Generally speaking, a center-pull gas turbine rotor consists of components such as the front hub, compressor disk, torque disk, turbine disk, mini-disk, rear hub, center-pull rod, and rod nut.

These components are connected in series by the center tie rod and engaged via the tightening of the nut and the meshing of the Hesse teeth, thereby achieving automatic centering, precise positioning, and torque transmission.

The production control of the disk is a critical step in the entire rotor manufacturing process, and the machining of the disk’s tenon slots is of paramount importance.

The tenon slots on the disk are typically distributed uniformly around the circumference, and their machining sequence follows the grinding of the Hesse teeth.

Many researchers have studied the machining methods for disc splines, concluding that broaching offers high efficiency and allows for easy control of surface quality.

However, broaching requires specialized broaches; a single set of broaches can only accommodate one profile, and these tools are expensive, with individual sets costing up to 2 million yuan.

For high-volume disc production, broaching can improve machining efficiency, meet production deadlines, and spread tooling costs.

However, for small-batch production of wheel rims, tool costs cannot be spread out, making broaching an uneconomical option.

Therefore, in response to the need to reduce tool costs, this paper proposes a milling method for machining heavy-duty wheel rim tenon grooves, aimed at lowering processing costs while ensuring machining accuracy.

Analysis of Tenon-and-Groove Structures and Machining Challenges

Figure 1 shows a schematic diagram of the 2nd to 4th stages of a reheat compressor disc manufactured by Shanghai Turbine Works (STW).

The third-stage impeller discs have identical structures and similar dimensions.

Both end faces feature Hess-type teeth, and the tenon slots are evenly distributed circumferentially on the outer circumference of the impeller discs.

Additionally, the symmetrical planes of the tenon slots are inclined at a specific angle relative to the centerline of the impeller discs, all at 22°.

• Dovetail Groove Design and Machining Challenges

The dovetail grooves of stages 2–4 on the rotor disc of this reheat compressor have identical profiles and feature a dovetail structure, as shown in Figure 2.

The dimensions of the dovetail groove profile are relatively large, with both width and depth exceeding those of conventional profiles.

In particular, the variations in width along the straight groove, throat, and bottom sections are especially pronounced, posing a significant challenge for machining and tooling design.

As shown in Table 1, this structural design imposes higher requirements on the geometric tolerances and surface roughness of the profile, as well as entirely new requirements for the contour accuracy of the profile.

Regardless of whether broaching or milling is used, the machining difficulty is significantly increased.

Table 1: Comparison of Processing Requirements for Normal and Heavy-Duty Wheel Hub Shafts

• Material Properties and Machining Difficulties

The operating temperature of the 2nd to 4th stage impeller discs in the reheat compressor is approximately 500 °C.

The design utilizes forged alloy steel components with high strength and good toughness.

The material grade is 26Cr2Ni4MoV, which is classified as a difficult-to-machine material.

During forming and milling, high cutting resistance leads to issues such as high cutting temperatures and tool wear.

As a disc-shaped component, the wheel has relatively low rigidity and is prone to vibration during machining.

These factors ultimately affect the dimensional accuracy and surface quality of the wheel’s tenon slots.

• Key Considerations for Tenon Slot Milling

In summary, to successfully complete the milling of the wheel’s tenon slots, it is necessary to select an appropriate tooling scheme to reduce cutting resistance and design a clamping method that ensures stable clamping to enhance the wheel’s rigidity during milling.

Selection of Machining Equipment

If milling is used, the engine plant has two equipment options to choose from:

1) CNC double-head milling solution.

This requires the design of a fixture to effectively secure the second through fourth stage impeller discs of the compressor together, forming a single integral rotor, and machining them using a method similar to that used for milling turbine rotor blade slots.

Based on the part’s characteristics, the fixture should consist of four components: a front shaft end, tension rods, a rear shaft end, and tightening nuts.

The advantage of this solution is that it allows for the simultaneous milling of the dovetail grooves on the third-stage impeller, resulting in high efficiency.

The disadvantage is that during tenon slot milling, the cutting force acts horizontally, resulting in significant axial stress.

