The machining process of CNC turning

As a highly efficient equipment, in order to fully utilize the characteristics of high performance, high precision, and high automation of CNC machine tools, in addition to mastering the performance, characteristics, and operation methods of the machine tools, detailed process analysis and determination of reasonable machining processes should be carried out before programming to obtain the optimal machining plan.

1、 The content of CNC turning processing technology

The CNC turning process is the sum of methods and technical means used when machining parts using CNC lathes. Its main content includes the following aspects:

（1） Select and determine the CNC turning content of the parts;

（2） Perform CNC turning process analysis on part drawings;

（3） Selection and adjustment design of tools and fixtures;

（4） Design of processes and steps;

（5） Calculation and optimization of processing trajectory;

（6） Writing, verifying, and modifying CNC turning programs;

（7） First piece trial processing and on-site problem handling;

（8） Develop technical documents for CNC machining processes; In short, there are many aspects of CNC machining technology, some of which are similar to ordinary machine tool machining.

2、 Analysis of CNC Turning Processing Technology

Process analysis is the preliminary process preparation work for CNC turning machining. The rationality of the process formulation has a significant impact on the programming, machining efficiency of the machine tool, and machining accuracy of the parts. In order to develop a reasonable and practical machining program, programmers are required to not only understand the working principle, performance characteristics, and structure of CNC lathes. Mastering programming languages and programming formats, as well as proficiently mastering workpiece processing techniques, determining reasonable cutting quantities, and correctly selecting cutting tools and workpiece clamping methods. Therefore, general process principles should be followed and combined with the characteristics of CNC lathes to conduct a careful and detailed analysis of CNC turning machining processes. Its main content includes: analyzing the machining requirements and rationality of parts based on drawings; Determine the clamping method of the workpiece on the CNC lathe; The processing sequence of each surface, the feed path of the cutting tool, and the selection of cutting tools, fixtures, and cutting parameters.

（1） Part drawing analysis

Part drawing analysis is the primary task in developing CNC turning processes. Mainly conducting analysis on dimension annotation methods, contour geometric elements, as well as accuracy and technical requirements. In addition, the rationality of the part structure and processing requirements should be analyzed, and the process benchmark should be selected.

1. Analysis of Dimensional Labeling Methods

The dimension annotation method on the part drawing should be adapted to the machining characteristics of the CNC lathe, and the dimensions should be annotated with the same reference or directly given coordinate dimensions. This annotation method is not only convenient for programming, but also conducive to the unity of design standards, process standards, measurement standards, and programming origins. If there is no unified design standard for the dimensions in all directions on the part drawing, it may be considered to choose a unified process standard without affecting the accuracy of the part. Calculate and convert various dimensions to simplify programming calculations.

2. Analysis of contour geometric elements

When programming manually, it is necessary to calculate the coordinates of each node. During automatic programming, all geometric elements of the part contour need to be defined. Therefore, when analyzing part drawings, it is necessary to analyze whether the given conditions for geometric elements are sufficient.

3. Analysis of Accuracy and Technical Requirements

Analyzing the accuracy and technology of the processed parts is an important part of the process analysis of the parts. Only by analyzing the dimensional accuracy and surface roughness of the parts can the processing method, clamping method, cutting tools, and cutting parameters be selected correctly and reasonably. Its main content includes: whether the analysis accuracy and various technical requirements are complete and reasonable; Analyze whether the CNC turning accuracy of this process can meet the requirements of the drawing. If it cannot meet the requirements, other processing methods are allowed to make up for it, and a margin should be left for subsequent processes; For surfaces with positional accuracy requirements on the drawing, it should be ensured that they are completed in one clamping operation; For surfaces with high surface roughness requirements, constant linear velocity cutting should be used (note: when turning the end face, the maximum spindle speed should be limited).

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（2） Selection of fixtures and cutting tools

1. Clamping and positioning of workpieces

In CNC turning, it is advisable to achieve the goal of machining all or most of the machined surfaces after one clamping, while minimizing the number of clamping cycles to improve machining efficiency and ensure machining accuracy. For shaft components, the outer cylindrical surface of the component itself is usually used as the positioning reference; For sleeve parts, the inner hole is used as the positioning reference. In addition to using universal three jaw automatic centering chuck, four jaw chuck, hydraulic, electric, and pneumatic fixtures, there are also various specialized fixtures with good universality for CNC lathe fixtures. Reasonable selection should be made in practical operation. The WeChat account for metal processing has good content and is worth paying attention to.

