Talking about how to control the quality of CNC machining

With the vigorous development of the automobile industry, the proportion of free-form surfaces in vehicle development is increasing, and the product quality requirements are constantly improving. Both of them put forward higher requirements on the manufacturing precision of curved surfaces, and also put forward higher precision and quality of numerical control machining. Requirements. The following metal processing online briefly talks about the measures that should be taken to control the quality of CNC machining from the aspects of machine tools, tool holders, tools, machining processes and software.

1. Use a suitable shank structure

When we use the equipment for a certain period of time, the influence of the tool holders and tools used in the machining on the processing quality becomes prominent. Our company's common tool holder structure has BT handle and HSK handle (see Figure 1, Figure 2). The connection between the BT handle and the machine tool spindle is a taper 7:24 taper shank. The shank of this structure is only suitable for low speed machining because the BT shank and the main shaft are only tapered. When the rotation speed is too high, the centrifugal force will cause The tapered surface fits the gap, which affects the quality of CNC machining. The HSK handle is coupled with the machine tool as an over-positioning structure. Under the action of the machine tool, the short cone and end face of the tool holder are closely matched with the machine tool to obtain better processing quality.

The clamping method of the shank to the arbor and the tool mainly includes side-fixing, elastic clamping, hydraulic clamping and thermal expansion. Through many years of application, comparison and summarization, the company has adopted the tool clamping scheme: BT spring chuck shank for roughing or large feed machining, BT hydraulic clamp for semi-finishing and finishing of three-axis machine tool. The head holder is a HSK type hydraulic chuck holder on a five-axis machine. The collet chuck has troubles in tooling, poor repeatability, and poor vibration absorption, so it is used for roughing or large feed machining. The hydraulic chuck shank used in finishing has a very high clamping rotation precision, and the clamping is very convenient. Moreover, the hydraulic chuck shank has obvious effect of absorbing vibration during processing, which effectively controls the surface processing quality and surface roughness of the mold. The HSK hydraulic chuck holder is used on the five-axis machine tool, which has the characteristics of simple structure, reliable clamping, high concentricity, high transmission torque and large radial force (see Figures 3 to 6).

2. Correct selection and use of tools

Carbide tools are becoming more widely used in the company. In the roughing process, the company uses large-diameter bull nose and spherical milling cutters as much as possible to achieve “more” roughing and chip removal. Semi-finishing uses insert spherical milling cutters to achieve "fast" semi-finishing. When finishing, try to use the integral alloy knife and the high-precision ball-end mirror blade to ensure the processing quality. The radius of the smallest tool used for "clearing the roots" in the mold finishing process should be less than or equal to the radius of the fillet on the machined part, so that the accuracy of the machining can be ensured and the finishing quality is good (see Figures 7 to 11).

3. Preparation of NC program

The preparation of NC programs can generally be divided into four stages:

(1) Preparation stage: According to the production task book, check the accuracy and timeliness of the data.

(2) Technical solution stage: According to technical requirements and manufacturing resources, the processing technology plan is prepared.

(3) NC programming stage: According to the 3D data and process files, the program is programmed according to the specified steps.

(4) Program finalization stage: The supervisory leadership review procedure, fill in the CNC machining program list after the qualification, and draw the processing sketch. And go to the scene to understand the program implementation, summarize the programming experience.

4. CNC process plan

Among them, the numerical control process scheme is difficult because it requires a lot of complicated information to be processed, mainly relying on the programmer's work experience. Therefore, the quality of the process plan is completely dependent on the level and experience of the technician. Principles of CNC process planning:

(1) The numerical control process should consider the processability of the machined parts, determine the clamping and positioning of the machined parts, select the tools, and formulate the process route, cutting methods and process parameters.

(2) The numerical control technology is mainly used to guide numerical control programming. Our company combines the duties of CNC technicians and programmers into one, responsible for the CNC machining process of the whole set of molds, and improves the work efficiency.

(3) CNC machining has a high degree of automation and many influencing factors. In CNC machining, quality and safety are essential and must be guaranteed.

(4) The preparation of numerical control technology must be strictly organized. In the event of a problem, it is difficult for the worker to correct it on the spot. The lighter causes processing defects and the heavy one causes a safety accident.

(5) The inheritance of the CNC process is good. Anything that proves to be a good CNC process in production can be used as a template to be saved as a file.

5. Important parameters of CNC machining software

(1) CNC machining software adopted by the company: UG NX4.0 and POWERMILL 8.0. These two softwares have very practical processing strategies, which are different and complement each other, which greatly improves the quality and efficiency of CNC machining. . The biggest difference between the two software is that UG can edit 3D data, and POWERMILL is just a single programming software. The parameters related to cutting quality in the CAM software are mainly Spindle speed, Cut feed, Lead in feed rate, Step over and depth of cut. (Step depth) and so on.

(2) The spindle speed is generally calculated based on the cutting speed, and the choice of cutting speed is closely related to the durability of the tool. Too low or too high cutting speed will cause the tool durability to drop sharply. When the mold is finished, the tool should be avoided in the middle to obtain a higher processing quality. Therefore, the cutting speed should be carefully selected in combination with the durability of the tool.

(3) The choice of feed rate directly affects the machining accuracy and surface roughness. The main parameter is the feed per tooth. The choice of feed per tooth depends on the mechanical properties of the workpiece material, the tool material and the milling cutter structure. In addition, we must pay attention to the choice of the amount of knife. The size of the knife is mainly limited by the rigidity of the machine tool, workpiece and tool. The selection principle is to use the largest possible knife under the condition that the process requirements and the rigidity of the process system are allowed. Quantity to improve processing efficiency. The feed rate when the tool is cut in should be less than the cutting feed rate.

(4) Stepover and Scallop are the two main parameters controlling the surface roughness value in the CAM software. When the step width is used to control the surface roughness value, the smaller the step width is, the smaller the surface roughness value is. When the residual height is used to control the surface roughness value, the step width is automatically adjusted according to the shape of the workpiece (see Figures 12 to 14). .