Reproducibility of cutting

In order to improve the machining efficiency of the machine tool, it is necessary to ensure the repeatability of the cutting process. In addition, the improvement of the machining efficiency largely depends on the dynamic characteristics of the machine tool. The stable spindle speed and allowable chatter-free depth of cut shown in the stable cutting area map depend on the natural frequency, stiffness, and damping of the entire machining system.

For most milling operations, the stable cutting area map reveals the range of stable machining intervals, and in particular, it is possible to achieve greater spindle speed without flutter depth. Tools, tool chucks, spindles, machine tools, workpieces and fixtures all have an impact on the stability of the machining interval. Once the cutting process has been optimized using a stable cutting area map, changing the machining parameter settings can have an adverse effect on cutting performance. However, there are tools and techniques that can be used by many machine tool users (such as tool length compensation adjustments and post-processing of CNC machining programs), which can help make machining parameter settings that can improve machining repeatability. So what are the most important factors?

The first thing to consider is the tool length. The CNC machine tool allows the user to roughly set the tool length, measure the length, and enter the correction value to compensate for the tool length. If the workpiece geometry is critical, then this size compensation will be very effective. However, achieving efficient machining does not just depend on the geometry of the workpiece. If a machine tool user sets a tool length that exceeds the nominal size, the tool stiffness will decrease and its natural frequency will be higher. Then, in the stable cutting area map, the stable machining interval will move to a lower spindle speed, and the flutter-free cut depth will also decrease. If the set tool length is shorter than the nominal size, the tool stiffness will increase and the natural frequency will be higher, so the stable machining area will move to a higher spindle speed. However, if the stable machining interval does not move, the only possibility is to use the maximum axial depth of cut available in the stable machining interval. In order to increase machining efficiency, the end user must control the tool length.

The number of teeth on the tool also affects the position of the stable machining spindle speed in the stable cutting area map. The most stable processing interval occurs where the pass frequency of the cutter tooth matches its natural frequency. So, for example, if you change a 2-blade cutter to a 4-blade cutter, it will reduce the spindle speed corresponding to the most stable machining interval by 50%. If you want to keep working in a stable machining area at all times, it means tightly controlling the spindle speed. Therefore, it also means that high spindle speeds must be avoided beyond the capability of the machine. The appropriate spindle speed must be selected and programmed into the machining program, and the spindle must be running at the programmed speed.

The type of tool chuck is also crucial. The chuck type, hydraulic type, hot-load type, Weldon side type, and other types of tool chucks all have different dynamic characteristics. By adjusting the workpiece machining program, it is possible to solve the problem of geometrical dimension change caused by the use of a new tool chuck. However, changing the tool chuck means that the position where the spindle speed is stabilized will move, and the allowable flutter-free depth of cut will also change. The effects of other factors on the reproducibility of processing may be less pronounced. For example, when using a collet chuck, the torque applied to the collet nut may affect the machining repeatability. Most collet manufacturers have restrictions on the amount of torque applied to the nut and should be set using a torque wrench or torque setting fixture. If the torque is too high, the tool chuck stiffness may increase, but its damping will decrease; if the torque is too small, the chuck stiffness may be reduced - even worse, the knife may be pulled out Chuck. In terms of torque, the torque acting on the tightening pin is also critical for ISO or CAT toolholders. Torque wrenches should also be used to set the torque. If the torque is too large, it will cause the end of the tool chuck to protrude, thus changing the match between the chuck and the spindle taper surface, directly affecting the rigidity and damping of the coupling.

Tightening forces are also critical for ISO and HSK type chuck connections. Some people seem to think that the bigger the tightening force the better, but this is not the case. A large amount of tension can indeed increase the stiffness of the collet connection, but it also eliminates the damping. The machine tool manufacturer specifies the allowable tightening force range. It is a good control method to use the tension force gauge to detect the tension force.

Although the choice of tools and chucks has a particularly important influence on the cutting performance, among other factors, the influence of the choice of machine tool and spindle on the cutting performance can not be ignored. The cutting performance of different machine tools is also different, even if they all use the same tool system. This means that a workpiece machining program should be programmed for a specific machine tool to take advantage of the machine-specific stable cutting area map. For the same tool, the stable cutting area map does not look the same on different machine tools.

CNC programming software now allows only the geometry of the workpiece to be considered when programming the workpiece. The compiled program is then post-processed so that it can run on a variety of different machine tools. Adopting this "geometry-only" programming strategy means that the machine tool's processing capacity has not yet been fully utilized and can benefit from a significant increase in processing efficiency.