Today's CNC machining centers and turning centers boast extremely fast rapid rates. It is not unusual to see current model smaller machines (especially those with linear guide ways) having rapid rates in excess of 1500 inches per minute. While rapid traverse rate is an extremely visible specification, and one that all CNC users want to know when purchasing machine tools, there is another important motion rate specification that is not nearly as publicized.
It's important to recognize that most machines do not allow you to actually machine at feeds as fast as the rapid rate. Though machine parameters control the machine's fastest feed rate (and these parameters can be modified to change a given machine's maximum feed rate), most machine tool builders initially set the maximum feed rate to about half the machine's rapid rate. A machine that can rapid at 800 ipm, for example, would have a maximum feed rate of up to about 400 ipm. Note, though, that this is just a rule of thumb, and machine tool builders vary with regard to how fast they allow their machines to feed.
Though the machine's maximum feed rate is usually more than adequate for the bulk of machining applications, there are a few times when it can be a limiting factor. When threading on a turning center, for example, it is possible to unwittingly exceed maximum feed rate. Say a turning center has a maximum feed rate of 300 ipm (600 ipm rapid rate). If machining fine threads in large diameters, it is likely that you'll never exceed the maximum feed rate. A 3.0-16 thread machined at 300 sfm, for example, would require a feed rate of 23.88 (3.82 times 300 sfm divided by 3.0 diameter times 0.0625 pitch). This relatively slow feed rate is allowable on all current model CNC turning centers.
On the other hand, you must be concerned with maximum feed rate when machining coarse threads at high speeds. And multiple-start threads present the most problems, since the lead of the thread (not the crest-to-crest pitch) determines the feed rate. Say, you must machine a four-start 1.25-4 thread having a lead of 1 inch (0.25 inch crest to crest) at 600 sfm. Now, the required feed rate becomes 1,833.61 ipm (3.82 times 600 sfm divided by 1.25 diameter times 1.0 lead). This feed rate exceeds the capabilities of even the fastest CNC turning centers.
Note that many turning center controls will not even warn you when you exceed the machine's maximum feed rate. They will simply let machining occur at the machine's fastest possible rate. When threading, of course, this will result in scrapped workpieces, since the setup person will not be able to catch the problem until after a workpiece is machined. Even then, the cause of the problem may not be obvious.
Another time when a machine's maximum feed rate may be the limiting factor is when doing 3D work. It is not unusual when machining electrodes made of graphite or molds made of aluminum to desire feed rates well in excess of 200 ipm. If performing these operations on older machines, it is likely that the machine's maximum feed rate is being exceeded.
For these reasons, all CNC users should know the maximum feed rate for each machine they own. Check in the machine tool builder's programming or operation manuals to find this important specification. If this information is not published (not all machine tool builders publish this value), you will have to contact the machine tool builder.