Numerical control is the automation of machine tools that are operated by precisely programmed
commands encoded on a storage medium, as opposed to controlled manually via hand wheels or
levers, or mechanically automated via cams alone. Most NC today is computer numerical
control, in which computers play an integral part of the control.
In modern CNC systems, end-to-end component design is highly automated using computer-aided
design and computer-aided manufacturing programs. The programs produce a computer file that
is interpreted to extract the commands needed to operate a particular machine via a post
processor, and then loaded into the CNC machines for production. Since any particular component
might require the use of a number of different tools – drills, saws, etc., modern machines
often combine multiple tools into a single "cell". In other installations, a number of
different machines are used with an external controller and human or robotic operators
that move the component from machine to machine. In either case, the series of steps needed
to produce any part is highly automated and produces a part that closely matches the original
CAD design.
History
The first NC machines were built in the 1940s and 1950s, based on existing tools that were
modified with motors that moved the controls to follow points fed into the system on punched
tape. These early servomechanisms were rapidly augmented with analog and digital computers,
creating the modern CNC machine tools that have revolutionized the machining processes.
Description Modern CNC mills differ little in concept
from the original model built at MIT in 1952. Mills typically consist of a table that moves
in the X and Y axes, and a tool spindle that moves in the Z. The position of the tool is
driven by motors through a series of step-down gears in order to provide highly accurate
movements, or in modern designs, direct-drive stepper motor or servo motors. Open-loop control
works as long as the forces are kept small enough and speeds are not too great. On commercial
metalworking machines closed loop controls are standard and required in order to provide
the accuracy, speed, and repeatability demanded. As the controller hardware evolved, the mills
themselves also evolved. One change has been to enclose the entire mechanism in a large
box as a safety measure, often with additional safety interlocks to ensure the operator is
far enough from the working piece for safe operation. Most new CNC systems built today
are completely electronically controlled. CNC-like systems are now used for any process
that can be described as a series of movements and operations. These include laser cutting,
welding, friction stir welding, ultrasonic welding, flame and plasma cutting, bending,
spinning, hole-punching, pinning, gluing, fabric cutting, sewing, tape and fiber placement,
routing, picking and placing, and sawing. Examples of CNC machines
Mills CNC mills use computer controls to cut different
materials. They are able to translate programs consisting of specific number and letters
to move the spindle to various locations and depths. Many use G-code, which is a standardized
programming language that many CNC machines understand, while others use proprietary languages
created by their manufacturers. These proprietary languages while often simpler than G-code
are not transferable to other machines. Lathes
Lathes are machines that cut spinning pieces of metal. CNC lathes are able to make fast,
precision cuts using indexable tools and drills with complicated programs for parts that normally
cannot be cut on manual lathes. These machines often include 12 tool holders and coolant
pumps to cut down on tool wear. CNC lathes have similar control specifications to CNC
mills and can often read G-code as well as the manufacturer's proprietary programming
language. Plasma cutters
Plasma cutting involves cutting a material using a plasma torch. It is commonly used
to cut steel and other metals, but can be used on a variety of materials. In this process,
gas is blown at high speed out of a nozzle; at the same time an electrical arc is formed
through that gas from the nozzle to the surface being cut, turning some of that gas to plasma.
The plasma is sufficiently hot to melt the material being cut and moves sufficiently
fast to blow molten metal away from the cut. Electric discharge machining
Electric discharge machining, sometimes colloquially also referred to as spark machining, spark
eroding, burning, die sinking, or wire erosion, is a manufacturing process in which a desired
shape is obtained using electrical discharges. Material is removed from the workpiece by
a series of rapidly recurring current discharges between two electrodes, separated by a dielectric
fluid and subject to an electric voltage. One of the electrodes is called the tool-electrode,
or simply the ‘tool’ or ‘electrode’, while the other is called the workpiece-electrode,
or ‘workpiece’. When the distance between the two electrodes
is reduced, the intensity of the electric field in the space between the electrodes
becomes greater than the strength of the dielectric), which breaks, allowing current to flow between
the two electrodes. This phenomenon is the same as the breakdown of a capacitor. As a
result, material is removed from both the electrodes. Once the current flow stops, new
liquid dielectric is usually conveyed into the inter-electrode volume enabling the solid
particles to be carried away and the insulating proprieties of the dielectric to be restored.
