Processing turbine engine components

Laser production systems effectively machine, weld, and repair turbine engine components

John Stackhouse, Martin Bull, and Mike Wakeham

Turbine engines, for both aircraft and industrial engines, are continually being refined in a drive to optimize performance. A significant contribution to this increase in performance is made possible by the use of non-conventional machining processes such as EDM, ECM, and laser. Laser machining is used more flexible than the other processes because it can drill, ablate, cut, weld, clad, and mark critical turbine components.

Typically CO2 lasers are used for cutting, welding, and cladding. Nd:YAG lasers are used for drilling, welding, and cutting. Q-switched lasers are useful in ablating and marking. Laser machines that utilize these sources are generally multi-axis CNC with technology modules that allow for part probing, airflow feedback, surface following, drilling on the fly, and breakthrough detection (see Figure 1). Also possible are data archival, bayonet-style nozzles, fixture read/write, and adaptive control using SPC. Inspection of laser machined features and shaped holes (diffusers, fans) laser machining capabilities are also available.

Part probing

Generally components are probed for eccentricity and out-of-roundness by contact or non-contact probes. In neither case is this done coaxially with the processing laser beam, eliminating restrictions forced by the nozzle orifice.

Airfoils, such as those on turbine blades and vanes are probed using a patented method to establish the “stacking axis” of the airfoil, precluding the need for expensive “staging” fixtures, used to orientate the part off-machine. A simple holding device allows the machine to establish the stacking axis in relationship to the machine datums, while giving complete access to the airfoil for machining. Information derived during part probing is used to establish the correct positions for holes to be drilled or other features to be machined.


FIGURE 1. An operator, holding an aircraft engine combustion liner, stands near a typical multi-axis CNC laser machine.
Click here to enlarge image

The CNC controller has built-in features that provide a unique probing and airfoil-fitting algorithm capable of accurately and quickly measuring alignment and using the resultant offsets to automatically offset machining positions in order to maintain positional tolerance.

Two fundamental stages of processing must be performed in order to correctly position holes on an airfoil. First the airfoil must be measured in order to determine its deviation from nominal and the results of this measurement stored and then the stored data is used to correctly “reposition,” using the CNC to give offsets, the airfoil in the machine prior to machining.

Air flow feedback

A common use of Nd:YAG lasers is the drilling of effusion cooling holes in combustion chambers, transition ducts, and similar components (see Figure 2). The holes in these types of components are usually dimensioned by airflows as well as by geometric requirements. It is clearly advantageous to incorporate airflow evaluations into the production control method.


FIGURE 2. In the double-airfoiled vane, each airfoil can be probed and appropriate offsets applied to both.
Click here to enlarge image

A procedure, fully integrated with the machine tool, uses a Component Based Data Archival System, which is described below. A further requirement is the integration of non-volatile tool management memory modules on each of the fixtures that will run airflow-sensitive hardware on this machine tool. These devices are re-writable memory modules, which allow a limited amount of information to be stored on a non-volatile module, which is sufficiently small and inexpensive, to become a permanent addition to each component fixture. The machine is configured with the ability to read and write to this device any time that the fixture is located in the machine. Multiple fixtures are used to optimize the machine’s productivity.

When the component and fixture are first loaded on the machine, the operator inputs part number, serial number, and operation number, and the machine automatically adds date/time stamp, currently registered part program, and part program name and revision date. At the completion of each row or group of holes further data on laser parameters used will be added.

Flow parameters are measured off-line

To support adaptive control techniques, the component and its fixture is removed from the laser machine at an appropriate point in the production cycle to be airflow tested.

To support and coordinate this activity, the memory module will be programmed and constantly updated with sufficient data to identify the current status of production throughout the manufacturing process for the particular component that is currently mounted in the fixture. This approach will require an airflow testing station with the ability to read and write to the memory module. In this way, the airflow testing station and the proposed laser drilling system will be able to share the production hardware for drilling and testing without any risk of losing or confusing the associated airflow data or laser drilling status for particular components. By this approach, the ability to have any number of in-process components active at any time will be available, the practical limit governed only by the number of fixtures that are available for simultaneous activity.

