Some of the devices that have been used to measure such small deflections by dynamometer are described below:
Device # 1. Dial-Indicator:
This is the simplest form of deflection measuring device and can measure accurately upto 25 x 10-4 mm when functioning properly. These should be rigidly mounted on some suitable stand. However, dial- indicators are subjected to striking and are not to be relied upon for absolutely static readings.
A lever system is frequently used in conjunction with a dial-indicator. In Fig. 25.1, the horizontal component of force is measuring the deflection of the over-arms of the machine to which the tool is attached; and the vertical component by measuring the deflection of the lever system attached to the work holder.
Device # 2. Hydraulic Pressure Cells:
In hydraulic pressure cells, the force experienced by the tool is applied though a diaphragm to an enclosed volume of oil, which gets pressurised on application of force. The pressure can be measured easily by the conventional pressure gauge which may be calibrated in terms of the force. In this method, it is possible to locate the pressure gauge at a distance from the sensing point, i.e. pressure cell.
Device # 3. Pneumatic Devices:
These include devices such as Solex micrometre. These operate on the principle that change in back pressure occurs when a flat surface is brought into closer contact with a sharp edge orifice; the back-pressure increasing if the flat surface is very much nearer to the orifice and decreasing if it is away from it. All this holds good within certain limits.
These are used to measure deflection (which may be produced due to tensile forces or compressive forces or due to bending) in tool dynamometers. Such systems are simple and reliable, if carefully supplied with clean and constant pressure air. There is however, a limited region over which they are linear and dynamometers of this type tend to be bulky. Initial settings, have to be done very carefully.
Device # 4. Optical Devices:
Several types of optical devices have been employed. For very precise linear measurements interferometry methods utilising monochromatic source of light are often used, but these are not found to be suitable for dynamic-deflection occurring in metal cutting. The dynamometers utilising optical devices include optical levers which though very simple in a principle or operation can be applied to metal cutting problems with considerable difficulty.
Very small angular deflections can be readily measured by reflecting a beam of light from the moving surface. In Fig. 25.4, a narrow beam of light travels from A to B where it is reflected and hence travels back to screen. When the surface B rotates through an angle α, the spot of light moves through a distance δ on the screen. If α is small, then δ = 2l x α.
This device acts as an optical lever and may be used to measure small angular displacements, if I is large.
Device # 5. Piezoelectric Crystals:
These can be used as dynamometers to measure the forces encountered during metal cutting, but it is found that the associated measuring equipment is often quite unwieldy because the piezoelectric crystals produce an electric charge rather than a current. Further the leakage effects can also be troublesome.
Device # 6. Electric Transducers:
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Due to so many inherent advantages in electric devices, electric transducers are often used in dynamometers to measure displacement which is proportional to the cutting force.
The various electric displacement measuring devices commonly used are:
(i) Electronic Transducer Tube (Triode, Vacuum Tube):
It contains a moving plate which is further connected to the deflection member of the dynamometer. The tube characteristics change as the plate moves.
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(ii) Differential Transformers:
It contain three transformer coils, the central one being known as primary coil which is supplied with an A.C. current and induces an e.m.f. in the two adjacent coils known as secondary coils. All the three transformer coils are on a common axis with a common movable core.
The output of the secondary coils are wired to oppose each other so that when the core is in the centre, there is no output. On displacement of the core, an e.m.f. is produced proportional to the displacement of the core, with a phase depending upon the direction of motion.
(iii) Strain Gauges:
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These are of two types:
(a) Unbonded type and
(b) Bonded type, and the latter type is commonly employed in dynamometers.
A strain gauge is a necessary length of gauge wire arranged in the form of a flat coil which is cemented between two insulating sheets of paper or plastic. This strain gauge is cemented to the member to be strained by properly cleaning the surface, applying suitable cement on both the surfaces of the member and the gauge.
During the cementing application there should be no air bubbles and the resistance between strain gauge wire and surface should be about 50 megohms. The two important parameters in a resistance gauge usually specified are the resistance of the strain gauge R which is usually about 120 ohms and the gauge factor F which varies from 1.75 to 3.5 and is defined as
where ΔR is change in original resistance R for strain ε.
The change in resistance of the strain gauge can be measured by a Wheatstone bridge. The unbalance in the bridge due to change in resistance can be measured either by a micrometre or by null instrument, the latter being more sensitive and accurate.