Producing components for modern gas turbines is not easy. Manufacturing professionals are often faced with the task of machining difficult-to-cut materials such as titanium alloys and superalloys to create complex shapes such as onion-shaped profiles and flutes. This difficulty is compounded by the fact that the turnaround time for machining the components must be as short as possible, while the process must be extremely precise.
Onion-shaped profiles are generally used in the compressor section upstream of modern gas turbines to affix 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 time-consuming and expensive. Looking for a more efficient solution, Walter has developed a process that uses profile mills.
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 dimensional accuracy to 20µm.
Walter unveiled this new process at the Starrag Technology Days exhibition in Germany by successfully milling 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 using indexable inserts means there is no need for subsequent grinding – necessary when using broaching tools – or the extra work that this grinding involves.
When roughing a flute measuring 3.9" x 3.9" (100mm x 100mm) – from the material 42CrMo4, the tools achieved a machining volume of just under 500cm³/min at a feed rate of 98.4ipm. (2,500mm/min). The standard tools included the new M4132 shoulder milling cutter and the M4002 high-feed milling cutter. Even when dry machining is employed, the M4002 effectively evacuates chip during plunge milling.
The three operations described below were conducted using Walter special bodies and standard indexable inserts. The finishing face mill achieved a dimensional tolerance of 0.03mm for the net shape. With a cutting speed of 260m/min and a feed per tooth of 0.18mm, the indexable insert milling cutters still achieved a surface finish of Ra = 0.35µm, significantly better than the required value of Ra = 1.6µm. If solid carbide milling cutters are used for the finishing operation, even smoother surface finishes are possible.
The concept was successful: A flute was fully machined in less than 11 minutes – including all tool change times. In addition, 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%.
During the demonstration, profile milling was also pitted against copy milling on the onion-shaped flute. 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 part, three special tools manufactured by Walter were used to mill the onion-shaped profile – two for roughing and one for finishing. The two profile mills for roughing the onion shape used 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 500cm3/min.
Walter engineers also employed the latest technologies for finishing the profile; specifically, using insert seats with a positioning accuracy of 5µm and high-precision special indexable inserts. The machining allowance for the last cut was a mere 0.3mm. All of the tools used were operated with internal cooling – an advantage for high-performance machining at cutting speeds of up to 310m/min. The initial results were positive, with the finishing face mill easily achieving the required dimensional accuracy of 20µm.
To enable comparison between the two methods, this alternative strategy was demonstrated in a second step using parallel strokes. Close contour roughing using standard tools was required for the profile to be completed with a maximum machining allowance of 0.5mm in the finishing stage that followed, using multi-axis machining.
Standard round insert cutters with the latest coating technology and up to four usable cutting edges were used. 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 proved suitable for semi-finishing and finishing operations using parallel-stroke milling or flank milling.
The bearing shoulder on an onion-shaped flute profile must meet strict requirements in surface finish and dimensional accuracy to withstand the forces exerted by a turbine as it rotates. During this demonstration, the bearing shoulder was produced with a single cut by the flank, and the rest of the profile was finished using parallel-stroke milling.
The Walter applications engineers’ findings are: 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 – with machining times reduced by about 50%.
Walter USA LLC
IMTS 2016 Booth #W-1700