Onion-shaped profiles are generally used in the compressor section upstream of modern gas turbines to fix the blades to the shaft. Depending on the design of the thermal shaft, a number of different machining processes may be used, such as broaching, a method that is both incredibly time-consuming and expensive. In response to these issues, the experts at Walter AG have developed a process that uses profile mills. When it was first presented publicly at the Starrag Technology Days exhibition in summer 2015, the new machining concept elicited a great deal of interest from those in the industry. One of the keys to the success of the new process is its use of innovative technologies, which include a Tiger tec Silver coating for roughing the profiles and high-precision profile mills with indexable inserts that can be used to achieve a dimensional accuracy of up to 20 μm.
Looking At The Cost-Effective Alternative
Walter AG accepted the challenge at the Starrag Technology Days 2015 exhibition and milled an onion-shaped flute. Four profile mills and three standard mills with indexable inserts were used in the demonstration. One advantage of the new process is that the use of indexable inserts means that there is no need for a subsequent grinding stage (as is necessary when using broaching tools) or any of the extra work that this grinding involves.
When roughing a flute measuring 100 x 100 mm (W x D) from the material 42CrMo4, the tools achieved a machining volume of just under 500 cm³/min at a feed rate of 2500 mm/min. The standard tools included the new M4132 shoulder milling cutter and the M4002 high-feed milling cutter. Even where dry machining is concerned, the M4002 boasts effective chip evacuation during plunge milling.
The three operations described in the following were conducted using Walter special bodies and standard indexable inserts. The finishing face mill achieved a dimensional tolerance of 0.03 mm for the net shape. With a cutting speed of 260 m/min and a feed per tooth of 0.18 mm, the indexable insert milling cutters still achieved a surface finish of Ra = 0.35 μm, significantly below the required value of Ra = 1.6 μm. If solid carbide milling cutters are used for the finishing operation, even smoother surface finishes can be achieved.
The concept was successful: A flute was fully machined in under 11 minutes – including all tool change times. This proves that, provided the right tools and technologies are used, milling is an ideal alternative to broaching when machining an onion-shaped flute. In addition to this, profile mills are able to perform the same task approximately twice as quickly as broaching tools, which translates to a productivity increase of around 100 percent.
Increasing Productivity In Milling
Onion-shaped flute on a gas turbine compressor disc.
Often machined using expensive broaching tools,
but milling cutters are now an option.
Profile milling was also pitted against copy milling on the onion-shaped flute profile machining station featured at the Starrag Days exhibition in Rorschacherberg. The challenge was to produce an onion-shaped flute profile for a blade using just three profile mills and exactly three cuts. For the profile milling demonstration, three special tools manufactured by Walter were used to mill the onion-shaped profile – two for the roughing and one for the finishing. The two profile mills for roughing the onion shape were operated with Tiger tec Silver indexable inserts: Standard tangential indexable inserts with four usable cutting edges were used in the first milling cutter for the top of the onion shape. Triangular indexable inserts with three cutting edges were used in the second roughing tool for the bottom of the profile. Both milling cutters operated at a machining volume of 500 cm3/min.
What was seen as the latest technologies were also applied for finishing the profile specifically, using insert seats with a positioning accuracy of 5 μm and high-precision special indexable inserts. The machining allowance for the last cut was a mere 0.3 mm. All of the tools used were operated with internal cooling – a decided advantage for high-performance machining at cutting speeds of up to 310 m/ min. The initial results were positive, with the finishing face mill easily achieving the required dimensional accuracy of 20 μm.
To enable comparison between the two methods, the alternative strategy was demonstrated in a second step using the parallel-stroke technique. Close contour roughing using standard tools was required in order to enable the profile to be completed with a maximum machining allowance of 0.5 mm in the finishing stage that followed, using multiaxis machining. Standard round insert cutters with the latest coating technology and up to four usable cutting edges were used here. The profile was roughed in a large number of small, individual steps until it came close to its final shape. Conical ball-nose end mills are ideal for semi-finishing and finishing operations using parallel-stroke milling or flank milling. This technology was employed here. The bearing shoulder on an onion-shaped flute profile must meet strict requirements in terms of surface finish and dimensional accuracy in order to withstand the forces exerted by a turbine as it rotates. The bearing shoulder was produced with a single cut by the flank, and the rest of the profile was finished using parallel-stroke milling.
