The Design of Plastic
Routing and trimming has
become one of the most common operations performed during the manufacture of
plastic components and finished goods during the last ten years. CNC routing
has taken those operations to the next level and allowed plastic fabricators
to put a finished edge on products that previously may have needed further finishing
operations. Interestingly, this ability was not an original intent of the routing
industry, conversely routing was historically a means of quickly shaping and
cutting wood and aluminum with occasional forays into the plastic and plastic
composites market. With the explosive growth of market demand for thermoformed
plastic components, POP displays, and thermoset plastic goods, one router tooling
manufacturer began to develop tooling that was dedicated solely to the machining
As stated before, router
tooling was originally segmented into two market areas - wood and aluminum.
Wood tooling is generally a carbide tipped or solid carbide tool with cutting
geometry that allows the fibrous material being cut to be sheared off cleanly,
leaving no chips or grain fuzzing. The designs were further refined as applications
began calling for faster material removal rates, better finishes, or as new
wood composites began to enter the market place. Even with the dozens of specialized
tooling lines that service the wood industry and the hundreds of refinements
in cutter body material and shape, the basic underlying cutting geometry remained
the same. A few specific combinations of rake angle and clearance angle (see
Fig. 1 for definitions) in conjunction with the helix angle of the cutter (0°
for straight tooling and up to 35° for spirals), combine to yield the results
that the wood fabrication industry desires.
Similar results are true
for the machining of aluminum. Whereas the wood industry sees only material
variations such as density and moisture content, aluminum machinists typically
need only to worry about the hardness and temper of the part being cut. Using
high speed steel or solid carbide spirals, a few specific cutter geometries
machined almost all of the product being produced.
Plastics machining, on
the other hand, has completely changed the router industry outlook on cutter
design. Each plastic manufactured can exhibit different cutting characteristics
and may respond differently to different cutting geometries. This has led to
an explosion in the number of cutter styles offered to cut plastics as well
as the development of new technologies used in the manufacture and development
of the router bits. Because of the immense number of variables associated with
routing plastic (composition, thickness, temperature) and the continuing importance
placed on the ability to produce a finished edge without secondary operations,
it has become necessary to design tools that are extremely specific in their
A general discussion on
plastic cutting tool geometry can be started by dividing plastic into three
general categories: hard, soft, and reinforced. The cutters geometries designed
for plastic vary widely, much more so than their counterparts in wood and aluminum.
For this reason, it is much easier for the sake of discussion to break the plastics
up into categories that reflect how they actually respond to machining.
Soft plastics are routed by removing long, curly chips from the face of the
material being machined. (See Fig.1) Normally the release of these chips is
quite easy and there is little or no instance of burring or fuzzing at the edge
as seen in the comparable release of similar chips from wood or aluminum. The
nature of wood and aluminum necessitates that the wedge angle (see Fig. 2) of
the router bit cutting edge be large. This translates to a lower rake angle
and a lower clearance angle. If the wedge angle is reduced, premature wear of
the cutting edge occurs due to the abrasiveness and/or hardness of the material
being cut. With soft plastics, however, the abrasive and impact wear is greatly
reduced and the rake angle can be increased significantly, resulting in a much
easier release of the chip from the material. This allows faster feed rates
and less movement of the part due to cutting pressure.
The tradeoff of high rake
angle in a cutting tool is that it becomes very aggressive. If anyone has ever
used dedicated CNC plastic tooling in hand routers, they can attest to the fact
that it wants to "run" and can sometimes rip the router from your
grasp. The solution for this aggressiveness has been to change both the angle
and type of clearance put on soft plastic tooling. By using a low angle radial
(or eccentric) relief grind on the clearance angle (see Fig. 3), it is possible
to "calm" the tool down and allow the high rake angle to cut freely
while still maintaining control of the cutting tool. This radial clearance is
designed to rub ever so slightly along the cut surface and provide some stability
to the cutting tool. One or two degrees of too much relief, and the cutting
tool will begin to chatter. The resultant knife marks along the cutting edge
produce a subsequent poor finish. One or two degrees of too little relief and
the router bit will rub too much, producing heat and melting the material.
Additional factors in the
design of soft plastic tooling involve the removal of the chips once they have
been cut from the material. If the chips clog the passageway on their journey
out they will heat up very rapidly and cause poor part finish and premature
tool wear. The tooling design solution has been to increase the flute area the
chips are allowed to flow in by reducing the number of flutes (thereby increasing
the allowable flute opening) and by using "O" flute geometry. "O"
flutes allow the chips to form naturally and follow the natural flow of the
cutting geometry without hitting sharp corners that might slow their exit from
the cut passage.
