In the CNC milling process, there are many reasons for machining errors. The error caused by the tool radial runout is one of the important factors, which directly affects the minimum shape error and the machined surface that the machine tool can achieve under ideal machining conditions. Geometry accuracy. In actual cutting, the radial runout of the tool affects the machining accuracy, surface roughness, unevenness of tool wear and the 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.
Causes Of Radial Runout
Manufacturing errors and clamping errors of the tool and spindle components cause drift and eccentricity between the tool axis and the ideal rotation axis of the spindle, as well as the specific processing technology and tooling, which may cause the radial runout of the CNC milling machine during processing.
1. The Influence Of The Radial Runout Of The Spindle Itself
The main reasons for the radial runout error of the spindle are the coaxiality error of each journal of the spindle, the various errors of the bearing itself, and the coaxiality error between bearings. Their influence on the radial rotation accuracy of the spindle varies with the machining mode. These factors are formed in the process of machine tool manufacturing and assembly, and the operator as a machine tool is difficult to avoid their influence.
2. The Impact Of The Inconsistency Between The Tool Center And The Spindle Rotation Center
When the tool is installed on the spindle, if the center of the tool is inconsistent with the center of rotation of the spindle, the tool will inevitably cause radial runout. The specific influencing factors are: the fit of the tool and the chuck, whether the method of loading the tool is correct, and the quality of the tool itself.
3. The Impact Of Specific Processing Technology
The radial runout of the tool during machining is mainly because the radial cutting force aggravates the radial runout. Radial cutting force is the component force of the total cutting force in the radial direction. It will cause the workpiece to bend and deform and produce vibration during processing. It is the main component that affects the quality of the workpiece. It is mainly affected by factors such as cutting amount, tool and work piece material, tool geometric angle, lubrication method and processing method.
Ways To Reduce Radial Runout
The radial runout of the tool during machining is mainly because the radial cutting force aggravates the radial runout. Therefore, reducing the radial cutting force is an important principle to reduce the 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 to reduce cutting force and vibration. Choose a larger clearance angle of the tool to reduce the friction between the main clearance surface of the tool and the elastic recovery layer of the transition surface of the workpiece, thereby reducing vibration. However, the rake angle and back angle of the tool cannot be selected too large, otherwise the strength and heat dissipation area of the tool will be insufficient. Therefore, it is necessary to choose different rake angles and back angles of the tool according to the specific situation. The rough machining can be smaller, but in the finishing machining, for the consideration of reducing the radial runout of the tool, it should be larger to make the tool sharper.
2. Use Strong Tools
There are mainly two ways to increase the strength of the tool. One is to increase the diameter of the tool holder. Under the same radial cutting force, the diameter of the tool holder increases by 20%, and the radial runout of the tool can be reduced by 50%. The second is to reduce the protruding length of the tool. The greater the protruding length of the tool, the greater the deformation of the tool during processing. When processing is in constant change, the radial runout of the tool will continue to change, resulting in a workpiece The machined surface is not smooth, too, if the extension length of the tool is reduced by 20%, the radial runout of the tool will also be reduced by 50%.
3. The Rake Face Of The Tool Should Be Smooth
During processing, the smooth rake face can reduce the friction of the chips on the tool, and can also reduce the cutting force on the tool, thereby reducing the radial runout of the tool.
4. Spindle Taper Hole And Chuck Cleaning
The spindle taper hole and the chuck are clean, and there should be no dust and debris generated during the processing of the workpiece. When selecting a machining tool, try to use a tool with a shorter extension length to load the knife, and the force should be reasonable and even, not too large or too small.
5. Choose A Reasonable Amount Of Knife
If the amount of cutting tool is too small, the phenomenon of machining slippage will occur, which will cause the continuous change of the radial runout of the tool during machining. When the machined surface is not smooth, the cutting force will increase accordingly. It will cause large deformation of the tool, increase the radial runout of the tool during machining, and also make the machined surface not smooth.
6. Use Up Milling In Finishing
As the gap position between the lead screw and the nut changes during down milling, it will cause uneven feed of the worktable. As a result, impact and vibration will affect the life of the machine tool and the tool and the surface roughness of the workpiece. When milling, the cutting thickness changes from small to large, the load of the tool also changes from small to large, and the tool is more stable during processing. Note that this is only used for finishing, and down milling is still used when roughing. This is because the productivity of down milling is high and the service life of the tool can be guaranteed.
7. Reasonable Use Of Cutting Fluid
Reasonable use of cutting fluid, mainly cooling water solution, has little effect on cutting force. The cutting oil whose main function is lubrication can significantly reduce the cutting force. Because of its lubricating effect, it can reduce the friction between the tool rake face and the chip and between the flank face and the transition surface of the workpiece, thereby reducing the tool radial runout.
Practice has proved that as long as the accuracy of the manufacturing and assembly of each part of the machine is ensured, and the reasonable process and tooling are selected, the influence of the radial runout of the tool on the machining accuracy of the workpiece can be minimized.