In the machining of gas turbine wheel disc, the machining of wheel groove is the most difficult and the largest workload, which is the key technology.
At present, several major gas turbine plants in China have introduced large-scale complex and high-precision wheel slot broaching machines through technological innovation, and the processing of wheel disc wheel slots has gradually changed from traditional special machine milling to broaching.
The rotor grooves of large gas turbine units are fir tree type blade root grooves with large profile. Therefore, the broach profile required is large (i.e. the tooth pitch of the broach is large and the machining depth of the broach is deep).
The cutting amount is large, the cutting load is large, and the tool wear is fast.
Moreover, the broach of the gas turbine is made of high-performance high-speed steel with high strength and poor cutting performance.
Therefore, the structure of the broach is complex and the machining is difficult Low efficiency.
（1） Technological characteristics of broaching fir tree tenon
Several major domestic steam turbine plants have imported large complex and high-precision wheel groove broaching machines from abroad, as well as a large number of precision combined Broaches for processing fir tree type wheel grooves of gas turbine discs.
Only a few major tool manufacturers in Europe and Japan can manufacture such broach abroad, such as Hoffmann in Germany, SMOC in France and Nachi in Japan.
Fig. 6-22 shows the fir tree tenon of the turbine disk.
Fig. 6-22 fir tree tenon of turbine disk
The broach for processing the blade root groove of the gas turbine disc is an external broach, which is composed of multiple groups of broaches.
For example, the longest is the third level disc broach of class F gas turbine, which is 21m long.
It is divided into seven groups, each with a length of 3M, and consists of six sections of broaches with a length of 500mm.
The shape of fir tree type tenon groove of a gas turbine impeller is a continuous curve, and the top and bottom of the tenon teeth are circular arcs.
The fir tree tenon broaching adopts a complete set of broaches composed of rough broaches and fine broaches to complete rough machining and fine machining respectively.
The broaching form is divided into non profile part and profile part.
The broaching process realized by the complete set of broaches is shown in Fig. 6-23.
The profile processing of the tenon and groove is realized through 12 processes of rough broaching and fine broaching.
Fig. 6-23 schematic diagram of broaching process with complete broach
（2） Precautions for selection of fir tree type tenon broach
Precision combined groove broach for machining fir tree type tenon and groove of gas turbine disc, hereinafter referred to as broach.
(1) The broach is made of high-performance high-speed steel.
Through the ratio of alloy elements and unique heat treatment process, the hardness, strength, thermal hardness, toughness and wear resistance of the product are improved, and the service life of the product is increased.
The selection of broach material needs to comprehensively consider the processing performance of wheel disc material, wheel disc accuracy requirements, tool cost, broach grinding and other factors.
It is recommended to select molybdenum cobalt and tungsten cobalt superhard high-speed steel materials, such as W2Mo9CrVCo8.
Cemented carbide blade is stronger than high-speed steel in hardness, wear resistance and cutting parameters, but cemented carbide blade can not bear large impact force, its strength is one third of that of high-speed steel, and it is difficult to heat treat.
Class K cemented carbides have good toughness and thermal conductivity, and can be used for rough machining.
Class m cemented carbides are suitable for finish machining, but the tool life is low.
(2) The precision combined broach for machining fir tree type tenon groove of gas turbine disc is composed of rough broach, semi fine broach, fine broach and finishing broach, including rough broach, rough broach, semi fine broach, fine broach, fine broach and finishing broach.
High speed steel broach is used for broaching tenon and groove of superalloy materials.
The back angle of rough broach is 5 ° ~ 6 °, and that of fine broach is 4 ° ~ 5 °;
The broach made of powder high speed steel has a front angle of 5 ° ~ 8 ° and a rear angle of 4 ° ~ 5 °.
(3) The deep broach of the rough broach wheel groove and the wide broach of the rough broach wheel groove in the rough broach set adopt a split structure.
The high-performance high-speed steel is selected for the broach teeth and the high-quality tool steel is selected for the tool body, which saves 30% of the raw material cost of the broach and greatly improves the cost performance ratio of the broach.
(4) The fine drawing wheel groove profile broach adopts the same profile wheel cutting structure to realize the same profile forming processing of the fir tree type mortise and groove, reduce the cutting force of the wheel groove broach, improve the tooth surface accuracy of the machined parts, ensure that the error value of the fir tree type mortise and groove tooth profile is <0.01mm, and ensure that the tooth pitch error of the fir tree type mortise and groove tooth surface is within 0.005mm.
(5) The broach tooth groove layered Multi Tool rough and fine milling and tooth profile NC grinding processing technology, and the design of the matched broach milling tooth groove and NC grinding device and other advanced special tooling fixtures are applied to the whole production process of the precision combined broach for the fir tree type tenon groove processing of the gas turbine disc, which increases the rigidity of the tool processing system and improves the machining accuracy of the tool.
Whether the profile is qualified or not mainly depends on the thermal deformation of the disc material or the elastic deformation of the disc material.
Generally speaking, the broaching speed is 1 ~ 2m/min, the tooth lift of the rough broach is generally 0.05mm, while the tooth lift of the fine broach is 0.01 ~ 0.02mm.