A while back we talked about our tooling geometry and how our end mills can have up to 3 rake face in the cutting edge of the tool. As the tool enters the cut and takes the first 3-5 revolutions (and remember you can be running up to 18,000 Revolutions Per Minute) there is a tremendous amount of cutting force placed upon the tool and you can notice your HP meter jump as the tool enters the cut multiple times. This entry is when the torque curve on your CNC machine is the most important. Older CNC machines where designed to quickly reach peak torque between 1200 - 3000 RPM and then drop off. Newer, high speed machining centers hold constant torque.
Think of it like this. When a hydrofoil starts off from the dock it's not up to speed so it requires both HP and torque until the hull of the boat lifts off the surface. At that point a couple important changes happen in power consumption because the ship has "lifted" from the surface: The coefficient of friction has dropped dramatically. Similarly, once our tools are "in the cut" the chip formation "lifts" from contact with the gullet of the tool (the 'core diameter"). Horsepower is not quite as important as maintaining torque in the cut especially as you interpolate around a corner in a full slotting application.
|Spindle Torque Efficiency|