Constant surface speed is a great turning center feature. It simplifies programming because the speed is specified directly in surface feet per minute (or meters per minute in the metric mode). The control constantly and automatically selects the correct speed in rpm based on the tool's current diameter. Constant surface speed ensures consistent finish as long as you program feed rate in per-revolution fashion. It also maximizes tool life, since the tool is always cutting at the appropriate speed.
For as good a feature as constant surface speed is, there are a couple of drawbacks. If it's not programmed wisely, for example, it can be a cycle time waster.
Another drawback that can have serious consequences has to do with a machine's maximum spindle speed. As you know, the machine will automatically select the appropriate spindle speed in rpm based upon the programmed speed in surface feet per minute (or meters per minute) and the tool's current diameter. When you face a workpiece to center, the tool will go to zero diameter, and the spindle will accelerate to its maximum speed in the current spindle range. If the high range is currently selected, the spindle will run up to its maximum.
In most cases (round, true workpieces), this is exactly what is desired. Truly, the spindle speed is matched to the diameter being machined. But there are times when allowing the spindle to run up to its maximum could be disastrous.
Consider, for example a large, unbalanced, raw casting. Say the machine's maximum spindle speed is 6,000 rpm. If you face this casting to center in the program as one of the first machining operations, the spindle will run up to 6,000 rpm. And because the workpiece is unbalanced, it's likely that the workpiece will be thrown right out of the workholding device.
Another time when allowing the spindle to run up to its maximum will cause problems has to do with bar feeding applications. Many bar feeders can hold very long bars (as long as 12 or 15 feet). It's likely that if the bar is not perfectly straight and/or the bar feeder is not perfectly aligned, the bar feeder may not be able to keep up with the spindle. With your largest bar, for example, you may not be able to run the spindle faster than about 2,000 rpm without serious vibration. (Note that many bar feeder manufacturers are overcoming this problem by providing bar feeders that hold and automatically load several shorter bars to achieve maximum spindle speed and still provide long periods of unattended operation. The only limitations with this style of bar feeder are that you must cut the bars prior to using them, and you'll have more wasted stock in remnants.)
All turning center control manufacturers provide a way to specify a maximum spindle speed in the program. Most use a G50 or G92 word to specify maximum speed. For example, the command N005 G50 S 1500 tells the control not to allow the spindle to run faster than 1,500 rpm, even if using constant surface speed and the current diameter requires a faster speed.
Unfortunately, determining the maximum speed for a given workpiece can be almost impossible to predict. In almost all cases, a test must be made during setup to determine this speed. And since this specification has so much to do with safety, you must err on the side of caution.
In the case of the large, unbalanced casting, for example, setup people must carefully test for maximum speed in rpm. After loading a casting, they'll start the spindle at a very slow speed in rpm. Slowly, and by small increments, they'll increase the speed until the machine begins to vibrate. Note that at this point they've already exceeded the maximum speed, so the actual maximum speed must be something slower. We recommend 20 percent slower. Since castings vary, we also recommend performing this test on a few workpieces to find the worst case scenario.
What some programmers forget, however, is that once the raw casting has been rough machined, it's likely that it will be running much more true than it does in its raw state. After roughing the first workpiece, the setup person can perform the maximum speed test again. While spindle speed will likely still have to be limited, the new maximum speed will probably be much faster than that for the raw casting. And since cycle time gets shorter as spindle speed is increased, incorporating a second spindle limiting command in the program improves cycle time. The same is true in the bar feeding application—as the bar gets shorter, the maximum spindle speed can be increased.