direction transverse to the radial plane of the grinding wheel the workpiece support means may also include one or more adjustable steady rests 24 as shown in fig1 a means 26 for moving at least one of the workpiece 12 or the tool 14 with respect to each other includes the aforementioned grinding wheel drive motor 16 and in the preferred embodiment a motor not shown for moving the grinding wheel into contact with the workpiece 12 and a workpiece drive motor 28 the workpiece drive motor 28 is connected to the workpiece 12 by a coupling 30 incorporated in the center support 20 to rotate the workpiece 12 in a counter clockwise direction as viewed in fig2 the apparatus 10 also includes a means 32 for establishing an eddy current in the workpiece 12 at an area 33 where the tool 14 contacts the workpiece 12 and sensing any change in the eddy current in response to change in the microstructure of the workpiece 12 resulting from the abrasive machining operation and generating an output signal responsive to the sensed change in microstructure in the preferred embodiment the means 32 for establishing an eddy current in the workpiece 12 includes an eddy current tester 34 and a probe 36 coupled to the tester 34 it has been found that a model m900 i verimet single channel hardness and alloy tester and a waterproofed model 15887 m100 0 625 inch 15 9 mm hardness and alloy probe both manufactured by k j law engineers inc of farmington hills mich usa are particularly suitable for incorporation into the.