Feature Stories Archive
for Plastic Routing
article is the first in a two-part series designed to help
fabricators improve their machining of plastics.
companies make the transition from the routing of wood or
aluminum to the machining of plastics, there are a number of
preliminary procedures and considerations that can help ease
the conversion and ensure a smooth transition.
factors in the routing of wood and aluminum can become some
of the most significant aspects of plastics machining. Good
planning and preparation can help ease these factors and the
costs associated with the startup of a new machining
article is the first of a two part series that discusses the
need to have active preparation when making the transition
from wood routing to plastic routing. Part 1 discusses the
CNC router and its associated hardware. Part 2 will discuss
tooling and material selection.
of the CNC Router
maintenance of CNC routers is a critical factor for ensuring
a high level of precision and repeatability in finished
parts. These maintenance operations are defined by the
router manufacturers and are absolutely essential when
plastic parts are to be machined. Minor spindle vibration,
gantry or bridge shake and servo positioning errors
frequently have minor or unnoticeable impacts in wood but
can result in scrap or expensive finishing operations in
plastic. The severity of these problems are the direct
result of machine quality and adherence to the manufacturers
recommended maintenance schedule.
preventative maintenance, there are additional steps that
fabrication companies can take to help ensure a successful
first run. Listed below are some recommended actions to
consider before machining plastic.
spindle, spindle mount, and colleting system should be
checked for the amount of TIR (Total Indicator Runout).
Tools required for verifying TIR are a 0.001 inch or better
dial indicator, a magnetic indicator base, a 6 inches or
longer indicator stand assembly and a long shank solid
carbide tool, a blank drill rod or blank solid carbide
first verification should be made inside the spindle taper.
The router or spindle manufacturer should be able to provide
you with an acceptable upper limit for TIR. An acceptable
value is typically 0.001-inch TIR or better on older
spindles and 0.0005-inch TIR or better on newer spindles.
There should be no play in the radial direction of the
spindle at any time.
second verification should be with a rod inserted into the
collet. Measure the TIR at the furthest point from the
spindle. This measurement needs to be taken multiple times
with the rod being re-chucked and rotated after each
measurement. TIR is an additive property and can vary
depending on how the taper, collet, chuck nut, and rod
align. The maximum reading is an indication or true TIR. The
colleting system should be better than .002" total TIR for
older machines and 0.001-inch total TIR for newer
third verification is dependent on whether the routing
involves any surface milling, pocketing, or lettering. If
these operations are performed and require a high degree of
surface finish, the spindle should be verified perpendicular
to the work surface. This typically involves removing any
spoilboards and setting the indicator as shown in Figure 3.
By finding the amount of tilt the spindle mount has in both
the X and the Y-axis, it is possible to determine the degree
of apparent machining marks that will be seen on the bottom
of a pocket cut.
the dial indicator is mounted in the spindle and zeroed on
the main table surface, rotate the spindle 180° by hand
and record the amount of TIV (Total Indicator Variance)
along both the X and Y-axis. The larger the TIV, the more
delineation will be seen during parallel pocketing cuts.
This effect is exaggerated by the use of larger diameter
collets and collet mating surfaces should be examined and
cleaned. Well-used collets should be considered for
replacement even if they are not showing obvious signs of
wear. The following are collet replacement time
After 400-600 hours of runtime
A tool has broken in the shank
A tool has spun in the collet
A tool has been "short shanked" within the collet
The collet has been sprung.
the machining of wood where collet condition typically has
the greatest affect on tool life and breakage, collet
condition becomes apparent much sooner in plastics machining
where the product's edge finish rapidly deteriorates. There
are specially designed felt and brass brushes shaped for
cleaning the insides of tapers and collets, and these should
be used during every shift change, every manual tool change,
and every time a collet is changed.
are various chemical cleaning products available for routine
collet maintenance, and they do a good job of removing
buildup that brushes cannot always eliminate. Petroleum
products should be avoided due to their ability to attract
and trap dust within the colleting system. Alcohol, citrus
cleaners, and other formulations are good
systems should be evaluated for their ability to hold small
or thin parts. Many plastic sheet parts and/or thermoformed
parts are much more difficult to hold due to their size,
shape, and comparably light weight. By taking steps to
increase the amount of usable vacuum, fabricators can reduce
the amount of time spent on custom fixtures and typically
achieve higher feed rates with better cycle
through systems should be evaluated for:
Pump Size &emdash; 800 cfm or greater for a 4-foot by 8-foot
Spoilboard &emdash; lightweight, porous MDF with a
reasonable thickness and the edges sealed to reduce air
Supply Lines &emdash; evaluate for diameter and quantity.
Flow through systems benefit greatly from multiple large
diameter supplies. Consider using two or more 4-inch or
larger supply lines for each table.
(or dedicated) systems should be evaluated for:
Pump Size &emdash; 25 in Hg or better vacuum at full sealed
Spoilboard - should be channeled to provide best vacuum
dispersion and sealed all around to prevent
Supply Lines - multiple 1Ú2-inch diameter or larger lines
is possible to have both too much and not enough dust
collection at the same time. Dust collection systems serve
two purposes: to remove the chips from the work area and to
keep the spindle and tool cool. Dust collectors that are
under-powered can reduce spindle life and produce poor
quality finishes by not extracting chips from the cut path.
These chips and their associated heat can ruin otherwise
acceptable finishes. Over-powered dust collection with rigid
dust brushes can overwhelm vacuum hold down fixtures for
small parts and cause part movement or part ejection. Care
should be given to the evaluation of the role of the dust
collector for each job.
mist or liquid coolants typically are an unacceptable
addition for most CNC plastic routing applications, air
coolant systems should be seriously considered. A simple air
nozzle directed at the cutting bit can dramatically improve
cut quality and tool life. The air serves to cool the tool
and cut path, remove warm chips, and reduce the instances of
chip wrap around the cutter.
option is to use a cooling nozzle. These devices go by
various names (cool gun, cold gun, Venturi gun) and use a
venturi orifice to significantly reduce the temperature of
the air flowing from the nozzle. By using chilled air,
cutter life and cut quality can be considerably extended. A
drawback is that the velocity of the air is significantly
reduced. This reduces the ability of the nozzle to remove
chip wraps and requires that it be placed closer to the
router bit to overcome the air dispersal associated with the
evaluating the above machine factors before beginning a
plastic routing operation, the chances of success and
profitability can be significantly improved. The next
article will cover the topics of tool selection and material
selection before actual machining takes place.
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: firstname.lastname@example.org.
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Plastics Machining & Fabricating
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