Adding new liquid dielectric in the inter-electrode volume is commonly referred to as flushing.
Also, after a current flow, a difference of potential between the two electrodes is restored
to what it was before the breakdown, so that a new liquid dielectric breakdown can occur.
Wire EDM Also known as wire cutting EDM, wire burning
EDM, or traveling wire EDM, this process uses spark erosion to machine or remove material
with a traveling wire electrode from any electrically conductive material. The wire electrode usually
consists of brass or zinc-coated brass material. Sinker EDM
Sinker EDM, also called cavity type EDM or volume EDM, consists of an electrode and workpiece
submerged in an insulating liquid—often oil but sometimes other dielectric fluids.
The electrode and workpiece are connected to a suitable power supply, which generates
an electrical potential between the two parts. As the electrode approaches the workpiece,
dielectric breakdown occurs in the fluid forming a plasma channel) and a small spark jumps.
Water jet cutters A water jet cutter, also known as a waterjet,
is a tool capable of slicing into metal or other materials by using a jet of water at
high velocity and pressure, or a mixture of water and an abrasive substance, such as sand.
It is often used during fabrication or manufacture of parts for machinery and other devices.
Waterjet is the preferred method when the materials being cut are sensitive to the high
temperatures generated by other methods. It has found applications in a diverse number
of industries from mining to aerospace where it is used for operations such as cutting,
shaping, carving, and reaming. Other CNC tools
Many other tools have CNC variants, including: Drills
EDMs Embroidery machines
Lathes Milling machines
Wood routers Sheet metal works
Wire bending machines Hot-wire foam cutters
Plasma cutters Water jet cutters
Laser cutting Oxy-fuel
Surface grinders Cylindrical grinders
3D Printing Induction hardening machines
submerged welding knife cutting
glass cutting Tool / machine crashing
In CNC, a "crash" occurs when the machine moves in such a way that is harmful to the
machine, tools, or parts being machined, sometimes resulting in bending or breakage of cutting
tools, accessory clamps, vises, and fixtures, or causing damage to the machine itself by
bending guide rails, breaking drive screws, or causing structural components to crack
or deform under strain. A mild crash may not damage the machine or tools, but may damage
the part being machined so that it must be scrapped.
Many CNC tools have no inherent sense of the absolute position of the table or tools when
turned on. They must be manually "homed" or "zeroed" to have any reference to work from,
and these limits are just for figuring out the location of the part to work with it,
and aren't really any sort of hard motion limit on the mechanism. It is often possible
to drive the machine outside the physical bounds of its drive mechanism, resulting in
a collision with itself or damage to the drive mechanism. Many machines implement control
parameters limiting axis motion past a certain limit in addition to physical limit switches.
However, these parameters can often be changed by the operator.
Many CNC tools also don't know anything about their working environment. Machines may have
load sensing systems on spindle and axis drives, but some do not. They blindly follow the machining
code provided and it is up to an operator to detect if a crash is either occurring or
about to occur, and for the operator to manually abort the cutting process. Machines equipped
with load sensors can stop axis or spindle movement in response to an overload condition,
but this does not prevent a crash from occurring. It may only limit the damage resulting from
the crash. Some crashes may not ever overload any axis or spindle drives.
If the drive system is weaker than the machine structural integrity, then the drive system
simply pushes against the obstruction and the drive motors "slip in place". The machine
tool may not detect the collision or the slipping, so for example the tool should now be at 210mm
on the X axis but is in fact at 32mm where it hit the obstruction and kept slipping.
All of the next tool motions will be off by −178mm on the X axis, and all future motions
are now invalid, which may result in further collisions with clamps, vises, or the machine
itself. This is common in open loop stepper systems, but is not possible in closed loop
systems unless mechanical slippage between the motor and drive mechanism has occurred.
