Heat Treatment Process for Independently Walking Guide Block of NC Double Column Vertical Lathe

The independent walking platform of CNC double-column vertical lathes produced by our company is equipped with a set of guide blocks (as shown in Figure 1). These guide blocks are arranged in a straight line and are mounted on the column, functioning similarly to two rolling guides. The walking platform can be raised or lowered by rolling along the guide blocks to meet specific operational needs. The design uses 45 steel material and features a flat butt-joint method. The hardness of the two rolling surfaces of the guide blocks must be between 52 to 57 HRC. The quenching process is performed as illustrated in Figure 2.

However, there are several challenges in the original design: First, the original design has a sinker hole located only 5 mm from the side. If the hole is drilled before quenching, it may cause stress deformation or even cracking due to the increased hardness of the counterbore surface, making it difficult to process afterward. Second, both ends of the quenched surface are right-angled, which makes them prone to overheating during the quenching process, potentially leading to cracking or deformation. Additionally, since the combined guide rail functions as a rolling guide, no soft areas, voids, or regions of low hardness are allowed between the rails.

First, the Solution

To address these issues, we implemented the following solutions:

(1) After reviewing the assembly drawing and understanding its usage, we advised the designer to move the hole inward so that the edge of the hole is at least 10 mm away from the side.

(2) We communicated with the technologist and requested that the hole be processed after countersinking.

(3) To ensure that the mating surfaces meet the required specifications, we increased the total length of the workpiece by 6 mm and added 3 × 45° chamfers at both ends. These chamfers are removed after quenching through machining.

(4) For the right-angled edges of the quenched surface, we used a partially inserted magnet to reduce heat concentration at the sharp corners (as shown in Figure 3).

Second, the Production Process

The production process for the guide block includes the following steps: forging → normalizing → straightening → thermal aging → machining → intermediate frequency induction quenching → straightening → tempering.

Normalizing

After forging, normalizing helps refine the grain structure, improve dimensional stability, and reduce the risk of deformation or cracking during subsequent quenching. If the forging blank bends more than 2 mm after normalizing, it should be straightened and then subjected to hot straightening (stress relief annealing) to maintain a flatness of within 2 mm.

2. Induction Quenching

(1) Prepare a flat sensor, contour horn, and plane quenching sample. Start the device and adjust the sensor water pressure to ensure proper spraying, with a pressure of at least 0.2 MPa.

(2) Based on the quenched area and equipment capacity, we adjusted the sensor length to allow simultaneous induction quenching of 2 to 3 workpieces. Adjust output voltage, current, moving speed, and distance from the workpiece to meet quenching requirements using a test block.

(3) Due to the large machining volume on both sides of the chamfer, the production cycle was long. We recommended removing the chamfer and quenching directly. Through several trial heating sessions, we found a solution: align the workpiece and make a mark, place the workpiece about 20 mm apart, adjust the gap between the sensor and the workpiece, raise the sensor by 2 mm using a wooden mat, and shift it 2 mm to the right, as shown in Figure 2. This way, the sensor heats the area 2 mm away from the sharp corner instead of the corner itself. Once the desired temperature is reached, remove the wooden mat, restore the original gap, and spray water to cool the sharp corner. Then, continue the quenching process. After the preheating ring passes, stop the induction heating. At this point, the terminal temperature reaches 860–880°C, and continuous water spraying completes the quenching of the terminal part.

(4) Flip the workpiece, realign it with the mark, and repeat the process to complete the quenching on the other side.

3. Inspection

After quenching, the hardness ranges from 55 to 61 HRC. There is no cracking or deformation at the sharp corners, and no soft bands are present. After tempering at 200°C for 3 hours, the hardness is between 53 and 55 HRC, which meets the design and usage requirements.

Third, the Conclusion

Through research into the heat treatment process of the guide block, we identified effective measures to prevent overheating at the corners—using locally inserted magnets and raising the sensor height to reduce heat concentration at the edges. At the same time, by optimizing the induction heating process, we reduced the machining workload on the machine guide, improved production efficiency, lowered costs, and ensured product quality. This approach not only solves the technical challenges but also enhances the overall performance and reliability of the guide blocks in CNC double-column vertical lathes.

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