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MATERIAL AND CUTTING SPEEDS
• The ease with which material can be drilled
is dependant on several factors including
tensile strength and abrasion resistance.
Whilst hardness and/or strength is the usual
criterion, wide variations in machinability can
exist among material showing similar physical
properties.
• The cutting conditions can be dependent upon
requirements for tool life and surface finish and
further restricted by the rigidity of the tool and
work piece, lubrication and machine power
available.
• The harder the material the lower the cutting
speed. Some materials of low hardness contain
abrasive constituents leading to rapid cutting
edge wear at high speeds. Feed rates are
governed by rigidity of set up, volume of material
to be removed, surface finish and available
machine power.
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• It is preferable to set and maintain a constant
surface speed (RPM) for a given material and
vary the feed rate within defined limits.
• Machine feed is measured in inches or millimetres
per minute and is the product of RPM x number
of teeth in the cutter x feed per tooth. Too light
or excessively high feed rates will both cause
premature cutter failure. Heavy feeds on hard
materials will cause chipping of the cutting edge
and excessive heat generation.
• Slender and long shanked cutters are restricted
in feed rate due to deflection, and wherever
possible the largest and most robust tool must
be used. This is important for harder materials.
Steel up to 400 HB is the potential limit for
conventional M2 HSS tools.
Above 300 HB, cobalt alloy cutters should be
considered for increased tool life. In softer
grades of material, cobalt alloy cutters may give
increased output by increasing speeds and feed
rates by up to 50%. Tungsten Carbide cutters
permit surface speeds and feed rates up to twice
those for standard cutters.
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