Spindle speed when turning external threads on a lathe

When NC lathe is processing thread, because of the change of transmission chain, in principle, the spindle speed should not be limited as long as it can ensure that the tool moves one pitch along the main feed axis (mostly Z axis) direction every revolution. However, when NC lathe processes threads, it will be affected by the following aspects:

1) The thread pitch (lead) value of the command in the thread processing program section is equivalent to the feed rate F expressed by the feed rate (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 frequency rise/fall of the servo drive system and the interpolation operation speed of the numerical control device. Because the frequency rise/fall characteristics can not meet the processing needs and other reasons, the "lead" and "lag" generated by the main feed motion may cause the pitch of some screw teeth to not meet the requirements;

3) Thread turning must be realized through the synchronous operation function of the spindle, that is, the spindle pulse generator (encoder) is required for thread turning. 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 workpiece thread to buckle disorderly due to "overshoot" (especially when the quality of the encoder is unstable).

Therefore, when turning the thread, the determination of the spindle speed should follow the following principles:

1) Under the condition of ensuring production efficiency and normal cutting, lower spindle speed should be selected;

2) When the lead length in the thread processing program section δ 1 and cut length δ 2 (as shown in the figure) It is sufficient to consider, that is, when the thread feed distance exceeds the length of the thread specified in the drawing, a 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, the spindle speed as high as possible can be selected;

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 NC system manual. The calculation formula is:

N screw ≤ n allowed/L (r/min), where n allowed - the maximum working speed allowed by the encoder (r/min);

L - pitch (or lead, mm) of workpiece thread.