Finding the Sweet Spot
There are a number of factors to consider when setting
up to machine plastics. Most importantly, plastic has
a higher thermal expansion rate than most other
materials, such as metal or wood
products. Thermal expansion is
the tendency of the material to
change volume in response to the
heat introduced during the formation
of chips created during
the cutting process. Reducing
thermal expansion is key to producing
clean and effective cutting
in any plastic cutting operation.
Following are some key tips to
reducing thermal expansion and improving your cutting
success. The first thing to consider is tool geometry and
quality. This is true for any type of cutting tool: router bits,
saw blades and wing-type cutters. For this article we will
be discussing router bits, but the techniques discussed
are applicable across the board.
Plastics fall into two main categories: hard and soft.
Harder plastics, or those with a higher durometer rating,
generally have a lower rate of thermal expansion. Softer
plastics will obviously have the opposite properties. Softer
plastics tend to be more difficult to cut due to the fact that
it will tend to “push” out of the way of the cutting edge of
the tool, increasing friction and thus heat, introduced to
the chip formation. When the threshold of heat exceeds
the melting point of the material the chips being formed,
the chips will melt causing a problem.
Generally you should select tooling with higher shear or
helix angle when cutting softer plastics. The higher shear
angle can reduce cutting pressure, thus reduce heat created.
However, factors such as material thickness and
hold down will limit the ability to introduce high
shear tools into the cut.
One thing Vortex Tool has introduced to their line
of plastics bits is a highly polished flute. High polish
dramatically reduces friction created during the cut
(Fig. 1). This will create cleaner cuts in most applications.
Selecting the proper chip load is another important
factor for proper cutting. Hitting the “sweet
spot” is one of the most critical factors for successfully
cutting. The window for the proper chip
load is much smaller for plastics than in any other
material. Typically, one or two thousandths of an
inch will make the difference for plastics where
wood, for example, can machine well across tens of thousandths
of an inch. Chips load is calculated using the following
Chip Load = Feed Rate (IPM)/Cutting Teeth X RPM
Chip loads (Fig. 2) for plastics are generally between .004”
to .008”. The important point is that plastics will have a very
small “window” of acceptable chip load and all conditions of
the cut will factor into what chip load will be successful in a
particular application. Learn to calculate chip loads in your
application and make small incremental adjustments to “dial
in” what works best for your particular application.
Tool deflection is
another factor that
needs to be addressed
in order to reduce friction
created at the cutting
site. Router tooling,
cutting tool diameter,
geometry and tool holding
all play a role. In
general, larger tool diameter will deflect less, thus reducing
tool deflection. Work with your supplier to select the best
tool geometry/diameter that will work for your application.
The tool itself is driven by the tool holding device. There are
numerous options for driving your cutting tool. The important
point is to realize that the more substantial your tool
holding is, the better the results you will see in cutting.
The final area to consider to improve your cut quality is
“Part Hold Down”. Again, the more rigidly the part is held,
the better your results will be. When fixturing parts it is
imperative that the part be held securely as close to the
cutting site as possible. This is especially true for thinner
materials. If the part can fluctuate due to part instability, it
can make the difference between a good cut quality and a
poor one. In a vacuum hold down situation you want your
gasket seal as close to the cut edge as possible. If you are
clamping parts, the same is true. Basically, the more you
invest in fixturing, the better your results will be.
In closing, to increase your cut quality in plastics there
are four things to concentrate on. First: select the proper
tool with the optimal tool geometry for your application.
Second: make sure that your tool is run at the optimal
parameters for your application (feed rate). Third: Review
your tool holding devices and make sure to optimize as
best you can. And fourth, make sure your parts are held
in the best way possible.
Written by Mike Serwa, Vice President of Vortex Tool Co.,
Inc. He has been designing and manufacturing cutting tools
for the wood and plastics industry for over 25 years. He
has written several articles for trade publications and regularly
gives tooling related seminars at Technical Colleges,
trade associations such as the AWFS Vegas and Cabinet
Makers Association as well as events sponsored by many
of the machine tool providers around the United States and
Canada. Vortex Tool Co., Inc. is a family owned and operated
cutting tool manufacturer located in Wisconsin and
produces a wide variety of cutting tools for the wood, plastics
and non-ferrous industries.
For more information, contact Vortex at
800-355-7708, Fax: 715-355-7353, E-mail: firstname.lastname@example.org, Web: www.vortextool.com.