Many turning centers, especially larger ones, have more than one spindle range. This allows power at low speeds for powerful machining operations and high speeds for finishing operations. Many beginning programmers are taught to rough in the low range and finish in the high range. While this is a good rule of thumb, there are exceptions to this rule that can make for inefficient machine usage.
Almost all turning centers that have more than one spindle range use M codes to select them. For your turning centers, you should know how much time it takes to change spindle ranges, which can vary. With some turning centers, the spindle must come to a complete stop before the range change can take place (taking as long as 15 seconds). With others, range change can occur while the spindle is still running (taking as little as one second).
On newer turning centers with double-wound high-torque spindle motors, range changing simply involves changing the flow of electricity from one set of windings to another. This change can occur almost instantaneously and while the spindle is running.
Regardless of your turning center's spindle range changing methods, the builder will publish in its operation manual a chart of your machine's power characteristics.
Only by knowing your turning center's power characteristics can you program spindle range changes wisely and efficiently. There will be times when changing ranges will be wasteful. Consider a workpiece less than one inch in diameter that you wish to machine at 600 sfm. At this diameter (or less) at least 2292 rpm will be necessary for every machining operation, including roughing. If you apply the rule of thumb and rough in the low range, the spindle would be limited to 1500 rpm. Since spindle rpm and cycle time are inversely proportional, this would increase cycle time. And since this machine has full power at 2292 rpm, even powerful roughing operations can be done. In this example, roughing in the low range would be wasteful. Not only would you see cycle time increased by inefficient cutting conditions, cycle time would be increased by the range changing function itself.
There may be times when it is more efficient to change ranges right in the middle of a machining operation. For the example machine, if you are roughing a large shaft down from eight inches in diameter to one inch in diameter and wish to run the spindle at 800 sfm, you must rough in the low range since insufficient power is available in the high range at 382 rpm (the rpm required for an 8-inch diameter at 800 sfm). In this operation, as roughing continues to machine smaller and smaller diameters, the machine will peak out at the top rpm of the low range (1500 rpm for our example machine). At 800 sfm, this will happen at a diameter of about 2.02 inches. Machining that occurs at smaller diameters will be limited to 1500 rpm, and cycle time will be wasted.
What you do in this case will depend on how much more machining is required versus how quickly your turning center can change ranges. If it can change ranges quickly, it will be wise to change ranges and perform the balance of the roughing in the high range. If not, you must compare the time it takes to change ranges to how much time can be saved if the balance of machining is done at proper speeds.