The US Department of Energy estimates that national electricity consumption will increase by approximately 30% by the year 2030. This drives today’s orders for a great number of gas and steam turbines for carbon and nuclear-based electricity plants, as well as specific components for renewable energy plants based on wind, solar, and hydroelectric power.

The factories producing machined components for turbines and generators face some unfavorable trends, however. Components are getting increasingly large, while the manufacturing tolerances get extremely tight. Take for example a hollow component that is 4′ tall and 4′  (1.22 x 1.22m) with two bores on opposite sides. The machined product must be almost completely parallel with a finish diameter tolerance of less than 0.001″ (0.025 mm). There are few tooling options available that can machine these bores successfully, especially if the quill of the machine is not capable of extending itself through the bore.

Vibration Dampening

Long tool assemblies are very common when machining steam and gas turbines. With long tool assemblies, the machinist must address how the cutting insert is brought to the extreme feature on the component. One option uses an integral (one-piece) steel or carbide shank with a steep taper or modular coupling on one side and the insert tip seat on the other side. A major risk with this design is that no part of the assembly addresses vibration that arises during the cutting process. Any vibration can degrade the surface quality of the component.

Another option is to use a modular tool assembly such as a Coromant Capto toolholder to achieve the required gage length. Such a tool assembly might include a steep taper adapter attached to extensions that are bolted together down to final adapter holding the insert(s). In this design, the segments act as dampeners when harmonics arise. When strong harmonics occur, however, the modular assembly will still vibrate.

Thirdly, the machinist can choose a dampened product. A dampened adapter is available both in steel or carbide (for reduced deflection), and length-to-diameter options ranging from 5 x D to 14 x D. A passive damper that sits inside a chamber at the adapter front, close to the insert, allows the tool to perform successfully at extreme overhang lengths. The damper consists of a heavy metal cylindrical slug surrounded by oil, which is attached to the inside of the chamber by a series of springs and rubber grommets.

When vibrations arise from the cutting action, the damper starts to move up and down at a frequency that counteracts the frequency generated at the tool tip.  The damper reacts very quickly and eliminates chatter marks that otherwise would be visible on the component, even under magnification. Machinists often find dampened products to be the best option for secure, long-overhang machining.

Cutting Deep Grooves

Many components in steam and gas turbines have features with deep grooves that vary in width. Some of these components include spacer disks, casings, and various shafts. Accessibility is a challenge when machining deep grooves. Chip control can also present problems since some of the materials involved could be long-chipping stainless steels or heat-resistant superalloys. Vibration poses a third problem since some grooves are very deep and narrow and the tooling lacks stability.

The first step in addressing deep grooving is to have a tooling platform that positions the blade and insert down inside the groove of the feature. The Coromant Capto toolholder offers high rigidity and quick change capability, including on vertical turning lathes (VTLs), where machine-adapted clamping units can bolt directly to the ram on most VTL models.

The other important feature of the tooling is the interface between the blade and the adapter. This coupling must be strong and precise so the insert location remains unchanged during exchange of a damaged blade for a new blade. The serration lock (SL) system offers mating serrated surfaces with mounting boltholes that ensure precision, repeatability, and strong bolt-on clamping for the blade. In particular, the SL70 coupling (oval shaped, serrated coupling 70-mm tall and 40-mm wide) was designed specifically to handle the cutting forces generated in deep grooving and deep profiling applications. An SL100 coupling is also available for extreme applications.

Machinists can solve chip-control problems using insert geometries that aid in steering and curling the chips to facilitate breakage. Deep grooves, however, offer a specific challenge because the materials can be long-chipping and groove walls must not be damaged by the chips. The use of high-pressure coolant (approximately 1000 psi or 6.9 mPa) can help control and break the chips as long as the force of the coolant is directed in a tight stream directly at the cutting zone. High-pressure nozzles can be installed on all SL70 and SL100 blades. The resulting chips are typically short and unable to damage the groove walls.

On some components, a groove might be 3/8″ (9.5 mm) wide and 6: (152 mm) deep. A standard steel grooving blade might not be able to cope with the cutting forces generated in this application because of deflections and vibrations. A possible solution to improve the performance of the blade involves mounting a large carbide insert to the face of the blade.  This setup can help stabilize the blade and allow it to successfully machine to the full depth of the groove. A dampened grooving blade uses an oval serration blade for best stability and accessibility. This 100-mm-high blade allows for direct coolant delivery. To remove the chips from the chamber without jamming the WCMX insert, the chip is split into three separate segments. The dampening design, with blades longer than four times the width of the blade, uses a patented dampening device to provide four times greater cut depths than would be possible without dampening.

Conclusions

The vibration dampening and deep grooving operations discussed are just a few of the many machining challenges that happen every day in the power generation industry. As components grow in size and complexity, companies are tasked with discovering innovative and dependable methods to machine them. Moving forward, even more refined tooling expertise will be required to meet the energy demands into the year 2030 and beyond.