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Grinding Critical Shafts for Helicopters
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TORRANCE, CA — Frank Robinson founded Robinson Helicopter Company in 1973. Its first business address was his home, where he designed the R22, a small, low cost two-seat piston-driven helicopter for civil use. The prototype was built in a small tin hanger at the Torrance Airport, and Robinson himself flew it on its first test flight in August 1975. The first production R22 was delivered in late 1979 and soon became the world’s top selling civil helicopter, holding every world record in its weight class, including speed, distance and altitude.
Such is the stuff of an American manufacturing success story: A man has a vision and molds and crafts the vision with care and tenacity into a successful reality; however, every step of the way is dogged by critical attention to detail.
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Safety in the air: steering components
Wayne Woodbury, manufacturing supervisor, turning, grinding and assembly, says the goal at Robinson is to be 100 percent self-sufficient, doing everything in house.
“Originally we made these parts on manual universal grinders,” he says, “but this limited us to only grind one diameter at a time. You couldn’t go from diameter to diameter in one setup. In fact, on the more critical shafts, we’d have to rough them, take them out, check them and put them back in to do the finish grind. And as our volumes kept rising, we knew we had to make a change.
Woodbury says the decision to go with Studer was made by the vice president of Production. The decision was simple: Word of mouth in the grinding business was and always has been that Studer makes the best universal cylindrical grinders in the business. So, in 1999 the company purchased its first Studer S21 CNC universal grinder to do the spools. “We can now do in four days what it once took four weeks,” Woodbury says. “And we’re grinding all six diameters in a single setup. Now that’s a really big difference in productivity. And as our volumes continued to increase, we had to buy a companion Studer S21 to keep up. We now use one machine to rough all the spools and then they go to the second Studer S21, which is in a climate controlled area for finish grinding.”
Woodbury says the tolerance on the spool and sleeve assembly is very, very tight — a mere 0.000060” clearance between the two pieces, which the Studer S21s have no trouble holding time and time again. The spools are very small cylindrical parts. The smallest has a critical OD dimension of 0.125" and it’s some 0.375" in length. The largest spool is 0.375" in diameter and 3" in length. The material of the spools is 440C stainless with a Rockwell hardness of 58-60. The required finish is 4 Ra.
“The important thing to keep in mind is that these cylindrical parts are flight-critical and safety-critical, so there is no room for error,” Woodbury says.
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Getting power to the rotors
The other part that fits the above category is the clutch shaft that delivers the power from the motor to the rotors. It drives the drive train that rotates both the top main rotor and the tail rotor. Woodbury notes that if this shaft fails there will be no power to the rotors. However, that’s not to say the helicopter will just fall like a stone. On the contrary, if the shaft fails, or if the motor loses power, there’s a spread clutch that will allow the main rotor to still turn. As the helicopter begins to descend, the upward force of air on the aerodynamics of the rotors will keep the blades spinning. It’s not exactly like gliding, but under good conditions the pilot should be able to land the helicopter safely.
“As demand for the R22s and R44s continued to grow, we realized that we couldn’t keep up production of the clutch shafts on the two manual machines that we were using, so we thought about locating an outside vendor for these parts,” says Woodbury. “But then we rethought that strategy and once again turned to Studer grinders, this time purchasing a pair of Studer S33 universal cylindrical grinders.
The Studer S33s are actually two different models, because the clutch shaft requirements for the R22 differ from those of the R44. “We’ve got one S33 with a 25" center length and the other has a longer 40" center length,” Woodbury says. “We can make the clutch shaft for the R22 on either machine, but the clutch shaft for the R44 is 34" long, so it must be done on the S33 with the 40" center length.”
Both shafts have six grinds and all six can be done in one setup for the R22. The shaft for the R44 is done in two setups because there are some large radii that can’t be ground without a wheel change. The material is 9310 with a Rockwell hardness factor of 60. The finish is 16 Ra. The tightest dimension on the shaft is 0.0003". And as with the hydraulic steering spools, they’re now able to produce in four days what previously took four weeks. And keep the job inside.
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So, now you’ve got the right technology . . .
Woodbury notes that no one in the grinding area had any CNC grinding experience. It’s true, however, that some operators had CNC experience with Fanuc controls, but that was limited to turning or milling applications (the shafts in question arrive in the grinding area with several diameters turned prior to grinding). But CNC turning is not the same as CNC grinding. So, where did Woodbury turn for experienced CNC grinder operators?
Working closely with the local Studer distributor, DMARK Machine and Tool Company, Robinson chose a plan suggested by DMARK. “We asked for volunteers from the shop,” he says. “Anyone who wanted to become a CNC grinder operator. And we got five volunteers, none of whom had even run a grinder before. No grinding experience at all.”
The next move was to send the “volunteers” to the DMARK facility in Cypress (CA) facility for two weeks of extensive training. The training was conducted by Ernst Loosli and Nick Schuetz from United Grinding. Woodbury personally went down for part of the training sessions, bringing prints and part samples, specifically of the clutch shaft.
“This way,” Woodbury says, “as part of their grinding training, these guys would actually be training on the very parts they’d be making. And let me add that Ernst trained them on the Studers exactly they way that he, Ernst, would run the machines. In that respect, I think it was wise to choose volunteer operators with no grinding experience at all. They came in without any preconceived notions on how grinding’s done, or should be done.”
He also notes that while these operators had some CNC experience, they were unfamiliar with the Studer Pictogramming, which allows operators simply to connect grinding cycles to one another and add necessary numerical values. The Fanuc 21i control then generates the machine program automatically. And if you alter a program, the control stores that as well as the original.
“Once these guys came back from training, they really took over,” Woodbury says. “Took the bull by the horns. Ernst has been in and out a couple of times, tweaking this and that, but these guys have really made all the difference. It just proves the old adage that you get what you pay for. Invest in the best grinding technology and in the best training and the payoffs are remarkable.”
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