wear parts are made from mechanical plastics. Common
ones include bearings, gears, material-handling parts
and machine components such as spacers and positioning
mounts where the reduction of vibration is essential.
Traditionally, these types of parts have been fabricated
from metal. But mechanical plastics are beginning to
replace metal because of their increased durability,
excellent machinability, and exceptional mechanical and
electrical properties. Common mechanical plastics
include acrylonitrile butadiene styrene (ABS), Acetal,
Delrin, Hydex, nylon, polycarbonate, polyurethane and
polyethylene terephtalate (PET).
Router bits for cutting mechanical plastics have
traditionally been run on CNC routers at high spindle
speeds and feed rates. Extensive testing and years of
field experience have shown that a tool with a high rake
and low clearance performs exceptionally well. It
machines mechanical plastics more productively than
tools with other geometries and imparts a finer surface
finish (Figure 1).
This kind of free-cutting geometry is rarely used by
shops to machine mechanical plastics. Most use endmills
running on CNC milling machines.
Mechanical plastics are characterized as either soft or
hard. By looking at the chip produced, a machinist can
easily determine the flexibility or rigidity of the
material being cut. Soft plastic produces a curled chip,
while hard plastic produces a splintered wedge.
Generally, O-flute tools are applied to soft plastic,
while V-flute tools are used with hard plastic (Figure
Most wear plastics are made from soft plastic.
Consequently, O-flute tools are recommended for
machining most mechanical plastics.
tools are manufactured in straight- or spiral-flute
configurations. The choice depends on which direction
the user wants the chips to flow. Straight tools have a
neutral effect, while spiral tools can influence the
chips either upward or downward. (For purposes of
clarification, a downcut spiral is a lefthand spiral,
while an upcut spiral is a right-hand spiral.)
For the most part, routers with upcut, or right-hand,
spirals are applied because they effectively evacuate
chips. Downcut, or lefthand, spirals tend to recut
chips, which is not advantageous when cutting mechanical
plastics where chip welding may be a problem. However,
for part hold-down considerations and through-cuts,
left-hand spirals are a standard item.
spirals are available as single- and double-edge tools
in diameters ranging from 1.16" to 3.4". When machining
mechanical plastics, the single- edge O-flute spirals
impart a finer finish than multiple-flute endmills. When
small tool diameters are necessary, the single-edge
design, with its more open flute, accentuates chip
evacuation. In terms of balance, a maximum cutting-edge
diameter of 3.8" is recommended for single-edge tools.
If cutting tool balance is an issue or a deeper cut is
required, double edge O-flute spirals and 3-flute
finishing tools are logical selections. Both of these
types of tools can machine materials up to 31.8" thick.
Excellent finishes can be achieved when deep cuts of two
to four times the cutting- edge diameter are made at
aggressive feed rates. The double-edge O- flutes are
available with a low- or high helix angle to accommodate
a range of horsepower requirements. Also, high helix
cutting tools are advantageous in materials over 1"
Once the correct tool geometry is chosen, the proper
chip load is the next consideration. In
mechanical-plastics machining, the recommended chip load
range is 0.004 to 0.012 ipt, which results in an
excellent finish and acceptable productivity rates
(Figure 3). This narrow range imparts the finest finish
through the continuous generation of properly sized or
curled chips. Inadequate chip load can lead to knife
marks, which adversely affect the finish. O-flute tools
with a high rake and low clearance help eliminate knife
marks by slightly rubbing the part during machining.
Today’s CNC milling machines are more than adequate to
achieve the proper feeds and speeds for router tools.
Spindle speeds of 10,000 rpm and higher, with feed rates
in excess of 600 ipm, are not uncommon. However, when
these kinds of capabilities are not available or
feasible, router tools toleranced for machining
mechanical plastics can perform at spindle speeds of
6,000 rpm and proportionately higher feed rates. The key
is maintaining proper chip load to enhance productivity
and part finish.