By Van Niser
Routing Acrylic Plastic can be classified as soft or
hard for routing purposes. The characteristics of
each classification necessitates the use of
different router tooling with varying geometry.
Acrylic has the attribute of being either soft or
hard depending on the manufacturing process.
Extruded acrylic tends to fall on the soft side of
the equation, while cast acrylic is hard.
Manufacturers have designations for each type of
acrylic and the user should be aware of the
differences because it makes a huge impact on the
type of router tooling utilized.
Once the condition of the acrylic is
established, tool selection becomes a matter of
choosing a router tool that satisfies the needs
of the user.
Generally speaking, "O" flute tools (Figure 1)
are used for soft acrylic, while "V" flute tools
(Figure 2) are appropriate for hard acrylic.
These tools are offered in straight and spiral
upcut and downcut configurations, (Figure 3 and
Figure 4) and may be manufactured from
high-speed steel, carbide tipped or solid
High-speed steel and carbide tipped tooling with
steel bodies are more appropriate for hand fed
routing, while solid carbide is best utilized in
the more rigid and accurate environment of the
CNC router. The choice of straight flute tooling
versus spiral flute depends on the application
and the way the user wants to influence the
Straight flute tools have no influence, while
the spiral flute tooling can move a chip upward
Thermoformers tend to use straight or downcut
spirals because of the neutral or downward
effect on the part being routed, while sheet
fabricators prefer the upcut spiral, which
enhances chip removal.
Besides chip influence, the diameter and number
of cutting edges of the router tool have an
important bearing on the productivity and the
ultimate finish of acrylic parts. Larger
diameter tools with more gullet area inside the
flute of the router tools provide for faster
feedrates and accentuate stability, which has a
positive effect on the finish. Increasing the
number of cutting edges also influences finish.
There can be a significant difference in edge
finish from a single edge tool to a multi-edge
However, there are points of diminishing return,
and the user should weigh all factors when
choosing diameter and number of cutting edges.
The diameter should be large enough to
accommodate finish requirements, dissipate chip
and maximize yield.
A case in point is the three edge finisher
(Figure 5), which does an excellent job in cast
acrylic, but must be utilized in 3/8" diameter
or above to provide enough gullet space in the
flute for adequate chip removal. At the same
time, single edge tools should not be applied in
diameters larger than 3/8" because of balance
concerns. These general guidelines for tool
selection in acrylic can be further quantified
by visiting www.plasticrouting.com. This website
provides specific tool recommendations based on
a variety of sheet manufacturers offering
Feeds, Speeds And Chipload
After tool selection, the question of proper
feed and speed to best utilize the tool becomes
essential. In CNC routing, the user manipulates
the feed and speed of the router to arrive at an
acceptable chipload. The chipload (or thickness
of the chip) influences the finish of the part,
the life of the tool and ultimately the
productivity of the routing operation. A thicker
chip removes heat, the natural enemy of a
cutting edge, but too much chipload can
adversely affect edge finish. The ideal chipload
for acrylic seems to be somewhere between
0.004-0.012 to accommodate the issues of tool
life and acceptable finish requirements. To get
an acceptable chipload for a specific
application, the user should determine the
feedrate to achieve a 0.006 chipload (Chipload =
feedrate/(rpm x # cutting edges) and examine the
routed edge. If the edge shows a white or chalky
appearance or chips are rewelding, the feedrate
should be increased. If the edge finish of the
part has a rough, crater-like appearance, the
feedrate should be reduced. Since chipload is
determined by feedrate and spindle speed, the
spindle speed can be reduced to achieve
appropriate chipload. Regardless, the ultimate
goal is to find the proper chipload to achieve
the best tool life at a profitable cycle time
with excellent finished parts.
Rigidity And Concentricity
Tool selection and proper chipload are all for
naught if the operation is not conducted in an
environment which insures rigidity and
concentricity. In terms of rigidity, this
applies equally to the machine itself and to the
fixturing of the components to be routed.
The machine, whether it be hand fed or CNC, must
be maintained to manufacturer specifications.
Collets, collet nuts and tool holders must be
cleaned after each tool change. For proper
collet maintenance, collets should be replaced
every 400-600 hours of run time, and collet nuts
should be replaced as they show wear. Proper
machine maintenance allows acrylic parts to be
routed with a tool that follows the correct tool
path in a concentric fashion.
While machine rigidity and concentricity are
vital to consistent performance, fixturing is
equally important to surface finish on each
routed part. Fixtures should be rigidly built
and mounted to the work surface. Vacuum supply
should be oversized whenever possible and hard
fixturing should be securely mounted without
opportunity for movement. Also, friction
enhancements such as rubberized coatings and
gasketing sheet foam may be utilized. All these
elements will keep vibration to a minimum.
Rigidity and concentricity are the key factors
to improve productivity and edge finish through
the stability of the machine and the parts.
Specific recom-mendations in these two areas can
be found by visiting www.allstaradhesives.com or
by ordering the soon-to-be-released CNC
Production Routing Guide by Onsrud Cutter LP,
Specialty Tools For Acyrlic
Refinements continue to be made in the area of
edge finishing, shaping and drilling acrylic.
the past, these operations were mostly
accomplished with tools designed for the wood-
or metal-working industry
and did not have the proper geometry to accommodate plastic. Today, tools
are readily available with plastic geometry to
provide specialty cuts with outstanding results.
Edge rounding router bits are available in "O"
flute, "V" flute (Figure 7), or spiral "O" flute
configurations with single or double edges. The
general guidelines mentioned in the first part
of this article would dictate the choice of
flute configuration. The tools provide a full
radius on the edge of the part for an excellent
The solid carbide rout and chamfer bit (Figure
8) was designed to provide up to 1/16" top face
chamfer and a finished side edge on acrylic
This multi-faceted design allows the CNC user to
perform what was ordinarily a two step process
and complete the task in one pass without a tool
The bottom surfacing router bit in solid
carbide upcut geometry (Figure 9) provides a
swirl-free bottom in pocketing or lettering
The tool utilizes a near flat point with
radiused corners to create a smooth bottom
with aesthetically pleasing results.
In the area of drilling, a drill with a
60-degree point and flat rake face (Figure
10) provides fast plunging with reduction of
chip wrap, tearout and minimized lifting of
parts. The point reduces the stresses
introduced into the hole walls and will
provide a clean hole surface without the
clouding typical in standard jobber drills.
Acrylic is a widely utilized material that
machines effectively in hand fed as well as
CNC routing applications. The key is
choosing the right-tool-for-the-job, which
is based on knowing the condition of the
material and operating in a rigid
environment with proper chipload. When these
parameters are met, the two important
factors of edge finish and productivity are
achieved with excellent results.
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Plastics Machining & Fabricating
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