Instead, in a closed loop system, the machine will continue to attempt to move against the
load until either the drive motor goes into an overcurrent condition or a servo following
error alarm is generated. Collision detection and avoidance is possible,
through the use of absolute position sensors to verify that motion occurred, or torque
sensors or power-draw sensors on the drive system to detect abnormal strain when the
machine should just be moving and not cutting, but these are not a common component of most
hobby CNC tools. Instead, most hobby CNC tools simply rely
on the assumed accuracy of stepper motors that rotate a specific number of degrees in
response to magnetic field changes. It is often assumed the stepper is perfectly accurate
and never mis-steps, so tool position monitoring simply involves counting the number of pulses
sent to the stepper over time. An alternate means of stepper position monitoring is usually
not available, so crash or slip detection is not possible.
Commercial CNC metalworking machines use closed loop feedback controls for axis movement.
In a closed loop system, the control is aware of the actual position of the axis at all
times. With proper control programming, this will reduce the possibility of a crash, but
it is still up to the operator and tool path programmer to ensure that the machine is operated
in a safe manner. However, during the 2000s and 2010s, the software for machining simulation
has been maturing rapidly, and it is no longer uncommon for the entire machine tool envelope
to be modeled accurately with 3D solid models, which allows the simulation software to predict
fairly accurately whether a cycle will involve a crash. Although such simulation is not new,
its accuracy and market penetration are changing considerably because of computing advancements.
Numerical accuracy vs equipment backlash Within the numerical systems of CNC programming
it is possible for the code generator to assume that the controlled mechanism is always perfectly
accurate, or that accuracy tolerances are identical for all cutting or movement directions.
This is not always a true condition of CNC tools. CNC tools with a large amount of mechanical
backlash can still be highly accurate if the drive or cutting mechanism is only driven
so as to apply cutting force from one direction, and all driving systems are pressed tight
together in that one cutting direction. However a CNC device with high backlash and a dull
cutting tool can lead to cutter chatter and possible workpiece gouging. Backlash also
affects accuracy of some operations involving axis movement reversals during cutting, such
as the milling of a circle, where axis motion is sinusoidal. However, this can be compensated
for if the amount of backlash is precisely known by linear encoders or manual measurement.
The high backlash mechanism itself is not necessarily relied on to be repeatedly accurate
for the cutting process, but some other reference object or precision surface may be used to
zero the mechanism, by tightly applying pressure against the reference and setting that as
the zero reference for all following CNC-encoded motions. This is similar to the manual machine
tool method of clamping a micrometer onto a reference beam and adjusting the Vernier
dial to zero using that object as the reference. See also
Binary Cutter Location Computer-aided technologies
Computer-aided engineering
Coordinate-measuring machine Direct numerical control
Design for Manufacturability for CNC machining Gerber format
Multiaxis machining EIA RS-274
EIA RS-494 G-code
Part program Wireless DNC
References
Further reading Brittain, James, Alexanderson: Pioneer in
American Electrical Engineering, Johns Hopkins University Press, ISBN 0-8018-4228-X.
Holland, Max, When the Machine Stopped: A Cautionary Tale from Industrial America, Boston:
Harvard Business School Press, ISBN 978-0-87584-208-0, OCLC 246343673.
Noble, David F., Forces of Production: A Social History of Industrial Automation, New York,
New York, USA: Knopf, ISBN 978-0-394-51262-4, LCCN 83048867.
Reintjes, J. Francis, Numerical Control: Making a New Technology, Oxford University Press,
ISBN 978-0-19-506772-9. Weisberg, David, The Engineering Design Revolution,
archived from the original on 03-09-2010. Wildes, Karl L.; Lindgren, Nilo A., A Century
of Electrical Engineering and Computer Science at MIT, MIT Press, ISBN 0-262-23119-0.
Herrin, Golden E. "Industry Honors The Inventor Of NC", Modern Machine Shop, 12 January 1998.
Siegel, Arnold. "Automatic Programming of Numerically Controlled Machine Tools", Control
Engineering, Volume 3 Issue 10, pp. 65–70. Smid, Peter, CNC Programming Handbook, New
York: Industrial Press, ISBN 9780831133474, LCCN 2007045901.
Vasilash, Gary. "Man of Our Age", Christopher jun Pagarigan Edmnton Alberta
Canada. CNC Infomatic, Automotive Design & Production. External links
Media related to Computer numerical control at Wikimedia Commons