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By Tony Moran
Vice President/Sales
ARRK Creative Network Corporation
San Diego, California
Combining stereo lithography apparatus (SLA)
with a CNC router makes it possible to produce injection molding
prototypes of a much higher quality than can be produced on SLA
alone. SLA is an outstanding method of producing geometrically
complex prototypes. The problem is that it's not capable of
producing fine details with the crispness associated with CNC
machining. Moving the part to a conventional CNC machining center
for touchup would be very expensive. So, ARRK uses a low-cost,
easy-to-operate CNC router to produce fine details on SLA parts
resulting in an unparalleled level of quality at an affordable cost.
The prototyping process is essential in
any industrial setting. Prototypes can be both visual and functional
or act as a visual medium from which marketing strategies can be
developed and product modifications can be made, prior to investing
thousands of dollars in a production tool. ARRK Creative Network is
the largest model maker and prototype manufacturer in the world.
Over 650 professionals take advantage of the latest technology to
provide outstanding quality, precision and service. The company's
services include stereo lithography, CNC machining, fabrication and
casting. ARRK has factories in San Diego, California; Troy,
Michigan; Thailand, Japan, and England, with sales offices
throughout the world.
While SLA has become an extremely
popular prototype manufacturing tool, it cannot produce highly
accurate models sometimes desired by industry. As a result,
prototypes produced by this method are often rejected for styling
reasons. CNC routers can dramatically improve the quality of
prototypes produced with stereo lithography. Modifications to the
SLA model can be made by machining the SLA material itself or
machining small pieces of ABS which can then be bonded into the
original model. These can be small logos or icons as well as any
overall modifications to the parts including the addition of ribs,
bosses, through-holes, etc. A typical example of this practice in
action is producing the bezel for an electronic component using
stereo lithography and using a CNC router to produce details such as
LED readouts. It should be noted that the option of cutting directly
into the stereo lithography part is preferable when it can be
accomplished because it eliminates the need to mount, flush and sand
the machined insert.
ARRK uses a low-cost easy-to-operate CNC
router, resulting in an extremely high level of quality.
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This technique can also be used to make
modifications to SLA parts in much less time than would be required
to produce a new part. In one case, ARRK made an SLA part from a
PRO/Engineer file. When the customer checked the fit of the
prototype, they decided to make several modifications and needed the
prototype returned the next day. This didn't leave enough time to
produce another SLA part. Therefore, ARRK bonded additional material
onto the prototype and machined it on the router to the new
specifications. The cost was only 15% of what it would have cost to
produce a new SLA prototype.
The key to the success of this process
is the recent availability of inexpensive, but very precise CNC
routers. ARRK has two three axis gantry routers from Techno, New
Hyde Park, New York. These routers have a positioning accuracy of
±.1 mm (.004 inches) in 300 mm and a repeatability of +0.01 mm. The
accuracy of the router is the result of several features inherent to
the table. Anti-backlash ball screws permit play-free motion that
makes it possible to produce circles accurate to the 0.0005 inch
machine resolution. These ball screws also make it possible to
produce parts as accurate as the machine resolution. The ball screws
have excellent power transmission due to the rolling ball contact
between the nut and screws. This type of contact also ensures low
friction, low wear and long life.
The Techno CNC router is able to finish
SLA parts and many other injection molding prototypes from scratch
at half the cost of doing the job on a conventional CNC machining
center. The router has a lower hourly rate because it requires a
small fraction of the investment of a machining center. Material
costs are dramatically reduced up to 50% by using a router for
modifications and for the creation of entire prototype models. The
models can be constructed by machining each wall and fabricating the
model together rather than machining the part of out of solid. This
method provides the same precision that can be achieved on a full
size CNC machining center.
ARRK typically receives geometrical data
from customers as either a native CAD file or an IGES or DXF format
neutral file. The company reads this file into one of several CAD
systems that they use including AutoCAD, Pro/Engineer and SDRC
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To produce an insert covering a detailed
area of a stereo lithography part, a section containing the detailed
area must be removed from the SLA geometry and transferred to a CNC
routing program. When the geometry is finished, the designers
produce an IGES or DXF file and import it into EZ/CAM, their CNC
machining software. This package produces a file that is read by the
MAC-100 controller used by the router.
Besides their use in finishing SLA
components, the routers also provide a unique method of producing
box-like prototypes such as bezels for electronic systems. Bezels
are normally produced on a CNC machining center from a solid block
of material. This is a very time-consuming process on an expensive
machine that typically requires a high degree of operator attention.
Machining parts from a solid block also is expensive from a material
standpoint.
The original model of the housing is
exploded into six separate walls. On a router, these walls are
produced from sheet material and bonded together to form a cube.
Material cost is typically reduced by 50% and machining time is cut
by about 33%. Finally, the hourly cost of the CNC router is
typically half the cost of the machining center. As a result, the
cost of producing the prototype is cut by about 50% without any
reduction in quality.
Producing bezels piece by piece also
provides the opportunity to enhance the finish of the prototype.
ARRK recently produced a business telephone bezel using this
approach. Making the bezel prototype in pieces made it possible to
polish the top panel so it was transparent and produce a second
panel that fits underneath it containing the buttons. This second
panel was silk-screened, adding a level of realism to the prototype
that would have been impossible to achieve with a solid block.
All in all, the use of CNC routers
provides major advantages for producers of injection molding
prototypes.
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