Solving Manufacturing Challenges for High-Helix Multi-Thread Groove Workpieces

To effectively address the technical challenges of machining multi-threads with a large helix angle in fragment slot products, a functional test was conducted. This involved selecting suitable machining methods and utilizing specialized auxiliary tools. Both the internal and external threads were machined in the same manner using a CNC lathe, and it was confirmed that the multi-thread produced by turning met the design requirements.

 

1. Introduction

The machining device and methods for multi-thread fragment slots are seldom discussed in the field. Due to their large helix angles and the structure of left- and right-hand multi-threads, machining these components is challenging. Although a four-axis machining center can perform the task, it requires the customization of special tools, resulting in low machining efficiency.

This paper focuses on the CNC turning method for multi-thread fragment slots with large helix angles, analyzing the difficulties associated with turning the multi-thread of a typical fragment slot product. By improving the process according to the structural characteristics of existing CNC lathe tool holders, we design a CNC lathe tool holder specifically suited for turning multi-threads. This design addresses the issue of blade back angle interference, which prevents normal cutting due to the long lead and large helix angle of the multi-thread during turning. This solution effectively overcomes the production bottlenecks associated with current CNC machining technology, significantly enhancing production efficiency while maintaining a high product qualification rate.

 

2. Analysis of Typical Parts

2.1 Analysis of Part Characteristics

Figure 1 illustrates a fragment of a typical large helix angle multi-start taper thread groove, made from 30CrMnSiA, a material known for its high strength and toughness. The mechanical properties of 30CrMnSiA are detailed in Table 1.

 

The multi-threaded fragmentation groove also requires quenching, with a final hardness target of 47 to 52 HRC. The high strength and hardness characteristics make the tool prone to wear and breakage during intermittent processing.

2.2 Processing difficulties and problem analysis

1) The large helix angle of the blade causes interference during turning, preventing normal cutting. The typical solution is to manually grind the blade’s back angle to meet processing requirements. However, this approach reduces the tool’s cutting strength, which not only affects its service life but also hinders the ability to achieve multi-thread processing on high-hardness surfaces.

2) The product features both left-handed and right-handed threads, necessitating the use of two different thread processing tools that must operate simultaneously. Additionally, two separate processing devices need to be designed for the corresponding multi-threaded fragmentation grooves. The manufacturing process also requires multiple intermittent turnings. Due to the product’s high surface hardness, there is a risk of tool tip breakage.

3) The multi-threaded fragmentation groove of this product is designed on a conical surface. The helix angle varies with the change in the outer diameter, presenting a significant challenge for the technology used in processing and measuring threads on conical surfaces.

 

3. Improvement methods and effects

3.1 Tool holder design

The thread pitch of this component is 300 mm, and the helix angle is significant. Therefore, the tool holder needs to be completely redesigned. The new tool holder will allow for the adjustment of the tool’s rotation angle based on the helix angle of the multi-start thread, enabling the processing of both left-hand and right-hand threads. Figure 2 illustrates the tool holder. A specialized tool holder is designed according to the specific helix angle of the multi-start thread to achieve the desired CNC manufacturing outcomes.

 

The lead calculation formula is P=πDtanθ, where P is the lead (mm), D is the diameter (mm), and θ is the thread inclination degree (°). The calculation formula for θ is: θ=arctan(P/πD).

The product features a tapered thread design, with a small end diameter of 48.4 mm and a large end diameter of 68 mm. The multi-start thread helix angle is calculated to range from 54.5° to 63.1° using the lead calculation formula. Given that the taper angle of the workpiece section is 8.6°, the back angle of the threading tool must exceed 8.6° to effectively prevent back angle interference during machining.

To achieve normal cutting operations, the angle of the tool holder needs to be adjusted. For this specific part, the adjustment angle is set to the arithmetic mean of the thread helix angles, which is calculated as: (63.1° + 54.5°) / 2 = 58.8°. Consequently, the left-handed helix angle is adjusted to -58.8°, while the right-handed helix angle is adjusted to +58.8°. This adjustment effectively mitigates interference from the helix angle and ensures the accuracy of the tooth angle.

 

3.2 Determine the processing technology and ideas

The overall processing flow for the part consists of the following seven steps: cutting → rough processing → heat treatment and tempering → semi-finishing → quenching → finishing → warehousing inspection.

Before finishing, the part undergoes vacuum quenching to achieve a hardness range of 47 to 52 HRC, with deformation limited to 0.1 to 0.2 mm. The shape is then finalized to meet the design specifications outlined in the drawing. A special tapered mandrel is employed for clamping and thread processing.

(1) Tool Selection: A composite external thread clamping toolbar is chosen for its high positioning accuracy and secure clamping. The blade selected is a full-tooth 60° thread blade known for its high positioning accuracy, robust wear resistance, and impact resistance, with a top angle radius (R) of less than 0.5 mm. The external thread toolbar is designated as CER 2525M16HD, and the thread blade is labeled 16ER3.0 ISO CP500. The tool shape is illustrated in Figure 3.

 

When machining cross threads, intermittent cutting can occur, which may lead to wear or chipping of the tool tip. Replacing the blade can help ensure greater positioning accuracy. Throughout the machining process, it is important that the coordinates of the tool tip for all threads at the tool position remain consistent to prevent random buckling.

(2) Optimization of CNC programs. Different thread machining requirements necessitate various machining methods. The three common techniques for thread machining are straight-in, oblique-in, and left-right alternating methods. Due to the high hardness of the material, larger cutting depths, and the intermittent nature of the machining process, the oblique-in and left-right alternating methods are preferred as they help reduce cutting resistance.

The CNC program can be compiled using software platforms such as UG and CAXA. It can also be directly input into the machine tool’s built-in thread fixed cycle or written as a macro program based on specific machining features. Each thread is processed sequentially, and during machining, the tool may experience chipping or wear.

The ideal approach is to machine one spiral groove at a time before proceeding to the subsequent grooves until the final depth is achieved. This method has the advantage of allowing only unqualified threads to be repaired, while qualified threads can be directly accessed, thus avoiding unnecessary repeated machining.

For example, processing can begin from the unqualified N5 program segment. Below is an example of a CNC lathe program for the Siemens 840D.

 

 

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