When machining threads on a CNC lathe, due to changes in its transmission chain, its rotational speed should not be limited as long as it can ensure that the tool shifts one pitch along the main feed axis (mostly the Z-axis) direction every revolution of the spindle. But when CNC lathes process threads, they are affected by the following aspects:
1）The pitch (lead) value of the instruction in the thread processing program segment is equivalent to the feed rate F expressed in mm/r. If the spindle speed of the machine tool is selected too high, the converted feed rate (mm/min) must greatly exceed the normal value;2）At the beginning and end of its displacement, the tool will be constrained by the servo drive system's up/down frequency and the interpolation operation speed of the CNC device. Due to reasons such as the up/down frequency characteristics not meeting the machining needs, it may be caused by the "leading" and "lagging" generated by the main feed motion, which may result in some screw pitches not meeting the requirements;3）Turning threads must be achieved through the synchronous operation function of the spindle, that is, turning threads requires a spindle pulse generator (encoder).When the spindle speed is selected too high, the positioning pulse emitted by the encoder (i.e. a reference pulse signal emitted by the spindle every revolution) may cause disorderly threading of the workpiece due to "overshoot" (especially when the quality of the encoder is unstable).Therefore, when turning threads, the determination of spindle speed should follow the following principles:1）It is advisable to choose a lower spindle speed while ensuring production efficiency and normal cutting;2）When the import length in the thread processing program segment δ 1 and cut out length δ 2 (as shown in the figure) Considering more abundant, that is, when the thread feed distance exceeds the specified length of the thread on the drawing, a slightly higher spindle speed can be selected;3）When the allowable working speed specified by the encoder exceeds the maximum speed of the spindle specified by the machine tool, a spindle speed as high as possible can be selected;4）In general, the spindle speed (n screw) during thread turning should be determined according to the calculation formula specified in the machine tool or CNC system manual, which is usually:N screw ≤ n tolerance/L (r/min) In the formula, n is the maximum allowable working speed of the encoder (r/min);L - Thread pitch (or lead, mm) of the workpiece thread.
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