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It's been an ongoing battle for years: cutting tool manufacturers introduce a new technology that speeds up milling and drilling operations and point to the machine tool builders to make machines that can keep up with the new cutting tool capabilities. So the machine tool guys do so and throw the gauntlet back at the cutting tool guys. And so on, and so on, and so on.
Right now, the upper hand seems to be with the machine tool guys with their high-speed spindles and high-feed rate capabilities. But Kennametal, Inc.(Latrobe, PA) has been looking into evening the playing field by focusing on—among other things—the link between the cutting tool and the spindle—the toolholder.
Balancing tools that will be rotating at high speeds is very, very important, and most experienced folk don't underestimate it. Out-of-balance tools cause chatter, gouging, and loss of part accuracy, not to mention uneven and premature wear of the cutting tool.
Anytime a new tooling assembly goes onto a machine; some sort of balancing operation needs to be done. What is sometimes overlooked is the fact that any change to the assembly, no matter how small it seems, requires a rebalance.
This includes anytime a cutting tool is tweaked or changed, or a part of the toolholder is adjusted or changed. These changes do throw the assembly off-balance, even if it is only an Nth of a micron off. And that Nth of a micron out of balance causes an Nth of a degree of oscillation, and the cutting tools will wear unevenly and more quickly than if the balancing operation had been done.
Another thing to keep in mind here is that with so many different toolholders and toolholder designs on the market, users have the added responsibility of making sure the toolholder is "balanceable." What does that mean? According to a paper written by David L. Lewis, a Kennametal engineer, it's one that is designed with features that enable "quick and accurate adjustment to a specified balance tolerance."
As an example, Lewis cites a toolholder with adjustable balancing rings. The matched pair of rings are in a symmetrical state of unbalance (they're both unbalanced to the same degree), letting users adjust the pair to counter any unbalance in the cutting tool and toolholder and lock them into place. Such a balancing application could be done with any number of commercially available balancing machines.
The state of being in balance is not merely subject to the whim of the machine tool operator here. "Balance" is given specific value in various quality standards. There's the ISO 1940/1 and the ANSI S2.19 standards, which are basically exact reflections of each other. These values, called "G" numbers, provide balance quality grades for different types of rigid rotors, as well as maximum unbalance values for the same rotors.
Forming an Attachment
Once balanced, tooling assemblies need to be adequately attached to the machine spindle. According to Lewis, this attachment is "truly the foundation that supports the cutting edge of any rotating tool machining system." As improvements are made in this spindle-to-holder connection, successfully machining at high speeds becomes easier.
Key to successful connection is taper rate accuracy. The more accurate the taper rate, the more accurately the toolholder shank will mate with the spindle socket. Following this logic through, the more completely the two elements are mated, the stronger the hold. And the stronger the hold, the more stable the tooling assembly. And the more stable the tooling assembly, the more accurately the part is machined.
This being said, it seems strange that such an essential piece of the toolholding pie would fall under the fire of debate (conflict, dispute), but it has. In his paper, Lewis points out the fact that ANSI (American National Standards Institute) and ASME (American Society of Mechanical Engineers) standards regarding taper fit (ANSI B5.10) is so outdated it can be considered obsolete. The standard sets forth a constant rate of 0.001 in. per foot as an acceptable rate of taper no matter what the cone length is, while the rest of the world conforms to the selectable specification method outlined by the ISO-1947 standard. What's the difference?
Well, first of all, the ANSI standard has not changed in a significant amount of years, even though tolerance capabilities and requirements have. Secondly, the 0.001 in. per foot rate is independent of the cone length, which means that as the shank gets longer or shorter, its fit into the socket changes because the rate of taper hasn't adjusted for length. The poorer fit means manufacturers have a harder time keeping parts within tolerance. Yet the toolholder still technically meets the standard.
On the other hand, the ISO 1947 standard uses 12 grades of possible accuracies to accommodate the manufacture of parts as variant as an ultra-precise gage to an ultra-imprecise sand casting. Further, with this standard, the taper rate changes with the cone length, so longer lengths have tighter tolerances. This sliding scale has resulted in the standardization of 10 cone lengths, giving machine users the opportunity to match toolholders more precisely to a machine's spindle without losing accuracy.
What this controversy has to do with the cost of tea in China is this: U.S. manufacturers are trying to make parts to designated tolerances using toolholders whose shank is not mating as accurately with the spindle socket as it could. So they are unable to machine at the higher spindle speeds without sacrificing accuracy.