Tool set comprising three standard and four
special profile mills for onion-shaped flutes
on a compressor disc for a gas turbine rotor.
The findings of the applications engineers from Tübingen can be summarised as follows: Parallel-stroke milling with standard tools is an economical solution but requires a multi-axis milling machine. For large-scale manufacturing, profile mills offer significant benefits in terms of productivity – machining times are reduced by about 50 percent.
Mobile, Faster & Smarter
In the age of Industry 4.0, tools can also be virtual. Apps and software solutions are simplifying daily work at the machine and increasing productivity. Tools can be selected at the click of a mouse and managed optimally – while on the move and at any time. Walter AG has a wide range of solutions which are tailored to the individual needs of its customers.
This means that they can always select the best tool and the most cost-efficient machining process. In addition, starting values and cutting data can be determined in a matter of seconds. The user benefits from process reliability and saves valuable time and money. These innovative solutions enhance Walter’s range of machining services and ensure a consistently smooth process – even from outside the production hall.
An overview of apps and software solutions:
• Dramatically reduce wear – with the new wear optimisation app
• Select customised tools – with Walter GPS
• Determine starting values easily – with Feeds & Speeds app
• Calculate cutting data accurately – with the machining calculator
• Access all the tool catalogues – with the e-library
• Select tools conveniently – with the Toolguide
Wear Optimisation App To Reduce Wear
This latest solution helps users to get the best use out of their tools because it reduces wear rates and therefore reduces the associated costs. The app can identify all wear patterns on indexable inserts and solid carbide tools in turning, drilling, threading or milling applications. Each wear pattern is shown with a zoom function, both graphically and in high-quality photos.
For every wear pattern, the app provides the user with recommendations on how to reduce wear, – and therefore increasing the service life of their tool and save costs. The app works offline on all current mobile devices, such as smartphones, notebooks and tablets, with iOS or the Android operating system, as well as on a Windows PC that operates on Windows 7 or a more recent version.
Selecting The Right Tools
One thing the device does is that it makes sure that one always has the right tool for turning, drilling, threading and milling. It takes just a few steps for you to find the best solution for the component to be machined: Whether online or offline – simply use the tool search or the application recommendation. The recommendations from Walter GPS are so precise that users can choose to search with or without a tolerance for the diameter, with or without a regrind length, or even according to complex pilot-drilling strategies.
This means that the user receives not just the best tool recommendation, but also the perfect machining strategy, including the cutting data. This can be printed out as a PDF document and then entered into the NC program on the processing machine. Thanks to an interface with the Walter ToolShop, recommended tools can be kept in a wish list and ordered at the click of a button.
Determine Starting Values Easily
The “Feeds & Speeds” app determines the starting values for milling, drilling and turning operations. Navigation is intuitive: From the application, workpiece material and tool type, to the cutting tool material, the feed and the cutting speed. The user always has the very latest data. This also saves considerable time since there is no need to search in the tool catalogue.
The Walter machining calculator can be used to calculate the cutting data for turning, drilling, threading and milling. This includes torque, drive output and machining volume, as well as operating time, main cutting force and chip thickness. The data can be exported, and it can now also be uploaded via CSV import. In addition, the calculated results can be shared via a smartphone.
With the app, one can view catalogues, find tools, set bookmarks, add tools to your basket – you can do it all with the e-library. This digital library contains full and supplementary catalogues as well as selected brochures. You can look for tools in a matter of seconds using categories or a search function. These can be transferred to the “ToolShop” via an interface. The app works offline on all current mobile devices, such as smartphones, notebooks and tablets, with iOS or the Android operating system, as well as on a Windows PC that operates on Windows 7 or a more recent version.
The Walter Toolguide app provides mobile access to all the tools you require from the Walter range. The user is also shown 2D and 3D tool models as well as a detailed tool data sheet. Content is displayed using responsive web design (RWD), which means that the content is automatically adapted to the screen size of the end device being used. For users of smartphones or tablets in particular, this makes using the website significantly more convenient.
