When Hole Drilling Precision Matters, Deliver It with Advanced Laser-EDM Combo

As aerospace manufacturers strive to improve engine performance and reduce fuel consumption, they need high-quality hole drilling on turbine engine components.

New jet engines are designed to operate at higher temperatures, which places greater importance on cooling film hole airflow. To minimize hot spots in the engine during operation, designers increase the number of cooling holes with complex geometries for improved airflow. Special ceramic thermal coatings also insulate the alloy turbine parts from extreme heat. Moreover, new materials are being introduced that improve engine performance and longevity — all of which present additional challenges to traditional hole-drilling approaches.

To meet these demanding requirements, a revolutionary system combines laser-cutting technology guided by a water jet with advanced, high-speed electrical discharge machining (EDM) in an automated cell to precisely drill pre-coated blades and vanes.

What’s Needed: High-Efficiency, Metallurgical Quality

Although the use of round cooling holes is prevalent, aerospace designers increasingly use more complex geometries such as diffuser holes. To enhance airflow, diffuser holes vary in shapes and depth in order to blend into the 3D shape of part details. Their geometries can range from tapered, round cones to squares or rectangles. The final through hole is often not centered to the outer diffuser shape.

Holes of Various Shapes and Forms Drilled in Blade Details

In aerospace turbine parts, metallurgical quality is critical. Two areas can impact the operating life cycle of the engine component:

• The recast layer is formed by molten material that adheres to the part during machining.
• The heat-affected zone alters the microstructure of the base material by heating as the result of machining.

To address these concerns about overall efficiency and metallurgical quality, hole-drilling processes require maximization of machining speed while improving control of the machining depth and location accuracy of each detail. Machining of the cooling film hole features is commonly performed before the ceramic thermal coating is applied, as the EDM process cannot machine the coating material. This means the features have to be EDM machined to an estimated larger size to accommodate the thickness of the final thermal coating. Such a traditional processing method may not provide the best possible cooling film-hole accuracy or location, which can result in lost engine performance and efficiency.

In response, Makino and Synova developed an automated cell comprised of Synova’s Laser MicroJet® MCS-500 series of 3- and 5-axis cutting machines along with Makino’s EDBV3 and EDBV8 EDM hole-drilling machines that process a pre-coated blade or vane detail. The laser system cuts diffuser shapes in the coating layer, and the EDM is used to drill the through holes. The EDBV system has an innovative solution to handle difficult-to-image “non-line-of-sight” holes. A sophisticated data-transfer scheme is also part of the cell, enabling high-accuracy hole drilling on both machines to produce complete components.

“The Laser MicroJet offers unique laser-cutting capabilities that complement our extensive machining and engineering portfolios. For example, coupling it with our EDBV series of EDM machines enables users to drill a complete blade with optimum output, including the drilling of non-line-of-sight holes,” said Mark Logan, director of Makino/SST (Single Source Technologies).

High Precision: Synova’s Laser MicroJet® for Complex Parts

Synova’s Laser MicroJet MCS-500 machines guide a cylindrical laser beam along a hair-thin water jet, resulting in perfectly parallel walls, tight kerf widths, smooth sawing surfaces and sharp edges. The technology has been used successfully for more than a decade for high-quality and high-precision micromachining of complex parts across a wide range of market segments. More recently, 5-axis machining systems have been developed to address needs in hole drilling in turbine machinery.

The water jet eliminates the complexity and process variations of maintaining the laser in focus typically associated with laser systems. Using water to guide the laser to a workpiece yields the following advantages:

• Water guides the laser – The application becomes insensitive to the focal plane of the laser. A cylindrical laser beam is created, resulting in perfectly parallel walls, tight kerf widths and enabling the user to cut thick or non-flat parts, without having to worry about being in focus.
• Water cools the material – Heat is generated during laser ablation. When using a conventional laser system, the surrounding material absorbs a lot of the laser energy, creating an unwanted heat-affected zone. With the Laser MicroJet, much of the energy dissipates into the water. There is little heat-affected zone on the workpiece. Stress-induced conditions such as micro-cracking, thermal damage or deformation are greatly reduced.
• Water cleans the surface – When using a conventional laser, a portion of a laser-ablated material tends to redeposit and solidify, creating unwanted slag. With Laser MicroJet, the water displaces that material before it solidifies, translating into much cleaner entrance, wall and exit surfaces without particle deposition or burrs. 

The MCS-500 laser system offers full 5-axis capability. Synova integrates its Laser MicroJet system in a base machine manufactured by Makino. The resulting MCS-500 offers high-accuracy 3-D micromachining for a wide range of applications.

