroing switch (24) is operated, connecting the output of the signal amplifier (18) to the input of the integration amplifier (21). The integrated output of the amplifier increases at a rate determined by integrating resistor (25), integrating capacitor (26) and the magnitude of the input error voltage. Part of the output of amplifier (21) is fed back to the input of the signal amplifier (18) via a potentiometer (27) in the sense required to reduce the output of the signal amplifier (18) via a potentiometer (27) in the sense required to reduce the output of the signal amplifier (18) and therefore the input to the amplifier (21).The integrating action continues until the output of the signal amplifier (18) is zero. At this point integration ceases because the input to the integration amplifier (21) becomes zero, and the circuit action automatically comes to rest at the desired zero point. Because the rate of rise of output of amplifier (21) is always proportional to the error voltage, the circuit will approach the zero point exponentially, making the control loop easy to set up. Stability is attained by adjustment of potentiometer (27) which effectively alters the zeroing loop gain. Measurements are made around the zero base thus obtained. After integrating action has ceased, the zeroing switch (24) is opened, leaving the integrator input floating.

Because the integration amplifier (21) is an F.E.T.

type, its input impedance is very high and input bias current very low (of the order of 30 x 1 0-12A). Its input current and voltage under these conditions can be supplied by the integrating capacitor (26), preferably a 10 iiF Polycarbonate type, without significant fall in the output of the integration amplifier (21 ) for long periods. The zero can therefore be maintained for periods up to about 10 minutes with accuracy sufficient for this purpose. To obtain a low droop rate in the zeroing voltage, the input offset current of the integration amplifier (21) must be accurately trimmed out, otherwise the circuit will sum this current and excessive drift in the zero point will result.On first setting up the circuit this may be accomplished by operating the zeroing switch (24), and adjusting the potentiometer (27) until no movement is seen on the indicating meter (19). This adjustment does not mean that the meter should be zeroed, but that movement should be stopped at whatever the reading happens to be.

The operation of the apparatus for measuring the internal diameter of a bore will now be described.

Firstly, the probe ends are set to a distance slightly greater than that to be measured by sliding the holders (1,2) towards or away from one another and then locking them in position. The actual value of this setting is relatively unimportant; however, a setting equivalent to approximately 2 mm greater than the size of the distance to be measured is preferred. The probes are then sprung into the bore and moved to the desired depth with the probe ends set by eye approximately along a diameter of the bore, and the line joining them at right angles to the axis of the hole. Initially the zeroing switch (24) is operated for about one second and then the probes moved across the bore, i.e. at right angles to the bore axis, whilst the meter (19) is observed. As the probes traverse a true diameter, a reversal will be seen in the movement of the indicating meter (19).

The peak of the reversal seen on the meter (19) indicates the probe position required to read the true diameter of the bore. At this point one can either (a) find the position of the peak by slight movements sideways of the probes (9) and by operating the zero switch (24) to transfer the peak to the centre of the meter (19) or (b) note the reading at which the peak occurred.

The probes (9) are then gently sprung out of the bore and the measuring head transferred to a measuring station including micrometers (28,29) (See Figure 4). The probe protruberances (12) are placed between the micrometer barrels, one of which acts as an adjustable zero stop for the other and their separation adjusted with the micrometer until either (a) the indicating meter shows zero, or (b) the meter shows the reading noted atthe peakofthe reversal, depending upon which choice was made from the options mentioned in the previous paragraph.

The measured diameter can then be read from the micrometer.

Because the probes (9) are sprung into the bore to be measured, no 'feel' is required on the part of the user. Also, the apparatus described measures by reproducing a system of stresses set up in the probes and fiats during the measuring process, in a second accurate system. This means that the probes can be made thin (to enter small diameter holes) and.

the fact that they bend in use is of no consequence so long as the bending is within the elastic limit of the material used to make the probes and flats.

If the reversal of the indicator at the maximum dimension of the hole is carefully observed, and the probe pair is kept square to the hole, there is little difficulty in obtaining accuracies to better than 25 Fm (0.001"). In well finished holes accuracies of 5 to 12 Rm (0.0002 to 0.0005") can be reached.

The apparatus described can alternatively be used in reverse, that is, the head can be set to some dimension in the micrometer assembly and the circuitry zeroed. The indicator zero then represents this setting size and measurements can be made around this size by observing indicator readings around zero. The indicator can be scaled in suitable units to read off variations around the nominal size (say .005 - 0 - +.005). These units are applicable only to a particular length of probe, a different calibration is required for another probe length. However the accuracy of the null method previously described is not affected by probe length (within reasonable limits). Probes up to 1 50mm long have been used on the prototype making the instrument useful for measurement down deep holes, cylinder bores, bearings, slots, etc.

It is to be understood that the apparatus described is capable of modification and can be employed, for example, for measuring diameters of very deep holes (metres in length), fixed inspection jigs, and external measurements, by reversing the procedures described.

The apparatus described has been found to be generally useful in setting up machine tools. For example, the apparatus can be clamped in a lathe tool holder, one of the probes allowed to touch the workpiece, and the indicator used to set up a workpiece for true rotation; alternatively, it can be used to set up the angle of a cross saddle top slide to a tapered hole in a workpiece held in the chuck. This is particularly useful in small tapered holes.

Claims (6)

1. Apparatus for measuring the distance between two surfaces comprising a pair of spaced probes each rigidly supported at one end and each adapted to flex about a portion of its length, strain gauges carried by the probes in the vicinity of the aforesaid length portions and operable to transmit electrical signals representative of compressive and, tensile stresses induced through flexing of the probes to electronic circuitry operable to provide an output representative of the true distance between the aforesaid two surfaces.

2. Apparatus as claimed in claim 1 wherein the probes are supported by relatively massive cylindrical holders mounted for sliding movement on a pair of parallel spacer bars.

3. Apparatus as claimed in claim 1 or claim 2 wherein each probe is located within a recess formed in one end of a cylinder carried by a respective holder, the strain gauges being mounted adjacent flat flexible central portions of the cylinders.

4. A method of measuring the distance between two spaced surfaces in which a compressive or tensile stress generated by flexing of elongate probes spanning the distance to be measured is translated into an electrical signal to provide an output representative of the distance between the aforesaid surfaces.

5. Measuring apparatus substantially as herein described with reference to Figures 1 to 5 of the accompanying diagrammatic drawings.

6. A method of measuring the distance between two spaced surfaces substantially as herein de scribedwith reference to Figures 1 to 5 of the accompanying diagrammatic drawings.