Drilling holes in turbine engines

Advanced control capability leads to faster, high-quality processing

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Laser drilling control has enabled designs that now incorporate more holes, tens of thousands per workpiece in some gas turbine engine combustor designs. Producing these holes by laser drilling has been made faster with higher quality results through advanced capability. Using integrated workpiece sensing and mapping, we have shown the ability to increase throughput in laser drilling by 1.5 to more than 2 times (see Figure 1).

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Figure 1. Modern control capability has increased the speed of laser drilling certain gas turbine engine components by up to 2 times.
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Laser drilling has had significant influence on the design and manufacture of gas turbine engines for aircraft, power generation and oil and gas pumping. It provides a productive way to add small, cylindrical and shaped, cooling holes at shallow angles in uncoated as well as thermal barrier coated (TBC) high-temperature metals and alloys.1

Throughput in laser drilling turbine engine components is influenced by several factors related to the following: programming, setup of the workpiece on the laser system, laser drilling process parameters, and motion/control system performance.

A program to evaluate the influence of these factors and to develop features and capabilities for increasing throughput in laser drilling was launched to make laser drilling even more cost effective. As we learned in this study, throughput is influenced to some degree by each of the above factors. Here we summarize some of the results of this program, in particular, those related to the laser system control.

Reducing setup time

Quite commonly, the start location and orientation of each workpiece must be adjusted to account for normal workpiece-to-workpiece variations, especially true for sheet metal components, but also true for machined and cast ones. This is because the tooling that is required to force the workpiece into position is complex and expensive. The alternative, manually aligning the workpiece and determining offsets, is, in most cases, too time consuming.

Workpiece sensing that is used to automatically find the surface of the part and maintain proper focal point location and assist gas nozzle standoff is used as the basis for automatic adjustment. By extending the capabilities of this workpiece sensing, the actual location of the workpiece is measured, offsets and rotations are calculated, and the actual path during drilling is adjusted to reflect the real part, all without needing to edit the program.

For example, to accurately drill shallow angle holes in a nozzle guide vane, it is necessary to locate the airfoil section prior to drilling. Advanced sensing capability that is part of the laser system is used to automatically locate (within the drilling program) the positions of the leading and trailing edges of the airfoil. Using this information, the orientation of the workpiece can be determined and a course of action chosen, including automatically adjusting the machine coordinate system for that workpiece before continuing the program and terminating the program and/or requesting operator intervention.

Increasing throughput

Turbine engine components are typically laser drilled by one of three methods depending on the hole size, shape, quality and throughput requirements: (1) trepanning, (2) percussion drilling by index and drill or (3) PosiPulse drill-on-the-fly percussion drilling. As shown in Figure 2, the latter method, which involves synchronizing the laser pulsing to the position of a rotary axis, has made significant contributions to increasing throughput.

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Figure 2. The integrated workpiece sensor used to maintain control of the laser beam focal point relative to the workpiece surface (focus control) is also used to determine the orientation of a localized region of the workpiece and orient the laser beam perpendicular (or at any user-programmed angle) to the surface before drilling the hole.
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Certain control features are required to realize the full benefit of this method for real-world parts. In most cases the shape of actual parts does not match the design. This results from normal manufacturing tolerances and the presence of, or changes in, the residual stress condition caused by forming, heating and coating. For example, cylindrical parts are seldom perfectly round and may not be referenced to the center of the rotary accurately. Failing to account for these differences results in long setup times or holes that are neither the correct size nor in the correct position.

The traditional approach for drilling angled cooling holes in combustors has been to move the laser system to the programmed hole location, activate an automatic focus control sensor typically based on capacitance, and to allow this sensor to 'find' the actual workpiece surface. Once the location of the actual surface has been determined, the laser system is positioned to the offset (de-focus) required to achieve the correct hole diameter and shape. The program continues with the laser shutter being opened to deliver the programmed number of pulses for percussion drilling the hole. This cycle is repeated for each hole.

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Figure 3. Cycle time comparison for three methods of percussion drilling shallow-angle cooling hole. The comparison is for a hypothetical cylindrical sheet metal combustor having 1000 holes per row.
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The cycle time shown in Figure 3 assumes that the part sensor has capability to move in a user-programmable axis (see Figure 4a) to maintain holes on a "waterline." With sensors that track the surface by moving the focusing lens only along the axis of the laser beam (see Figure 4b), the error in hole location (Abbe error) can be quite significant because it is proportional to the run-out (deviation of actual shape from the design) and angle of the hole to the surface. If the focus control is of the type pictured in Figure 4b, then additional time is required to re-position the focusing lens to approach the part perpendicular to the surface.

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Figure 4. Maintaining the correct location for angled holes requires offsetting along a user programmable axis (a). Out-of-roundness of the cylinder will produce an error, referred to as an Abbe error if the laser beam focus point is positioned along the axis of the laser beam (b).
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It is obvious from Figure 3 that a drawback to the sense and percussion drill sequence is that it is slow—it can take significant time to locate the workpiece surface and position the laser beam at the correct location for drilling.

Pre-scanning the workpiece to measure the actual workpiece shape significantly increases throughput, as shown in drill Sequence 2 in Figure 3. In this sequence, the time for sensing the workpiece surface and moving to the correct location at each hole is replaced by a routine in which the shape of the part is first mapped.

For example, for a cylindrical component, the part is rotated in front of the focus control sensor (now acting as a measurement device) and linear axes position data that reflects the actual part surface coordinates is mapped to specific rotary positions; the data is subsequently used to position the laser system to compensate for out-of-roundness during the drilling portion of the process. The surface irregularities only need to be determined once per region of the workpiece.

With this sequence, the time to determine the actual part location and move to the correct offset is substantially reduced. Instead of sensing at each hole location, the system positions at rapid traverse speeds to the correct location using data from the mapping routine and then opens the shutter and delivers the desired number of pulses for drilling the hole.

Drilling time can be even further reduced by mapping and then continuously rotating the workpiece beneath the laser beam (see drill Sequence 3). One pulse is delivered to each programmed hole location per revolution of the part. In this case, the time for sensing and moving to the correct location and the time for opening and closing the shutter at each hole location is effectively eliminated (it is actually amortized over the total number of holes within a row).

Summary

Manufacturers and subcontractors are looking for ways to reduce costs by reducing the floor-to-floor time in laser drilling. Integrated workpiece sensing and mapping, standard features of modern multi-axis laser drilling systems, are contributing to this goal by reducing the hole-to-hole drilling time by 1.5 to more than 2 times.

References

  1. Baxter, Richard, "Turbine Drilling", Industrial Laser Solutions, Volume 17, No. 2, p16.

The authors are all with Laserdyne Systems Division of Prima North America Inc. in Champlain, MN. Terry VanderWert can be contacted by e-mail at Tvanderwert@laserdyneprima.com.

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