Metal removal rates are faster today than ever before. What was considered high-speed machining a few years ago is now regarded as conventional. Two major factors in high-speed machining are sfpm (surface feet per minute) for the machine tool and ipt (inches per tooth) or inches per revolution for the cutting tool.For heavy metal removal, a high sfm and ipt rate will maximize metal removal. A good surface
finish is achieved by decreasing the ipt but maintaining a high sfm.
High-speed machining requires that the workpiece be rigidly held and supported. Positive stops can prevent movement in response to cutting forces. Cutter-induced vibrations have to be considered and eliminated, and additional mass in workholding can give the desired damping effect.
With the high cutting speeds of today, tooling also has to be safe at these speeds. The high centrifugal forces at 10,000 or 20,000 rpm can rip an insert away from its seat and turn it into a bullet. Whereas a simple setscrew clamp is sufficient to hold an insert at conventional speeds,
at high speeds a pin locking the insert to the seat is necessary. At such high M13_KIBB5087_09_SE_C13.QXD 6/3/09 1:52 AM Page 707
cutting speeds, coolant may not be needed. Under ideal cutting conditions the cutter creates the chip, and the heat created
is transferred to the chip. The chip carries the heat away and leaves the workpiece relatively cool. Because the chips are ejected from the cut so rapidly, heat does not have time to transfer to the workpiece. Often, an air blast directed at the cutting interface is effective in evacuating chips. Thermal shock needs to be considered when ceramic or harder cutting tools are used.
Coolant applied intermittently will cause premature tool failure. Effective high-pressure coolant systems operate between 1000 and 5000 psi; special nozzles direct the coolant to penetrate between the cutting edge and the chip, where temperatures often reach At these high temperatures, low-pressure coolant evaporates before it can reach the cutting edge.
Normally, stringy chips cooled by high-pressure coolant are more brittle and break up into the desired chip form. Benefits of high-pressure coolant use are extended tool life and improved surface finish. When applied through the tool, as in drilling operations, high-pressure coolant blows the chips out of the hole and cools the cutting edge. Pecking cycles to break the chips or clear the hole become unnecessary.
Because the feed rate in drilling is determined by the size of the tool, increasing production requires the increasing of cutting speeds. If the resulting elevated temperatures cannot be controlled, productivity suffers. Besides high pressure, a sufficiently high volume of coolant is needed. Volume required is approximately 10 gallons per minute (gpm) per inch of hole diameter. Pressures used depend on hole diameter:smaller-diameter holes use higher pressures.
Other factors critical in high-speed machining are the toolholder and the machine spindle. At speeds up to 5000 rpm the notches or setscrew on a toolholder normally do not affect the balance of the tool. At higher speeds the tool has to be balanced in the toolholder; even tool wear affects the balance and may cause vibration. Standard toolholders and spindles that meet design specifications and tolerances still
create problems. If the taper of the toolholder is at the minimum limit of its tolerance and the mating taper of the spindle is at the maximum
limit, the resulting difference will cause the tool to wobble. New dimensional standards use tighter tolerances and give a much better fit between spindle and toolholder.
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