High Speed Machining and NC Post-Processing
High Speed Machining has today become
one of the leading methods to improve machine productivity. The term High Speed Machining
(HSM) embraces high spindle speeds, high feed rates, high acceleration and deceleration
rates as well as high rapid traverse rates. Emphasis though, should not only be placed on
high speeds but also on high levels of precision and accuracy, working in conjunction with
the high speeds.
HSM (specifically milling) is, in some sense,
not very different from conventional machining. It involves the same process components as
conventional machining: feeds, speeds, depths of cut, cutting tools, etc. The difference
lies in the application of these components. In HSM practice, one is basically replacing a
few slower, heavy cuts with cuts that are faster and lighter, without stretching out but
rather shortening the production cycle time.
The goal for each cutting method
remains the same - to produce a work piece that is accurate and precise.
Using high spindle speeds and feed rates, however, can create some potential problems.
These must be addressed first before we can achieve our goal of manufacturing products
faster, while maintaining the desired precision.
With traditional spindle speeds at low revolution rates, the tool
can be seen almost as a stiff body ("almost" covers issues like deflection,
which is not analyzed here). However, this does not hold when the spindle speed is high.
At a particular revolution rate (characteristic to a specific, machine, spindle, tool,
etc.,) the tool will exhibit a self-excited vibration (chatter) and resonance
vibration. The result can be at minimum a machining
error and in the worst-case tool breakage or damage to the machine. To avoid errors
resulting from spindle and tool vibrations, one must ensure that the spindle is well
balanced. The chatter frequencies for particular machine settings should be known, in
order to avoid programming at such spindle speeds.
Let’s concentrate now on the situations related
to machining at high feed rates. As the machining velocity increases, the forces required
to quickly accelerate or decelerate moving machine components are increasing as well. The
result of this may be an increase of tool position errors, decreased motion repeatability
and increased wear of the machine's moving parts. Depending on the type of controller and
machine, errors of a different nature can emerge. With increased feed rates, an older
machine will tend to overshoot at sharp corners. A modern machine, equipped with an
adaptive controller, will produce undercuts at a sharp motion corner. As the feed
increases, an adaptive CNC will reduce the radius of the circle being cut (in fact the
radius change happens not only along circular arcs, but on any curved move, including
spline interpolation or a series of very short linear moves representing long, curved
motion). Any sudden changes in feed rate or in acceleration (jerk) will result in machine
body vibrations and some random position errors.
At each stage of the manufacturing process, there are certain solutions that can be
applied to facilitate HSM. For example:
1) Part Geometry Design.
When working with CAD systems and designing part geometry, try to create smooth surfaces
whenever possible.
2) Machining Process Design.
When working with CAM systems, to generate curved interpolation (NURBS, spline, 2D or 3D
arcs), use smooth transitions between zigzag pattern passes, etc as much as possible. Most
of the modern CAM systems can optimize the shape of the tool path for HSM purposes.
3) NC post-processing.
When generating an NC tape, apply all available features related to HSM.
4) Cutting the Part.
Use machines, controllers that support high-speed cutting and curved interpolation. If
this is not possible (e.g.: somebody owns already an older machine), use a NC
post-processor that will, off-line, prepare HSM-ready tape.
What is an NC
Post-Processor?
An NC post-processor is software responsible for
translating neutral instructions from a CAM system into the specific instructions required
by the NC machine. This software responds to the unique requirements, features and
limitations of the CAM system, NC machine and manufacturing environment.
High Speed Machining and NC Post-Processing
Since the NC post-processor converts CAD/CAM data
into machine controlling codes, it plays a pivotal role in the HSM process. First, the NC
post-processor must be able to generate NC codes controlling CNC HSM. While in HSM mode,
the controller will analyze the tool motions ahead. Based on the gathered information, it
will constantly adjust the feed in such a way that the amount of the position error is
always below requested minimum, while the machining time remains as short as possible.
An HSM enabled NC post-processor should be capable of outputting
NURBS interpolation (including linear and rotary axes, feed rates, spindle speeds and 3D
circular interpolation). With curved interpolation the cutting process is smoother,
causing less vibrations in the system, thus allowing for faster feed rates.
Curve fitting is another important feature of the HSM capable NC
post-processor.The fitting can be used either to convert piecewise linear interpolation to
curved motions or, to convert CAM generated curved motions to the type of interpolation
supported by the machine's controller. The last point is especially important with regards
to 5-axis machining. When converting a CAM motion (X,Y,Z tool tip part position and I,J,K
tool axis vector) to the NC tape (e.g.: X,Y,Z machine linear axes position and A,B rotary
heads) the application of inverse kinematics makes the output curve completely different
from one generated by the CAM system. The fitting process can come to help here, by
producing NURBS or other types of curves that can be understood by the controller. In
addition, with regards to better cutting conditions using NURBS or Arc Fitting has other
beneficial effects. First, it significantly shortens generated NC tape, thus making any
data transfer easier. Second, the possibility of arriving at data starvation conditions
decreases while sequences of short motions are combined into one longer motion.
Now, somebody can ask the question: "What can be done when your vintage machine and controller simply does not support
HSM?"
When this is the case, the NC program must be modified
off-line in order to emulate HSM or, even better, an HSM enabled NC post-processor can
generate HSM-ready tape. In the similar way as a controller does it, the NC post-processor
looks ahead to analyze tool path curvature, tool velocity, acceleration and other
parameters. Based on the analysis, information about the controller parameters, if
available - experimental data (overshoot table) and the required cutting tolerance, the
distribution of machine axes velocities is adjusted before the tape output is generated.
The adjusted feed rate allows the machine to maintain the required cutting accuracy while,
at the same time minimizing machining time.
At its basic level the greatest benefit of High Speed
Machining is to allow manufacturers of all sizes to produce a greater number of parts in a
shorter period of time, thus improving the all important bottom line. NC post-processing
plays an essential part in this process. It ensures that the high speed machining
functions of an HSM capable machine are utilized to the maximum. Through emulation,
machines that are not equipped with HSM capabilities are still able to take full advantage
of this feature.
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