2. Tool selection

The service life of a tool is not only related to the tool material, but also to the diameter of the tool. The larger the diameter of the tool, the greater the cutting amount it can withstand. So, when the shape of the parts allows, using the largest possible tool diameter is an effective measure to extend tool life and improve productivity. The commonly used cutting tools for CNC turning are generally divided into three categories. Namely pointed turning tools, arc-shaped turning tools, and formed turning tools.

(1) Sharp turning tool. A cutting tool characterized by a straight cutting edge is generally referred to as a pointed cutting tool. Its cutting edge is composed of linear main and auxiliary cutting edges, such as outer circular offset cutters, end face turning tools, etc. When machining parts with this type of turning tool, the contour shape of the part is mainly obtained by the displacement of an independent tool tip or a straight main cutting edge.

(2) Circular arc turning tool. In addition to being able to turn inner and outer circular surfaces, it is particularly suitable for turning various smooth connected forming surfaces. Its characteristic is that the shape of the cutting edge that constitutes the main cutting edge is a circular arc with very small roundness error or contour error. Each point of the circular arc edge is the tip of the circular arc cutting tool, so the cutting point is not on the circular arc, but on the center of the circular arc.

(3) Forming turning tool. The contour shape of the processed part is completely determined by the shape and size of the cutting edge of the turning tool. In CNC turning, commonly used forming turning tools include small radius arc turning tools, groove turning tools, and thread turning tools. In order to reduce tool changing time and facilitate tool alignment, and to achieve standardization in mechanical processing. In CNC turning, it is recommended to use a clamp type turning tool as much as possible.

（3） Selection of cutting parameters

The cutting parameters in CNC turning include the back cutting amount ap, spindle speed S (or cutting speed υ), and feed rate F (or feed rate f).

The principle of selecting cutting parameters is crucial for improving the machining quality of CNC lathes. Reasonable selection of cutting parameters is essential. When determining the cutting parameters of a CNC lathe, it is necessary to choose according to the requirements specified in the machine manual and the durability of the cutting tools. It can also be determined by using the proportional method based on actual experience. The general selection principle is: when rough machining, the first consideration is to choose the maximum possible back cutting amount ap within the allowable stiffness of the machine tool; Secondly, choose a larger feed rate f; Finally, determine a suitable cutting speed υ based on the allowable lifespan of the tool. Increasing the back cutting amount can reduce the number of cutting times, improve processing efficiency, and increase the feed rate is beneficial for chip breakage. When precision machining, it is important to consider how to ensure machining quality and improve machining efficiency as much as possible. Therefore, it is advisable to use smaller back cutting and feed rates to maximize machining speed. The spindle speed S (r/min) can be calculated based on the cutting speed υ (mm/min) using the formula S=υ 1000/π D (D is the diameter of the workpiece or tool in mm), or it can be determined by looking up a table or based on practical experience.

（4） Divide the process and formulate the processing sequence

1. Principle of process division

There are two commonly used principles for dividing processes when machining parts on a CNC lathe.

(1) Principle of maintaining accuracy. The process generally requires as much concentration as possible, and rough and fine machining are usually completed in one clamping.

To reduce the impact of thermal deformation and cutting force deformation on the shape, positional accuracy, dimensional accuracy, and surface roughness of the workpiece, rough and fine machining should be carried out separately.

(2) Principle of improving production efficiency. To reduce the number of tool changes, save tool change time, and improve production efficiency, all parts that need to be processed with the same tool should be completed before using another tool to process other parts. At the same time, empty strokes should be minimized as much as possible.

2. Determine the processing sequence

The formulation of processing sequence generally follows the following principles:

(1) First rough, then refined. Follow the sequence of rough turning, semi fine turning, and fine turning to gradually improve the machining accuracy.

(2) Close first, then far. Process the parts closer to the tool setting point first, and process the parts farther away from the tool setting point later, in order to shorten the tool movement distance and reduce the idle travel time. In addition, turning from near to far is also beneficial for maintaining the rigidity of the blank or semi-finished product and improving its cutting conditions.

(3) Internal and external intersections. For parts with both inner and outer surfaces that need to be machined, rough machining of the inner and outer surfaces should be carried out first, followed by precision machining of the inner and outer surfaces.

(4) Base surface first. The surface used as a precision reference should be processed first, and the more accurate the positioning reference surface, the smaller the clamping error.