We all know that when I receive a CNC machining project, I must conduct a process analysis of the machining project in order to carry out the next practical operation, otherwise it will be difficult to continue. Below we come to the specific process of turning parts for in-depth analysis.
- Analysis of the NC machining processability of the part drawing
Clarify the processing content and technical requirements of the parts, analyze whether the dimensions, tolerances and surface roughness standards of the drawings are complete, and analyze whether the required processing accuracy and dimensional tolerances of the parts can be guaranteed, whether there are extra dimensions that cause contradictions or affect the process The closed size of the arrangement, etc.
In actual processing, the feasibility of realizing these technical requirements should be analyzed in combination with existing production conditions. Review and analyze whether the dimensions in the part drawings are suitable for the characteristics of CNC machining. For CNC machining, it is best to cite the dimensions with the same datum or give the coordinate dimensions directly. In manual programming, the base point or node coordinates must be calculated. Numerical control processing technology particularly emphasizes positioning processing, especially for parts that are processed by numerical control on both sides. It is very necessary to position on the same reference, otherwise it is difficult to ensure the coordinated position and size of the two sides after two installation and processing.
- Arrangement of CNC machining sequence:
- The principle of base first:
The surface used as a precision datum should be processed first, because the more precise the surface of the positioning datum, the smaller the clamping error. For example, when machining shaft parts, the center hole is always processed first, and then the outer circle and end face are processed based on the center hole.
- The principle of rough first and refined:
The processing order of each surface is carried out in the order of roughing —- semi-finishing —– finishing —– smoothing, gradually improving the processing accuracy of the surface and reducing the surface roughness.
- The principle of first priority and second priority:
The main working surface and assembly base surface of the part should be first, so that possible defects on the main surface of the blank can be found early. The secondary surface can be interspersed, placed on the main surface to a certain degree, before the final finishing.
- The principle of near first and far away:
Generally, the parts close to the tool setting point are processed first, and the parts far away from the tool setting point are processed later, in order to shorten the tool movement distance and reduce the idle travel time.
Choose the appropriate CNC program according to the tool path. When determining the tool path, pay attention to the reasonable selection of the cutting-in and cutting-out direction. When determining the path of the tool, the cutting-in or cutting-out point of the tool should be along the tangent direction of the part contour, not Cut in and out along the normal direction of the part to avoid cutting in and out of the cutting tool to affect the surface quality, and to ensure the smooth transition of the contour curve of the part. In practical applications, it is often necessary to select a suitable tool path according to the specific processing conditions.
Three, the choice of CNC machining fixture
In CNC machining, the basic principle of workpiece clamping is the same as that of ordinary machine tools. The positioning datum and clamping plan must be selected reasonably according to the specific situation. The following points should be noted:
- Strive for the unification of design benchmarks, process benchmarks and programming calculation benchmarks.
- Minimize the number of clamping times and auxiliary time of the workpiece, that is, as much as possible to process all the surfaces to be machined in one clamping of the workpiece.
- Avoid using a clamping scheme that takes a long time for manual adjustment of the machine in order to give full play to the effectiveness of the CNC machine tool.
- The point of application of the clamping force should fall on the part with better rigidity of the workpiece.
Four, CNC turning tool selection
The choice of cutting tools is one of the important contents in the CNC machining process. The selected tools on CNC machine tools often use tool materials suitable for high-speed cutting (such as high-speed steel, ultra-fine-grained cemented carbide) and use indexable inserts. Machine clamp indexable tools are widely used in CNC turning, which is an important means to improve the productivity of CNC machining and ensure product quality. There are many types of indexable turning tool blades, the most widely used is diamond blades, followed by triangular blades, round blades and grooving blades. Rhombus blades are divided into three categories: 80 °, 55 ° and 35 ° according to their diamond sharp angles. Commonly used tools are as follows:
- Select the outer contour knife:
The pattern is processed with a vertical surface, and the entering angle of the outer contour processing tool must be greater than 90 degrees. Choose the appropriate tool nose radius according to the surface roughness of the workpiece. Generally, the surface roughness is r1.6um, and the blade with the nose radius of 0.4mm is selected, and the surface roughness is r3.2um, and the blade with the nose radius of 0.8mm is selected. Under normal circumstances: Choose a carbide diamond insert with an entering angle of 93 °, a nose angle of 80 °, and a nose radius of 0.4 mm for rough and fine machining of the outer contour.
- Select the grooving knife.
The general principle of grooving knife selection is: the amount of knife
- The choice of concave arc turning tool In order to avoid tool interference when machining concave arc, the main deflection angle and secondary deflection angle of the outer contour machining tool are restricted, and the tool nose angle must be less than a certain angle. The 35 ° diamond blade is mostly used for turning concave arc workpieces or workpieces with complex profiles because of its small nose angle and less interference.
Five, determine the cutting amount
When selecting the cutting amount in CNC machining, it is to give full play to the performance of the machine tool and the cutting performance of the tool under the premise of ensuring the quality of the processing and the service life of the tool, so that the cutting efficiency is the highest and the processing cost is the lowest.
- The amount of back-grabbing: Under the condition that the rigidity allows: the machining allowance should be completed with the least number of feeds to improve labor productivity.
- Cutting speed: Increasing cutting speed is also a measure to improve productivity, but the relationship between cutting speed and tool life is relatively close. The control Panel of the CNC machine tool is generally equipped with a spindle speed adjustment switch, which can be used to adjust the spindle speed during processing.
- Feed rate: The feed rate should be selected according to the machining accuracy and surface roughness requirements of the parts, as well as the tool and workpiece material. The increase in feed speed can also improve production efficiency. When processing workpieces with high surface quality requirements, high-speed and small-feed processing methods should be used.
- Tool setting point, tool change point selection and tool nose radius compensation establishment in CNC turning processing program
The tool setting point is the reference point used to determine the position of the workpiece coordinate system in the machine tool coordinates after the workpiece is positioned and clamped on the machine tool. Generally speaking, the tool setting point should be selected at the origin of the workpiece coordinate system, generally at the intersection of the right end surface of the workpiece and the axis of rotation. The tool setting process is generally carried out from each coordinate direction, which can be understood as realizing by aligning the tool and a point with a certain position in the workpiece coordinate system. When the tool needs to be changed during the machining process, the appropriate tool change point should be considered during programming. The tool change point should be set outside the workpiece or fixture, and the tool holder does not touch the workpiece and other parts during indexing.
The purpose of tool nose radius compensation is to solve the machining errors that may be caused by the tool nose arc. When the tool tip is an imaginary tool tip, the cutting process will be executed according to the shape specified by the program without any problem. However, the real cutting edge is composed of circular arcs. When machining oblique lines or circular arcs, there will be positional deviations between the actual cutting point and the ideal tool tip in the x and z axis directions, and there will be a phenomenon of undercutting or overcutting. , Resulting in processing errors. The greater the radius r of the tool nose arc, the greater the machining error. In order to enable the system to correctly calculate the actual motion trajectory of the tool center, in addition to the tool nose arc radius r, the ideal tool nose position number t of the tool must be given. The tool nose arc radius compensation and its compensation direction are determined by The g41, g42, and g40 instructions are implemented.
g40: Cancel tool nose arc radius compensation.
g41: Left compensation of tool nose arc radius.
g42: Right compensation of tool nose arc radius
CNC machining programmers must conduct a full and comprehensive process analysis of the parts, and then work out a reasonable process design, including the processing sequence and processing route, the clamping method, the selection of cutting tools, and the selection of cutting quantities. Only on this basis can we compile the best numerical control processing program, and process parts that are economical and can meet the quality requirements.