|
What is Post-Processing?
In the early days of post-processing, a
post-processor was considered an interface tool between computer-aided manufacturing (CAM)
systems and numerically controlled (NC) machines - a mere translator, reading the
manufacturing instructions issued from a CAM system and writing an appropriate rendition
for a target NC machine. Today however, post-processing has evolved to include a dynamic
range of code optimization tools which are responsible for outputting the most efficient
and productive machine tool code possible.
NC post-processing is responsible for joining two very different
technologies, and it often serves to compensate for weaknesses on either end. Therein lies
the crux of the issue: a post-processor can enhance technology, or it can inhibit it,
depending upon its application.
To understand how a post-processor can enhance technology, it
helps to understand how and why post-processing evolved, how it has been traditionally
applied, and how the emergence of advanced post-processing systems has changed the way it
is used today. This article will show how post-processors can be used as key components in
factory automation
What is a Post-Processor?
Most CAM systems generate one or more types of neutral language files
containing instructions for a CNC machine. These are either in a binary format called
CLDATA or some ASCII readable format tailored after the APT language. APT is an acronym
for "Automatically Programmed Tools," software that accepts symbolic geometry
and manufacturing instructions, and generates CLDATA describing the manufacturing
operation in absolute terms. Some CAM systems provide a large degree of flexibility,
allowing just about anything to be included in the neutral file, others are quite strict
about what can and cannot be included.
At the other end of the equation sits the NC machine. It requires input
customized for the controller being used and arguably to a lesser extent, the operator
running the machine. Most important, the NC machine must be driven in a manner that
satisfies shop floor criteria, which are primarily based on safety, efficiency and
tradition.
Between these two lies the post-processor. The post-processor is
software responsible for translating neutral instructions from the CAM system into the
specific instructions required by the NC machine (Figure 1). This software responds to the
unique requirements and limitations of the CAM system, NC machine and manufacturing
environment. Therefore, post-processing is an important part of factory automation, as is
anything that lies on the critical path between the design engineer and the shipping
department.
A Historical Perspective
People often ask if post-processors are really needed, wondering if
perhaps the whole issue has been perpetrated on the unsuspecting by unscrupulous software
houses! In fact, there really isn't a conspiracy, just a lot of practicality.
International standards (ISO) as well as national standards (ANSI, EIA) define both an
output format for CAM systems and an input format for NC machines. These two formats,
output and input, differ significantly.
Why not one standard, one format? Standards are more often than not
based on existing practice. They serve to define a single method from a host of possible
choices, all of which are generally rooted in actual practice. Standards that go against
common practice do appear from time to time, but they are hard to justify, difficult to
create and slow to be accepted. They also require a lot more dedication and effort than
most people are willing to volunteer.
So when the proliferation of competing APT systems warranted a standard
to help define and control the format of its inputs and outputs, standards were created
defining the core elements required for manufacturing. Similarly, the proliferation of
controllers also demanded some uniformity, and NC control language standards were created
defining the core practices of industry.
What Might Have Been
But let us suppose for a moment that a single unifying solution had
been created in a reasonable time frame, and that a significant number of CAM companies
and NC controller manufacturers agreed to do things differently for the common good. What
then?
Time passes and CAM and NC vendors soon realize that a single unifying solution does
not account for competitiveness. There are at least three ways a new feature (such as
probing) can be brought to market in this environment. One is to revise the standard
first, then provide this feature to customers at a suitable point after the standard is
next published. The second is to provide the feature to customers first, then press for
standardization later. The third is to ignore any effort to standardize company
proprietary information and get the feature to market as quickly as possible.
No contest. The feature goes to market as quickly as possible.
Now things get a little more complicated. If the feature is an NC one,
how will the customer's CAM system access it, and vice versa? The standard has to be
extended on both sides of the interface to make the feature work. The CAM and NC vendors
must both agree to incorporate nonstandard functionality to allow access to this new
feature. Who will profit? Will both profit equally?
It would be more likely that some sort of pre-processor would be
required to change the output of the CAM system to satisfy the input requirements of the
NC machine. Besides, a pre-processor is probably already needed to handle binary format
conversions between the CAM system computer and the NC controller. Initially the
conversion will be simple, but as time goes on and deviations from the standard continue,
the conversion will become more complex, perhaps to the point where different
pre-processors might be required for different NC machines.
Who will provide the pre-processor, especially if both the CAM system
output and the NC machine input contain extensions to the standard? What happens when a
revised standard appears, or a CAM vendor leaves the market, or the computer manufacturer
tells you that the computer you are using is obsolete and not object-compatible with the
newest model?
Is this all starting to sound familiar?
It really makes no difference if the interface between CAM and NC is
unified or not. Market pressures will ultimately create incompatibilities, and software
will be necessary to bridge the gap. The only question left to answer is, what software?
Enter Post-Processing
Post-processors can do many other things besides translating CLDATA to
NC machine codes. For example a post-processor may summarize axes travels, feed and speed
limits, job run-time and tool usage information, which enables better selection and
scheduling of resources.
More sophisticated post-processors may validate the program before it
is run by the machine tool. There are many simple rules that a post-processor can follow,
with warning messages displayed when these rules are violated. Some examples: Noting if a
tool is not selected near the start of the program, warning when motions at feed rate are
done with a stopped spindle, flagging long series of positioning moves, or conversely,
flagging feed moves at or above the program clearance plane, or noting if diameter or
length compensation switches are not changed when a tool is.
Beyond simple validation comes correction. There are many situations
where a post-processor can detect an error and correct it. Examples include: cycles left
active during a tool change (they should be temporarily cancelled), selecting an incorrect
or nonexistent spindle gear range (the post-processor should select a range that supports
the speed), or specifying an unavailable coolant type (the post-processor should select
the next best type).
The best post-processors maintain a global picture of the entire job at
all times, using upcoming events to help make decisions about current ones. The NC
programmer uses this information to optimize the job without intervention. For example:
pre-selecting the next tool as soon as physically possible, segmenting a tape at a tool
change if the entire upcoming tool path will not fit on the current reel, selecting a
spindle gear that best fits the current and subsequent speed requirements, or switching
intelligently between parallel axes (Z and W) based on the types of upcoming operations
and available travel limits.
Post-processors can also work around limitations and bugs in the CAM
system or in the machine tool. It is generally far easier to change the post-processor
than it is to get a new revision of the CAM system, or a new executive revision for the NC
controller.
The important point to be made here is that the NC programmer should
not be concerned about machine tool or machine operator idiosyncrasies that do not
directly affect the production of a job. Wherever possible, good post-processors should
hide these details within.
Standard CAM systems, standard NC machines, standard CLDATA and
standard post-processor vocabulary can not all be mixed together to instantly produce a
working system. There are too many variables in the real world, and standards are too
restricted in scope, to achieve integration with off-the-shelf components.
Post-processors tie it all together, and good post-processors can do
this with a minimum of effort.
Post-processing works best when it is "transparent," in
other words the best post-processors are those that the user neither knows about nor cares
about.They quietly go about their work, only raising an alarm when warranted.
|