Since the individual components are held together via the tightening force of the central tie rod and tie rod nut through Hesse tooth engagement, the tightening force must exceed the milling force to prevent axial movement of the parts during machining.

However, the meshing sections of the parts may deform due to excessive tension, leading to part damage and posing a relatively high overall risk.

2) CNC boring and milling machine solution.

This requires a high-precision rotary table and the design of a clamping fixture to reliably secure the parts.

The wheel is mounted horizontally, and the entire setup primarily withstands the cutting forces generated by the tool moving downward.

Consequently, the clamping force required for part mounting is not high, and the fixture design is simpler.

Additionally, this clamping method is similar to the fixture used for subsequent drilling of the disc, making it easier to achieve fixture versatility and significantly reducing fixture costs.

After comprehensively considering the risks, difficulty, and cost of machining the tenon groove on the disc, the CNC boring and milling machine solution was ultimately selected.

Milling Tooling Strategy

As mentioned earlier, to successfully complete the milling of the disc tenon groove, it is necessary to develop an appropriate tooling strategy to reduce cutting resistance; therefore, when designing the tools, one should avoid forming milling cutter configurations with large cutting allowances as much as possible.

Unlike the solid form-milling cutters used for turbine rotor tapered grooves, the profile of the gas turbine disc tenon groove is divided into upper, middle, and lower sections for separate machining based on tolerance requirements, as shown in Figure 3.

The tenon groove can be divided into three parts: the upper section, the middle throat, and the bottom profile.

During rough milling, a standard end mill is selected to remove most of the stock in a segmented milling process, thereby reducing cutting forces and preventing blade vibration; during semi-finishing, a contour milling method is used to mill the overall contour, ensuring uniform finishing allowances, which helps guarantee the final machining accuracy of the dovetail groove.

• Process Breakdown and Tooling Plan

Based on the structure of the tenon and mortise, the entire milling process can be broken down into six steps, as shown in Figure 4.

Given the machining characteristics of each operation, there are numerous tool options available.

Following an initial review of the proposed plan, the tool combination scheme shown in Table 2 was established.

However, for operations 2 and 5, there are still two tool options each, and a final decision must be made based on the results of cutting tests.

Table 2: Tooling Plans for Each Process Step

• Key Tool Selection for Critical Steps

Step 2 involves rough milling the bottom straight groove. Options include a 29 mm diameter insert-type end mill or a 25 mm diameter wavy-edge end mill.

The former has a diameter closer to the dimensions of the middle throat, so it does not need to be milled in two passes, theoretically resulting in higher cutting efficiency.

The latter uses a wavy-edge design, which allows for smoother cutting.

As a solid-body tool, it offers better rigidity; however, it requires milling from both sides, resulting in a longer travel distance and lower machining efficiency.

Step 5 involves finishing the middle throat.

Two options are available: profile finishing milling using a profile finishing cutter, or contouring eccentric milling using an offset profile finishing cutter.

The former is similar to the milling method for turbine rotor spindle-shaped grooves, cutting on both sides during milling, which is highly efficient but involves a large cutting volume, leading to rapid tool wear, inevitable tool deflection, and high surface roughness.

The latter uses an eccentric milling cutter, whose tool profile has a certain eccentricity relative to the tenon groove profile.

During machining, single-sided milling is performed over two passes, resulting in a longer travel distance; however, the single-pass cutting volume is small, there is no tool deflection, and the surface roughness is low.

Figure 5: Test machine and test disc

• Experimental Setup and Verification Method

Based on the differences in tooling schemes for steps 2 and 5 described above, four tooling scheme combinations were formed.

This paper conducts cutting tests to compare each scheme and identify the optimal combination.

The Wuzhong 200 CNC boring and milling machine, which will be used for the final product, was selected as the test equipment.

Compressor wheel test specimens made of the same material grade were chosen to ensure consistency between the test results and the final machining outcome.

The test machine and test disk are shown in Figure 5.

The Wuzhong 200 CNC boring and milling machine features a large-diameter work table and a rigid spindle, making it highly suitable for machining the tenon grooves of the disk.

Additionally, the test disk has a diameter close to that of the final product and sufficient thickness to accommodate the cutting tests.

