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There is an important reason why the term "high speed machining" cannot be defined in terms of any particular speed. To pick a number—whether rpm, sfpm or ipm—would be to imply that just reaching that number is sufficient to unlock high speed machining's benefits. That's not the case. The "speed" in high speed machining is the means and not the end. The speed isn't the finish line; speed is the starting point.
Modern Machine Shop has been covering the evolution and acceptance of high speed machining for several years now. This is our third special issue on the topic. Watching the many ways that many very different shops have taken advantage of various increases in available speed has shown us one recurring theme. What all of these applications have in common suggests the most useful definition we've found for the phenomenon we're talking about:
High speed machining (n.): machining at a spindle speed or feed rate high enough to allow a cumbersome and/or complex process to be replaced with a more flexible and efficient process in which the machining center plays a larger role.
For example, instead of using different machines and different stations for rough milling, finish milling, EDM and polishing, a mold maker might produce a core or cavity from start to completion using fast, light cutting on a single machining center.
Instead of using assembly to build a large aircraft component from smaller pieces, an aerospace manufacturer might mill the component in one piece out of a solid block of metal.
And instead of customizing a machining line around a particular part whose design may change, a volume production facility may use standard machining centers capable of high speed and feed rates to achieve productivity comparable to the custom line without losing the flexibility to change the process later.
These are among the classic high speed machining applications. In each of these examples, speed is the enabling factor that makes it practical to realize the more efficient process. The machining center takes on more work, and the critical speed is the threshold at which the machine can handle this extra work within a cycle time that remains cost effective.
But even when the speed is there, the process has to be developed that can put it to use. This is the challenge of high speed machining. Using the higher speed and feed rate effectively requires fundamental aspects of the process to change. Equipment may change, and many assumptions may change as well. Shops that have had the most success at high speed machining have been the ones that are most willing to begin again. Leaving the old process behind is both a benefit and a cost.
Fortunately, these successful shops have now shown the way. There has now been enough practical experience with machining at very high speeds and feed rates to say with confidence what the realities of faster machining must be. Not every shop will have a need to cut fast enough that all of the following considerations will apply. However, the shop that does want to apply ever-faster rpm and ipm toward continually more efficient machining will eventually face each item on this list. Consider these the facts of life on a fast machining center:
1. The process is at least as important as the machine tool.
A fast machining center won't be enough if the rest of the process is not also up to speed. At the very least, the machine will need tools capable of high speed cutting and toolholders that keep the load on those tools steady. Where milling of any complexity is involved, the machine will also need to be supported by CAM features and CNC functionality that allow accuracy and efficiency at high feed rates. (Our HSM Electronic Supplement has much more to say about all of these points.) Considerations such as these mean the initial move to high speed machining usually requires an investment over and above just the cost of the machine.
2. Handbooks have much less to say.
Shops are used to finding their machining parameters using general-purpose data tables found in handbooks or provided by the cutting tool manufacturer. But at the high rpm, high feed rates and low depths of cut characteristic of high speed machining, processes become more individualized and the usefulness of general-purpose data tables breaks down. Process factors such as toolholding, entry and exit strategy, smoothness of interpolation, or even the symmetry of the tool itself can all cause small fluctuations in the load on the tool, and high speed cutting conditions dramatically exaggerate the influence of these small fluctuations. By contrast, machining data tables are based on a more universal effect: the rate of steady, repeatable tool wear. To the extent that these data tables do address high speed machining, their recommendations tend to be conservative because the room for variation from process to process is so great.
3. Vibration may set the pace.
Vibration presents yet another reason why machining parameters will vary from process to process. Different processes vibrate at different frequencies. At low speeds, generally the only way to overcome vibration is to work against it, perhaps by making the process more rigid. But high speeds make it possible to work with the vibration for even more dramatic results. The higher end of the speed range may contain one or more stable "sweet spot" speed zones that permit considerably heavier cutting than either faster or slower speeds would allow. Sweet spots vary for different combinations of spindle and tooling, but they often occur in the range of 500 to 800 cycles per second. That means that if spindle speed is high enough that the rate of cutting edge impacts reaches well into this range, then it may be possible to realize more effective cutting by tuning the speed up or down in search of this inherently more stable value.
4. Experimentation is a necessary investment.
Finding the most productive parameters is just one reason why experimentation may be essential to using high speeds effectively. Trial and error may also be needed to find the best way to achieve the particular goal that brought you to high speed machining in the first place. If the goal of high speed machining is to create a smooth surface through milling instead of polishing, then one toolpath pattern may make it possible to realize the required finish where another pattern falls short. Or if the goal is to mill a delicate form out of solid metal, then one approach to removing the stock may be dramatically more successful than other approaches at leaving the delicate features undeflected and intact.
The effectiveness of these specific approaches can vary from process to process just as the optimum parameters can. And even where there is one clearly superior approach, high speed machining is new enough that the best approach for a particular application may not yet be widely known. As a result, shops committed to high speed machining generally must devote some machining time just to finding their own way.
The purpose of this special issue is not to advocate. High speed machining offers a better way to produce a great many parts, but in the end, it's a tool and only that—a tool that various shops will use and develop to varying degrees, and a tool for which your shop may or may not have a need. With the articles in this issue, and with the past articles on high speed machining we've archived on our Web site, our purpose is only to let you be informed.
The pages that follow offer success stories in which shops contrast high speed machining with the less efficient processes they used to use.
In addition, for shops already adept at high speed machining, some articles suggest the next step. These stories include:
- The Efficiency Test - How a mold maker uses the machines and practices of high speed electrode production for fast cutting directly in steel.
- Getting The Time Out Of Titanium - How Boeing is applying its knowledge of high speed machining of aluminum to the challenge of faster cutting in titanium.
- Standard Jobs Beyond Standard Speed - How a production job shop familiar with high speed machining at 15,000 rpm now identifies which jobs to run on its new machines capable of 50,000 rpm.