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The following are
a list of questions that are 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 manufacturers recommendations on cutter selection,
cut direction, and feeds & speeds, why will acrylic sometimes still
show heavy knife marks after cutting?
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 .002 and .010 of material in each
revolution and the desired surface finishes are in the neighborhood
of 20mInches. (That means the average differential in height between
knife marks is .00002!) 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 throughout 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 wreak havoc.
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?
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 differing
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 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 area for evaluation
is 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.
What is the expected lifetime of spindles and other components before
they affect the quality of the cut?
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 5HP to 10HP 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 its better to be safe
and 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?
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 setup so that their need
for large surface areas to hold on to is accomplished through the intelligent
use of scrap and appropriate 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 holdown 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.
The most important factors when designing sealed systems is 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. If the parts are very small or intricate, using channels as small
as 1/16 with 1/16 walls between them can provide adequate support with
good vacuum distribution. 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.
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