POWER AND STIFFNESS REQUIREMENTS FOR ROTARY DRESSERS

A review of some commonly available dresser spindles, sold to machine tool builders and as retrofits for grinders previously using single point dressing, gives a good indication of the power require­ments and hence stiffness. Table 15.2 lists the capacity of various Wheel Dressing and Norton (Divisions of Saint-Gobain Abrasives) dressing spindles.

The first observation is that the dressers for roll applications tend to operate at speeds around 7 to 8 m/s, which for conventional applications gives a crush ratio of the order of -0.2. It is also possible to extrapolate guidelines for dresser power requirements for traverse and plunge rotary dressers. It should be noted that spindles handling up to 300-mm-wide rolls will in fact have a series of rolls and spacers and so the diamond-contact width will be considerably less. Based on this, one can estimate approximately 50 W/cm for counterdirectional plunge dressing of conventional wheels, which is four to five times higher than that predicted for a flat roll from Table 15.2 and Table 15.3, but in line with the expected higher power required for profiles and necessary safety factor. Based on this and additional data from the field, the recommendations in Table 15.2 for power requirements can be made.

Taking these data, estimates can now be made for spindle stiffness requirements and the limits thereof. For example, for a 10-cm-wide roll dressing a CBN wheel might draw up to

1.5 kW running at 25 m/s. This would generate a normal force, based on F/Fn = 0.2, of up to 300 N. If deflection is to be kept under 2 pm, then the system stiffness must be greater than 150 N/pm or 1.2 m lbf/in. Precision dressing spindles for form-roll applications have maximum stiffness values in the range of 1 to 2 m lbf/in. (125 to 250 N/pm) due to the nature of the bearing designs available and required arbor diameters. However, the sum of the remaining machine components is rarely 1 m lbf/in. except on a small number of specifically designed grinders.

For crush-forming, TVMK [1992] offers a hydraulic unit with a power of only 25 W/cm max, but a torque of 10.8 N. m rated for rolls up to 35 mm wide. Since most crush-forming operations occur at roll speeds of 1 to 2 m/s, the normal force created would be expected to be as high as 30 N/cm roll width. This is 50% higher than those for diamond roll dressing conventional wheels with a +0.8 crush ratio, and comparable to the highest dressing forces for CBN wheels indicated above. The advantage of the hydraulic drive is to provide the necessary high torque at low rotational speeds. The results also illustrate why crush forming is uncommon and limited to narrow forms for applications such as thread grinding. Most standard grinders do not have the stiffness capability. Also note, crush forming has a form accuracy of only about 25 to 50 pm, which is an order of magnitude less accurate than diamond roll dressing.