(1) Selection of processing methods
The selection of the processing method is to ensure the processing accuracy and surface roughness of the processed surface. Because there are generally many processing methods to achieve the same level of accuracy and surface roughness, it is necessary to separate the shape, size, and heat treatment requirements of the parts when selecting in practice. For example,
Machining methods such as boring, reaming, and grinding for IT7-level precision holes can meet the accuracy requirements, but the holes on the box body generally use boring or reaming instead of grinding. Generally, reaming should be selected for small-scale box holes, and boring should be selected when the hole diameter is larger. In addition, we should also consider the demand for consumption rate and economy, as well as the practical situation of the factory's consumer equipment. The economic processing accuracy and surface roughness of common processing methods can be found in the relevant process manuals.
(2) A rope ruler with a certain processing plan
Machining of comparable precision surfaces on parts is often gradually achieved through rough machining, semi-finishing and finishing. It is not enough for these surfaces to select the corresponding final processing method based on the quality requirements, and the processing plan from the blank to the final shape must be correctly determined. When determining the processing plan, first of all, according to the requirements of the accuracy and surface roughness of the primary surface, it must be the processing method required to reach these requirements. For example, for holes with a small diameter of IT7 precision, when the final processing method is fine reaming, it is generally processed through drilling, reaming, and rough reaming before the fine reaming.
Fourth, the distinction between processes and steps
(1) The distinction of process
Processing parts on a CNC machine tool can be more concentrated in the process, and most or all of the processes can be completed as much as possible in one setup. First of all, according to the part drawing, think about whether the processed part can complete the processing of the entire part on a CNC machine tool. If not, you should decide which part will be processed on the CNC machine tool and which part will be processed on other machine tools. The processing steps of the parts are discontinued.
(2) The division of work steps
The distinction of working steps should be considered mainly from the aspects of machining accuracy and power. It is often necessary to select different tools and cutting parameters in a process, and stop processing on different surfaces. In order to facilitate the analysis and description of the more complicated process, the process is subdivided into process steps. The following takes a machining center as an example to illustrate the rope ruler for process steps:
1) The same surface is finished by roughing, semi-finishing, and finishing in turn, or all processed surfaces are separated by roughing and finishing.
2) For parts with both milled faces and boring holes, the face can be milled first and then boring. According to this method to distinguish the work steps, you can advance the accuracy of the hole. Because of the high cutting force during milling, the workpiece is prone to deformation. The face is milled and then the hole is bored to allow a period of recovery to reduce the impact on the accuracy of the hole caused by the deformation.
3) Divide work steps by tool. The reversal time of some machine tools is shorter than the tool change time. You can choose to divide the work steps by tool to reduce the number of tool changes and advance the processing power.
In short, the distinction between processes and steps should be comprehensively considered based on the structural characteristics of specific parts, skill requirements and other conditions.
Five, the selection of equipment and fixtures for parts
(1) The base rope ruler of the positioning device
1) Strive to be consistent with the benchmarks of planning, process and programming accounting.
2) Try to reduce the number of clamping times, and process all the surfaces to be processed after positioning and clamping once as much as possible.
3) Avoid using manual adjustment processing schemes that occupy the machine to give full play to the effectiveness of CNC machine tools.
(2) Choosing the base rope ruler of the fixture
The characteristics of
CNC Machining put forward two basic requirements for the fixture: one is to ensure that the coordinate direction of the fixture is relatively fixed with the coordinate direction of the machine tool; the other is to harmonize the scale relationship between the part and the machine tool coordinate system. In addition, we must consider the following four points:
1) When the batch of parts is not large, modular fixtures, adjustable fixtures and other general fixtures should be used as much as possible to shorten the production preparation time and save consumption costs.
2) Only think about the use of special fixtures when mass consumption, and strive to have a simple structure.
3) The loading and unloading of parts should be fast, convenient and reliable to shorten the machine stop time.
4) The parts on the fixture should not hinder the machining of the surface of the parts by the machine tool, that is, the fixture should be opened and its positioning, and the clamping mechanism components should not affect the cutting during processing (such as bumps, etc.).
Six, the selection of tools and cutting parameters are determined
(1) Selection of tools
The selection of cutting tools is one of the important contents in the CNC machining process. It not only affects the machining power of the machine tool, but also directly affects the machining quality. When programming, the selection of tools generally requires consideration of factors such as the processing capabilities 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 also requires stable dimensions and convenient equipment adjustments. This requires the use of new and high-quality materials to manufacture CNC machining tools, and optimization of tool parameters.
When selecting a tool, the dimensions of the tool should be compatible with the surface dimensions and shape of the workpiece to be processed. In the process of production, end mills are often used to process the peripheral contours of flat parts. When milling planes, you should choose cemented carbide insert milling cutters; when processing bosses and grooves, choose high-speed steel end mills; when processing rough surfaces or rough machining holes, you can choose cemented carbide insert milling cutters. When selecting an end mill for processing, the relevant parameters of the tool are recommended to be selected according to the experience data. Ball-end milling cutters are often used for surface processing, but when processing flat surfaces, the cutter is cut with the top edge of the ball-end, and the cutting conditions are poor, so ring cutters should be used. In single-piece or small batch production, in order to replace multi-coordinate linkage machine tools, drum cutters or conical cutters are often used to process some variable bevel parts on the aircraft plus insert tooth disc milling cutters, which are suitable for CNC machine tools with five-axis linkage. For upper processing of some spherical surfaces, its power is nearly ten times higher than that of a ball-end milling cutter, and good processing accuracy can be obtained.
