AFFORDABLE CAD/CAM SERIES
Sorinson, Technology Teacher, Lake Washington HS. Bob Koll,
Technology Teacher, Junita HS Kirtland, WA
First, Boeing's designers create the plan,
then the rest of the company works on the challenge of bringing their
vision to life.
For example, Boeing decided the way to make
the strongest possible wing spars (the internal structure of the wing) and
wing skins (the outer surface) was to do each as a continuous piece. This
design can withstand constant stress better than many previous ones.
Traditionally, wing spars and wing skins have been manufactured in many
small parts and then assembled. Every attachment point has a possibility
of fatigue and failure that is higher than the rest of the structure.
In theory, wing spars and wing skins
made in one continuous piece sound great, but they are two of the largest
parts on some of the largest airplanes built today. They require raw
blanks of metal measuring up to 105 feet by 21 feet. That's a lot of
aluminum! To get a perspective on the size of blank needed, a college
basketball court is 90 feet long and 50 feet wide.
To compound the difficulty, spars and skins
are also highly sculptured 3-D surfaces requiring precision tolerances.
The solution to the problem of accurately machining these huge pieces of
raw material took some innovative thinking and resulted in one of the
world's largest Computer Numeric Control (CNC) machines.
A standard CNC mill has a fixed length arm
extending over a table. The table can move the stock in two directions,
referred to as the X- and Y- axes. To cut a wing skin, the spindle would
have to hang over 12 feet from the column, losing precision and strength.
Boeing's answer was to build a completely different CNC mill
configuration, the gantry mill.
A gantry mill from Techno-isel. Note
that the Z-axis is supported by both sides, thus eliminating the problem
of deflection due to large countilevered overhangs found on standard CNC
Boeing contracted Ingersoll Rand to build
these gantry machines. Instead of one vertical column supporting the
spindle and cutter, there are two on the gantry. The spindle rides back
and forth and up and down on the cross bar, and the whole gantry rides
back and forth on rails embedded in the platform. The gantry's lack of
vertical column gives it much more flexibility for cutting and fixturing
various stock sizes. Gantry mills have long been a standard in industry
for cutting large parts, but monsters of this scale had never been made.
This design is so successful that 22 of these giant machines reside in
Boeing's Auburn Valley plant alone.
Theory and design are exciting, important, and
are often pushed to the forefront of manufacturing; yet the most important
issue that drives manufacturing is the product itself. The finished part
in hand generates the reward, and makes the whole process worth the
effort. The goal of producing the best plane on the market drove the
designers to make the specifications that forced a new manufacturing
When this attitude is applied to
educational projects, the results can be just as fruitful. Creation and
ownership are two successful student motivations. Instructors are
consistently successful in coaxing their students through long educational
processes when the end result is viewed as desirable by the student.
Over 1500 secondary and postsecondary
schools annually purchase tabletop CNC milling machines. Some schools have
five or six machines. Why? Because students want to own what they design
and produce. Many things that students can imagine and design are
difficult or impossible to make manually. Computer- controlled machines can bring
their ideas to reality. Teachers can effectively harness this tool to
motivate students and drive many divergent curriculums.
What Are Schools Doing With CNC Milling
Machines? First, the student uses CAD to create the geometry for a design.
Then they generate the toolpath and NC-Code for the CNC mill in the CAM
software. Finally, when the NC-Code is communicated to the CNC Mill, the
student's design is accurately cut out of an acrylic blank. Beyond
providing simple aesthetic satisfaction, these activities expose students
to high tech manufacturing job opportunities and are good practical
problem-solving experiences. Each week, for example, students can
concentrate on a different subject to make more sophisticated parts.
Pete Sorinson and his colleagues at Lake Washington HS, Kirkland, WA use
their CNC machines in a number of different courses. CAD class uses
it to produce the prototypes that are designed in the mechanical CAD
curriculum. Designing and producing a part on a CNC machine gives real
application experience for a mechanical CAD student. It is comparable to
architectural CAD students building a balsa stick frame house.
The Technology Exposure class is given the
challenge of making an assembly out of Legos that will perform a specific
task. But the solution requires the design and production of a missing
part. This part must interface with the standard Lego components. For this
exercise, students work in teams to learn group dynamics and
Or consider Mr. Bob Koll, of Junita HS,
Kirtland, WA. This year, he and his class were dissatisfied with the
wheels provided in their CO2 car kits, so they designed new wheels using CAD/CAM
software. They cut the wheel pattern out of wax on the CNC mill and
used a cold mold process to produce the wheels. They tried using plastics
of different resiliencies to get the performance they wanted.
and instructors get excited with the possibility of producing commercial
quality products on the CNC mill, and are creating articulations among
marketing, CAD design, and technology classes for the purpose of
establishing student companies to sell student creations. The CNC
mill allows intricate items to be mass-produced from a single design.
