Preparing for Plastic
Routing - Part 1
As 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. Periphery 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 process. This 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
Preparation of the CNC
Routine 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. Besides 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
The spindle, spindle mount,
and colleting system should be checked for the amount of TIR (Total Indicator
Runout). Tools required for verifying TIR are a .001" or better dial indicator,
a magnetic indicator base, a 6" or longer indicator stand assembly, and
a long shank solid carbide tool, a blank drill rod or blank solid carbide round.
The first verification
should be made inside the spindle taper (see Figure 1). The router or spindle
manufacturer should be able to provide you with an acceptable upper limit for
TIR. An acceptable value is typically .001" TIR or better on older spindles
and .0005" TIR or better on newer spindles. There should be no play in
the radial direction of the spindle at any time.
The second verification
should be with a rod inserted into the collet. Measure the TIR at the furthest
point from the spindle (see Figure 2). 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 of true TIR. The colleting
system should be better than .002" total TIR for older machines and .001"
total TIR for newer machines.
The 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.
Once 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
All 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.
Onsrud Cutter recommends collet replacement at the following times:
- After 400-600 hours of
- 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.
Unlike 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 products 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.
There 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 alternatives.
Vacuum 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 times.
Flow through systems should
be evaluated for:
- Pump Size - 800cfm or greater for a 4x8 table
- Spoilboard - lightweight, porous MDF with a reasonable thickness and the edges
sealed to reduce air leakage
- Supply Lines - evaluate for diameter and quantity. Flow through systems benefit
greatly from multiple large diameter supplies. Consider using 2 or more 4"
or larger supplies for each table.
Discreet (or dedicated)
systems should be evaluated for:
- Pump Size - 25 inHg or better vacuum at full sealed vacuum conditions
- Spoilboard - should be channeled to provide best vacuum dispersion and sealed
all around to prevent leaks.
- Supply Lines - multiple ½" diameter or larger lines are recommended.
It 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
While 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 (see
Figure 4) 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.
Another 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 dust collector.
By 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