Optimized CNC Strategies for Reliable Thread Production on Raw Surfaces
This paper presents a new design scheme for chamfering threaded holes on non-machined surfaces. This scheme is utilized in CNC machine tools (machining centers) to process parts, ensuring consistent drilling depth, thread tapping depth, and chamfer size at the hole mouth. As a result, it significantly enhances product processing quality and boosts production efficiency.
01 Introduction
With the development of the automotive industry, the variety of engine exhaust manifolds has increased, leading to a greater range of unprocessed boss end faces on pipe bodies. These bosses require drilling and tapping holes, as well as chamfering the hole mouths. Due to the high precision required for these holes, standard drilling equipment often struggles to maintain accuracy, so CNC machines are utilized for this purpose.
In this series of products, the boss end faces are ensured through casting and do not require further processing. However, due to limitations in casting conditions, the sizes of the bosses for different product parts vary, measuring between 150 mm and 151.6 mm. CNC machining is performed with a target median tolerance of 150.8 mm, which is standardized through programming.
To ensure that most products achieve proper chamfering, a smaller set value is usually chosen. As a result, when the boss height exceeds the machine tool’s set height value, the chamfer may be too deep, which can lead to product scrappage, as shown in Figure 2a. Conversely, if the workpiece blank is lower than the set minimum value, the chamfer may be inadequate or too small, as illustrated in Figure 2b. This inconsistency can lead to out-of-tolerance chamfers on threaded holes or, in some cases, no chamfer at all, resulting in scrap.
Currently, about 30% of submitted products fall out of tolerance due to these issues, which significantly impacts the needs and costs for the main engine manufacturers. There are also challenges related to the varying depths of the bottom hole and the thread tapping, contributing to the production bottleneck in this process.
To address the quality issues that arose during processing, we conducted an analysis of the fixture, production process, equipment characteristics, and the causes of size deviations. As a result, we designed a new device optimized for CNC machine tools to process threaded holes in workpiece blanks. This device is ideal for mass production in CNC machining. It can be integrated with the machine tool’s handle and tool-changing system, allowing for automatic tool changes and the selection of different tools based on the size of the thread being processed.
02 Advantages of the technical solution
(1) Consistency of processing quality
Achieve consistency in the quality of thread processing on the non-processing surface of the product on the CNC machine tool.
(2) Applicable to products with different apertures
The device can realize thread processing of different thread apertures by simply matching different corresponding tools, which is easy to operate.
(3) Efficient processing
The device is paired with the CNC machine tool handle to enable automatic tool changes. The workpiece can perform drilling, reaming, chamfering, and thread processing all in one clamping, achieving high-efficiency processing.
03 Structural design and working principle
In conjunction with the CNC machine tool handle structure, the technical device was designed, as illustrated in Figure 3.
The main components of the device include a guide sleeve, which houses a cutting compression spring, a compression spring adjustment screw, and the device body. A symmetrical guide pin is installed at the upper part of the device body, designed to work in conjunction with the symmetrical U-shaped slide groove on the guide sleeve. The device body also contains a chamfering drill bit, a chamfering limit screw, a drill bit locking screw, and a locking nut.
At the lower end of the device body, a positioning sleeve is installed. This sleeve contains several components, including a one-way thrust ball bearing, a deep groove ball bearing, and retaining rings. During assembly, the thrust ball bearing and the shaft retaining ring are inserted into the device body in sequence. Next, the deep groove ball bearing is installed into the positioning sleeve. Both the deep groove ball bearing and the positioning sleeve are then added to the device body, followed by the installation of the retaining ring and the dust cover.
The size of the chamfering drill bit can be adjusted according to the chamfering specifications of the workpiece, using the locking screw and the chamfering limit screw. The tool is then secured with the locking nut. After that, the cutting clamping spring and the device body are inserted into the guide sleeve using the guide pin. The axial cutting force is adjusted via the cutting clamping spring adjustment screw to ensure adequate axial force during chamfering.
