In terms of their complexity, bladed disks (blisks) are surely be amongst the most difficult of components to machine. Fortunately for a Russian manufacturer looking to optimise the process, help was on hand from Renishaw. PES reports.
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 which designs, manufactures and services high performance products for both fixed and rotary wing aircraft and gas and oil producing sectors.
The customer wanted to increase the accuracy and efficiency of its blisk milling process. The complex high curvature surfaces of blisks used in gas turbine engines make them challenging components to manufacture.
To overcome this conundrum, Technopark applied Renishaw’s OSP60 on-machine 3D scanning probe with Sprint technology and the Productivity+ scanning suite.
Introduced in the mid-1980s, a blisk is a single component comprising a rotor disk and multiple curved blades. By removing the need to attach separate blades to a bare rotor, blisks have helped transform turbine design, substantially reducing component counts, increasing reliability and maximising engine efficiency.
Blisks are machined from very hard high-value metal, typically titanium or nickel-based alloys. Milling is by far the most important machining process during their manufacture, and due to the high curvature surfaces, multi-axis CNC machine tools and advanced software are required.
Blisk milling usually involves rough slot-milling and semi-finishing to create a near net component, followed by fine milling to arrive at the finished high-precision blade and rotor surfaces.
Despite using a touch-trigger probe for on-machine disk 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. This process needed to be repeated several times and was subject to human error.
The company established that the off-machine inspection and milling process was accounting for anywhere between 30% and 60% of the total labour cost involved in blisk production. In addition, statistical analysis of the dimensional deviation in blades (after leading and trailing edge machining) identified the presence of errors.
Deviations in blade cross section were observed as: residual allowance fluctuation ±0.064mm, offset from nominal profile 0.082mm. Deviations in the longitudinal section were similar: residual allowance fluctuation ±0.082mm, offset from nominal profile 0.111mm.
The main reasons for deviations arising during edge machining were concluded to be kinematic errors in the machine during 5-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 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 blisk milling.
Developing a CNC machining process for the milling of blisks included the following requirements: 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.
“In the case of this application, a clear need arose for software development based on a scanning probe, so we approached Renishaw for a solution. Their Sprint 3D scanning measurement technology for machine tools provided all the technical capabilities we required.”
The 3D scanning and measurement of blisks on the machine tool and at very high-speed has generated significant time savings in the production process resulting in a marked increase in CNC machine productivity.
With respect to milling accuracy, a comparison of the respective deviations in blisk 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 on the new.
On the implications for machine manning, Mr Starovoytov affirms: “The control system’s execution mode ensures automatic adjustment of the CNC control program based on the 3D 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.”
He concludes: “The combination of Sprint 3D scanning technology and Productivity+ CNC software means that even the most minor deviations in blisk form are always identified in real-time. With touch-trigger systems these would have gone unnoticed.
“The results we have gained from the investment have far exceeded our expectations. Our accuracy in fine milling of bladed disks is more than four times better and the related labour costs have been halved.”
As the probe stylus tip precisely follows a bladed disk surface, it allows high-resolution stylus deflection data to be recorded with exceptional accuracy, registering all sub-micron X, Y and Z movements at the ultra-responsive stylus tip.
The OSP60 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 disk surface data, which is used to calculate precise feature position, size and form.
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.
Blisk 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.
Renishaw www.renishaw.com
