CNC machining, as an advanced manufacturing method, is widely used in the aerospace, shipbuilding, and electronics industries for the production of high-precision and complex parts. In CNC machining, the process from a part drawing to producing a qualified finished part is a tightly integrated and complex sequence of steps. If any link in this chain encounters a problem, the entire machining process will be interrupted.
This article analyzes some key factors in CNC machining across three stages, based on specific machining examples, to enhance understanding of CNC operations and provide guidance for production.
1. Process Planning Stage
Most CNC machines do not have built-in process planning capabilities. The objective of CNC process planning is similar to that of conventional machines; however, since every detail of CNC machining must be predetermined and executed automatically, it has its own characteristics compared to traditional process preparation.
1.1 Tool Setting and Tool Change
In conventional machining, the position relationship between the tool and the workpiece is established manually using measuring tools and handwheels. If the tool position is incorrect, the operator can adjust it at any time.
In CNC machining, however, multiple tools are often used in a single setup. Each tool’s parameters must be entered into the computer, which establishes the tool–workpiece relationship through coordinate transformations. Unlike conventional machining—where tool setting is not considered a major factor—CNC machining requires accurate tool setting before program execution, or serious consequences may occur.
Tool changes in conventional machining are often based on the operator’s feel, but in CNC machining, the tool change position must be carefully considered to avoid collisions.
1.2 Selection of Clamping Method
Since each clamping in CNC machining requires re-setting the tools, multiple setups will greatly increase auxiliary machining time and reduce efficiency, while underutilizing the CNC machine’s capabilities. Therefore, it is preferable to machine all required surfaces in a single clamping, fully leveraging the CNC machine’s potential.
Example:
- Equipment: Schaublin CNC machining center
- Tools: (1) Rough turning tool, (2) Finish turning tool, (3) Rough grooving tool, (4) Finish grooving tool, (5) Center drill, (6) Drill bit, (7) Boring tool, (8) Cut-off tool
- Fixture: Pneumatic three-jaw chuck
With CNC machining, the above process can be completed in a single clamping, while conventional machining would typically require four.
1.3 Tool Design
Compared with conventional machining, CNC tooling has the following characteristics:
-
Simplification of Tool Design
For example, when processing a part surface (as shown in Fig. 2), conventional machining would require a double-edged form tool to ensure positional and dimensional accuracy. CNC machines, however, can accurately control tool positions, allowing the use of single-edged tools instead. A specially designed single-edged grooving tool successfully completed the machining. -
Design of Special Tools
While CNC machining simplifies some tool designs, it often requires special tooling for parts that are difficult or impossible to machine on conventional equipment. For instance, machining a continuous curved surface (Fig. 3) may require a custom-designed tool, considering the surface composition, toolpath, minimum radius, convex/concave transitions, and potential interference. Since curved-surface machining is a key trend in CNC, three tool designs were developed for this purpose (Figs. 4–6), with Figs. 4 and 5 successfully manufactured and tested, achieving continuous machining results.
2. Mathematical Processing Stage
2.1 Calculation Work
The coordinate system in part drawings often differs from that used in machining programs, requiring conversion. Additionally, the dimensions given in drawings may not match the program requirements, so the necessary coordinates must be calculated according to machine characteristics.
For complex surfaces composed of lines and arcs, calculations must include line start/end points, arc start/end points, and arc center coordinates.
2.2 Analysis of Tool Center Path
Most modern CNC systems have cutter compensation functions that allow programming directly from the part contour. However, in some special cases—particularly in continuous surface machining—tool–workpiece interference may occur, rendering compensation unusable. In such cases, the tool center path must be analyzed manually.
Example:
- Equipment: Schaublin machining center
-
Path of a Semi-Circular Tool
Analysis of tool movement and cutting path shows that the tool center path corresponds to a continuous curve composed of parallel segments, concentric circles, and eccentric circles relative to the part contour (Fig. 7). -
Path of a Double-Arc Tool
The analysis is the same as for the semi-circular tool. Tool center path points are shown in Fig. 8.
3. Programming Stage
A CNC program is the only “language” a CNC machine can understand, sending it step-by-step commands to control every operation. The quality of the program directly affects machining accuracy and efficiency. This requires not only a thorough understanding of machine performance and each machining step, but also continuous practice to improve programming skills.
3.1 Effective Use of Built-in Programs
Modern CNC machines are increasingly powerful in both mechanical and software functions. Many systems come with mature built-in machining programs for common operations. Selecting and applying these built-in programs effectively is an important part of programming work.
3.2 Program Management
Machining programs—created from toolpath data and processed coordinates using specific CNC codes—are valuable technical documents containing machining methods, techniques, and even reflecting a shop’s technical level. They should be carefully stored and archived.
Frequently used programs are often stored in the CNC control system, with part names recorded. Less frequently used programs should be documented in written form, with notes added for potential problem areas to assist future use.
3.3 Using Parametric Programming to Solve Practical Issues
Example:
- Part: Internal bore with an arc-shaped bottom surface
- Machine: Shanghai CNC precision lathe
In general arc machining, the G66 canned cycle is often used (Fig. 9). However, in practice, weak tool-tip strength can lead to poor tool life when the tool is subjected to bidirectional forces at sharp corners. Process analysis suggested an alternative toolpath (Fig. 10).
Direct programming for this path would require recalculating tool coordinates for each pass, which is time-consuming, especially when multiple shallow passes are needed due to poor tool rigidity. Parametric programming was used instead, allowing quick adjustment of cutting depth without recalculating all coordinates, effectively solving the machining problem.
Conclusion
Analyzing key factors in CNC machining provides a practical basis for improving CNC machine utilization rates. Applying these insights can effectively ensure machining accuracy and achieve high-quality results.
