A recent report by machine tool company Makino found the average vertical machining center (VMC), even when it’s in cycle, isn’t cutting 30% of the time. And when it is cutting, it’s likely doing so much slower than it could. Factor in loading/unloading, setup, cleaning, tool maintenance, etc… and Makino found the typical VMC is only cutting 34% of the time.
JM Performance Products Inc. (JMPP), maker of the High Torque Retention Knob, says a major reason for the reduction in cutting time is the angle of toolholder expansion at the spindle.
Toolholder expansion generates vibration and chatter, increased run-out, poor finishes, tolerances, repeatability, increased machine and spindle wear and tear, and increased setup times. The collective impact of these issues is directly related to the number of parts that can be produced efficiently.
The average amount of toolholder expansion is 1/7th the width of a human hair. This nominal amount of expansion is enough to inhibit complete toolholder taper-to-spindle contact and reduce tool life significantly. The lack of contact is equivalent to the motion of a bell clapper in that the holder moves randomly within the spindle.
A toolholder should make 85% to 90% taper contact with the spindle for maximum rigidity, with solid contact at the gauge line. Retention knob induced expansion will prevent even dual contact toolholders from properly seating with the spindle. If it doesn’t fit the spindle precisely, a critical loose-tool factor occurs that can create the perfect storm for chatter, poor tool/spindle life, and decreased productivity.
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