Fixturing for High-Speed Machining
Let's face it: no matter what kind of bearings are used in the spindle and no matter whether it is a linear motor or a highly capable ballscrew system, how parts are fixtured for high-speed production machining makes all the difference in the world when it comes to productivity.
High-speed machining for production operations is generally figured to be that metal removal which is performed by a machine equipped with a spindle that's capable of spinning a tool at 25,000 rpm or more. Clearly, a driver for using equipment that can machine at this swift rate is cycle time reduction. More chips in less time.
But what about the elements of that high-speed machining system? Work is being done in a different way compared to the norm, otherwise we wouldn't bother with the compound adjective. What are the effects on subsystems of the machine? Specifically, what about the fixturing?
People have been building fixtures for conventional production machines for years. But within the past couple of years, high-speed machines have begun to make inroads in powertrain machining operations. Which means that things are changing and are likely to continue to do so. To find out what some of the fixturing issues are, we talked with people at a few of the growing multitude of companies that are offering auto makers and suppliers the wherewithal to go incredibly fast in their machining operations...
"One of the myths that surround the high-speed machining of parts," said Jeff Porter, general manager of the High Velocity Machines Div. of Ingersoll Milling Machine Co. (Rockford, IL), "is that the cutting forces are a lot weaker than in conventional machining and therefore the requirement for rigid fixtures is a lot less." That is, it could be thought that the cutting tool is whizzing along on the workpiece surface, rapidly trimming away material with a light touch. But Porter pointed out that the real issue is chip loads, not the surface speed. Assuming that the goal of high-speed machining is to get higher productivity, then backing off on the feedrate, which will reduce the chip load, is not the way that higher productivity can be achieved, Porter explained.
"If higher productivity is what you are after," he amplified, "your chip loads won't go down all that much.
"If your chip loads don't go down, then you are going to be faced with the same requirements in fixturing, such as minimizing deflections."
So if you think that high-speed machining will mean less-robust fixtures, you might want to think again.
But what about machining aluminum? Certainly that must mean that clamping requirements are reduced compared with cast iron, right? Well, maybe. Porter admitted that although the cutting forces may be lower with aluminum, the Ingersoll fixture designers keep in mind the same issues they deal with when producing a fixture for a cast iron part. Porter noted that some aluminum parts—such as a transmission case—are comparatively light and flimsy, so it is necessary to provide plenty of support—such as around the bell housing of the transmission case. Which means that the fixture must provide the required beefiness.
"A lot of fixtures we've built for projects weigh 2,000 to 3,000 pounds," Porter said. Now, not all of this mass is related to providing a rugged, durable fixture. A good percentage of it is related to another desired characteristic of high-speed machining, which is high spindle utilization. If you have a spindle that can remove metal in a veritable blur, then you want to keep that spindle in the cut as much as you can.
This means minimization of part-changing time. Which means—whenever possible—multiple part-handling fixtures (i.e., Porter noted that this is not a likely alternative for engine block machining). So it might be a case where there are four cylinder heads per fixture or 16 transmission pumps or whatever. In order to accommodate the number of parts, the fixture tends to be bigger, which contributes to the mass.
There is a related consideration. The parts must be fixtured dead-on. Otherwise, you'll be producing high-speed scrap. So, Porter said, dowel crowders and air-checking devices can be necessary in order to assure that the parts are seated right and located properly. Which can add some complexity and mass.
And although building fixtures for handling single parts is easier in some respects than building a fixture that can handle four parts, Porter pointed out that there is something that has to be kept in mind with regard to the single-part fixtures: Consistency from fixture to fixture. That is, if you are machining cylinder heads one to a fixture rather than four, then each of the four fixtures had better be like the other three. Variations can introduce all manner of part-tracking concerns. Yes, this is no different than is the case when machining at conventional rates, but remember: High-speed machining should mean more parts per hour.
With regard to fixturing, Porter commented: "Some people think life gets easier. That's not been our experience."
The message from Lamb Technicon (Warren, Michigan), delivered by Steve Arksey, director of Engineering, and Mark Tomlinson, director of Market Planning, is to do as much as possible to a part per fixturing in high-speed machining operations. "The most critical issue," Arksey said, "is how much time is required for tool changes versus the amount of time in the cut." The objective is to keep removing metal for as much of the cycle time as possible.
On the one hand, high-speed machining with a programmable-spindle machine means that there are fewer fixtures required than are necessary for the comparable amount of work done with a fixed-spindle transfer line. So whereas for a transfer line five or six spindles may need to be designed and built, for a high-speed machining module it may take only one or two to complete the same amount of operations. However, given the amount of work performed on the part in a given setup, the given fixture design may be more demanding, as it is configured to minimize interference for the multiple cutting operations.
While high-speed machines from many machine tool builders tend to be single-spindle units, Lamb is really concentrating on high spindle utilization for its Mach 1 machine, as it features two independent 28-kW, 24,000-rpm spindles. In some instances, one spindle can be machining while the other is undergoing a tool change. In other instances, it could be that two workpieces are being machined independently but simultaneously. The Mach 1 is equipped with an X-axis worktable so that it can be used as a stand-alone machine or as a part of a machining system.
The XHC 240 high-speed machining center from Ex-Cell-O North American Operations (Sterling Heights, MI), explained Tom Tourangeau, vice president of Engineering, has a self-contained hydraulic fixturing unit so that there are no external devices holding the part in place on the tombstone fixture once the system is energized. Which is different than is the case on some other machine tools.
"Overall," he noted, however, "a fixture has to hold the part so that the part doesn't move during machining. So I really don't see a great difference between fixturing for high speeds compared with conventional machining."
In the case of employing the aluminum-cutting XHC 240 the approach is to take lighter cuts. Tourangeau talked about a couple of tenths of material per pass. This, he said, means that the forces and loads are kept down. Which means that the clamping forces are actually smaller than are necessary for cutting at conventional rates.
Which brings us to the conclusion that when you are talking to an equipment vendor about high-speed machining as it relates to the fixturing requirements, be sure that you don't assume that they are greater or lesser, simpler or more complex. All high-speed machining may be fast. But that's about the only generalization that you can make.
The front bumper for the 1997 Dodge Ram trucks are being aligned with a pneumatically operated fixture designed and built by Peak Industries (Dearborn, MI). This modular fixture is actually moved into place via an overhead-rail mounted lifting and handling arm. The truck arrives in the station with the bumper hanging in place; the fixture is swung into position, and once location is secured, the bumper is secured to the chassis by operators wielding nutrunners.
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Sometimes you want mechanical clamping. Other times, a vacuum system is desirable. Big Kaiser Precision Tooling (Elk Grove Village, IL) is offering the KPT/Haberle modular workholding system that features a base plate that facilitates either mechanical or vacuum clamping.
Fixture Design, Too
As some machine tool vendors are looking for ways to (a) add value and (b) provide a point of differentiation, they are going beyond simply determining that a given machine has the size and capacity to handle the given part(s). They are actually providing full processing information.
For example, at Mazak Corp.'s National Technology Center (Florence, KY), engineers are using the SDRC I-DEAS Master Series 3 CAD/CAM system to provide part process validation, including fixture design recommendations.
According to Chuck Birkle, general manager, Marketing, Performance Div., Mazak Corp., the implementation of solids modeling and finite element analysis is permitting the design of fixtures that can be comparatively lighter in weight than might otherwise be engineered, and which provide better machining access (e.g., it may be possible to punch a window through a portion of the fixture, which not only results in access, but also lightens the fixture).GSV
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