Because the spindle speed and range of CNC machine tools are far higher than ordinary machine tools, and the output power of the spindle is large, compared with the traditional processing methods, higher requirements are put forward for the CNC machining tools, including high precision, high strength, good rigidity, high durability, and the size stability, convenient installation and adjustment, and the characteristics of the spindle drive system of CNC machine tools. This requires that the structure of the tool is reasonable, the geometric parameters are standardized and serialized. NC tool is one of the preconditions to improve the machining efficiency. Its selection depends on the geometry of the parts to be processed, the material state, the fixture and the rigidity of the selected tool. The following aspects should be considered:
(1) The cutting tool is selected according to the cutting performance of the part material. If turning or milling high strength steel, titanium alloy, stainless steel parts, it is recommended to select indexable carbide tool with good wear resistance.
(2) Select the tool according to the hardware parts processing stage. That is to say, in the rough machining stage, the cutting tool with better rigidity and lower precision should be selected; in the semi finishing and finishing stage, the machining accuracy and product quality of the parts should be guaranteed; the tools with high durability and high precision should be selected; the precision of the tools used in the rough machining stage is the lowest, while that in the finishing stage is the highest. If the same tool is selected for rough machining and finish machining, it is recommended to select the tool eliminated from finish machining during rough machining, because most of the tool wear out of finishing machining is slight wear on the edge, and coating wear and polishing. Continuous use will affect the quality of finish machining, but the impact on rough machining is small.
(3) According to the characteristics of NC lathe processing area, the tool and geometric parameters are selected. The tool with large diameter and small ratio of length to diameter should be selected if the structure of the part is allowed; the end edge of the cross center milling cutter for cutting thin-walled and ultra-thin-wall parts should have enough centripetal angle to reduce the cutting force of the cutter and cutting parts. When processing aluminum, copper and other soft material parts, the end mill with larger front angle should be selected, and the number of teeth should not exceed 4.
When selecting the tool, the size of the tool should be compatible with the surface size of the workpiece to be processed. In production, end milling cutter is often used for peripheral contour processing of plane parts; when milling plane, carbide blade milling cutter should be selected; when machining boss and groove, high-speed steel end milling cutter should be selected; when hardware processing blank surface or rough machining hole, corn milling cutter with cemented carbide blade can be selected; Ball end milling cutter, ring milling cutter, conical milling cutter and disc milling cutter are often used in the machining of solid surface and variable bevel profile.
In the process of free-form surface machining, because the cutting speed of the ball end cutter is zero, in order to ensure the NC machining accuracy, the cutting row spacing is generally very small, so ball end milling cutter is suitable for surface finishing. The end milling cutter is far better than the ball end milling cutter in both surface processing quality and processing efficiency. Therefore, under the premise of ensuring that the parts can not be cut, the end milling cutter should be selected as far as possible when rough machining and semi finishing surface machining. In addition, the durability and accuracy of cutting tools are closely related to the price of cutting tools. It must be noted that in most cases, although the selection of good tools increases the cost of cutting tools, the improvement of processing quality and efficiency brought about by this can greatly reduce the whole processing cost.
In the machining center, all the tools are pre installed in the tool library, and the corresponding tool change action is carried out through the tool selection and tool change instructions of the NC program. It is necessary to select the corresponding standard tool holder suitable for the machine tool system specification, so that the tool for NC machining can be quickly and accurately installed on the machine tool spindle or returned to the tool magazine. The programmer should be able to understand the structure size, adjustment method and adjustment range of the tool holder used by the machine tool, so as to ensure that the radial and axial dimensions of the tool can be determined and the arrangement order of the tool can be arranged reasonably.
The surface geometric characteristics of NC machining include surface roughness, surface waviness and surface texture. Surface roughness is the basic unit of geometric characteristics of machined surface. When machining workpiece surface with metal cutting tools, surface roughness is mainly affected by geometric factors, physical factors and NC machining process factors.
(1) Geometric factors
From the geometric point of view, the shape and geometric angle of the tool, especially the radius of the tool tip arc, the main deflection angle, the secondary deflection angle and the feed rate in cutting parameters have a great influence on the surface roughness.
(2) Physical factors
Considering the physical essence of the cutting process, the metal material plastic deformation is caused by the corner of the cutting edge and the extrusion and friction behind the tool, which seriously worsens the surface roughness. When the plastic material is processed by numerical control lathe to form strip chip, it is easy to form chip accretion with high hardness on the rake face. It can replace the rake face and cutting edge for cutting, so that the geometric angle of the cutter and the back feed can be changed. The contour of the chip accretion is very irregular, so that the depth and width of the workpiece surface are constantly changing. In CNC machining, some chips are embedded in the workpiece surface, which increases the surface roughness. The vibration during cutting increases the surface roughness parameters.
(3) Technological factors
From the perspective of technology, the influence on the surface roughness of metal parts is mainly related to cutting tools, workpiece materials and CNC processing conditions.
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