ROUTING: Plastic Routing FAQs
The following are questions commonly
asked during the setup or actual
machining of plastics.The topics
discussed relate to machine condition
and fixture design.The answers are
general in nature,but should provide
a starting point for further
optimization of the cutting processes.
After following the manufacturer’s
on cutter selection, cut
direction, and feeds and speeds,
why will acrylic sometimes still
show heavy knife marks after
A: The number of factors
that must be in perfect
alignment for a high quality cut
in acrylic is truly staggering. Consider the fact that a
router bit is typically gouging out between 0.002 inch and
0.010 inch of material in each revolution and the desired
surface finishes are in the neighborhood of 20 inches.
(That means the average differential in height between
knife marks is 0.00002 inch!) The geometry of the cutter
is what allows the knife mark heights to be reduced from
the chipload amount down to an acceptable surface finish,
but unless all other factors are perfectly dialed in, there is
little chance of success.
There are two critical factors
in maintaining the operational
characteristics required for
routing acrylics: fixturing and
machine condition. Unless the
fixturing is rock solid through-out
the entire cutting cycle,
any tiny movements of the part
can cause erratic finishes
throughout the cut. When
attempting to maintain a finish measured in the millionths
of an inch, a few thousandths of movement can
Machine condition is a more difficult factor to evaluate
and possibly fix. A thorough evaluation will involve examining
each portion of the machine and determining its
impact upon the cutting cycle.
Can machine condition be objectively evaluated
without the having the manufacturer visit or using
expensive tools such as laser alignment devices?
A: Absolutely. There are many checks that can be performed
that will give a good indication of whether
further action needs to be taken.
The first area to examine is almost always the tool clamping
system. Collets need to be in perfect condition and
replaced on a regular basis. These are not merely devices to
hold the tools during cutting, but are the primary means of
aligning router bits along a perfect centerline. Any out-of-round
condition will cause multi-fluted tools to remove dif-fering
amounts of material with each cutting edge. When
this occurs it is impossible to form a homogenous finish
across a machined surface. Single edge tools will also show
degraded cutting characteristics and will be extremely sensitive
to feed rate fluctuations due to the constantly changing
chipload present with an out-of-round tool.
All mating surfaces from tool-changing taper through clamping
nut need to be cleaned every shift and examined for wear
indications. Broken shanks on tools or severe crashes should
always be reason for a detailed examination of the collet and
probable replacement. Collets should be considered perishable
tooling just as the cutting tools are, and should be
replaced on a regular basis. Regular evaluations of the concentricity
of the clamping system should be performed with a
dial indicator and plug gage or cylindrical round. Remember
to take multiple concentricity readings and to disassemble and
reassemble the chuck between each reading. This will ensure
an accurate determination of repeatability and accuracy.
If the colleting system is in good condition the next
areas for evaluation are the spindle and drive systems.
There should be no play within the spindle bearings and
the spindle mount should be solid to the back plate. Any
movement here should be cause for concern and may
require remounting of the spindle or a rebuild of the bearing
packs. If the spindle is in good shape, check the axis
movement next. A quick check is to turn the drive motors
on and check for play. There should be no room for movement
in any of the axes when the servos are engaged. If
there is any play, most likely the nuts on the ball screws
are showing signs of wear and there will be backlash present
in the system. This can lead to unsteady movements
particularly during acceleration and deceleration that will
show up as poor finishes on the part.
A final check that can be an effective demonstration of
machine condition is to make a series of cuts at various
locations on the vacuum table. Straight cuts along each
axis, a diagonal cut across both the X and Y-axis, and a
large diameter and small diameter circular cutout can provide
a very good indication of how smoothly the table and
head are traveling as well as how well the axes are performing
in simultaneous movement. If any of these cuts
appear significantly worse than the others, a more
detailed examination of the involved axes is required.
Making these cuts at various locations on the table will
allow the operator to see if there are problems at the
extreme ends of the travel distances
or if one particular portion of the
ball screws or ways has been worn
out due to repeated use.
Q: What is the expected life-time
of spindles and other
components before they affect the
quality of the cut?
A: This is an impossible question
as it pertains to machine
components. Ways, ball screws and
ball nuts are constantly being
improved and are showing longer
and longer service life. The same
can be said of spindle components
and the bearings within them. The
one item that has remained relatively
constant over the last five years is
the collet. Shops that have the best
success typically replace their collets
every 400 – 800 hours. If this
number seems extremely low,
remember it is the only flexible
piece of equipment in the entire
machine and it is expected to maintain
tolerances in the range of a few
ten-thousandths of an inch over
repeated cycles. Using 5-HP to 10-HP
motors to drive solid carbide
tooling with a piece of spring steel in
between seems like an awfully tenuous method of achieving
premium finishes. For this reason it is better to be safe
and to maintain all of the clamping components in optimum
conditions at all times.
What is the best way to fixture small or intricate
parts such as letters or logos?
A:This is always a difficult question without specifics,
but there are some options to try. First decide
whether to use universal (flow-through) or dedicated (discreet)
vacuum. This will determine the best methods of fixturing.
Universal vacuum systems can be set up so that
their need for large surface areas to hold onto is accomplished
through the intelligent use of scrap and appropri-ate
tool diameters. Typically small diameter tools (those
causing less lateral pressure on the part) in conjunction
with skin cutting or tabbing have had the best success
rates. Low vacuum loss is essential in this type of configuration
and it is not uncommon to see problems with hold-down
as a large panel is gradually machined into smaller
parts. Skin cutting to the masking can eliminate vacuum
loss, as can the use of scrap pieces to gradually cover
cutouts and reseal the cut paths as the program advances.
It is also essential to seal the edges of the universal vacuum
spoilboard and any unused areas on the board face.
Dedicated systems offer some challenges to small part
routing but can be a more effective method than universal
systems if designed properly. It is important when designing sealed systems to completely eliminate any areas of
vacuum loss, increase the frictional contact area of the part
to the board, and to have a spoilboard that distributes vacuum
over as much of the part surface as possible.
To reach the optimum operating conditions, it is a good
idea to use some type of sealant on the spoilboard surfaces
to prevent vacuum loss. Good results can be
achieved with enamel or latex primers and paints as well
as rubberized coatings. The rubberized coatings can also
add additional frictional hold down capability with minimal
effort. Using channels is typically the best method of
distributing vacuum throughout the part while maintaining
an adequate amount of material/spoilboard contact
area. Using spoilboards without channels or using gasket
tape on top of the board without recessing it can cause
long-term headaches. The tape will typically wear faster
due to the higher compression amounts and the lateral
stresses placed against it. In many cases the part will also
warp toward the vacuum supply and slightly deform itself
or seal the vacuum supply hole.
If you would like to contribute a question or topic for a
future article, please submit it to firstname.lastname@example.org
or fax it to 847-362-5028.
Van Niser is Director of Plastic Application Engineering
at Onsrud Cutter. Readers are invited to send questions
to Van Niser at Onsrud Cutter, 800 Liberty Drive,
Libertyville, IL 60048, E-mail: email@example.com.
For more information, click on the Author Biography link at the top of this page.