In a back-door sort of way, though, this situation is working itself out. First of all, as U.S. manufacturers work to achieve ISO certification, they are switching from the ANSI standard to the ISO 1947 standard because they have to. Also, many U.S. toolholder manufacturers are making and selling lines of toolholders to accommodate this shift, including Kennametal, whose HSK and KM lines of shanks are designed to the ISO standard. To make it official on the home front, a new ASME technical committee has been formed to rewrite the taper gage standard, and it is expected that the new standard will closely resemble—if not mirror—the ISO 1947 standard.
While correctly using the proper toolholder improves efficiency and gets users closer to machining at the highest speeds possible, all cutting tool makers acknowledge that new innovations in terms of geometries and materials need to be introduced to give users the best possible tool. And to this end, there's certainly a lot of work going on. No one was willing to discuss anything specific, but spokespeople from Kennametal, WidiaValenite (Madison Heights, MI), Iscar (Irving, TX), Sandvik (Fair Lawn, NJ), and elsewhere all indicate that their research and development centers are all busting tail to find new alternatives for the industry. We'll all just have to stay tuned.
What Do You Mean By "High Speed"?
The first thing that Andy Pitsker, product manager, Tooling Systems, Sandvik Coromant Co. (Fair Lawn, NJ), points out with regard to the subject of "high speed machining" is that there doesn't seem to be a whole lot of agreement as to what those three words within the quotation marks signify. He says that he's come across at least five different types of "high speed machining":
High cutting speed machining
High rpm machining
High feed machining
High speed and feed machining
High productivity machining.
All of which is to say that there are different understandings as to what constitutes going very fast. And even within some generally accepted notion of what high speed machining is (e.g., using a spindle running at a rate of approximately 25,000 rpm), Pitsker points out that there are still problems because there is the issue of the tool diameter to take into account, to say nothing of the material being cut. He points out, for example, that cutting Inconel at 400 m/min would probably be considered to be high speed machining—but what happens when it is revealed that the spindle rpm for that task is 12,000 rpm?
Blasting through aluminum is one thing. Going through even cast irons can be problematic. Consider, for example, any inclusions, such as sand. Pitsker says that there is an increase in the use of ceramic tools for cast iron machining, but there is a concern: the material uniformity is such that there isn't sufficient predictability. A grain of sand could lead to the failure—to the expensive failure—of a ceramic-tooled face mill in the event that said tool is rotating at a high rate.
But before going any further, some of the advantages of high speed machining should be noted. Among them are:
1. Better surface quality: Generally, there are shallower depths of cut and the cuts have high widths. There is less heat input into the surface, which minimizes distortion. And there is less tool force on the workpiece.
2. High chip removal rates: Sometimes chips are being generated at such a rate that there is a tendency for people to use coolant to wash away the chips and to keep the dust down. But Pitsker says that when the coolant hits the carbide it can lead to thermal shock...which causes tool failure. So while there are plenty of chips produced, dealing with them effectively is important.
3. Shorter machining times: This is especially the case when the tool is in the cut for a longer amount of time than, say, when there are setups that have long tool or workpiece changes.
4. Ability to make thin walls: Because there are comparatively low pressures and quick passes involved ("reduced time of engagement"), it is possible to machine walls that otherwise would be distorted by the cutting process.
5. Reduced burrs: Burrs tend to be sheared off, not merely pushed over.
With all that said, there are several considerations that must be taken into account for successful high speed machining. For example:
1. What about the bearings? Pitsker provides an analogy. Consider the life of wheel bearings on a conventional car driven at conventional speeds. Next, consider the life of wheel bearings in a race car that goes 200 miles per hour. Which will last longer?
2. What about maintenance? There is more wear experienced by the equipment because of the high spindle speeds and axis feeds, so are the resources in place to deal with it?
3. What about runout? When the spindle is rotating the tool at a high speed, runout and unbalance conditions can lead to excessive wear on one side of the tool, which can result in a variety of problems. Tool balancing is a must.
4. What about the toolholder? There must be a solid connection between the toolholder and the spindle—as well as between the toolholder and the insert(s). Otherwise, there could be a dangerous projectile flying through the plant.
"High speed machining is not very forgiving," Pitsker says.
His recommendation is that companies that are thinking about going the high speed machining route do a whole lot of careful planning. Small variations tend to be greatly amplified when the speeds and feeds are turned up. So careful consideration of all aspects is essential.
Which leads us to a fundamental question of whether the capabilities of machine tools have outstripped the performance capabilities of the cutting tools. Perhaps not surprisingly, Pitsker doesn't think that's the case. He argues that given the right planning, it is a draw between what the machines can do and how the tools can perform.-GSV