Which of the following forces are measured directly by strain gauges or force dynamometers during metal cutting? 1. Force exerted by the tool on the chip acting normally to the tool face 2. Horizontal cutting force exerted by the tool on the workpiece 3. Frictional resistance of the tool against the chip flow acting along the tool face 4. Vertical force which helps in holding the tool in position Select the correct answer using the codes given below. Correct Answer 2 and 4 Explanation: Strain gauge type dynamometers These are considered superior to mechanical dynamometers and are most widely used. A typical cantilever type strain gauge turning dynamometer is shown in fig. The device works in conjunction with a wheat stone flow bridge circuit. The forces being measured are Ft and Fa. It is well known that the electric resistance of a wire changes if it is stretched. Different pairs of strain gauges are cemented on the four flat surfaces (section at AA'). During the experimentation, different pairs of strain gauges are subjected to tension or compression depending on the force applied and the surface on which they are mounted. It will be seen that under the effect of cutting force (Ft), strain gauges T1 and T2 are subjected to tension but the bottom strain gauges and C1 and C2 will be subjected to compression. Likewise, force Fa puts strain gauges T3 and T4 under tension and C3 and C4 under compression. The arrangement of one set of strain gauges (T1, T2, C1, C2) in wheat stone bridge circuit is shown in Fig. The other set comprising gauges T3, T4, C3, C4 are arranged in another bride circuit. Strain gauges subjected to tension show an increase in their resistance (due to increase in length) and those subjected to compression show decrease in their resistance (due to decrease in length). Changes in the resistance are measured by wheatstone bridge. The strains and stresses corresponding to the changes in length (and so the resistance) in the strain gauges can be found using standard formulae and so the corresponding forces responsible for causing these changes. [ alt="F1 Savita Engineering 13-4-22 D19" src="//storage.googleapis.com/tb-img/production/22/04/F1_Savita_Engineering_13-4-22_D19.png" style="width: 425px; height: 196px;"> [ alt="F1 Savita Engineering 13-4-22 D20" src="//storage.googleapis.com/tb-img/production/22/04/F1_Savita_Engineering_13-4-22_D20.png" style="width: 167px; height: 206px;"> Therefore, the following forces are measured directly by strain gauges or force dynamometers during metal cutting. The horizontal cutting force exerted by the tool on the workpiece The vertical force which helps in holding the tool in position