Feature Stories Archive
Machining Plastics: Optimizing
Cutting Tool Productivity
Use of the correct tool design and
material can turn production processes into
Editor's note: This is the first
in a series of articles on cutting tool optimization. Later
articles will address troubleshooting techniques and cutting
tool solutions for specific materials.
Productivity -- it's the name of the game. Because of
this, routing has become the preferred method of material
removal with practically all plastics. But to achieve
productivity, two things must occur: The process must be
performed quickly and efficiently, and the finish must be
acceptable. To achieve quality production, factors such as
the type of plastic being cut, cutting tool materials and
design, tool wear, fixturing and colleting, need to be
Cutting tool materials
Four groups of cutting tool materials will be considered
for this analysis: high-speed steel, carbide, ceramic and
High-speed steel (HSS) has been a low-cost, yet highly
productive tool material for cutting both rigid plastics and
flexible plastics. It is particularly well suited for
hand-held air or electric routers because of the shank
strength and resistance to breakage.
Typically, HSS's tool life is not as long as carbide,
particularly in cutting Group R materials. However, there
are several factors that can work in HSS's favor if the
setup is difficult to fixture and the operating environment
It is worth noting that HSS will not perform on
Carbide, on the other hand, is a much more dense and
harder cutting tool material than HSS and must be used with
Group F materials. Carbide-tipped tools -- where carbide is
brazed to tool steel at the cutting area -- are also used to
cut Group R materials. Solid carbide tools -- where the tool
is ground from a single piece of carbide-- can be used to
cut all groups effectively.
The downside of carbide is that it is more expensive than
HSS; solid carbide bits are roughly three times more
expensive than HSS, and carbide-tipped is roughly twice the
price. However, the trade-off of when to use HSS or carbide
with Group R is one of tool life and feed rates.
When machining Group X, the decision between carbide and
HSS also becomes one of price/performance and whether CNC or
hand-held machines are used.
Ceramic and diamond tooling is worth consideration when
carbide's tool life is not acceptable. This is typically the
case with more exotic formulations of Group F. (See figure
There is a theory that states, "If a cutting tool is
tough enough to cut steel or wood composites, certainly it
will cut plastics."
Wrong. This fallacy is perhaps the single biggest reason
there are so many routers cutting plastic materials today at
less than 50 percent potential productivity. Each plastic
category, as well as many specific sub-categories, demand
different tool geometry for optimum router performance and
The design of the flute and the included angle of the
cutting edge are what differentiates a plastic router bit
from a generic cutting tool. Plastics have a unique
characteristic in that chips have the ability to reweld or
stick themselves back on to the piece part or cutting tool
once cut. Tool geometry must be designed to exit the chips
away from the cut, not only to resolve this problem, but to
protect the finish.
Multi-flute solid carbide spiral tools are often
preferred for machining fiber-reinforced materials, if a CNC
router is being used. If hand routers are used, the
least-cost alternative is a carbide-tipped tool with plastic
Rigid plastics are most often cut with straight flute
solid carbide or high-speed tools with Z flute plastic
geometry. Carbide-tipped tools are also used where cost
effective. Tipped tools, however, are limited in their
geometry when compared with HSS or solid carbide. Acrylics
can be cut very effectively with a new concept in plastic
router tooling called three-flute finishing tools.
Flexible plastics are often most productively cut with
single-edge O flute tools made from HSS or solid carbide.
These flutes may be straight, shear or with a very slow
Heat is the enemy of tool life. Preventing the tool from
becoming hot will lead to longer life.
The first step in accomplishing this is to match the
proper tool material to the type of plastic to be routed.
The second is to select a cutting tool with the proper
plastic geometry for the material. It is essential that this
tool has a sharp edge to reduce heat. As the tool edge
dulls, it will slow down in the cut and the finish will
The third principle is to take big chips -- get the heat
out with the chip. This is accomplished by increasing the
feed rate and/or lowering the spindle speed until the finish
is no longer acceptable. Then, slow the feed rate and/or
lower the spindle rpm until the finish is acceptable again.
Do not dwell in the cut. Always feed the tool when it is in
contact with the plastic being cut.
Slow moving tools recut the same surface and further cut
the chips as they exit. Both generate heat. And each time
the tool impacts against the material, it has one less
revolution in its life.
Other tool life considerations are the rigidity of the
router, the rigidity of the setup and fixturing. Material
that moves when it is cut, for whatever reason, will cause
premature tool failure. Rubbing will cause both vibration
and heat. Stacking thin materials also makes an investment
in good fixturing pay off, provided the volume is
Many users select tools without regard to the importance
of adequately holding them in the collet. Both CNC and air
routers need 80 percent of the collet to be filled,
preferably with a full shank, if the tool is to perform
consistently. Stay off the flute fadeout area of the bit as
much as possible.
Collets should be changed every 500 to 700 hours run
time, on the average. Over time they can lose elasticity and
harden, which in turn leads to runout, and in time, uneven
cutting action and poor finish.
Equally as important as changing the collet is cleaning
it with a wire brush, each and every time a tool is changed.
Chips, chemicals and resins, all made fluid by heat, can
migrate up the slits in the collet and deposit themselves
wherever they find an opening. If not removed at each tool
change, these deposits will continue to build up, resulting
in runout and tool failure. The key clue is this: If brown
marks on the shank of the tool become visible at the mouth
of the collet -- problems are about to begin.
CNC routing has made possible new advances in tool life.
Feed rates, multi-axis cutting action, spindle balance,
rigidity and horsepower all have been improved. However, the
right tool must be in the spindle to achieve design
Van Niser is director of Plastic
Application Engineering at Onsrud Cutter. Based in
Libertyville, IL, Onsrud Cutter is a manufacturer of a wide
range of cutting tools for the plastics industry. For more
information, Niser can be reached at (847)
Plastics Machining & Fabricating
F: (847) 362-5028