In the cutting process of CNC milling machines, there are many reasons for machining errors, and the error caused by tool radial runout is one of the important factors. It directly affects the minimum shape error that the machine tool can achieve under ideal machining conditions and the geometric accuracy of the machined surface. In actual cutting, the radial runout of the tool affects the machining accuracy, surface roughness, uneven tool wear, and cutting process characteristics of multi tooth tools. The larger the radial runout of the tool, the more unstable the machining state of the tool, and the more it affects the machining effect.
2、 Reasons for radial jumping
Manufacturing errors and clamping errors of cutting tools and spindle components can cause drift and eccentricity between the tool axis and the ideal rotation axis of the spindle, as well as radial runout of CNC milling machine tools during machining due to specific processing techniques and fixtures.
1. The impact of radial runout on the spindle itself
The main reasons for the radial runout error of the spindle include the coaxiality error of each journal of the spindle, various errors of the bearings themselves, coaxiality error between bearings, spindle deflection, etc. Their impact on the radial rotation accuracy of the spindle varies with different machining methods. These factors are formed during the manufacturing and assembly processes of machine tools, and as operators of machine tools, it is difficult to avoid their impact.
2. The impact of inconsistent tool center and spindle rotation center
During the installation of the tool onto the spindle, if the center of the tool is not aligned with the rotation center of the spindle, it will inevitably cause radial runout of the tool. The specific influencing factors include the fit between the tool and the chuck, whether the cutting method is correct, and the quality of the tool itself.
3. The impact of specific processing techniques
The radial runout generated by cutting tools during machining is mainly due to the increased radial cutting force that exacerbates the radial runout. Radial cutting force is the component of the total cutting force in the radial direction. It will cause bending deformation of the workpiece and generate vibration during processing, which is the main component affecting the quality of workpiece processing. It is mainly influenced by factors such as cutting parameters, tool and workpiece materials, tool geometry angle, lubrication method, and machining method.
3、 Methods to reduce radial runout
The main reason for the radial runout of cutting tools during machining is that the radial cutting force intensifies the radial runout. So, reducing radial cutting force is an important principle for reducing radial runout. The following methods can be used to reduce radial runout:
1. Use sharp knives
Choose a larger tool rake angle to make the tool sharper and reduce cutting force and vibration. Choosing a larger tool rake angle can reduce the friction between the main rake face of the tool and the elastic recovery layer on the transition surface of the workpiece, thereby reducing vibration. However, the front and back corners of the tool cannot be selected too large, otherwise it will result in insufficient strength and heat dissipation area of the tool. So, it is necessary to choose different tool front and back angles based on specific situations. For rough machining, a smaller angle can be used, but for precision machining, to reduce the radial runout of the tool, a larger angle should be used to make the tool sharper.
2. Use high-strength cutting tools
There are two main ways to increase the strength of the cutting tool. One is to increase the diameter of the tool holder. Under the same radial cutting force, increasing the diameter of the tool holder by 20% can reduce the radial runout of the tool by 50%. The second is to reduce the extension length of the tool. The larger the extension length of the tool, the greater the deformation of the tool during processing. During processing, it is constantly changing, and the radial runout of the tool will also change accordingly, resulting in an uneven surface of the workpiece. Similarly, reducing the extension length of the tool by 20% will also reduce the radial runout of the tool by 50%.
3. The front cutting surface of the tool should be smooth
During machining, a smooth rake surface can reduce the friction of chips on the tool and also decrease the cutting force on the tool, thereby reducing the radial runout of the tool.
4. Cleaning of spindle taper hole and clamp head
The spindle taper hole and chuck should be clean, without dust or debris generated during workpiece processing. When selecting machining tools, try to use tools with shorter extension lengths. When cutting, the force should be reasonable and even, not too large or too small.
5. The selection of knife quantity should be reasonable
When the cutting amount is too small, there will be a phenomenon of machining slippage, which leads to continuous changes in the radial jumping amount of the tool during machining, making the machined surface uneven. When the cutting amount is too large, the cutting force will increase, resulting in large deformation of the tool. Increasing the radial jumping amount of the tool during machining will also make the machined surface uneven.
6. Use reverse milling during precision machining
Due to the changing position of the gap between the lead screw and nut during forward milling, it can cause uneven feed of the worktable, resulting in impact and vibration, affecting the service life of the machine tool, cutting tools, and the surface roughness of the workpiece. When using reverse milling, the cutting thickness increases from small to large, and the load on the cutting tool also increases from small to large, making the tool smoother during processing. Note that this is only used during precision machining. When performing rough machining, it is still necessary to use sequential milling because it has high productivity and ensures the service life of the tool
7. Reasonably use cutting fluid.
The reasonable use of cutting fluid with cooling effect as the main water solution has little effect on cutting force. Cutting oil mainly used for lubrication can significantly reduce cutting force. Due to its lubricating effect, it can reduce the friction between the front cutting surface of the tool and the chips, as well as between the rear cutting surface and the transition surface of the workpiece, thereby reducing the radial runout of the tool.
Practice has proven that as long as the precision of manufacturing and assembly of various parts of the machine tool is ensured, and reasonable processes and fixtures are selected, the impact of radial runout of the cutting tool on the machining accuracy of the workpiece can be minimized to the greatest extent possible.