Hole machining is a common cutting process in the field of machining. It is an essential part of the process for machining the geometric elements of parts.
The machining accuracy of the hole directly determines the assembly accuracy between the shaft and the hole.
In aerospace, weapons equipment manufacturing, automotive manufacturing, and other fields, hole machining is a critical part of the manufacturing process.
The reasonable selection of hole machining methods directly affects the quality of the hole products.
Poor cutting process and chip removal will lead to a decline in hole machining accuracy, and even cause the scrap of the workpiece, has long been the machining manufacturing technology to overcome the difficulties.
Hole processing difficulties mainly exist in the following aspects:
1) The processing of a large number of holes.
The machining process is mainly concentrated in the drilling process, for higher precision holes also need to be completed with the aid of other processes.
(2) For the processing of larger size holes, step holes and blind holes, the traditional process needs to be completed in multiple processes and multiple tools.
(3) The dimensional accuracy and geometric tolerance of the holes are required to be high.
The machining accuracy of the hole affects the assembly of the part and the overall accuracy of the equipment.
Therefore, it is essential to ensure the machining accuracy of the hole.
Traditional hole machining methods
Traditional hole machining methods mainly include drilling, expanding, boring, reaming, and countersinking.
The main tools used in these processes are center drills, twist drills, boring tools, and other related tools, as shown in Figure 1.
With the development of tool materials, the hole machining process is also continuously evolving. However, the development space for drilling and boring hole processing methods is gradually shrinking.
This is mainly due to the need for a large number of tools, which leads to high costs.
In particular, fine boring tools are relatively expensive. Traditional hole machining methods can no longer meet the requirements of high-speed machining.
Milling
Traditional hole machining methods often require multiple drills and boring tools of different diameters to process a single hole.
When the hole diameter varies significantly across a workpiece, the need for additional cutting tools increases.
This results in higher processing costs and longer tool change times, which seriously affect processing efficiency.
Hole milling is a new direction in hole processing research in recent years.
By preparing milling cutters with different arc radii and using the tool radius compensation function, it is possible to machine holes of various diameters.
This greatly improves machining efficiency, reduces the number of cutting tools required, and lowers the overall cost of workpiece processing.
Conventional Hole Milling
The machining is done with an end mill, rotating the tool at high speed.
Firstly, the tool moves in the negative Z-direction, starting from a pre-drilled hole. It then mills a hole with a diameter larger than the tool itself in the XY plane of the material.
The principle of conventional hole milling is illustrated in Fig. 2 on the following page.
Due to the impact of the tool’s radial cutting force, the existence of tool retraction phenomenon.
The processed hole produces a cone with a large upper hole diameter and a small lower hole diameter.
Conventional hole milling typically involves milling one full circle per a fixed Z-axis increment.This method often results in the formation of joint marks, leading to very poor hole wall roughness.
As a result, it is only suitable for roughing operations or hole machining with low precision requirements.
Spiral milling
Spiral milling (dynamic milling) is a newly emerged hole machining process in recent years. Compared to traditional hole machining methods, the tool design has been optimized.
The feed method is straightforward, and the machining mechanism is both refined and reliable.
When machining, an end mill is used, and the tool rotates at high speed.The tool is then fed along a helical trajectory to mill a round hole in the material.
The principle of helical hole milling is illustrated in Figure 3 on the following page.
The most important feature of spiral milling is that the tool performs cutting and machining by milling along a helical path.
On a CNC milling machine, the spiral feed motion is a combination of two independent movements.
One is the tool’s rotary feed movement around the axis of the machining hole, known as the rotary movement.
The other is the tool’s straight-line feed movement along the axis of the machining hole, referred to as the axial feed movement.
In addition, the tool’s own high-speed rotary motion, i.e., the main cutting motion, also known as the rotary motion.
Spiral milling hole processing mainly relies on the endface edge of the milling cutter as the primary cutting edge.
Due to the high spindle rotational speed during processing and the shallow depth of cut, the side cutting edge engages only a small portion of the material.
As a result, the radial cutting force is very low, which helps minimize tool deflection. This ensures that the straightness and perpendicularity requirements of hole machining are effectively met.
Spiral milling hole efficiency is very high, can maximize the material removal rate, and reduce tool wear.
Ensure constant tool load, which can prevent tool breakage during machining;
Chip removal is smooth, most of the heat is taken away by the chips, the workpiece machining temperature rise is very small, and the tool heat dissipation is faster.
The deformation effect on thin-walled hole parts is very small.
The hole accuracy of spiral milling is dependent on the motion accuracy of the machine tool, the rigidity of the tool, and the accuracy of circular interpolation to ensure.
Therefore, optimizing the cutting elements and selecting a reasonable tool is a necessary condition for spiral milling hole processing.
With the rapid development of high-speed cutting technology and advancements in tool materials, the rational application of spiral milling has become increasingly effective.
When combined with high-performance tools, spiral milling can achieve holes with high dimensional and morphological accuracy, gradually realizing the concept of ‘milling instead of boring’.
Process Comparison
Compared to traditional hole processing, spiral milling is a milling method with a unique form of feed.
It offers significant technical advantages over conventional hole machining methods, as shown in Table 1.
Table 1 Comparison of Processing Technology
Conclusion
Spiral milling is the current stage of machining field hole processing technology, and its performance is outstanding, especially in large diameter hole processing.
It has outstanding performance in the processing of large-diameter holes, with high processing efficiency, low processing cost and stable processing accuracy, and has been widely used.
In recent years, scholars have carried out in-depth research on spiral milling technology, which has been continuously enriched and improved in practice.
In the future, with the continuous improvement of spiral milling technology itself and the use of high-performance tools, the scope of its application will gradually expand.
Eventually, spiral milling will achieve the goal of replacing boring with milling.