Spindle range changing on most machining centers is somewhat more transparent than it is on turning centers. With turning centers, the programmer must specify an M code to select the desired spindle range. But with almost all machining centers, the spindle range selection is done automatically with the spindle speed word (the S word).
Say, for example, a machining center has a low range that goes from 1 to 1500 rpm and a high range that goes from 1,501 to 4,000 rpm. With this machine any S word under 1500 rpm will cause the machine to automatically select the low range. Likewise, any S word over 1500 rpm will cause the machining center's control to automatically select the high range. This makes it possible for a programmer to be completely unaware that the machine he's programming has more than one spindle range!
As with turning centers, changing spindle range on machining centers takes time. The spindle must stop (it is normally stopped at this point anyway for the upcoming tool change), the new range is selected, and then the spindle is restarted. Note that the time it takes to change spindle speed ranges varies from one machine to the next. With older machines, mechanical transmissions are involved. With newer machines, the machine tool builder may be using a double-wound, high-torque spindle motor. With these newer motors, range changing time is almost instantaneous.
Consider this set of tools and the rpm they require for our example machine:
- Face mill - 1,490 rpm
- Center drill - 2200 rpm
- 1/4 drill - 2,000 rpm
- 3/16 drill - 2100 rpm
With this process, the face mill is the only tool requiring the low spindle range (cut-off point for range changing is 1500 rpm). What is the face mill doing? If it is rough machining in a hard material, of course the low range is necessary to attain the required power. But if this face mill is finish milling in aluminum (no power is required), the range change could be eliminated by moving its speed up just a tiny bit to 1,501 rpm.
Even if range changes are required, try to group tools in such a way that you minimize the number of range changes needed within a single part program. For our example machine, this means running all tools requiring under 1500 rpm at one time. Of course, you can only do this if it doesn't jeopardize the quality of your process.
Make special note of the fact that excessive range changing may go unnoticed. Since it happens during the tool change, and since the spindle is already stopped, the time required for range changing may appear to be part of the tool change. This is why we said that you may have more than one spindle range and not even know it. If your tool changing time is inconsistent, it is likely that spindle range changing is the cause.