On the machining center, various tools are separately mounted on the tool magazine, and the tool selection and tool change operations are stopped at any time according to the program rules. Therefore, it is necessary to have a set of connecting rods for connecting common tools, so that standard tools used in drilling, boring, expanding, reaming, milling and other processes can be quickly and accurately installed on the spindle or tool magazine of the machine tool. As a programmer, you should understand the structural dimensions and adjustment methods of the tool holder used on the machine tool, and adjust the scale so that the radial and axial dimensions of the tool must be determined during programming. At present, my country's machining center uses the TSG East-West system, and its shank has two types: straight shank (three specifications) and tapered shank (four specifications), including a total of 16 knives for different purposes.
(2) The cutting amount is determined
The cutting amount includes the spindle speed (cutting speed), the amount of back cutting, and the amount of feed. Regarding different processing methods, different cutting parameters need to be selected, and they should be compiled into the program list. Reasonable selection of the cutting amount of the rope ruler is that during rough machining, the forward consumption rate is generally the main factor, but economy and processing costs should also be considered; semi-finishing and finishing should be coordinated with the premise of ensuring the quality Power, economy and processing cost. The specific value should be determined according to the machine tool manual, cutting parameter manual, and separate experience.
Seven, the tool setting point and the tool change point are determined
When programming, you should correctly select the orientation of "tool setting point" and "tool change point". The "tool setting point" is the starting point of the tool relative to the movement of the workpiece when
Machining Parts on the CNC machine tool. Because the program segment is initially executed from this point, the tool setting point is also called "program starting point" or "tool starting point".
The rope ruler for selecting the knife point is:
1. Facilitate the use of digital processing and simplify programming;
2. It is easy to align on the machine tool and easy to check during processing;
3. The processing error caused is small.
The tool setting point can be selected on the workpiece or on the outside of the workpiece (for example, on the fixture or on the machine tool), but it must have a certain dimension connection with the positioning datum of the part. In order to advance the machining accuracy, the tool setting point should be selected as far as possible on the planning reference or process reference of the part, such as the workpiece positioned by the hole, the center of the hole can be selected as the tool setting point. The orientation of the tool is aligned with this hole, so that the "tool position point" and the "tool setting point" coincide. The common calibration method in factories is to install the dial indicator on the machine tool spindle, and then roll the machine tool spindle to make the "tool position point" different from the tool setting point. The better the incompatibility, the higher the accuracy of tool setting. The so-called "tool location point" refers to the tip of a turning tool and a boring tool; the tip of a drill; the center of the bottom surface of the end mill and end mill head, and the center of the ball end of a ball end mill. After the parts and equipment, the workpiece coordinate system and the machine tool coordinate system have a certain scale connection. After the workpiece coordinate system is set, the initial coordinate value of the first block from the tool setting point; the coordinate value of the tool setting point in the machine tool coordinate system is (X0, Y0). When programming by absolute value, regardless of whether the tool setting point and workpiece origin can coincide, they are X2 and Y2; when programming by incremental value, when the tool setting point coincides with the workpiece origin, the coordinate value of the first block is When X2 and Y2 do not overlap, it is (X1 + X2), Y1 + Y2). The tool setting point is not only the start of the program, but also the end of the program. Therefore, it is necessary to consider the repeated accuracy of the tool setting point in batch production. The accuracy can be checked by the coordinate value (X0, Y0) of the tool setting point away from the machine origin. The so-called "machine origin" refers to a fixed limit point on the machine tool. For example, for a lathe, it refers to the point of intersection between the center of reversal of the main shaft of the lathe and the end face of the head chuck. When the tool needs to be changed during the machining process, the tool change point should be ruled. The so-called "tool change point" is the orientation of the tool holder when it is indexed and changed. This point can be a fixed point (such as a machining center machine tool, the orientation of the tool change manipulator is fixed), or an arbitrary point (such as a lathe). The tool change point should be located outside the workpiece or fixture, and the tool holder does not touch the workpiece or other parts when it is indexed. The set value can be determined by practical measurement methods or accounting.
8. The processing route is definitely determined
In CNC machining, the path of the tool position point related to the movement of the workpiece is called the machining path. When programming, there are the following points to determine the length of the processing route:
1) The processing path should ensure the accuracy and surface roughness of the processed parts, and the power should be high.
2) Make numerical calculation simple to reduce the amount of programming work.
3) The processing path should be the shortest, so that it can reduce the program segment and the time of the empty tool. In the case of degree and so on, it must be a single pass, or multiple passes to complete the processing, and in the milling process, whether to choose down milling or up-cut milling, etc.
For point-controlled CNC machine tools, only high positioning accuracy is required, and the positioning process is as fast as possible, and the movement path of the tool relative to the workpiece is irrelevant. Therefore, such machine tools should arrange the tool path according to the shortest idle distance. In addition, the axial movement scale of the tool must be determined. The size is mainly determined by the hole depth of the machined part, but some auxiliary scales should also be considered, such as the introduction distance and overrun amount of the tool.
