The Pros And Cons Of Constant Surface Speed

Constant surface speed (specified by G96 on most machines) is a turning center feature that automatically determines the appropriate spindle speed in revolutions per minute (rpm). It does so based on a speed specified in surface feet per minute (sfm) in inch mode or meters per minute (mpm) in metric mode as well as the cutting tool’s current diameter position. It automatically applies the rpm calculating formula (rpm = sfm × 3.82 ÷ cutting diameter) and updates the spindle speed in rpm accordingly.

Constant surface speed is a great feature. It provides at least four advantages:

1. It simplifies programming. Speed can be directly specified in sfm or mpm—units used by most cutting tool manufacturers for recommending speed. This eliminates the need to calculate rpm.

2. It provides consistent workpiece finish. When used in conjunction with feed per revolution feed-rate mode (G99 on most turning centers), finish will almost always be consistent, regardless of diameters machined.

3. It optimizes tool life. Tools will always machine at the appropriate speed.

4. It optimizes machining time. Cutting conditions will always be properly set, which translates to minimal machining time.

Though constant surface speed is a good feature, there are times when it cannot be used:

1. During center cutting operations—Constant surface speed cannot be used when the tool is machining along the Z axis while at the center of the workpiece/spindle. This is the case during drilling, tapping, reaming and other such operations. At spindle center, a speed of even 1 sfm will cause the spindle to run at its maximum speed.

2. During threading—Threading requires perfect synchronization between spindle revolution and Z-axis motion to allow precise chasing threads. For this reason, almost all machine tool builders require that the rpm mode be used for threading.

3. During live tooling operations—Constant surface speed applies only when the main spindle is used in the normal turning mode, not in the live tooling mode.

Constant surface speed is most appropriately used when the machining operation requires a change in the machined diameters. Rough and finish facing; rough and finish turning; rough and finish boring; and grooving are among the most popular applications for constant surface speed.

There are also some problems that constant surface speed can cause. All CNC programmers should be aware of them:

1. When programming a given machining operation, it is not obvious what rpm should be used. Mostly, this tends to trouble people with manual (engine) lathe experience. An engine lathe operator is comfortable working with spindle speed in rpm, but on CNC turning centers rpm is not as predictable when constant surface speed is used.

2. Speed can increase rapidly. If a workpiece is faced to center in the constant surface speed mode, for example, the spindle will turn at its maximum speed in the current spindle range. If the workpiece is perfectly round and balanced, this won’t present any problems (other than being a bit scary). But if the workpiece is unbalanced (as is the case with many castings and forgings), this can present a dangerous situation. For this reason, there is a feature that allows programmers to specify the maximum speed at which the spindle will run (G50 for most turning centers).

3. If there is a minimal change in diameter during the machining operation, even for the kinds of operations previously mentioned, constant surface speed is not very helpful. Many programmers elect not to use it for these applications.

4. Without special program structure considerations, constant surface speed can be a cycle time waster. With constant surface speed, the spindle will respond to all diameter changes, including those that retract the cutting tool to a safe tool-change position—and those that cause a tool to approach the workpiece. To recognize this problem, listen to the machine during tool changes. The spindle will slow during retract only to speed up again after the tool change and as the next tool approaches. This condition also wastes electricity and causes more wear and tear on the machine.

Again, there is a way to eliminate this problem through proper program structure. At the end of each tool’s operation, the rpm mode can be temporarily selected (G97 for most machines) with the speed required for the next tool’s approach position. After the tool change and approach, the spindle will be ready for the next tool. At this point, the constant surface speed can be reselected.

I’m not trying to discourage the use of constant surface speed. Again, it is a great feature. Just remember its limitations—and be ready to deal with them.