Fast
track
Gautam
Doshi
highlights key cutting strategies for the successful implementation
of high speed machining to improve tool life and reduce tool breakage
Many
Indian companies have started purchasing
machines with high speed machining (HSM) capabilities to enhance performance
levels at the shopfloor. While, some have successfully implemented the
system, others have failed. In this regard, the machine tool buyer and
the process engineer of a company can ensure successful execution by
adhering to certain key strategies.
Success of HSM depends on several factors – high rapid and contouring
rates, accurate and stiff spindle, precisely balanced tool holders,
high performance cutting tools, and a high end CNC. In spite of these
factors being correct, sometimes efforts fail. One of the reasons behind
this is an improper understanding of correct cutting strategies.
MACHINING
STRATEGY
The machining strategy
in HSM involves making light cuts at high speed. This removes material
faster than heavy cuts at lower speeds. Light cuts reduce cutting forces,
distortions and vibrations. A high cutting speed results in hard materials
being cut. The chips carry away the energy and reduce thermal distortion
of a workpiece.
Failure in implementing HSM primarily results in poor tool life or tool
breakage.
A machining strategy often adopted is to run current tool paths with
higher speeds and feeds when implementing HSM. More often than not this
leads to failure.
To prolong tool life in HSM, the load on the cutter must be kept constant.
At the same time one must avoid changing direction of cutting because
this leads to slowing the feed rate due to acceleration and deceleration........
CUTTING
TOOLS AND TOOL HOLDERS
Cutting tools and
tool holders are critical and affect the tool life. The grain structure
of the carbide used gravely affects tool life. Several manufacturers
offer a sub micrograined carbide. This results in an increased structural
strength of the tool, greater resistance to chipping, and a greater
wear resistance as well. Though these tools appear to be costlier than
normal carbide end mills, the productivity increase justifies their
cost. A correct coating like aluminum nitride forms a protective aluminum
oxide layer on the rake face.
The effect of coolant is also important to understand.
When hard metals are machined, dry cutting is preferred.
Wet cutting invariably results in premature tool failure, as the coolant
never reaches the cutting zone in HSM.
Coolant pressure is also very critical; flood coolant simply does not
work. The tip fails due to thermal fatigue. The preferred solution in
cutting hard materials is dry cutting with air blast or oil mist..........
........CONTD
