How to Select a
Here are some questions to investigate prior to
making a buying decision.
the machine cast iron, aluminum, or polymer composite?
Cast iron construction offers a higher
level of rigidity and longerwear, but is
heavy. Will you move the machine around a lot? If you
will, consider aluminum, it is lighter and almost as rigid. The
polymer composites are light, also.
it use industry standard ISO G&M codes?
Fanuc® is currently industry standard in
the US and many parts of the world.
or servos, what's the difference?
The axis motor drive types on the market are called stepper and
servos. Servos are more accurate than steppers
and cost much more. The true servo
system strength is that the system checks its
position at each move against an independent measuring device,
such as a glass scale. This is a closed loop system. Steppers
are open loop systems executing a chain of commands
without checking their position against an independent
device. There is no question that servos are more accurate,
however, steppers could be adequate, it depends on how
repeatable and accurate your final product needs to be.
it provide Unlimited Program Lengths through drip feed
machining may call for more complicated, longer programs.
Drip feed allows longer programs to be run.
big is the work envelope?
This is the total area
that the mill can possibly cut. Perhaps a more
accurate definition could be the largest possible part that could
be cut. Is it big enough to accommodate the work you envision?
Many small CNC machines boast Y-axis travels of 4+ inches,
when in reality it is much less if a vice or stock over 2" high
is used. If you want to use clamps, t-nuts, vices, fixtures,
vacuum tables, etc., make certain they fit in the work
is the axis feed rate?
Feed rate is how fast a
machine can move while cutting stock. High
feed rates might be crucial to the success of your program, as
the production schedule usually must fit into a 50 minute class
period. For example, the Techno DaVinci's maximum machining
feed rate is 140 IPM (inches per minute), while some small
CNC mill's maximum machining feed rates are in the 16- 30
IPM range. You need to determine how long it will take to mill
the pieces you plan to make. If a CO2
car body takes 15-20 minutes to machine
at 80 IPM; at 16 IPM, one car could take well
over a class period to complete.
about spindle speed?
For nonferrous metals,
wood plastics, and prototype material, high
spindle speeds are recommended. Without high spindle speeds
on soft materials, the flutes on the endmills will load up with
stock and ruin the part. The only way to avoid gumming up the
cutters in soft materials at low spindle rpm is to lower the feed
rate. Is that a problem? See #6 above to determine if it is.