used in conjunction with very small diameter tools in order to reach ridiculously fast speeds. It’s not unheard of to achieve upwards of 50,000 RPM in some applications. The only catch is, of course, you are using very small tools. Again, when it comes to tool holders, the max speed largely depends on balance. Therefore, a high-speed machinist will be well-acquainted with the fickle art of balancing tool holders and the standards that are used. As tooling technology and speed progress, so too does the need for a proper standard by which to measure unbalance. When good enough doesn’t cut it As cuts get faster, tools get smaller, and tolerances get tighter, there is no longer any more room for the good-enough attitude. Traditionally, tool holder balance has always been measured based on the archaic ISO 1940-1 balancing standard (yes, we’re talking about the year 1940). ISO 1940-1 is all-encompassing and applies to pretty much any type of rotating machinery, large or small. There’s nothing wrong with this old standard. In fact, it’s done an outstanding job up until this point. But the modern problem is twofold. First, the standard is so generalized that it doesn’t account for many important variables unique to spindle/tool holder systems such as ATC repeatability, dynamic cutting forces and modular components. Second, holders have gotten so small and are rotated so fast that, in some cases, achieving a balance benchmark like G2.5 is nearly impossible from a practical
Exploring a new frontier Once you start exploring this new frontier of speed, complications besides balance come into play. When someone uses a dial indicator to gage runout on a test bar, they are measuring what’s known as static runout. This gives you an idea of the level of dimensional precision involved in the manufacturing of the tool, holder and spindle assembly. Let’s say you measure 5 tenths of runout with an indicator on a high-performance drill, which is within your tolerance range. If you run it at 3,000 RPM or so, you wouldn’t really notice anything unusual in the generated hole that would dispute the earlier reading. However, you feel like you can run it faster. At first you may feel satisfied with the shorter cycle times, but at a certain point after ramping up the spindle speed, you will gage the holes and realize that they’re suddenly oversized. Naturally, you go to measure the runout again and are dumbfounded when it still reads 5 tenths. You’ve just become the latest victim of dynamic tool runout. This type of runout is mainly caused by centrifugal forces associated
with faster spindle speeds. These inertial forces will make the tool want to bend away from the centerline, so it will appear to fan outward at the tip when rotating. It’s hard to detect and even harder to accurately measure – it typically requires some sort of noncontact laser/light sensor device. You wouldn’t encounter it under most standard application conditions because the forces require exceptionally fast speeds, but it can also be an issue with small diameter micro-machining tools that are easier to bend. Ideally, you want to keep the mass of the tool holder as close to the centerline of rotation as possible. You also want to keep your tool as short as possible. This is the reason why there is general trend of higher max speed the smaller your tool holder is. You simply don’t have that much mass to throw around, so a higher speed can be attained safely. HSK-E style tool holders take this principle to the extreme. In addition to having an HSK taper that’s already well-suited to higher speeds, this form of the standard aims to be as close to perfectly symmetrical as possible by the removal of all notches and drive key slots. These are commonly
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