A series of progressive turning operations leading to a final hard stage profile turn of a high strength, nitriding steel aerospace shaft is being performed on a flat bed lathe supplied by XYZ Machine Tools

A series of seven progressive turning operations leading to a final hard stage profile turn of an aerospace, high strength, nitriding steel shaft destined for use in the rotor assemblies of helicopters is being performed on an XYZ Proturn SLX CNC lathe incorporated into a special machining cell at Mollart Engineering.

Within the past 12 months almost 1000 shafts have been produced on the Proturn SLX 1630 CNC flat bed lathe at the Chessington subcontract facility of Mollart. Most are produced in split batches of 10, largely due to the number of operations required and to help maintain a consistent schedule of deliveries. Staged processes on the lathe which take around 40 minutes include profile and taper turning, grooving, recessing, facing and chamfering of the difficult to machine, safety critical, S132 corrosion resistant, 3% chrome-moly-vanadium toughened material.

Final profiling

Critical in the application is the final profiling of the 50 mm nominal diameter, heat-treated shaft which includes a 34 mm diameter waist around 180 mm long, a 7.5 deg taper, various stepped diameters and a recess in the OD.

All tolerances have to be maintained not only within 0.05 mm but also to achieve a final stage concentricity of 20 micronTIR to the finished bore and counterbore over the 490 mm overall length of the part. The 400 mm swing by 760 mm between centres machine with a 52 mm diameter spindle bore is installed in a dedicated cell comprising a heavy duty Mollart BTA deep hole drilling machine and a bore honing machine that contribute to some 14 operations. Also included are two heat treatment processes before the parts are returned to the customer for final grinding.

“The level of quality we can maintain in the final profiling is so important, not only due to the build-up of costs from the number of progressive operations but in order to ensure we maintain the geometry involving concentricity and straightness between the bore, counterbore and the outside profile,” explains setter operator Bartosz Lenczowski. “The Proturn SLX has proven to be very reliable and consistent in holding our process tolerances and any program adjustment at the ProtoTRAK control is very straightforward. This means the flexibility of the machine is put to good use in producing one-offs such as inspection gauges and recently to produce the soft turning for special thread gauges.”

The hard turning operation has proven to be highly successful in allowing us to maintain the consistent level of quality and frequency of delivery”

The final hard stage profiling of the helicopter rotor shaft involves a depth-of-cut of 0.2 mm. The lathe is run at 720 revs/min with 100 mm/min feed.  However, prior to the finish cut the solid bar length goes through several operations involving gundrilling on a Mollart BTA machine to create an initial datum through bore of 20 mm.

Ensuring concentricity

The OD of the bar is then turned on the Proturn to a nominal 46.5 mm diameter and faced before being sent for initial heat treatment. Back on the Proturn, the diameter is further reduced and the part taken to the BTA machine for internal diameters to be rebored and a counterbore created using a special Mollart-developed, combination ballnose style BTA tool to ensure concentricity.

The part is then loaded again to the XYZ machine before being re-skimmed which includes the creation of the waist and adjacent diameters and taper prior to final heat treatment. The part is then re-bored on the BTA machine, honed and then returned to the Proturn SLX 1630 for the critical finish machining cycle.

The subcontract operation of Mollart Engineering is located on two sites in Chessington Resolven, South Wales. Both are currently being extended to meet increasing demands from the aerospace, semiconductor, deep sea, medical and oil and gas sectors. The family-owned company under managing director Guy Mollart has two other successful divisions involved in building deep hole drilling machines and the provision of tooling for hole processing and deburring that created sales of well over £16 million in 2012.

“To achieve the final process size and geometry prior to the customer carrying out the final grind, the rotor shaft has to be taken progressively to size,” concludes operations director Mike Pragnell. “The hard turning operation on the Proturn SLX 1630 has proven to be highly successful in allowing us to maintain the consistent level of quality and frequency of delivery.”

XYZ Machine Tools

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