Makita HB350, Original Instructions Manual

Page: 7 / 155

6
MATERIAL AND CUTTING SPEEDS
•
The ease with which material can be drilled
is dependent 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.
•
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
millimeters 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
thecutting edge and excessive heat generation.
•
Slender and lo ng shank e d 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