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Pete 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 mills
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
solution.
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.
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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.
Mr. 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
problem-solving.
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.
Students 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 Machine
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 need answers.
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 and
science?
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.
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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 run.
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 is.
Something simpler?
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 important features.
Don't forget the computer program
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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