CNC lathe machining selects suitable machine tools based on the material, contour shape, machining accuracy, etc. of the workpiece being machined, formulates machining plans, determines the machining sequence of parts, and uses cutting tools, fixtures, and cutting quantities for each process.
1、 Selection of CNC lathe:
There are generally two situations when machining parts on a CNC lathe.
1. If there are part drawings and blanks, it is necessary to choose a CNC machine suitable for processing the part.
2. There are already CNC machine tools available, so it is important to choose parts that are suitable for machining on the machine.
In any case, the main factors considered when machining parts on a CNC lathe include the material and type of the blank, the complexity of the part contour shape, size, machining accuracy, number of parts, and heat treatment requirements
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There are three points to summarize:
1. To ensure the technical requirements for machining parts and produce qualified products.
2. Beneficial for improving productivity.
3. Minimize production costs (processing expenses) as much as possible.
2、 Process analysis of CNC lathe machining parts
The analysis of CNC machining process involves a wide range of aspects, and here we will only analyze the possibility and convenience of CNC machining.
（1） The provision of dimensional data on part drawings should comply with the principle of easy programming
The dimension annotation method on the part drawing should be adapted to the characteristics of CNC machining
On the CNC machining part drawing, the same reference should be used to indicate dimensions or coordinate dimensions should be provided directly. This annotation method is not only convenient for programming, but also facilitates the coordination between dimensions, bringing great convenience in maintaining consistency between design standards, process standards, inspection standards, and programming origin settings. Due to the fact that part designers generally consider assembly and other usage characteristics more in dimension annotation, they have to adopt a partial and scattered annotation method, which will bring a lot of inconvenience to process arrangement and CNC machining. Due to the high precision of CNC machining and repetitive positioning, the usage characteristics will not be compromised by significant accumulated errors. Therefore, the local scattered annotation method can be changed to a annotation method that uses the same reference annotation size or directly provides coordinate dimensions.
2. The conditions for the geometric elements that make up the contour of the part should be sufficient
When manually programming, it is necessary to calculate the coordinates of the base point or node. In automatic programming, all geometric elements that make up the contour of the part need to be defined. Therefore, when analyzing part drawings, it is necessary to analyze whether the given conditions for geometric elements are sufficient. Like arcs and lines, arcs are tangent to each other on the drawing, but according to the dimensions given on the drawing, they become intersecting or separated when calculating the tangent condition. Due to insufficient conditions for the geometric elements of the components, it is difficult to start programming. When encountering such a situation, it is necessary to consult with the part designer to resolve it.
（2） The structural processability of each machining part of the component should conform to the characteristics of CNC machining
It is best to use a uniform geometric type and size for the inner cavity and outer shape of the parts. This can reduce tool specifications and the number of tool changes, making programming easier and improving production efficiency.
The size of the inner groove fillet determines the diameter of the tool, so the inner groove fillet radius should not be too small. The quality of part processability is related to the height of the processed contour, the size of the transition arc radius, and other factors.
When milling the bottom plane of the part, the fillet radius r of the groove bottom should not be too large.
4. A unified benchmark positioning should be adopted. In CNC machining, if there is no unified reference positioning, it will cause the contour positions and dimensions on the two surfaces after machining to be inconsistent due to the reinstallation of the workpiece. Therefore, to avoid the above-mentioned problems and ensure the accuracy of their relative positions after two clamping processes, a unified reference positioning should be adopted. It is best to have suitable holes on the parts as positioning reference holes. If not, process holes should be set as positioning reference holes, such as adding process lugs on the blank or setting process holes on the remaining amount to be milled off in subsequent processes. If it is not possible to create process holes, at least the surface that has been precision machined should be used as a unified reference to reduce errors caused by two clamping processes. In addition, it is necessary to analyze whether the required machining accuracy and dimensional tolerances of the parts can be guaranteed, and whether there are any unnecessary dimensions that cause conflicts or enclosed dimensions that affect the process arrangement.
