Quill Designs for CBN

The preferred approach is to ensure that the quill design has been optimized for use with vitrified CBN as illustrated in the Figure 18.6. The quill length should be reduced to a minimum and the

mounting diameter increased. The standard wheel screw used for conventional wheels should be eliminated and the wheel glued directly to the quill. This allows the wheel bore size to be maximized (note the lack of stiffness of the wheel bond itself). Stiffer quill materials may also be necessary and several materials are listed above. Carbide is up to four times stiffer than steel but is very brittle and is usually only used for the smallest wheels with very tight tolerances. Molybdenum is twice as stiff as steel and tough but expensive. Ferro-TiC is a less expensive alternative. Tungsten alloys with trade names such as No-Chat and Wolfmet have the same stiffness as molybdenum but cost a third as much. However, they are almost twice as dense and any runout from mistreatment will reduce spindle life.

For a cylindrical bar of radius, r, and length, l,

Discussions with machine tool builders suggest that if the calculated deflection during the rough feed portion of the grind cycle exceeds 80% of part tolerances, then the quill should be redesigned as indicated above. A crude estimate can be made using the equation above by either direct measurement of power or by estimation, that is,

• From spindle power (Assume Fn/Ft = 0.33)

W (newtons) = 3 x Power(watts)/ Wheel speed (m/s)

• From typical values Q = 5 mm3/mm/s and specific energy ec = 60 J/mm3

W(newtons) = 3 x 5 x 60 x part length (mm)/wheel speed (m/s)

Once the CBN wheel length-to-diameter ratio exceeds about 1.5, then increased attention has to be paid to the level of quill deflection. Especially for wheels under 10 mm it may be necessary to resort to a shorter wheel and stroking the bore as with alox wheels.

Quill length can also be a problem. Titanium has been used occasionally for extremely high rpm applications because its low density moves the resonant frequency of the quill above the rotational frequency. However, in general, the gains are relatively small. Figure 18.7 graphs the maximum recommended quill length as a function of spindle rpm. The center line represents steel while the slightly higher line is for titanium, and the lower line is for tungsten.