When the partly machined component is delivered to the airflow testing station, the system will be required to read the memory module to determine the current status of the component. Once the airflow measurement has been made the result of the test should be recorded, so that when the component is next returned to the machine tool the laser processing can continue taking account of the current airflow error. This technique will also ensure that the correct part program and laser parameters are selected each time that the component is replaced on the laser machine.

Throughout the entire process, the workstation controller will acquire and record the production data that is generated for each component that is processed. This data is automatically stored in the Component Based Data Archival system and provides record-keeping for traceability and quality assurance.

Surface following

Through a long history developing practical surface following techniques in the turbine industry, a range of solutions to the problem of auto-focus and surface following have been developed.

In the case of high incidence angle holes, the ability to follow the surface of a component and adapt to the changes in size between consecutive components is of paramount importance. For high incidence angle holes however it is recommended that the technique of pre-logging of the component profile, perpendicular to the surface, prior to the commencement of laser processing is used.

For speed and precision an optical gauging method is preferred with on-the-fly logging capability. Using this technique, one complete 360-degree rotation of a typical circular component with a diameter of 20.00 inches can be logged in less than 10 seconds. Because this data can usually be used for multiple rows on the same part, the “per row” time penalty can generally be reduced to negligible proportions.

Drilling on-the-fly

In drilling on-the-fly (DOF) mode the CNC sends information to the controller, which then generates a table with all the desired hole positions before machining commences (see Figure 3). The CNC will set up the registers with a number corresponding to the desired position of the next hole. The axis continuously moves; when the encoder position matches that of the registers an output is given to the laser to fire once. The controller then loads the next position to the registers and the whole process repeats again and again. Several laser pulses may be required to give full penetration and so the part is rotated or moved linearly as many times as necessary, with the laser firing at the precise location each time until the holes are completely machined.


FIGURE 3. Shown is a workpiece that is processed by drilling on the fly (DOF).
Click here to enlarge image

In this way, it is necessary for the component to rotate or move linearly several times before the holes are complete. However, because acceleration, deceleration, and settling in position are eliminated, the whole procedure is much faster.

Data archival system

The component-based data archival system provides record-keeping for all process-related variables and characteristics of machined performance that occur during part processing. If airflow adaptive control (described above) is also implemented, this data will also be automatically included in the database.

For each new component, a database record is created by scanning an appropriate barcode that defines the part type and serial or by manual entry of pertinent data. This information allows the process controller to create a unique database record, and a comprehensive database of laser and process performance characteristics will be recorded during the entire in-process cycle of the component. As a minimum, the following variables would be monitored and recorded: date and time of program execution, cycle time, laser parameter selection against time, and memo entries.

In this way, a comprehensive database of machine performance and component characteristics for every component processed would be archived within the process controller and could be recalled for immediate review by selecting the appropriate part record from the database. The results database is also archived in such a way that the data can be uploaded to a host via an Ethernet link.

Fixture read/write system

This is designed to automatically read information about components that have already been fixtured, where this information can be read and manipulated by the machine’s software.

Information about the component is stored on a chip and used when there is a need to track a component that is being passed from one machine to another. Standard features of this unit are a read-only function to take information about the part with respect to the fixture. The write function of this unit will allow the first machine to write any information about the machining process prior to unloading the fixture from the machine.

In multiple-machine cells, whether robotized or manually loaded, it is useful to “probe once and machine more than once,” with data derived using the probing described herein downloaded to the respective machine, whether it be another laser, an EDM machine, or a conventional machining operation.

Conclusions

Laser processing is utilized widely on critical aircraft engine and industrial turbine components and has proven to be more flexible than other non-conventional processes.

Combining the advantage of a laser machining center, which requires no more tooling than a workpiece holding fixture, and the various technology modules for process control and quality assurance gives the user a cost-effective and efficient production tool.

John Stackhouse (jstackhouse@winbrogroup.com) is president, Martin Bull is a senior project engineer, and Mike Wakeham is a project manager all with Winbro Group Technologies, Coalville, U.K., and Woburn, MA.

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