CNC Tip:

Q & A

Disclaimer: CNC Concepts, Inc. accepts no responsibility for the use or misuse of techniques shown in this web page. We simply publish information we feel will be of interest to CNC users. In all cases, the reader is totally responsible for considering the implications, good and bad, of implementing one or more of the techniques we show.

We are a small machine shop located in Ohio. I had read an article sometime back in your newsletter on this subject but I cannot remember which issue it was in. We do a lot of turning of parts that have changes in diameter. I have been trying to get the guys on the floor to use G96 for these parts but they have always programmed with G97. I have tried to explain the advantages as far as the increase of tool life as long as G96 is used properly but they continue to reject this mode. I was wondering if you could send me a list of the benefits between using G96 instead of G97. Also if you could include how this affects the feedrate on the part. I appreciate anything you might be able to send me on this topic. Thank you. Mike Stephens

Response:

Mike,

For Fanuc controlled turning centers, G96, of course, specifies constant surface speed mode, while G97 specifies rpm mode. In constant surface speed mode, the spindle speed in rpm is automatically determined by the CNC control based upon the diameter a tool is currently cutting and the speed specified in surface feet per minute (or meters per minute in metric mode). This mode is only used for single point turning tools (boring bars, turning tools, grooving tools, etc.), when diameters to be machined change substantially throughout the workpiece. There are (at least) four benefits to using constant surface speed mode for appropriate applications.

1) Easier programming

The correct rpm for a cutting tool is based upon this formula: RPM=3.82*SFM/DIAMETER. If an accurate speed in rpm is to be used, the person calculating the rpm must first determine the tool's appropriate speed in surface feet per minute (note that the formula is different when working in the metric mode). Before a person can even begin to make the calculation, they must know the desired speed in sfm. This speed is published by cutting tool manufacturers and is based upon the material being machined and the material of the cutting tool's cutting edge. When using constant surface speed mode, the programmer simply enters the speed in the program directly in sfm (no need for any calculations). The machine will constantly and automatically perform this calculation, updating the spindle speed in rpm.

One common reason why setup people and operators on the shop floor continue to use rpm mode -even though it is easier to use sfm mode - is that they have a background in manual machining. With manual lathes, there is no such thing as constant surface speed. Everything must be done in rpm. When a manual machinist is exposed to constant surface speed on a CNC turning center for the first time, the spindle will probably appear to be running much too fast (most manual machinists cannot run at optimum spindle speeds for obvious safety reasons). Frankly speaking, many manual machinists determine spindle speed in rpm by the seat of their pants, causing the spindle to run at a speed that looks good. Indeed, I've talked to many that never use the rpm formula given earlier - and some that have never heard of it. Many continue using the seat-of-the-pants approach even on CNC turning centers when it is safe to run optimum spindle speeds.

2) Consistent workpiece finish

As long as you work inches- (or millimeters-) per revolution feedrate mode (G99 on a Fanuc control), witness marks left on the workpiece by the cutting tool will remain consistent throughout the cutting tool's machining of the workpiece if you use constant surface speed. Since the feedrate is tied to spindle speed, as the spindle speeds up and slows down in rpm, so will the feedrate in inches per minute. Again, this causes the tool to create a consistent finish throughout the workpiece.

3) Optimum tool life

Since the spindle is constantly running at the appropriate speed in sfm, tool life will be at it's maximum. When compromising speed as must be done when working in rpm mode, the spindle seldom runs at the appropriate rpm, and tool life may suffer.

4) Optimum cycle time

Again, feedrate is directly tied to spindle rpm if you're working in per revolution feedrate mode with constant surface speed. The faster the spindle runs, the faster the tool will machine. So when you're working in constant surface speed mode, cycle time is inversely proportional to spindle speed in rpm. The faster the spindle runs, the shorter the cycle time. Since most machinists working exclusively in rpm mode tend to error on the side of caution (commonly choosing an rpm that is much slower than it must be), cycle time will be longer than it should be.

A few cautions

Out of round workpieces - If you machine a lot of castings or forgings (or any other kind of workpiece that doesn't run true in the spindle), you must be very careful with the constant surface speed mode. One of the reasons why manual machinists have trouble getting used to constant surface speed mode is that they cannot predict what rpm the spindle will be running. And if, for instance, you program the cutting tool to go to the spindle center in X, as is done when facing a workpiece to center, the spindle will run up to its maximum speed in the current spindle range. If working in the high spindle range, the spindle will run up to its maximum. For round, true running workpieces, this is acceptable. But if workpieces are at all out of round, the vibration set up by running too fast can be very dangerous. It can cause the workpiece to be thrown from the workholding device. Note that there is a special function that lets you set a temporary maximum rpm (G50 with Fanuc). But you must remember to use it (and know the maximum rpm a workpiece can be rotated without vibration).

Poor workholding setups - Since speed in rpm is difficult to determine when working in constant surface speed mode, it should only be used when making adequate setups. Another reason why manual machinists tend to prefer working in rpm mode (and running at slower than optimum speeds) is that the workholding devices they've used on manual lathes (commonly manual three-jaw chucks) could not apply the gripping force necessary to run at optimum spindle speed. Most CNC turning centers use hydraulic three-jaw chucks that can supply the gripping force needed to run at maximum speeds. But if you're workholding device cannot grip with adequate force, of course, you cannot run at optimum speeds.

Cycle time waster - While one of the benefits of constant surface speed is improved efficiency, it must be programmed wisely. If you work exclusively in the constant surface speed mode, the spindle will always be rotating at the appropriate speed - matching the diameter the cutting tool would be cutting. While this is great during each tool's cutting operation, consider what happens during tool changing. The cutting tool will rapid to a tool change position (usually a large diameter in X). During this motion, the spindle will slow down. Depending upon your spindle drive system's response time, it will likely take longer for the spindle to slow down than for the rapid motion to occur. If it does, cycle time will be wasted. The same is true during each tool's approach. As the tool rapids to a smaller diameter, the spindle speed will increase in rpm. This constant increase and decrease in rpm not only wastes cycle time, it wastes electricity and causes undue wear-and-tear on the spindle drive system. To program around this problem, most programmers will temporarily switch to the rpm mode just before the rapid motion to the tool change position, using a speed in rpm that is appropriate for the next tool's first position. In this way, the wasted change in spindle speed will not occur, and when the next tool is in position, the spindle will be running at its correct rpm. At this point the constant surface speed mode is re-selected with the proper speed in sfm.

Minimal diameter changes - If you are machining but one diameter on a workpiece, or if there is but a small difference from one diameter being machined to the next (say, under 1 inch of diameter change), constant surface speed will not help you much. In this case, many programmers will calculate the appropriate speed in rpm and run the machine in rpm mode. This also eliminates the cycle time wasting problems caused by constant surface speed (just mentioned).