What’s the Best Speed for Turning External Threads on CNC Lathes?
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What’s the Best Speed for Turning External Threads on CNC Lathes?
When machining threads on a CNC lathe machine, due to the change of its transmission chain, in principle, its speed can ensure that the tool moves one pitch along the main feed axis (mostly Z axis) every time the spindle rotates, and it should not be restricted. However, when machining threads on a CNC lathe, it will be affected by the following aspects:
- The pitch (lead) value commanded in the thread processing block is equivalent to the feed speed F expressed in feed (mm/r). If the spindle speed of the machine tool is selected too high, the converted feed The speed (mm/min) must greatly exceed the normal value;
- At the beginning/end of the tool’s displacement, it will be restricted by the up/down frequency of the servo drive system and the interpolation speed of the numerical control device. Because the up/down frequency characteristics cannot meet the processing needs, it may be due to the main feed The “advance” and “lag” produced by the movement cause the pitch of some threads to fail to meet the requirements;
- Thread turning must be realized through the synchronous operation function of the spindle, that is, thread turning requires a spindle pulse generator (encoder).
When the spindle speed is selected too high, the positioning pulse sent by the encoder (that is, a reference pulse signal sent every time the spindle rotates) may be caused by “overshoot” (especially when the quality of the encoder is unstable). Lead to random buckling of the workpiece thread.
Therefore, when threading, the determination of the spindle speed should follow the following principles:
- Under the condition of ensuring production efficiency and normal cutting, a lower spindle speed should be selected;
- When the lead-in length δ1 and the cut-out length δ2 in the thread processing block are considered adequate, that is, when the thread feed distance exceeds the specified thread length on the drawing, a suitable higher spindle can be selected Rotating speed;
- When the allowable working speed specified by the encoder exceeds the maximum speed of the spindle specified by the machine tool, the highest possible spindle speed can be selected;
- Under normal circumstances, the spindle speed (n screw) during threading should be determined according to the calculation formula specified in the machine tool or numerical control system manual. The calculation formula is mostly: n screw≤nallow/L(r/min) In the formula, n allows the maximum working speed allowed by the encoder (r/min); L-the pitch (or lead, mm) of the workpiece thread.
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