Knowledge of both technologies is important for the selection of the right tool for the job.
"Perspectives" is a column co-authored by Terry Wohlers. The
following was published
in the January/February 2003 issue of Time-Compression
Technologies magazine.
When comparing the merits of rapid prototyping (RP) and CNC machining, opinions abound. RP and CNC machining both have staunch supporters that tout the benefits of their preferred technology. However, those who are familiar with both technologies generally agree that each has its own merits.
In the past 15 years, prototype development has changed significantly. In the early days, most RP technologies offered an obvious speed advantage, but were limited by accuracy and material property issues. Since the launch of RP, CNC has advanced, in part due to the competitive threat, to become faster while offering many well-known benefits. Likewise, RP has improved in areas such as accuracy, material properties and surface finish. As the technologies come closer in capabilities, the selection process becomes increasingly difficult.
Today, knowledge of both technologies is important for the selection of the right tool for the job. The following guidelines will aid in the selection process.
RP: Limited
Material development has come a long way. A broader range of materials and
material properties are available. These now include metals, plastics,
ceramics and composites. Yet, the selection is still very limited. And in
most cases, properties do not exactly match those of machinable, moldable
or castable materials.
CNC: Nearly unlimited
Machining centers are capable of milling nearly every material imaginable.
RP: 24 x 36 x 20 inches (600 x 900 x 500 mm)
Although commercially available units may not be able to handle an
instrument panel or bumper, the available build envelopes are suitable for
the majority of consumer and industrial products. Should a part be too
large for the system, it can be constructed in sections and glued. An
important consideration is the impact of size on time. Larger parts take
longer to build.
CNC: Large enough to handle aerospace parts
From small desktop units to large gantry systems, CNC machining can
fabricate parts and molds of virtually any size. Practically speaking,
size is only limited by the capacity of the available machine tools.
RP: Unlimited
If a prototype can be modeled with design software, it is possible to
build it with little impact on time or cost. The ability to quickly and
cost effectively produce complex parts is one of RP's biggest benefits.
CNC: Limited
CNC machining must deal with every feature in a part, and this can add
time and cost. As the complexity of the part rises, so do the number of
setups and tool changes. High aspect ratio features, deep slots and holes
and square corners can challenge the most expensive CNC milling machine.
While a five-axis mill and some ingenuity can overcome challenging
features, something as simple as an undercut can produce problems.
RP: Varies
There are situations in which RP can produce features that CNC cannot. For
example, RP can produce sharp inside corners and features with high aspect
ratios, such as deep, narrow channels or tall, thin walls, ribs and posts.
CNC: Varies
For many features, CNC has the advantage over RP. Sharp edges, smooth
blends and clean chamfers are among the details where CNC excels. This is
especially true when evaluating detail in terms of accuracy and surface
finish.
RP: 0.005 to 0.030 inch (0.125 to 0.75 mm)
While it is possible to achieve dimensional accuracy better than 0.005
inch (0.125 mm) on individual dimensions, 0.005 to 0.030 inch (0.125 to
0.75 mm) is the typical range of deviation. Accuracy is a function of the
RP system and the size of the dimension. As a dimension increases, so does
the inaccuracy.
CNC: 0.0005 to 0.005 inch (0.0125 to 0.125
mm)
With the right equipment, it is possible to machine at high precision.
While CNC in general is more accurate than RP, precision is usually
related to the cost of the machine.
RP: Low
Build any part on two different days and the results may vary. RP is
sensitive to many factors that affect the quality of the prototype.
Temperature, humidity, orientation and placement are just a few of the
parameters that can affect the repeatability of the output.
CNC: High
CNC is much more repeatable than RP. If the toolpath, tool and materials
are unchanged, the output is highly repeatable. Note, however, that
environmental conditions and the human factor can affect results.
Temperature and humidity, for some materials, can alter the outcome as can
the accuracy of the machinist's setup.
RP: Ra 100 to 600 ìin. (2.5 to 15 microns)
Without any secondary operations, some, if not all, surfaces will be
rough. While RP has progressed to layer thicknesses of 0.0005 to 0.001
inch (0.0125 to 0.025 mm) for some technologies, the layer striations and
stairstepping still affect surface finish. If desired, secondary
operations can improve the smoothness to any level, but this can alter the
dimensional accuracy of the part. Also, these operations can add time and
cost to the project.
CNC: Ra 20 to 200 ìin. (0.5 to 5 microns)
Unlike RP, machining can produce surface finishes suitable for prototypes,
patterns and tooling. As with RP, secondary operations (sanding,
polishing) can further improve the surface finish, but they may adversely
affect accuracy, time and cost.
RP: Moderate
For most technologies, reliability increases as the product matures. The
age of RP technologies ranges from one to 15 years, which means that one
can expect varying degrees of reliability. With less time and fewer
resources, some of the RP manufacturers have not had sufficient time to
refine the components of the systems to improve reliability.
CNC: Moderate to high
With more than 30 years of research and development, CNC is a mature
technology that is dependable and reliable. Over the years, continuous
improvement has eliminated system elements that diminish reliability.
RP: Minimal
With the exception of secondary operations (benching), RP requires little
labor. Within a few minutes, it is possible to prepare files for part
production and start the build. During the build, there is little or no
operator attendance.
CNC: Significant
CAM software applications have improved, but in most cases, they have not
eliminated the need for human intervention. Machine setup and operation
require an experienced machinist and it is rare that prototypes are made
in a lights-out mode.
RP: Minimal
The technology is certainly not a minimum wage position, but when compared
to machining, the demand for highly skilled talent is much less. This is
true in part because less labor is required. Additionally, RP has improved
in ways that take the art out of the process.
CNC: Moderate to high
Machining takes skill, creativity and problem-solving abilities. From
designing toolpaths and machining strategies to operating and monitoring
the cutting, machining is the work of experienced craftsmen. With
organizations getting leaner and the availability of machinists declining,
there could be a lack of available resources for prototyping work.
RP: Short to moderate
With less labor, fewer steps and an insensitivity to design complexity, RP
reduces leadtimes not only for the physical build, but also for the entire
process. Overall, the RP process is more efficient in both time and labor.
With RP, data can be received at 4:30 p.m. and parts delivered the next
morning; unthinkable for CNC machining unless there is more than one
shift. Yet, this does not mean that RP is the fastest for all parts.
CNC: Moderate
A lot goes into machining. Labor, toolpaths, fixturing, machine time and
materials are required. The result is that many jobs will take longer than
those done in RP. However, CNC can be faster when the design is simple and
straightforward, and when high spindle speeds and feedrates are an option.
Practically speaking, the selection of the best technology may come down to machine time availability. Often, a looming deadline will override any accuracy or material property requirements. This, in turn, reveals the true measure of what is important to the prototyping application.
So, what is important to that next prototype - time, quality or cost? Balancing these factors requires a thorough knowledge of both RP and CNC. With it, informed decisions - not biased opinions - can be made for the selection of the best technology for the application.