Hitchiner [1998, 1999] gives typical parameters for dressing of CBN wheels for grinding aircraft blades and vanes. Crush ratio values of +0.6 to +0.8 are used as for conventional wheels but infeed rates are limited to 0.1 to 0.25 mm/min or 0.03 to 0.20 pm/rev, which are 10 times less than for conventional wheels. On stiff, purpose-designed machines, the dwell time is kept to a minimum, that is, zero dwell time is programmed in the CNC control, while the actual value is limited by the machine control response and inertia, which is perhaps 0.1 s. For a modern stiff grinder, the relaxation time is as little as 0.35 s, while for older, weaker grinders a dwell time of up to 2 s has been necessary to generate a round wheel. More comment is made on this in the discussion on dressing spindles (Chapter 15).
Efforts have been made to employ acoustic sensors for touch dressing. Care has to be taken to allow for the fact that contact has to be resolved in two planes. The dressing arrangement in Figure 7.40 was presented by Landis (Waynesboro, PA) at the IMTS (Tool Show) in Chicago in 1996 for dressing vitrified CBN angle-approach wheels. The acoustic sensor is mounted to the left
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of the diamond roll with diamond-free areas to relieve dressing pressure. The roll shape provided two orthogonal planes on which to touch the wheel. An alternative approach on more complicated forms is to have a wheel edge and side from which to touch.
Issues with system relaxation times and programmed dwell times, whether it is with conventional or CBN abrasive, inevitably lead to situations where the wheel has less than ideal sharpness. To overcome this, several methods have been applied to give a very brief period of contact. These rely upon sweeping the roll past the wheel in a sliding motion analogous to using a stationary block dresser. The method, with the dresser mounted on a linear slide, has been used on, for example, matrix grinders for ballnut grinding for the last 30 years. An alternative method is to have the dresser spindle mounted on a swing arm. The technique had advantages of offering the minimum contact time, but field reports suggest stability problems rough dressing conventional wheels. A problem with this approach is that numerous spark-out passes must be made to ensure the entire wheel face has been contacted and to eliminate chatter. This then becomes analogous to traverse dressing with a wide impregnated roll. Certain parts of the wheel are hit numerous times while other areas could go almost untouched.