Aerospace Machining Solves Challenges Big and Small
From machining monolithic parts for airplane wings to turning small jet engine parts, machining continues to advance the art of creating better, more affordable parts.
Aerospace machining encompasses machines
small and large. These range from the Tornos SwissNano to the Makino
MAG3, as Rich Sullivan put it. He is the OEM manager for Iscar Metals
Inc., Arlington, Texas. The first machine has a footprint under 8 sq.
ft. (0.74 sq. m) and makes parts under 4 mm in diameter. The latter
could put eight SwissNanos on the pallet.cnc precision machining parts
Yet for any size part, cutting tool and machine tool suppliers are developing technology to make those parts faster and better. The challenge for users is finding a machine that meets tomorrow’s needs as well as today’s.Aerospace manufacturers have two fundamental conflicts: increasing material removal rates (MRR) while meeting tighter tolerances. That is the perspective of Scott Walker, chairman of machine tool builder Mitsui Seiki (USA), Franklin Lakes, N.J. The need for high MRR is obvious when considering large aircraft structural parts. But even small Swiss-turn type parts often need tough machines and tools.
Brian Such, vice president of customer support for Swiss machine builder Marubeni Citizen Cincom Inc., Allendale, N.J., said the “most common fact” in aerospace machining is the use of corrosion-resistant materials. Materials like Hastelloy, Waspaloy, and MP35N, all of which have “high chrome characteristics and high tensile strength,” he said. “These parts require very rigid machine setups and rigid cutting methods. A cheap machine will quickly fail with these materials!”
What’s more, tolerances are tight. Parts are expensive. Aerospace manufacturers want to keep scrap to a minimum. Maintaining a stable, predictable process that produces a perfect part every time is a virtual religion in aerospace.
But Walker cautioned that some machines that deliver high MRR “don’t lend themselves to those rates on a long-term basis.” Likewise, he said, some machines achieve required tolerances over a short period of time, but then the machine settles in and can drift on certain features on tight-tolerance parts. “As a result, they really don’t have a robust process for either tight-tolerance parts or for the high MRR applications for an extended period of time.”
Also, Walker said OEMs “have to make a part for 20-plus years. A Tier 1 or Tier 2 supplier may sign a 10- or 20-year agreement. Some have 40-year agreements. We’re seeing a lot of customers potentially putting themselves at long-term risk because they’re looking at a machine that’s capable today, but they’re not determining if it’s going to be capable for the long-term.”
As Walker
explained, the end user needs to gamble on a machine that makes the part
for a relatively short-term program or invest in something that lasts
for years and can repurpose at the end of the program—a long-term asset
that delivers both precision and high MRR means stiffness, rigidity,
torque, and thrust. But, Walker added, it is not as simple as comparing
numbers in a brochure. Just because a spindle has a certain horsepower
and delivers a certain torque, it may not do so in the speed range
required.
You really need to analyze the spindle characteristics.
And when you get into really heavy cutting, the spindle might be
adequate but do the motors driving your linear and rotary axes also have
sufficient torque to perform the operation? It all needs to work
together,” he said. Walker added that the machine must adequately handle
the harmonics created in the anticipated operating range.
But he also pointed out that excessive vibration sometimes occurs at lower than ideal operating speeds. The counter-intuitive solution is to run harder and faster to find the sweet spot. Finally, he suggested prospective buyers consider a machine builder’s reputation in meeting tolerances and MRRs. Examine what machines the OEMs and their suppliers are using for the same applications. “A machine is usually aimed at doing tight tolerance work or it’s capable of high MRRs, but not necessarily both. With Mitsui, you can have both for a very long period of time,” said Walker.
