Feature Stories Archive
Troubleshooting CNC Router
The full productivity of CNC
machines will not be realized unless proper tooling is
Plastic fabricators purchase CNC routers only after a lot
of thought and a reasonably thorough study of what is
available in the marketplace. With many options and
configurations to choose from, the whole process can be
exhausting and intimidating for the first-time buyer.
Unfortunately, the process often stops as soon as the
machine decision is made, with no consideration given to
tooling. Once the CNC router is delivered, the tooling
decisions become critical to meet performance expectations.
It is far more expedient and productive to plan tooling
requirements when the CNC router is ordered.
Putting non-CNC cutting tools on a CNC router is much
like buying cut rate gas for a new Porsche. Sure it will
run, but it won't run well.
CNC router bits are a must for good CNC router
performance. There are a vast array of CNC router bits to
choose from depending on the plastic material to be cut.
Fine tuning the geometry of the solid carbide tool to find
the best tool for the job is often a matter of examining
results from various alternatives. Make several cuts with
spiral and straight flute bits with 1, 2 or 3 flutes and see
which one gives the best finish. Spindle speeds and feed
rates, also discussed previously, can be varied to provide
the best finish and productivity.
Problems may still occur, particularly when changing to a
new set up or upon receipt of a new batch of material. Here
are some troubleshooting suggestions that many have used
Proper Collett Maintenance
Heat is the biggest enemy of the tool, and the first place
the heat goes from the tool is into the collet. It is also
important to note that collets are made of spring steel that
will, over a period of time, lose its elasticity and harden,
making it increasingly tougher to tighten adequately. As
this hardening takes place, the steel does not fatigue
evenly and often causes the collet to grip tighter on one
side than the other, creating runout in the tool.
It is important to understand that if they are overrun
enough, this over tightening will eventually damage the
internal spindle taper, resulting in costly repairs. Because
it takes place over a period of time it is very hard to
notice, but a safe recommendation for collet life is in the
range of 400-600 run-time hours. This is about three months
in a two-shift operation of normal run-time averages,
probably much more frequent than one might expect. This is
not the absolute, but it stands to reason if you're running
a $75 tool in the machine and you're only getting half the
true tool life due to runout caused by bad collets, it
doesn't take many tools to justify the added expense of
replacing the collet. If collets are not changed, they will
eventually become brittle enough to crack or break,
potentially causing permanent spindle damage that could have
been avoided. Just like changing the oil in your car, this
is good preventative maintenance that should be done
Just as replacement is important, equally as important is
cleaning the collets each and every time the tools are
changed. Collets are in a brutally dirty environment and are
expected to perform a very accurate task while undergoing
extremes of heat and dirt.
As material is routed, whether it be plastic, wood,
aluminum, or another type of man-made board, the chips carry
with them many resins that migrate up the slits in the
collet and deposit themselves onto the inside end of the
collet ears, usually nearest the mouth of the collet (see
illustration below). This minute vibration is often the
cause for tool breakage when seen in the actual shank area
of the tool instead of down by the cutting edge. The
resinous deposit acts as pressure points, gripping the tool
tighter at the mouth of the collet. These pressure points
often distort the grip on the tool, creating runout. This
resin heats up as does the tool, and actually ends up
depositing itself onto the shank of the tool, often almost
gluing the tool into the collet, leaving brown marks at the
mouth of the collet contact on the shank.
These brown marks are a sure sign of collet neglect. To
prevent this problem, the resin must be removed from all
surfaces that it is prone to build up onto, using a
non-abrasive brass tube brush for the inside of the collet
and a mild solvent and rag for the external surfaces of the
collet and inside spindle taper. It is important to point
out that blowing out the collets does not get rid of the
resin, nor does soaking them overnight in thinner. A brass
brush is the best thing, along with some of the
non-flammable cleaners available. Do not use a
petroleum-based lubricant for cleaning as it will only act
as a magnet for all the dirt and dust by the residue it
Proper Colletting of the Tools
Collets come in two basic types, and their attributes and
peculiarities are important in the way in which they secure
the tool in the machine. Think of the spindle/collet system
as a chain. Just like a chain is only as strong as its
weakest link, so too is the collet's relation to the tool. A
high-performance tool can only perform if the collet is
properly maintained each and every time the tool is changed.
There are a number of things that are important to the
system. The first to be discussed are the two basic types of
collets themselves. Both types are not always available,
depending on the spindle type itself.
1) Half-grip collets (see below) are identified by their
slits that run from the bottom or mouth to about 80 percent
of the way to the top. This allows them to squeeze the tool
with a force primarily directed at the mouth or bottom of
the collet. These collets are often counterbored at the top
so as not to require the tool to fill the entire length of
the collet. This type of collet is the simplest of the two,
and is ideal for use in cases where tools sometimes don't
have a long-enough shank to fill the entire collet length.
2) Full grip collets (see above) are identified by their
slits that run from both ends, almost cutting the collet in
pieces. This type of collet tends to have more flexibility
and often comes in what is termed as "range collets," which
allow gripping a range of shank sizes. For example: 12-13mm
is used for 1Ú2-inch shank tools. This full-grip type allows
gripping over the entire length of the collet and requires
that, in order to be properly used, it should be 75-80
The most important portion of the collet is the mouth
which is at the bottom. This area is important because all
the lateral pressure taken by the tool must be evenly
distributed on all ears of the collet for it to cut true or
concentric. It is very critical that the 80 percent rule be
followed when using a full-grip collet due to the ability of
the collet to flare at the back if not full. The collet can
then allow tool movement in very minute amounts, oftentimes
resulting in tool breakage. There are times that this 80
percent is not possible due to shank lengths available, so
it is necessary to fill this void in the back of the collet
with a filler plug that is of the same size as the shank to
avoid the collapsing problem.
Equally as important as filling the collet properly, it
should also be understood that it is possible to overcollet
as well. This is when the "flute fadeout" portion of the
tool is allowed to extend up inside the collet. This does
not allow a firm equal grip by all ears of the collet at the
mouth. The tool can then have uneven support at the most
critical area and, oftentimes with solid carbide or
high-speed steel tools, the tool material is hard enough to
actually scar the inside of the collet, causing permanent
damage to the collet.
This can also be a common cause for tool breakage.
Breakage often results in permanent damage to the collet due
to intense pressures exerted, often either "burring" or
"mushrooming" the mouth of the collet.
Proper Tool Colletting
The only way to solve problems and make them go away is to
understand more fully what the real cause of the problems
truly is. Following the above prescribed tips and
information on collet maintenance and programming tips can
have a dramatic effect on part quality and consistency in
any plastic routing application.
The productivity promise of CNC routing can be realized
when all parts of the process are taken into consideration.
Proper tool selection, fixturing, spindle and collet
maintenance are all major links in the CNC routing chain.
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) 362-1560.
Plastics Machining & Fabricating
F: (847) 362-5028