To verify the accuracy of the final milling dimensions, this test employed a dual-inspection method using both a test cylinder and a product blade root.

The machined tenon groove profiles were subjected to go/no-go inspections to ensure the reliability of the tooling scheme.

Photographs of the test cylinder and product blade root inspection site are shown in Figure 6.

• Comparative Results and Optimal Scheme Selection

The final test results are shown in Table 3. Although all options passed the test on the sample column and the product blade root, there were certain differences in terms of cutting stability, machining efficiency, and machining quality:

1) Step 2 Cutting Performance Comparison of Two Milling Cutters

In Step 2, the 29-mm-diameter indexable insert milling cutter exhibited poor rigidity and was used for full-cut milling, with a cutting depth of only 1 mm per pass; the 25-mm-diameter fluted end mill performed relatively smoothly during rough milling, with a cutting depth of 5 mm per pass, resulting in significantly higher actual machining efficiency than the former.

2) Step 5 Surface Quality Improvement via Eccentric Milling Strategy

In Step 5, the form milling on the up-cut side has protrusions; single-pass milling cannot ensure that the surface roughness on both sides meets the requirements; eccentric milling allows for adjustment of the feed direction on both sides, ensuring machining quality on both sides and achieving a lower surface roughness.

After comparison, Scheme 4 yielded the best overall machining results; therefore, it was selected as the final machining scheme for this tenon slot milling operation.

Table 3: Results of Tooling Plans Combined for Each Process Step

Clamping and Indexing Device

Designing a stable and reliable clamping and indexing solution for the worktable of a CNC boring and milling machine is also a key focus of this study.

The clamping and indexing device designed in this paper is shown in Figure 7.

It consists of seven components. The entire turntable clamping and indexing device is powered by an indexing turntable and supports manual indexing.

A transition plate is mounted on the indexing turntable; care must be taken to adjust the concentricity between the two during installation.

The adapter plate is pre-drilled with holes and slots, allowing it to be securely fastened to the indexing turret via bolts.

The upper end face of the adapter plate features a Hesse tooth structure identical to that of the wheel.

The wheel and adapter plate engage via the Hesse teeth on their end faces, enabling automatic centering, accurate indexing and positioning, and torque transmission.

The structure is shown in Figure 8.

• Installation and Alignment Process

To ensure the stability of the turntable during placement and to facilitate its hoisting and centering, a circular mandrel with a diameter slightly smaller than the turntable’s inner bore is secured at the center of the table surface prior to the actual hoisting of the turntable.

During hoisting, the center hole of the turntable is fitted over the circular mandrel, then slowly lowered while fine-tuning the turntable’s angle to ensure that the Hesse teeth on the lower end face of the turntable fully engage with those on the upper end face of the transition disc, thereby completing the turntable hoisting.

At this point, the turntable remains concentric with the rotary table.

After verifying with a dial indicator that the runout of the turntable does not exceed 0.02 mm, place the circular clamping plate on top and tighten the nuts to complete the installation of the entire turntable clamping and indexing device.

• Indexing Monitoring System and Accuracy Control

After adjusting the circular runout of the entire indexing device, the indexing monitoring system must be installed.

This system consists of an indexing transmission rod, a circular grating, and a mounting bracket, enabling real-time monitoring of the turret’s indexing position to prevent part scrap caused by deviations during the indexing process.

The final on-site installation is shown in Figure 9.

This solution employs a high-precision indexing table manufactured by Nihon Kenkyu.

While it offers high indexing accuracy, its indexing deviation and repeatability under heavy loads require further verification.

Therefore, prior to machining each turret, the indexing deviation must be calibrated using a circular encoder and properly recorded.

Additionally, the repeatability of the same tenon slot across different machining steps must be compared.

Taking the third-stage compressor disc as an example, this disc has 35 tenon slots evenly distributed around its circumference.

Before and during machining, the difference between the indexing reading of the rotary table and that of the circular grating monitoring device is recorded each time indexing is confirmed.

The digital display screen of the monitoring device and the results are shown in Figures 10 and 11.

