CNC Machining of Single-Start Turbine Threads: Achieving Ultra-Precise Steam Turbine Performance
A steam turbine is a rotary power machine that converts the thermal energy of high-temperature, high-pressure steam into mechanical energy.
It is now widely used in numerous fields, including thermal power generation, nuclear power generation, marine propulsion, and petrochemicals.
Within the turbine’s structural composition, the tooth-shaped single-start thread serves as the core connection between the turbine body and valves.
The machining quality of this thread significantly determines the sealing performance between the high-pressure outer cylinder and the main steam valve.
The effectiveness of this sealing directly impacts the overall performance and operational efficiency of the entire turbine unit.
Therefore, ensuring the precision and quality of tooth-shaped single-start thread machining holds critical and irreplaceable significance for the stable operation and high-efficiency performance of steam turbines.
Analysis of Machining Challenges
The dimensions of single-start threads encompass multiple specifications, including pitch sizes of 5 mm, 9 mm, and 20 mm.
Corresponding profile dimensions and tolerance requirements vary across these pitches.
Figure 1 illustrates the thread dimension parameters, and Table 1 provides the detailed specifications.
Note:
P₁ = single-start thread pitch
d₃ = thread root diameter
t₁ = tooth depth
d, e₁, e₂, r₁, r₂, and r₃ = see details in the table
Compared to conventional thread machining, single-start threads present the following machining challenges:
1) The profile surface structure of single-start threads is complex, with thread contours composed of multiple curves, increasing machining difficulty and complexity.
Additionally, the larger thread dimensions and substantial machining allowance of the workpiece render conventional threading methods unsuitable.
Consequently, engineers must develop multi-depth machining strategies to ensure both efficiency and precision throughout the process.
2) Single-start tooth-shaped threads demand exceptionally high surface quality.
Manufacturers must strictly control the surface roughness (Ra) of the bearing surfaces, as well as the inner and outer thread diameters, to 1.6 μm.
Meeting the Ra 1.6μm requirement demands meticulous tool selection and cutting parameter configuration.
3) To accommodate multiple pitch specifications, CNC programming must prioritize versatility, enabling adaptation to diverse thread dimensions.
4) Thread pitch tolerances of ±0.02 mm impose stringent demands on machine tool precision and machining techniques.
Processing Technology Plan
Standard series thread machining typically employs multiple passes with forming tools, utilizing the tool’s contour to complete tooth profile machining.
However, the tooth structure of single-start threads is unique, rendering this method unsuitable.
For such specialized tooth profiles, manufacturers must adopt rotary machining methods such as turning and boring.
Multiple incremental passes using a copy-milling process produce the desired thread shape.
Based on detailed process analysis, this study developed a layered helical turning process plan.
It comprises roughing and finishing operations, with the roughing/finishing boundaries for the single-start thread shown in Figure 2.
The black area indicates the roughing zone, where a 0.2mm roughing allowance is set.
During roughing, layered cutting removes excess material layer by layer.
For finishing, a single-pass turning method ensures final thread precision and surface quality.
Tool Selection
The material for the high-pressure outer cylinder of steam turbines is chromium-molybdenum-vanadium cast steel, a high-performance alloy cast steel with a typical hardness of 25–40 HRC.
Due to its high hardness, tool life during machining is reduced by 30%–50% compared to machining ordinary carbon steel.
Additionally, this material exhibits work-hardening tendencies and poor thermal conductivity, characteristics that increase machining difficulty and complexity.
The process selected carbide-coated tools to enhance wear resistance and thermal shock resistance.
Simultaneously, heat dispersion reduces localized tool temperatures.
Considering the high surface quality requirements for single-line thread profiles, contouring radius inserts were chosen to effectively improve machining accuracy and reduce surface roughness values.
After a comprehensive evaluation, the study selected a 35° rhombic turning tool from a specific brand (see Figure 3), as its performance and geometric parameters meet the requirements of high-precision machining.
Following tool selection, the study conducted a thorough interference verification of the tool’s motion trajectory to ensure that no interference occurs between the tool and the workpiece during machining (see Figure 4).
For thread features with pitches of 9 mm and 20 mm, the root radius r1 = 0.80 mm, and inserts with a tip radius of 0.8 mm can be selected.
For thread features with a pitch of 5 mm, the root radius r1 = 0.45 mm, and inserts with a tip radius of 0.4 mm can be selected.
Development of CNC Programs for Vertical Serrated Thread Machining
Calculation of Key Program Parameters
The key parameter calculation involves determining the X and Z coordinates of the starting point.
The process divides the X-axis into equal-distance layers, and calculates the X-coordinate by incrementing it according to the specified step size.
The Z-coordinate calculation involves first determining the Z-axis start and end points corresponding to the X value.
After establishing these points, the process calculates the coordinates for each starting point based on the Z-axis step increment.
◊ Tooth Profile Segmentation
According to the tooth profile characteristics, the tooth shape can be divided into five segments L1 to L5 for separate calculation (see Figure 5).
Among these, the root radius r1 represents the tool tip clearance and does not require calculation, so segment L3 can be disregarded.