When operating, the device is mounted onto the CNC machine tool handle, which transmits torque. First, confirm the height variation range of the end face of the threaded hole on the workpiece from the fixture on the machine tool worktable. Adjust the tool so that the desired chamfer is less than or equal to the minimum height of the threaded end face.
The processing program is compiled based on the workpiece requirements. While the CNC machine tool performs the chamfering, the device approaches the end face of the threaded hole when the product’s boss height is at its minimum value. Once the positioning sleeve contacts the workpiece, the device body stops descending, completing the chamfering process. Here, the positioning sleeve contacts the upper end face of the workpiece’s boss. Due to friction, the positioning sleeve ceases to rotate, preventing any scratches on the workpiece boss’s end face. Meanwhile, the chamfering cutter continues to turn, completing the chamfering.
If the CNC machinery parts blank boss height exceeds the minimum value set on the machine tool, the device still approaches the threaded hole’s end face. When the positioning sleeve makes contact, the device body ceases to move down. The positioning sleeve stops rotating due to friction, further preventing scratches on the workpiece boss’s end face. The chamfering cutter continues to rotate beneath the machine tool spindle’s drive, completing the chamfering. Throughout this process, the machine tool spindle parameter Z value has not yet achieved completion (as the machine tool is configured with a minimum value), and it will continue moving downward until the parameter reaches the specified minimum.
During this motion, the compression force of the cutting clamping spring within the device is overcome. This forces the device body to drive the tool and the positioning assembly upward along the guide sleeve. The positioning assembly can only move axially up and down along the symmetrical U-shaped groove in the guide sleeve due to the action of the guide pin. This continues until the remaining Z value is completed—the minimum value set by the machine tool. Throughout this process, the positioning sleeve maintains a consistent distance between the chamfering cutter and the end face of the threaded hole in the workpiece boss, ensuring uniform chamfering depth.
By replacing tool 15 in the device with a drill bit, the depth of the bottom hole of the thread can be precisely controlled, ensuring consistent drilling depth across CNC custom cutting products.
Alternatively, if tool 15 is replaced with a tap for processing threads, the moving length of body 4 within guide sleeve 1 must be increased (as illustrated in Figure 4, indicated by 2mm). This distance refers to the axial movement of body 4 in guide sleeve 1 and is determined by the height variation of the processed boss, with a minimum moving distance of 1.5mm. Consequently, body 4′s length must reach at least six times the pitch of the thread. When setting the tapping parameters, the initial setting for tapping depth should be at least twice the pitch of the boss surface for the threaded hole being processed.
During tapping, the tap can advance into the threaded hole under the force of spring 3, which helps to mitigate the risk of losing step in the servo mechanism. If the boss surface of the threaded hole being processed is either higher or lower than the standard height, the tap drives body 4 to move up and down within guide sleeve 1. This adjustment addresses variations in tapping depth caused by imperfections in the boss surface, allowing for precise control of the thread depth and ensuring consistency throughout the tapping process.
This device works in combination with a CNC machine tool to create threaded holes on the end faces of workpieces with different diameters, specifically on faces that do not need machining. It ensures consistent drilling depth, tap depth, and chamfer size. Additionally, the CNC machine tool’s tool magazine and quick-change tool holders allow for automatic tool changes, making it highly suitable for mass production and demonstrating great versatility.
04 Conclusion
The machine tool operates smoothly and reliably during the commissioning and cutting of workpieces, thanks to its well-designed processes. The threaded hole machining device rotates steadily, which effectively prevents any tool vibrations. The rigidity and strength of both the machine tool and toolholder fully satisfy the machining requirements. This device cuts efficiently, allowing for smooth chip removal. Key specifications such as threaded hole depth, tap depth, chamfer depth, and surface roughness all meet the necessary drawing requirements. Consistent product quality is maintained throughout use, and the commissioning and acceptance processes were successfully completed on the first attempt. This advancement has significantly enhanced the quality of thread machining and increased production efficiency, effectively addressing the quality issues associated with threaded holes on end faces of workpieces that do not require machining, achieving the desired outcomes.
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