10/20/2014 | 2 MINUTE READ

Applying “Dengeling” to Finish Turbine Blades

Facebook Share Icon LinkedIn Share Icon Twitter Share Icon Share by EMail icon Print Icon

This alternative to grinding, polishing and shot peening enables turbine blades to be machined and finished on one five-axis machine in one setup.

Loading the player ...


Facebook Share Icon LinkedIn Share Icon Twitter Share Icon Share by EMail icon Print Icon

Related Suppliers

You might know “dengeln” (German for sharpening or honing) to be a manual process whereby a hammer and anvil are used to smooth and sharpen the blades of scythes or sickles. Today, a more advanced version of this concept is being applied to finishing turbine blades on the same five-axis machine that mills them.

Machine tool builder Starrag has developed what it calls its “dengeling” process for its LX series turbine blade machines to eliminate secondary polishing, grinding or shot peening operations, producing a ready-to-install blade in a single clamping. The technique can also eliminate manual polishing for dies and molds.

The dengeling process is performed after five-axis roughing and finish machining operations that create the blade profile. It uses an electrically powered head installed in the machine’s spindle that oscillates a tungsten carbide tool such that the tool’s spherical tip repeatedly impacts the blade at a rate as high as 600 Hz. As shown the video above, the process takes advantage of the five-axis movement provided by the machine (in this case, a Starrag LX 051 turbine blade machine). Dengeling cycle time is comparable to a finishing milling operation and delivers a surface finish of 0.2 micron Ra. The dengeling head can be stored in the machine’s toolchanger magazine like any other tool when not being used.

Michael Koller, Starrag product manager, says the dengeling process changes the original structure of the boundary layers on the part’s surface to a depth of 10 mm. As in a hardening process with quenching phase, a “distortion” of the atomic lattice occurs as the tool impacts the part. Therefore, the internal compressive stress (thus endurance strength) can be increased at specific areas of the part with precise control. The amount of increased hardness depends on the type of material (see Table 1), and the process can be applied on virtually every material that can be processed by means of plastic deformation.

The dengeling process is said to offer a number of blade performance advantages. In terms of material fatigue, crack initiation and propagation can be suppressed through the residual compressive stress that’s generated combined with the smooth surface. This differs from a grinding process that creates a smooth surface by simply cutting the scallops that remain following milling, because the surface cracks will remain. The dengeling process closes those cracks while compressing, hardening and smoothing the surface.

Material wear resistance can be significantly improved, too, because of higher surface hardness and better finish, while the threat of stress corrosion cracking commonly due to surface tensile strain is minimized. This means parts that are exposed to changing dynamic loads will have better fatigue resistance and a longer life.

Although shot peening is also widely used for finishing and hardening turbine blades, Mr. Koller says initial tests demonstrate that the dengeling process is more targeted and more controlled, with all programming performed using Starrag’s RCS 7 dedicated blade CAM system. Therefore, there is no need for covering or masking part areas that are not to be treated, and it is possible to hone critical thin areas at a blade’s edge more selectively.


  • Bringing Anodizing In-House

    What’s it going to cost? How much space do I need? What environmental hassles will I encounter? How steep is the learning curve? Exactly what is anodizing? Here are answers to preliminary questions shops have about bringing anodizing in-house. 

  • A Model Camshaft Grinding Process

    Optimizing a camshaft lobe grinding cycle has traditionally been based less on science and more on educated guesswork and numerous test grinds. Now, computer thermal modeling software can predict areas where lobe burning is likely to occur, in order to determine the fastest possible work speed that won't thermally damage lobes and greatly reduce the number of requisite test grinds.

  • How To Machine Aircraft Titanium: The 8-To-1 Rule For Finishing Walls And Ribs

    Part of a series of articles on more efficient machining of pockets in titanium parts, this article makes the case for a tool with many cutting edges, and describes how best to apply it.