Hard plastics machine much differently from their soft plastic counterparts.
The largest difference is in their production of chips. Those machining wood,
aluminum, or soft plastic are used to the sight of large chips ejecting from
the router bit path and having enough weight to actually carry for some distance
before landing on the router table. Hard plastic chips appear very different
and are normally very small shards that resemble crystalline fragments or dust.
Unlike soft plastic chips, hard plastic waste is formed by frequently breaking
small, individual chunks of material from the base material. (See Fig. 4) This
necessitates different cutter geometries from that seen in any other application.
Like soft plastics, hard
plastic tooling benefits from an increased rake angle that allows the material
to be broken away much easier than if you were using a lower rake wood or aluminum
tool. Unlike soft plastic tooling, however, the need for a dramatically increased
rake angle is not present. Because of the willingness of most hard plastics
to release their bonds in response to a sharp cutting edge, a moderate increase
in rake angle will usually produce the best results. Commensurately, the clearance
angle does not need to be lowered as much to control the tool and frequently
a straight relief angle is all that is required to control the tool and prevent
Hard plastic suffers from
the same chatter and melt problems as soft plastic and it must be controlled
through the same tight tolerances for rake and clearance angles held by soft
plastic cutting tools. Hard plastics also exhibit a cutting effect that is rarely
seen in softer materials which is "cratering". Because of the manner
in which hard plastic is machined, if the rake angle becomes too high, the tendency
for the material to break and release its bonds is greatly exaggerated and the
chips will actually pull additional material from within the cut edge leaving
a "cratered" or dimpled surface along the finished edge. By tightly
controlling the designed wedge angle of the cutting tool, this can normally
be prevented for a reasonable range of cutting speeds.
Whereas soft plastics respond
best to "O" flutes, hard plastics generally rout best with modified
"O" flute or straight rake face geometry. This combined with the smaller
chips produced, allow multi-fluted spirals to effectively cut the material with
a superior finish and good chip extraction.
Reinforced plastics are frequently a polyester, epoxy, or phenolic base with
either a fibrous or glass material woven or otherwise embedded to add rigidity
to the composite. While this can add significant strength to the material itself,
it causes it to be extremely difficult to machine.
There are two different
methods for attacking the tooling design problem associated with machining abrasive
plastics. The first involves using a high rake angle and high clearance angle
to allow the bit to cut freely and aggressively and reduce the amount of heat
built up during the cutting operation (this heat is a major factor of accelerated
tool wear in these operations). The adverse side to this is that the resultant
wedge angle is very small and a weak cutting edge is continually presented to
the reinforced plastic which can lead to chipping of the tool and a general
break-down of the cutting edge.
The other method employed
in the design of these special cutting tools is to present a very strong cutting
edge to the material by greatly lowering the rake angle and slightly decreasing
the clearance angle. This method reduces the chipping of the cutting edge but
can lead to tremendous heat buildup. The best application of these tools requires
decreased spindle speeds to reduce the material heating but this can lead to
increased cutting forces and cause part movement.
Machining of reinforced
plastics requires that great care be made when choosing one of these two tooling
types and that the spindle RPMs and feed rates are matched to the cutting tool
selected, as each requires different cutting properties and heat characteristics
to function best. Cutting tools typically consist of spirals and straight rake
face tools with either radial clearance (for low RPMs, strong cutting edges)
and straight clearance (for high RPMs, free cutting action).
The general groups listed before are just the beginning of the categories that
plastic cutting tools are designed for. There are many sub-groups that require
modification of the basic cutting geometry formula to take into account thickness,
temperature, fixturing concerns, as well as the combination of multiple materials
such as acrylic/ABS (a hard plastic and a soft plastic used in the many bathtubs
and liners), laminated phenolics (desktops and lab table tops), and co-extruded
PVC/ABS (fence posts).
CNC and non-CNC router
tooling for plastics has increased in both breadth and depth in the market place.
Router bit manufacturers must attempt to stay ahead of both the burgeoning plastic
development industry and the focused attention that CNC router manufacturers
have given the plastic fabricators. This trend will continue and the number
of application specific tooling will increase correspondingly along with the
growth of the demanding market. Continue to look for new innovations from the
leader in the router tooling market as both the quality and the speed of the
cut is increased in the next few years.