Based in Ufa, Russia, Technopark Aviation Technologies — a provider of educational and scientific research and engineering services — works closely with the country’s largest provider of gas turbine engines for both fixed-wing and rotary-wing aircraft, as well as the gas- and oil-producing sectors.

This manufacturer wanted to increase the accuracy and efficiency of its bladed-disc milling — a process that can be challenging, such is the complexity of the blades’ curved surfaces.

In the assembly that comprises the compressor, combustor and turbine, bladed discs play an important role in minimising drag and optimising airflow — and thus the amount of thrust an engine produces. Introduced in the mid-1980s, a bladed disc is a single component comprising a rotor disc and multiple curved blades.

Moreover, by removing the need to attach separate blades to a bare rotor, bladed discs substantially reduce component counts, increasing reliability and maximising engine efficiency.

Bladed discs are typically machined from titanium or nickel-based alloys. Milling is by far the most important machining process during their manufacture, and it is one that requires multi-axis CNC machine tools and advanced software to produce the complex curved surfaces — usually via rough slot-milling and semi-finishing to create a near net component, followed by fine milling to achieve the finished high-precision blade and rotor surfaces.

For this Russian engine manufacturer, the fine milling process for a wide variety of bladed discs was a labour-intensive and costly process; and despite using a touch-trigger probe for on-machine disc measurement, each workpiece needed to be removed from the CNC machine after milling for offline measurement and inspection, and then remounted for any further machining — a process that usually needed to be repeated several times and was subject to human error.

Labour savings
Moreover, the company calculated that the off-machine inspection and milling process was accounting for anywhere between 30% and 60% of the total labour cost involved in bladed-disc production, while statistical analysis of the dimensional deviation in blades (after leading and trailing edge machining) identified the presence of errors.

For example, the deviations in blade cross section observed were: residual allowance fluctuation ±0.064mm, and offset from nominal profile 0.082mm. Deviations in the longitudinal section were similar: residual allowance fluctuation ±0.082mm, and offset from nominal profile 0.111mm.

The main reasons for deviations arising during edge machining were concluded to be: kinematic errors of the machine during five-axis operation; elastic deformation of the blade during the cutting process due to its low rigidity; and elastic deformation of the tooling during metal cutting.

The company said: “The large number of operator interventions necessary for this process simply increased the risk of irreparable rejects due to human error. There was an obvious need to develop a fundamentally new solution for faster, high-precision bladed disc milling, one that included semi-finishing using a parameterised control program, on-machine workpiece inspection, correction of the parameterised control program based on inspection results, and fine milling of the workpiece using the corrected parameterised control program.”

Technopark was selected to develop and implement the process controls needed. Semen Starovoytov, the company’s head of the Innovation Department, said: “We have worked successfully with Renishaw for many years, equipping machines with Renishaw touch-trigger probes to achieve flawless measurement accuracy.

For this application there was a clear need for software development based on a scanning probe, so we approached our partners from Renishaw. Their Sprint 3-D scanning measurement technology for machine tools provided all the technical capabilities we required.”

Increased productivity
The introduction of Renishaw’s Productivity+ PC-based probe software for machining centres and the OSP60 probe has had a significant impact on the bladed-disc manufacturing process in terms of machining accuracy, speed and labour costs.

Indeed, Mr Starovoytov says the 3-D scanning and measurement of bladed discs on the machine tool and at very high speed has generated exceptional time savings in the production process resulting in a marked increase in CNC machine productivity.

“With respect to bladed-disc milling accuracy, a comparison of the respective deviations in bladed disk cross section and longitudinal cross section after edge machining showed a dramatic improvement, from 0.082mm and 0.111mm with the former process to 1µm and 28µm with the new.”

Regarding the implications for machine manning, he added: “The control system’s execution mode ensures automatic adjustment of the CNC control program based on the 3-D blade scanning data delivered by the OSP60 probe. This means there is no longer a need for an engineer to be at the machine constantly.

“Furthermore, the combination of Sprint 3-D scanning technology and Productivity+ CNC software means that even the most minor of deviations in bladed-disc form are always identified in real time. With touch-trigger systems these would have gone unnoticed.”

The OSP60 on-machine 3-D scanning probe is equipped with Renishaw’s Sprint technology. As the probe stylus tip follows a bladed-disc surface, it allows high-resolution stylus deflection data to be recorded, with X, Y and Z movements registered at sub-micron levels. It sends a thousand XYZ tip-centre data points to the OMM-S receiver every second using a high-speed noise-resistant optical transmission link.

Advanced algorithms then process the probe deflection data together with machine tool position encoder data to produce accurate bladed-disc surface data, which is used to calculate precise feature position, size and form.

Furthermore, with the Productivity+ CNC plug-in, scanning speeds of up to 15,000mm/min can be achieved, providing on-machine measurement that is sometimes six-times faster than traditional touch-trigger systems — and scanning on the machine tool eliminates the need to remove workpieces during processing.

In conclusion, Mr Starovoytov said: “The software displays high accuracy measurement results on-screen in real-time and uses them to automatically adjust machine tool settings for the ensuing fine-milling process. Measurement reports can be exported to file for analysis or quality assurance purposes.

“Bladed-disc inspection programs can be generated quickly and easily from solid-model geometry using existing off-machine graphical programming tools, while the interactive front end of Productivity+ is used for editing and running probe inspection routines with easy-to-understand graphical screens, instead of complex NC codes to display the program to the user.”