Keeping choppers up & maneuvering

Robinson Helicopter's grind shop tackles demanding cylindrical parts for the company's helicopters.

CNC universal grinding machines increase productivity for flight and safety-critical helicopter parts.

To keep police on top of troublemakers and the reporting crews that provide traffic reports and dramatic news updates in the air, the Robinson Helicopter Co., a maker of one of the world's top selling civil helicopters — the type used by police and news crews — relies on precision grinding.

The company makes two models of piston-driven helicopters, the R22 two-seater and the R44 four-seater. It grinds flight and safety-critical components, such as clutch shafts for power transmission and spools for steering, to strict FAA standards and tolerances with the help of two Studer S33 and two S21 CNC universal cylindrical grinders from United Grinding Technologies (UGT) (www.grinding.com).

For clutch shafts, one S33 sports a 25-in. center length, the other a 40-in. center length to handle the R44's 34-in.-long shafts. Clutch shafts, made of 9310 steel with a Rockwell hardness of 60, require surface finishes of 16 Ra and tolerances held as close as 0.0003 in. Both shaft types require six grinds, all of which Robinson Helicopter does in one setup for R22 shafts and in two setups for R44s. The clutch shafts for the R44s are designed with large radii and require a wheel change.

Besides holding tight tolerances, the S33 grinders have increased productivity significantly for clutch-shaft production. Robinson Helicopter now can produce the shafts in four days rather than four weeks.

"Previously, we ground clutch shafts on manual universal grinders, but this limited us to one diameter at a time," says Wayne Woodbury, a manufacturing supervisor at Robinson Helicopter. Because the grind shop was unable to go from diameter to diameter in one setup, it had to rough grind the shafts, unload and check them, then reload them to finish grind. The company found that keeping up with increasing production volumes was impossible.

The S33 grinders help to reduce part setups because they can grind part faces, O.D.s and I.D.s. The machines feature high-precision, Hirth-coupling B axes with 1-degree indexing and turret heads that repeat within 0.08 arc seconds, ensuring accurate concentricity and less I.D.-to-O.D. runout.

Studer's UniDrive MFM highfrequency internal-grinding spindles transform the S33s into internal grinders that provide high torque over a wide speed range. Grinding speeds up to 120,000 rpm help shops such as Robinson Helicopter handle bores as small as 0.125 in. in diameter.

Spool shafts, the other flight and safety-critical component that requires precision grinding at Robinson Helicopter, are part of a helicopter's steering mechanism. The small, cylindrically shaped spools go into spool-and-sleeve assemblies and feature critical O.D. dimensions of 0.125 in. to 0.375 in. that must be held constant along lengths up to 3.0 in.

Robinson Helicopter uses its two Studer S21 CNC universal grinders to produce the spool shafts from 440C stainless steel with a hardness of 58-60 Rc. A rough grind is done on one machine, and finish grinds are done on the other in a climate-controlled area. The two-step process and climate control help the company to achieve the extremely tight tolerances — 0.000060 in. of clearance — between the two pieces in the spool-and-sleeve assembly. Woodbury says the Studer grinders consistently hold the demanding tolerances and produce 4-Ra surface finishes those parts require.

To meet FAA and its own ISO-certification requirements, Robinson Helicopter conducts numerous in-process and post-process inspections and measurements on clutch shafts and spool-andsleeve assemblies, and maintains full traceability records on them as far back as production heat data from the steel mill.

Robinson Helicopter's grind shop receives shafts already heat treated, hardened and turned, and immediately checks them for cracks. After grinding, the shop uses gage blocks and indicating micro meters to measure the shafts and performs magnetic-particle inspection.

Then the shafts are submerged in a nitric-acid bath to check for burn spots that could become brittle and crack under stress. Last, the shop bakes the shafts to prevent hydrogen embrittlement and performs a 100-percent dimensional inspection on each, both manually and with coordinate measuring machines (CMMs).

Woodbury says this rigorous and timeconsuming testing is essential in keeping the news crews, police forces and countless others that fly Robinson Helicopters safe on a daily basis.

When Robinson Helicopter incorporated CNC grinders, the company asked for volunteers to operate the equipment. Five employees responded, but none had previous CNC-grinding experience. The company sent its volunteers to UGT's Cypress, Calif., facility for two weeks of training.

Woodbury attended part of the training sessions, and provided print and part samples of clutch shafts for the training courses. Ernst Loosli, a UGT service engineer who conducted the training, used the part samples to instruct Robinson Helicopter's machinists on the Studers, showing them the way he would run the machines. Woodbury said that finding inexperienced volunteers was beneficial because they came to the training without preconceived notions on how grinding should be done.

The machines are equipped with Fanuc 21i controls and grinding software developed by Studer. The software is simplified to allow users to in-put grinding cycles and to add custom information for specific jobs. The controls then automatically generate machining programs. When programs need to be changed, the controls store the new data and machining programs with the original information and programs.

"Once back from training, these new operators jumped in and took the bull by the horns," says Woodbury. "Loosli (the UGT trainer) has been in and out of the shop a couple of times to tweak a few things, but these guys have really made all the difference."