As shown in Figure 11, the maximum absolute deviation throughout the entire circumferential indexing process is 0.004°, which meets the machining requirement of a maximum error of 0.005° specified in the drawings.

• Repeatability Requirement and Error Control

Table 1 shows the dimensional tolerances for the tenon groove profile of a re-milled tenon.

The minimum contour tolerance requirement for the tenon groove is 0.03 mm.

To meet this geometric tolerance requirement, the repeatability deviation across the six machining steps during the milling of a single tenon groove must be strictly controlled.

Assuming E is the repeatability deviation of the rotary table and R is the radial positioning dimension of the tenon slot, then E must satisfy:

Ｅ ≤ （０． ０３∗１８０）／（Ｒ∗π）

In the above equation, R is 760 mm; calculation shows that E ≤ 0.002°.

Therefore, during the actual machining process, it is also necessary to record the repeatability of the same tenon slot at different processing stages to ensure that the repeatability deviation does not exceed 0.002°.

Verification of the Milling Process Plan

After the indexing table, transition plate, and other fixtures have been installed and commissioned, and the workpiece clamping has been calibrated and confirmed to meet requirements, the indexing monitoring device is installed, and formal milling of the product can begin.

Before tool cutting begins, record the indexing values of the rotary table and the deviation monitored by the circular grating in real time according to the aforementioned requirements, ensuring the deviation does not exceed 0.005°.

Simultaneously, record the monitoring values for each tenon slot at different processing stages to ensure that the repeatability positioning deviation does not exceed 0.002°.

Only after confirmation is the tool installed, and the corresponding CNC program is called to perform the milling operation.

The angle, structural dimensions, and positioning dimensions of the tenon groove are guaranteed by the machine tool’s accuracy and the CNC program.

• Machining Issue and Cause Analysis

However, a new issue arose during the formal milling of the wheel rim.

To ensure down-cut milling, the machining quality on the right side of the groove—where the eccentric-edged precision cutter moves upward from the bottom—was inferior to that on the left side.

Analysis suggests that under actual machining and clamping conditions, the wheel groove is left-handed.

Upward feed is required during right-side milling to maintain the down-cut direction, thereby generating upward cutting forces.

This reduces the rigidity of the clamping system.

The workpiece tends to “lift” upward, which causes tool vibration during milling and leads to a decline in the surface finish quality of the tenon groove.

• Optimization of Milling Strategy

To resolve this unexpected issue, the project team consulted the literature and decided to optimize the feed strategy for the eccentric finishing cutter.

The left side maintained the original down-to-up conventional milling, while the right side changed to down-to-up reverse milling.

This adjustment ensured that forces on both sides were directed downward, improving the rigidity of the clamping system.

• Final Machining Results

Final machining results confirmed that the optimized finishing strategy significantly improved the surface finish quality of the right side of the dovetail groove, as shown in Figure 12.

Results demonstrated excellent application performance.

After resolving the above issues, milling of the dovetail grooves for stages 2–4 of the compressor disk was successfully completed.

Machining quality fully met the drawing requirements. Product successfully passed the owner’s acceptance inspection.

Machining task reached successful completion.

Conclusion

This paper presents the design and optimization of a milling cutter for the dovetail groove profile on the disc of a reheat compressor.

By adopting a “milling instead of broaching” machining approach, the process reduces cutting resistance while ensuring dimensional accuracy of the dovetail groove, meeting geometric and positional tolerance requirements, and significantly reducing tooling costs.

At the same time, the following measures were implemented to ensure the positioning accuracy of the dovetail groove:

1) A transition disc fixture was designed utilizing the self-centering characteristics of the Hess teeth on the disc’s end face.

Serving as a transitional positioning device between the rotary table and the disc, it enabled precise positioning of the disc during clamping.

Furthermore, the engagement of the Hess teeth transmitted torque from the rotary table.

The torque determined the indexing for dovetail groove machining.

A circular grating indexing monitoring device was installed.

The device monitored the wheel indexing in real time. It ensured the indexing accuracy of the tenon slot machining.

The solution proposed in this paper reduces the machining costs of tenon slots on gas turbine compressor wheels.

It also provides a reference for the milling of tenon slots on large-sized wheels.