For consistently accurate and reliable measurement, the following requirements are considered during design and construction of any tool force dynamometer’s.

Sensitivity : The dynamometer should be reasonably sensitive or precision measurement.

Rigidity : The dynamometer need to be quite rigid to withstand the forces without causing much deflection which may affect the machining condition.

Cross sensitivity : The dynamometer should be free from cross sensitivity such that one force (say $P_z$) does not affect measurement of the other forces (say $P_x$ and $P_y$)

• Stability against humidity and temperature.

• Quick time response.

• High frequency response such that the readings are not affected by vibration within a reasonably high range of frequency.

• Consistency, i.e. the dynamometer should work desirably over a long period.

Construction and working principle of some common tool – force dynamometers.

The dynamometers being commonly used now – a – days for measuring machining forces desirably accurately and precisely (both static and dynamic characteristics) are

Either – strain gauge type.

Or – piezoelectric type.

Strain gauge type dynamometers are inexpensive but less accurate and consistent, whereas, the piezoelectric type are highly accurate, reliable and consistent but very expensive for high material cost and stringent construction.

Turning dynamometers.

Turning dynamometers may be strain gauge or piezoelectric type and may be of one, two or three dimensions capable to monitor all o $P_x \ P_y \ and \ P_z$.

For ease of manufacture and low cost, strain gauge type turning dynamometers are widely used and preferably of 2 – D (dimension) for simpler construction, lower cost and ability to provide almost all the desired force values.

Design and construction of a strain – gauge type 2 – D turning dynamometer are shown schematically in figure, 10.8 and photographically in figure. 10.9. two full bridges compromising four live strain in terms of voltage which provides the magnitude of the cutting forces through calibration. Figure. 10.10 pictorially shows use of 3 – D turning dynamometer having piezoelectric transducers inside.

Drilling dynamometer.

Physical construction of a strain gauge type 2 – D drilling dynamometer for measuring torque and thrust force is typically shown schematically in Fig. 10.11 and pictorially in Fig. 10.12. four strain gauges are mounted on the upper and lower surfaces of the two opposite ribs for $P_x$ - channel and four on the side surfaces of the other two ribs for the torque channel. Before use, the dynamometer must be calibrated to enable determination of the actual values of T and $P_x$ from the voltage values or reading taken in SMB or PC.

Milling dynamometer.

Since the cutting or loading point is not fixed w.r.t. the job and the dynamometer, the job platform rests on four symmetrically located supports in the form of four 0-rings. The forces on each O-ring are monitored and summed up corresponding for getting the total magnitude of all the three forces in X, Y and Z direction respectively.

Fig. 10.13 shows schematically the principle of using 0-ring for measuring two forces by mounting strain gauges, 4 fr radial force and 4 for transverse force.

fig. 10.14 typically shows configuration of a strain gauge type – 3 D milling dynamometer having 4 octagonal rings. Piezoelectric type 3 – D dynamometers are also available and used for measuring the cutting forces in milling (plain, end and face)

Grinding dynamometer.

The construction and application of a strain gauge type (extended O-ring) grinding surface dynamometer and another piezoelectric type are typically shown in Fig. 10.15 and Fig. 10.16 respectively.

Unlike strain gauge type dynamometers, the sophisticated piezoelectric type (KISTLER) dynamometers can be used directly more accurately and reliably even without calibration by the user.