◊ CNC Variable Definitions
Using common variables in CNC machine tool systems, set the straight edge of the thread profile as the Z-axis reference, with an X-coordinate value of R24.
The tool tip radius is R11, and the pitch is R40. Define t1, e1, e2, r1, r2, and r3 as R41, R42, R43, R44, R45, and R46 (refer to Figure 1 for details of t1, e1, e2, r1, r2, and r3).
The calculation program is as follows.
Setting Program Parameters
The program categorizes parameters into four types: primary input parameters, machining parameters, tooth profile parameters, and calculation parameters.
Engineers must specify the primary input parameters during programming based on drawing dimensions.
Machining parameters derive from trial cuts and actual machining experience and generally require no modification.
Engineers define tooth profile parameters according to vertical sawtooth thread standards, and these parameters are typically non-modifiable.
Calculation parameters represent variables used in program computations and do not require initial value specification. The program is as follows.
◊ Main Program Parameter Specifications
The main program for thread machining must be modified according to different thread types, while subroutines remain fully fixed.
Simply compile the main program based on specific requirements and call the fixed subroutines.
Program Refinement and Supplementation
After completing the above tasks, the process programs the X and Z axes cycles according to the predefined interval divisions, ultimately finalizing the machining program for vertical sawtooth threads.
When machining external threads, processing proceeds from outer to inner diameters, starting with a larger diameter and ending with a smaller one.
For internal threads, machining proceeds from inner to outer diameters, beginning with a smaller diameter and concluding with a larger one.
The process divides the machining program into roughing and finishing operations.
For simplified usage, engineers consolidate these operations into a single subroutine.
During program execution, decisions are made based on the allowance parameter: if Allowance > 0, the program jumps to the roughing segment; if Allowance = 0, it jumps to the finishing segment.
In special cases, it may be necessary to machine only a specific section of the thread rather than the entire tooth profile.
To accommodate this, the subroutine includes a section selection parameter.
Based on this parameter, the program can directly jump to the desired section for individual machining.
Program Simulation and Actual Machining Results
Simulation verification was conducted using simulation software for the completed CNC macro program.
The simulated machining process is shown in Figure 6, while the simulated results for the single-line thread profile are presented in Figure 7.
These simulation results further validate the accuracy of the CNC macro program.
The actual machining results of the single-line thread profile are shown in Figure 8.
Inspection and measurement confirm that all machined dimensions meet the technical requirements specified in the drawings, validating the effectiveness of this machining method.
Conclusion
To address the challenges of machining single-start threads for turbine teeth, engineers have proposed a contouring machining process based on CNC macro programs.
By rationally selecting cutting tools, optimizing machining parameters, and utilizing macro programs to achieve precise machining of complex thread profiles, both machining efficiency and quality have been effectively enhanced.
This approach meets the high-precision machining requirements for single-start threads in turbine teeth.
What are single-start threads in steam turbines, and why are they important?
Single-start threads are tooth-shaped threads that connect the turbine body to valves, forming the primary seal between the high-pressure outer cylinder and the main steam valve. Their machining quality directly affects turbine sealing performance, operational efficiency, and long-term reliability, making precise manufacturing critical for safe and efficient turbine operation.
What machining challenges are associated with single-start threads?
Single-start threads present several challenges:
Complex thread profiles with multiple curves increase machining difficulty.
Large dimensions and substantial material allowance make conventional threading methods unsuitable.
High surface quality requirements, with Ra tolerances of 1.6 μm on thread surfaces and diameters.
Tight pitch tolerances (±0.02 mm) demand exceptional machine precision.
Versatile CNC programming is needed to accommodate varying pitch sizes of 5 mm, 9 mm, and 20 mm.
What processing strategy is used for single-start thread machining?
Engineers adopt a layered helical turning process comprising roughing and finishing operations. Roughing removes excess material incrementally, while finishing ensures precise thread geometry and surface quality. Rotary machining methods, including turning, boring, and copy-milling, are used instead of standard multi-pass forming tools due to the thread’s unique tooth structure.
How are cutting tools selected for machining turbine threads?
Due to the high hardness and work-hardening tendency of chromium-molybdenum-vanadium cast steel, carbide-coated tools are selected to improve wear resistance and thermal stability. Contouring radius inserts and a 35° rhombic turning tool are used to enhance accuracy, reduce surface roughness, and prevent interference with the workpiece during machining. Tool tip radii are matched to thread pitches for optimal performance.
How does CNC macro programming enhance thread machining precision?
CNC macro programs calculate X and Z coordinates, layer divisions, and tool paths for complex thread segments. Parameters are categorized into primary input, machining, tooth profile, and calculation variables, allowing roughing and finishing operations to be automated or manually adjusted. This ensures precise machining of complex thread profiles and supports selective section machining when necessary.
How is thread quality verified after machining?
Thread quality is confirmed through simulation, inspection, and measurement. CNC program simulations validate the machining process, while physical inspection of finished threads ensures compliance with dimensional and surface roughness requirements. This approach guarantees high-precision machining suitable for single-start threads in turbine teeth, meeting strict operational and efficiency standards.