Today’s CNC turning centers allow very fast spindle speeds. It is not uncommon for a machine having an eight inch diameter hydraulic chuck to provide 6,000 rpm or more. Since these machines allow spindle speed to be programmed with a feature called constant surface speed, it is not obvious just how fast the spindle will run for a given machining operation. With constant surface speed, of course, you specify speed in surface feet per minute (in the inch mode) or meters per minute (in the metric mode).
You can apply a formula to determine rpm for a given operation:
rpm equals 3.82 times speed in sfm divided by the diameter being machined
Note that if you will be facing a workpiece to center – a very common operation for turned workpieces – the spindle will run up to its maximum rpm in the current spindle range. This is because you are sending the cutting tool to spindle center – a diameter of zero. And even if you specify a speed of one surface foot per minute, the spindle will run up to its maximum speed.
When you have small, completely concentric and round workpieces, this is seldom a problem.
The workpiece will run true in the spindle and maximum rpm can be achieved without vibration.
But when you are running larger workpieces, and especially when the workpiece is not perfectly round and balanced (castings and forgings are notorious for this problem), it’s likely that the machine will not be able to achieve its maximum rpm with out – at the very least – some vibration. Worse, the vibration may be severe enough to cause problems with machining. At worst, the out-of-balance condition may cause the work holding device (chuck) to fail – and the workpiece will be released at a very fast rpm. Few machine doors and windows can hold in such a workpiece, and this of course makes for a very dangerous situation.
For this reason, control manufactures provide you with a way to program the maximum spindle speed. With Fanuc and Fanuc-compatible controls it is done with a G50 command. If the control executes this command
it will not allow the spindle to rotate faster than 1,500 rpm – even if constant surface speed is being used. Said another way, if you are facing a workpiece to center with constant surface speed, the spindle will stop accelerating when it reaches 1,500 rpm. It will be as if the machine’s maximum rpm is 1,500.
Determining how fast the spindle can run without vibration can be difficult. Even within a lot of workpieces, it is possible that some parts will experience vibration at lower speeds than others. Because this is such an important safety issue, we recommend erring on the side of caution (slow). That is, set up your maximum speed for the worst possible workpiece condition.
One way to determine maximum rpm is to physically test it (frankly, this is the only way we know of). Place the workpiece in the spindle and start the spindle in manual data input (MD) mode at a very slow rpm. Then, in small increments, increase the rpm until you either achieve the machine’s maximum or vibration starts to occur. You should repeat this test with several workpieces. When a maximum rpm is determined, we recommend decreasing it by another 10% or so, just to ensure safety.
The spindle range surprise
Remember that many turning centers have multiple spindle ranges. This can cause some real problems for out-of-balance workpieces. Consider, for example, a turning center that has two spindle ranges. The low range runs from 0 – 2,000 rpm. The high range runs from 0 – 6,000 rpm.
When roughing the workpiece, the programmer uses the low spindle range to get the power needed for machining. And of course, the spindle will be automatically limited to 2,000 rpm. So if a face-to-center operation is done, the spindle will not accelerate past 2,000 rpm. But when they do the finishing operations, the programmer will switch to the high range. If a face-to-center operation is done, the spindle will accelerate to 6,000 rpm.
When you do your test to determine maximum spindle speed, you should do it in the high spindle range. Doing it in the low range will render false results. The spindle may peak-out in the low spindle range prior to vibration. But in reality, it cannot achieve the machine’s true maximum rpm without creating a dangerous condition.
Is efficiency an issue?
One last point. There may be times, especially when quantities are high, when you will want to fine tune your spindle limiter in the program. The situation may exist when a workpiece is terribly out-of-round in its rough state (requiring a severe spindle limitation), but after roughing, will be in a more balanced state. So after roughing in this case, you will be able to increase the maximum spindle rpm to allow more efficient machining.