CNC: Cycle time reduction by controlling what the spindle does at tool change
On a CNC lathe, what you do with the spindle during tool change can make a difference to the cycle time, and help in cycle time reduction. The spindle takes some time to accelerate to the programmed speed. The larger the lathe, the more time it takes, because of the higher size and inertia of the spindle. A typical machine would take 1 sec. for every 1000 RPM. E.g., 3 secs. to reach 3000 RPM.
You can do one of 3 things:
1. Stop the spindle before moving to the tool change position. Restart it after the tool change.
Result: When the tool approaches the next operation, the machine will wait at the destination position for the spindle to accelerate to its programmed speed. To save time you can program the spindle start during the approach to the operation from the tool change position, but even then the rapid motion will take less time than the spindle acceleration, and the machine will wait. E.g., if you are turning a 50 dia. part at 300 m/min, the spindle accelerates to 1900 RPM each time, taking approx. 2 seconds – for 5 tool changes that’s 10 seconds.
During spindle acceleration the power consumption is momentarily very high (you can see the power meter going beyond 120 %). This increases the power bill, reduces the motor and spindle life.
2. Keep the spindle running in CSS (Constant surface speed)
mode, with the same surface speed Result: The spindle slows down to a very low speed as the tool moves to the tool change position (because the diameter at the tool change position is very large), then speeds up again to a high RPM when the tool approaches the operation area. Almost the same effect as option 1, but slightly better – the spindle starts from some speed instead of from zero. Taking the same example, if the tool change position is at 200 dia., the spindle decelerates to 475 RPM each time and then accelerates to 1900 RPM, taking approx. 1.5 secs.
3. Keep the spindle running with CSS OFF
before tool change, switch back to CSS ON after tool change. Change only the mode, not the surface speed. Result: The spindle is running at the same RPM as when cutting was going on, does not decelerate or accelerate during the motion to and from the tool change position. There is no time lost.
Action point
Option 3 is the best option: Switch to Constant spindle speed mode before tool change, back to CSS mode after tool change.
Cadem’s CAPSturn CNC lathe programming software reduces cycle time drastically by taking care of this automatically. It is a CNC offline programming software and conversational CAM software.