Makino’s EDM Solution for Hole Drilling

The drilling of diffuser shapes on Makino EDBV3 or EDBV8 EDM machines is performed using a tube electrode for the final through hole. If needed, a contouring tool-path pattern, similar to that of a milling process, can be used to machine funnel diffuser shapes. To accomplish this, the process involves performing a series of changing X- and Y-axis contours while stepping down in the Z-axis.

Blade and vane components, which are commonly casted, typically feature a hollow interior for saving weight and increasing the internal airflow. During the EDM drilling process, the cooling holes break into these internal cavities. To preserve the correct airflow, it is critical that no back wall impingement or back striking takes place during the drilling process. Back striking occurs when the drill/electrode comes in contact with the opposite internal cavity wall, which can cause disruptions in airflow and subsequently the cooling efficiency of the engine component.

To ensure best part quality, the EDBV is equipped with advanced control technologies to improve depth accuracy and prevent back striking. To improve geometric form accuracy, the EDBV uses a highly sensitive dynamic feedback circuit within its electrical generator to sense when the tube electrode is in the correct position for machining. This intelligent processing minimizes the non-value-added “air cut” time that is commonly seen when processing high-engagement angles of 3-D diffuser shapes. The EDBV automatically self-optimizes and increases the machine feed rate when it detects areas of non-machining or air-cut time when processing diffusers. When this occurs, the machine also disables the electrode wear depth compensation, which results in improved 3-D form accuracy.

The EDBV machines also employ dedicated detection circuitry to sense when the electrode penetrates through the material and into the inner cavities of blades or vanes to prevent back striking. This dedicated detection is achieved through the enhanced generator control and improved flushing that is provided by the EDBV’s fully submerged operation. The machine is able to detect breakthrough within 1 second or within 0.040 inch (1.0mm) of depth, allowing for proper levels of quality control and safety while operating at maximum machining speeds.

The EDBV machines are designed around Makino’s proven EDM platforms, incorporating a programmable rise-and-fall drop-tank design that has open and easy access for operators or automated equipment. A dielectric water reservoir has been integrated into the base casting of the machine, which saves on valuable floor space and offers greater thermal stability for improved accuracy.

Onboard filtration and resin systems consist of two large-capacity filters and a separate deionization resin cylinder to control water conductivity. These proven systems ensure consistent water quality, to further enhance the reliability and repeatability of machining results.

All EDM drilling on EDBV machines is performed fully submerged under water. This approach enables faster machining speeds, improves part quality, and creates more stable and consistent conditions during cavity wall penetration. The most significant improvement is seen in the machining speed, which in recent test validation has demonstrated up to 10 times faster processing than conventional technologies.

An additional benefit to submerged machining is the elimination of water splashing and potential slip hazards common in other EDM drill designs. To further improve productivity, EDBV machines use a single-electrode processing approach, which avoids the high cost of custom multi-electrode holders and standardizes the tool holders with a more flexible, cost-efficient system.

Complex Hole Drilling: Two Machines Better Than One

Combining the Synova MCS500 Laser MicroJet with a Makino EDBV3 or EDBV8 in an automated cell leverages the strength of each machine and its underlying technologies to deliver a superior hole-drilling solution.

The unique combination of machining processes gives manufacturers new capabilities to produce pre-coated turbine engine components with exacting details that achieve near-perfect levels of engineered cooling airflow. However, it is critical to combine the attributes and capabilities of both machining processes to achieve a reliable, practical solution. Advanced data transfer between the two machines ensures high precision with minimal setup. The result: a flexible automation cell in which work can be shared between laser and EDM machines in an optimum way for each product.

The MCS-500 Laser MicroJet can be used to remove any non-conducting thermal barrier coating, serving as a path to pre-coat parts with a thermal barrier layer. It can also be used as a stand-alone solution for complex piece-cutting applications of non-conductive materials.

The EDBV3 or EDBV8 can be used for drilling through holes, diffusers and also non-line-of-sight holes. The key limitation to the EDM process is that the workpiece surface must be electrically conductive, which has limited the EDM process to the machining of uncoated thermal barrier components. As part features become deeper, the EDM process gains a significant speed advantage over Laser MicroJet.

By combining Makino’s breakthrough EDM technology found in the EDBV-Series of machines with Synova’s innovative MCS-500 Laser MicroJet, customers benefit from eliminating process steps typically associated with pre-coating drilling. By using each machine for what it does best, an optimum cell throughput is achieved, overall costs are improved and processes are simplified.