How To Buy An Entry-Level CNC Milling
So, your supervisor has given you the
go-ahead to purchase a piece of CNC equipment. Of course, as the euphoria
of the news wears off, you realize there are some serious questions that
First of all:
1. What are the educational objectives?
2. Will it meet the objectives?
3. Will it fit into the budget?
To help answer those questions, here are
some things to consider: Is this to be a precision machining program or do
you just want to explore the basics and integrate the curriculum with math
1. Is the machine cast iron, aluminum, or
polymer composite? Cast iron construction offers a higher level of
rigidity and longer wear, but is heavy. Will you move the machine around a
lot? If you will, consider aluminum, it is lighter and almost as rigid.
The polymer composites are light, also.
2. Does it use industry standard ISO
G&M codes? Fanuc® is currently industry standard in the US and many
parts of the world.
3. Stepper or servos, what's the
difference? The axis motor drive types on the market are called stepper
and servos. Servos are more accurate than steppers and cost much more. The
true servo system strength is that the system checks its position at each
move against an independent measuring device, such as a glass scale. This
is a closed loop system. Steppers are open loop systems executing a chain
of commands without checking their position against an independent device.
There is no question that servos are more accurate, however, steppers
could be adequate, it depends on how repeatably accurate your final
product needs to be.
4. Does it provide Unlimited Program
Lengths through drip feed capability? Precision machining may call for
more complicated, longer programs. Drip feed allows longer programs to be
5. How big is the work envelope? This is
the total area that the mill can possibly cut. Perhaps a more accurate
definition could be the largest possible part that could be cut. Is it big
enough to accommodate the work you envision? Many small CNC machines boast
Y-axis travels of 4+ inches, when in reality it is much less if a vice or
stock over 2" high is used. If you want to use clamps, t-nuts, vices,
fixtures, vacuum tables, etc., make certain they fit in the work area.
6. What is the axis feed rate? Feed rate is
how fast a machine can move while cutting stock. High feed rates might be
crucial to the success of your program, as the production schedule usually
must fit into a 50 minute class period. For example, the Techno DaVinci's
maximum machining feed rate is 140 IPM (inches per minute), while some
small CNC mill's maximum machining feed rates are in the 16- 30 IPM range.
You need to determine how long it will take to mill the pieces you plan to
make. If a CO2 car body takes 15-20 minutes to machine at 80 IPM; at 16
IPM, one car could take well over a class period to complete. How many
students do you have?
7. How about spindle speed? For nonferrous
metals, wood plastics, and prototype material, high spindle speeds are
recommended. Without high spindle speeds on soft materials, the flutes on
the endmills will load up with stock and ruin the part. The only way to
avoid gumming up the cutters in soft materials at low spindle rpm is to
lower the feed rate. Is that a problem? See #6 above to determine if it
If you are planning an exploratory program into CNC technology, the
questions you need to ask are somewhat different:
1. Are there easy-to-use and complete
curriculums available? The curriculum needs to be something you and your
students feel comfortable with and that will meet sound educational
objectives. Does it integrate math, science and technology concepts? Are
the suggested activities engaging to students? It might be a good idea to
recruit some student evaluators for this part.
2. Is the machine easy-to-use? Does it have
a "Machine Hard Home"? Does it require additional interface
cards to be installed or is it a direct RS-232 connection? A machine that
does all this will be easier for the instructor to supervise. Part offsets
can be saved as files and recalled quickly when the machine is turned on.
This will save valuable "on-task" time for students and
instructors. Besides, that's how it works in industry. Additional cards
that have to be installed in the computer limit the flexibility of being
able to use other computers to drive the mill.
3. Are limit switches on each axis for
greater safety and control? Is it fully enclosed with an interlocking
guard? Is it well lighted? Can you see the work in progress and still be
protected? Obviously, safety for the operator and for the machine are
Don't forget the
One final note: the CAM system should be full 3-D and include full 3-D CAD
functionality. It should be an educational and industrial standard, so
that support is available from other teachers and book publishers.
Carefully examine the CAM package. It is the interface to the machine.
Remember, the machine can only run what is sent to it, and that the
students will spend more time on the CAM package than any other component
of the system. Finally, while selecting your program's CNC mill, make sure
to talk to another teacher who is using that specific machine and find out
what works and what doesn't in his or her program. If you are having
trouble finding a teacher using the mills you are considering, ask the
manufacturer for schools that use their product.
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