The CNC Router market runs the gamut from hobbyist machines at
$5000, up to huge CNC machining systems that cost in excess of
$200,000. The idea is to find the CNC Router with the right price to
performance ratio within your budget.
Size and space requirements should be
decided upon before other more complex CNC features. Allotted shop
space in relation to a router’s work envelope can determine whether
a 14" x 19" tabletop model or a 59" x 120" CNC system with a moving
gantry are the right machine specifications to research.
After deciding on a machine’s footprint
(e.g., 109" x 149" x 60"), the element that greatly determines the
quality, durability and overall performance of a CNC Router is found
in its drive components. Basically, what method is used to move the
machine’s axes. Techno’s CNC Routers utilize THK rails and ball
screw drives, which provide smooth play-free motion, require minimal
maintenance, provide excellent accuracy and long life.
The placement of the ball screw is
in the center of the axis of travel, which eliminates the
possibility of racking (i.e., when the system twists due to
misalignment). This also ensures that the Techno machine does not
need to be realigned ever, causing no wear on the drive or carriage
system. Eliminating the downtime spent repairing damage from
racking, results in increased productivity and profits.
Some CNC Routers use other drive
systems, such as the rack-and-pinion gear drive. The racks are
typically installed on the outside of the machine, thus exposed to
the elements. As the machine cuts, debris collects on the rack.
These foreign materials get ground into the racks and gears, causing
more friction in the drive system which, in turn, causes wear and
makes the machine less accurate and unstable.
In a rack-and-pinion system, there are
typically two drive motors required to run the one axis (one on the
right side and one on the left side of the machine). The two motors
must stay completely in sync with one another. When these motors get
out of sync, racking occurs. Racking deforms the gears within the
system, wearing down the components, and the unit itself can be
jolted out of square.
The choice between what drive motors to
use first comes down to either servo or stepper motors. Servos are
typically the more expensive motor, but certain Micro- Stepper motor
options bring parity to the purchase price. The big difference
between the two motors is in how they run. Steppers, as the name
implies, have a set number of steps per revolution. Movement is
measured assuming that each commanded step has been completed. Most
steppers are run in what is called an open-loop configuration. This
means that the location of an axis is not verified on an ongoing
basis. The motor is commanded to move a certain distance and it is
assumed the move is successful without verification. This can cause
problems when excessive vibration or resonance from the
motor/machine construction can cause the stepper motor to lose steps
or even stall.
In contrast to steppers, servomotors
have constant feedback from the optical encoder. This device sits on
the back of the motor and keeps the controller informed of how far
the motor has actually moved. This constant feedback is used to
correct any discrepancy between a desired and an actual position.
This automatic corrective action results in faster cuts (up to three
times the throughput), and increased power (up to three times the
torque) at high speeds. The closed-loop nature of the servo also
ensures that stalling cannot occur unless there is an immovable
object in the path. When such an obstacle is encountered, the
closed-loop system would communicate to the machine’s controller to
shut down rather than lose position.
Servomotor resolution depends upon the
encoder used. Typical encoders produce positional signals (or
pulses) per revolution, and encoders range from 500 to 200,000
pulses per revolution. The more pulses there are, the finer the
resolution capability of the motor. Servos can perform high- speed
continuous motion much more reliably because of the constant
feedback from the encoder, making them much better suited to
applications requiring a high-end quality finish.
Geometrics
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