The optimum speed of turning external thread on nc lathe
When CNC lathe is machining thread, due to the change of its transmission chain, in principle, the rotation speed can only ensure that when the spindle rotates for one cycle, the cutter can move one pitch along the main feed axis (mostly Z-axis), and it should not be limited. However, when CNC lathe is used to process thread, it will be affected by the following aspects:
1) The screw pitch (lead) value of the command in the thread processing program section is equivalent to the feed speed F expressed in the feed rate (mm/r). If the spindle speed of the machine tool is selected too high, the converted feed speed (mm/min) must greatly exceed the normal value;
2) At the beginning and end of the tool's displacement, the tool will be constrained by the servo drive system's increasing/decreasing frequency and the interpolation operation speed of the CNC device. Because the increasing/decreasing frequency characteristics can not meet the processing needs and other reasons, the "leading" and "lagging" caused by the main feed movement may cause the screw pitches of some threads to fail to meet the requirements.
3) The thread turning must be realized through the synchronous operation function of the spindle, that is, the thread turning requires a spindle pulse generator (encoder). When the spindle speed is too high, the positioning pulse sent by the encoder (that is, a reference pulse signal sent by the spindle every revolution) may cause the thread of the workpiece to be disorderly buckled due to "overshoot" (especially when the quality of the encoder is unstable).
Therefore, the spindle speed shall be determined according to the following principles when threading:
1) Under the condition of ensuring production efficiency and normal cutting, lower spindle speed should be selected;
2) When the import length in the thread processing program segment δ 1 and cut out length δ 2 (as shown in the figure) When the feed distance of the thread exceeds the length of the specified thread on the drawing by a large amount, the spindle speed can be selected to be higher appropriately;
3) When the allowable working speed specified by the encoder exceeds the maximum speed of the spindle specified by the machine tool, the spindle speed can be selected as high as possible;
4) In general, the spindle speed (n screw) during thread turning shall be determined according to the calculation formula specified in the machine tool or CNC system manual. The calculation formula is mostly:
N screw ≤ n allowable/L (r/min), where n allowable - the maximum operating speed allowed by the encoder (r/min); L - thread pitch of workpiece thread (or lead, mm).