3、 Selection of Processing Methods and Determination of Processing Plans
（1） Selection of processing methods
The principle of selecting processing methods is to ensure the machining accuracy and surface roughness requirements of the processed surface. Due to the fact that there are generally many processing methods that can achieve the same level of accuracy and surface roughness, when selecting in practice, comprehensive consideration should be given to the shape, size, and heat treatment requirements of the parts. For example, for holes with IT7 level accuracy, machining methods such as boring, reaming, and grinding can all meet the accuracy requirements, but holes on the box are generally boring or reaming, rather than grinding. Generally, hinge holes are selected for small-sized box holes, while boring holes should be selected for larger hole diameters. In addition, the requirements for productivity and economy, as well as the actual situation of the factory's production equipment, should also be considered. The economic processing accuracy and surface roughness of commonly used processing methods can be found in relevant process manuals.
（2） Principles for determining processing plans
The machining of more precise surfaces on parts is often achieved gradually through rough machining, semi precision machining, and precision machining. It is not enough to select the corresponding final processing method based solely on quality requirements for these surfaces. It is also necessary to correctly determine the processing plan from blank to final forming. When determining the processing plan, the first step is to preliminarily determine the processing methods required to meet the requirements of the accuracy and surface roughness of the main surface. For example, for IT7 precision holes with small aperture, when the final processing method is precision reaming, drilling and rough reaming are usually required before precision reaming.
4、 Selection of Cutting Tools and Determination of Cutting Parameters
（1） Selection of Cutting Tools
The selection of cutting tools is one of the important contents in CNC machining technology, which not only affects the machining efficiency of the machine tool, but also directly affects the machining quality. When programming, the selection of cutting tools usually takes into account factors such as the machining capacity of the machine tool, the content of the process, and the material of the workpiece. Compared with traditional machining methods, CNC machining has higher requirements for cutting tools. Not only does it require high precision, good rigidity, and high durability, but it also requires stable dimensions and easy installation and adjustment. This requires the use of new high-quality materials to manufacture CNC machining tools and the optimization of tool parameters.
When selecting a cutting tool, the size of the tool should be adapted to the surface size and shape of the workpiece being processed. In production, end mills are often used to process the contour around flat parts. When milling flat surfaces, a hard alloy blade milling cutter should be selected; When processing protrusions and grooves, choose high-speed steel end mills; When processing rough surfaces or rough machining holes, corn milling cutters with hard alloy inserts can be selected. When choosing an end mill for machining, it is recommended to select the relevant parameters of the tool based on empirical data. Ball end milling cutters are commonly used for surface machining, but when machining flat parts of the surface, the tool cuts with the top edge of the ball end, which results in poor cutting conditions. Therefore, a ring cutter should be used. In single piece or small batch production, in order to replace multi coordinate linkage machine tools, drum shaped or conical cutters are often used to process some variable angle parts on aircraft with toothed disc milling cutters. They are suitable for machining some spherical surfaces on five coordinate linkage CNC machine tools, and their efficiency is nearly ten times higher than using ball end milling cutters, and can achieve good machining accuracy.
On the machining center, various cutting tools are installed in the tool magazine and selected and changed according to the program requirements at any time. Therefore, there must be a set of connecting rods for ordinary cutting tools, so that standard cutting tools used in drilling, boring, expanding, reaming, milling and other processes can be quickly and accurately installed on the machine tool spindle or tool magazine. As a programmer, one should understand the structural dimensions and adjustment methods of the tool holder used on the machine tool, as well as the adjustment range, in order to determine the radial and axial dimensions of the tool during programming. At present, China's machining centers adopt the TSG tool system, which has two types of handles: straight handle (three specifications) and tapered handle (four specifications), including 16 different types of knives for different purposes.
（2） Determination of cutting parameters
Cutting parameters include spindle speed (cutting speed), back cutting amount, and feed rate. For different processing methods, different cutting parameters need to be selected and should be included in the program sheet. The principle of selecting cutting parameters reasonably is that during rough machining, the main focus is generally on improving productivity, but economic efficiency and processing costs should also be considered; During semi precision machining and precision machining, cutting efficiency, economy, and machining cost should be considered while ensuring machining quality. The specific values should be determined based on the machine tool manual, cutting parameter manual, and combined with experience.
