When
plans for North America's largest five-axis gantry machine were set in motion,
the challenge of establishing a massive machine with an X-axis three-quarters
of a football field in length was daunting, to say the least. This massive
machine was developed by Ingersoll
Machine Tools
for use W Industries, a metal
products manufacturer based in Detroit.
Since the spring of 2009, W Industries has been using the
Cybermill to machine huge, high-precision parts for aerospace and defense
applications, with quotes now out on many alternative energy jobs. Using the giant
Cybermill, W Industries is currently working on a program for the AirBus A350
for Spirit Airlines. This high-definition, high-tolerance project consists of
two long tools for the fuselage, both 16 x 70 ft, which utilizes about 1/3 of
the travel of the Cybermill.
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Big, But Precise
When
designing a gantry machine that is 20 ft wide by 204 ft long (with 8 ft under
its gantry), accurate, reliable linear scales at those long lengths are not
easily found. Heidenhain Corp. was approached
by Ingersoll to deliver these scales. The company delivered two, 72m linear
scales, which Heidenhain claims are the world's largest, to run down the length
of Cybermill's guideway, helping to enable around-the-clock operation.
To put the Cybermill's size into perspective, staff at W
Industries estimate that one could stack 78 Hummer H2 sport utility vehicles
(without tires) two-high, three-wide, end-to-end within its table. Within that
amount of space, many kinds of large structures can be machined, including
large molds, jigs, fixtures and composite tooling. All three of the
axes on the mill include linear measurements provided by Heidenhain's LB 382C
sealed linear scales (X axis is 200 ft, Y axis is 26 ft and Z axis is 8 ft).
Linear Measurement
Long-length machining jobs are fast becoming a forté of W
Industries, enabled by the company's precision manufacturing capabilities. "The
positional accuracy on these long Heidenhain linear scales is ±5 microns and
the repeatability is exceptional," says Martin Honer, a controls project engineer at Honer. "One of the reasons we chose
these scales were because of the LB scales' ability to carry distance-coded
reference marks."
The distance-coded reference marks (semi-absolute) on
the LB scales allow it to ascertain its position at startup very quickly. At
every startup, subsequent electronics find the absolute reference ("home"
position) after traversing a very small distance. Conventional machine
referencing involves physically travelling the machine axes to a fixed home
position that could potentially be many meters away. Distance-coded reference
marks speed and simplify such reference runs.
"On W Industries' Cybermill the X-axis homing cycle with
distance-coded reference marks can save more than three minutes compared to a
typical system (with a home limit switch at one end of the travel and the
gantry at the other end)," explains Honer.
Since
a combined total of more than 450 ft (150m) of these two LB distance-coded
linear scales had to be installed at W Industries, technicians from Heidenhain
assisted in their installation. W Industries staff mounted the housings and
each 72m scale tape, bearing surface and protective sealing lips over a
five-day period. The housings are dial-indicated and had to be within 0.004
(four thousandths) flatness and parallel. In addition, the scanning head and
bracket had to be mounted correctly providing for a 0.06 (60 thousandths) air
gap over the entire 72m.
Spindle Accuracy
"It
is important to note that the tools on the spindle of the gantry are moving on
some pretty big arcs and they too need to be very accurate," says Honer.
The
Cybermill's spindle head operates at 16,000 rpm with 50 kW of power. "This
amounts to cutting very accurately at approximately 1,000 inches per minute,"
explains Dave Hislop, W Industries' NC programming specialist.
To
handle the measurement of the angles within the spindle, two absolute
Heidenhain RCN angle encoders are used: one on the C axis (the joint between
the RAM and the red spindle head) and on the B axis (the joint at the tool that
allows the tool to angle). The B axis must move ±100 degrees.
"The
angular positional accuracy on the spindle head is 20 arc seconds with
repeatability of 10 arc seconds," says Honer.
Kathleen
Stoneski is the PR manager and technical writer for Heidenhain.