For example, what about combining milling and grinding?
Grinding, of course, is typically thought of as a finishing operation.
John Richard, senior sales manager for Studer, explains why the two processes are complementary. Combining the two operations would not seem to be an obvious choice. This is exactly the solution that GE Gas Turbines (Greenville, South Carolina) adopted for machining large turbine wheels. If the argument for multitasking has to do with reducing both the delay and the potential for error that come from transporting parts between different machines, then that argument gains strength if the multitasking machine can combine operations that are even further removed from one another in the production process. The bigger the part, the better it is to Socks Machine Manufacturers
set it up in just one place. Specifically, it relies on a toolchange arm like that of a machining center, along with toolholders (or wheelholders in this case) that use the HSK interface of a machining center's toolholders. Large parts magnify the savings in floor space that might result from combining grinding and machining center operations into one machine. Accepting this seeming inefficiency within the machining cycle, for the sake of a more efficient process overall, is part of the recipe for implementing these machines effectively. The MFP-TC grinding machine pictured, from Magerle (represented in the United States by United Grinding Technologies, Miamisburg, Ohio), provides an example.
A traditional manufacturing mindset looks at a part's features and tolerances to see the particular machine tools best suited to produce all of that geometry. To support an increasing role for creep-feed grinding (a higher-metal-removal rate process than grinding just for finish), this machine features a powerful spindle and stiff hydrostatic ways. In an even larger number of plants, they are performed by different employees who have different skills. Even on a grinding machine equipped with multiple wheels and capable of high metal removal rates, there are plenty of features that simply cannot be ground. In many plants, these two operations are performed in different locations.
For a part to benefit from milling or drilling on a grinder, the cycle should consist of mostly grinding.
On the other hand, the turning insert can deliver a performance advantage that a grinding wheel can't. It typically involves a rigid lathe and a hard cutting tool material such as PCBN. Labor savings might also be more significant. Further, because wheel types such as conventional abrasive, plated CBN and vitrified CBN all excel in different applications, the machine has been equipped with the capability to store different wheels and automatically switch between them. By contrast, a turning insert can withstand a slightly varying depth of cut for the sake of beginning the machining at the same diameter every time, to make the machining time consistent from piece to piece. Taken together, this machine's tool changing, spindle power and rigidity produce the equivalent of a highly capable machining center. Milling is more about removing a stock envelope.When the word "multitasking" is applied to machine tools, the term generally refers to a machine that is capable of both turning and milling. It also requires a particular mindset about production. As a result, the machine can't compete economically against a machining center if traditional metalcutting makes up most of the work. Holes, pockets and slots that don't run completely through the workpiece are examples. The S242 machine from Studer (represented by United Grinding Technologies) is the latest machine from this company able to perform OD grinding and hard turning within a single cycle. The part has to be produced to meet a certain price, after all of the costs both on and off the machine tool have been factored in. That definition is fine as far as it goes. Overall, there has been a 30 percent cycle time reduction. In this view, the efficiency with which each feature is machined is less significant as long as the overall process is as efficient as it can be. Stine says that the range of potential operations expands to include turning.
Turning On A Grinding Machine
"Hard turning" usually refers to the turning of workpieces that have Rockwell C hardness values in the range of 50 or 60.
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However, certain profile grinding machines have developed in the direction of this type of multitasking, even when combining operations was not necessarily the goal.
This latter feature relies on proven technology. When the part has one or more features such as these, performing these cuts on the grinder can save considerable cost by eliminating the need to set up on multiple machines.
Chris Stine is a vice president of United Grinding Technologies. However, turning and milling are two fairly similar operations. If heat treating introduces variations in the geometry of a rotating part, then a grinding wheel is going to begin by finding the high spot of the workpiece every time. The MFP-TC machine cited above remains a grinder first and foremost, offering precision beyond what might be associated with even a higher-end machining center, he says. If the tolerance is tight enough, he says, then turning will not do.
However, many ground parts do feature a small but critical amount of this metalcutting. This part-centered perspective may optimize machining time and machining cost, but it may do so at the expense of less easily quantifiable costs related to handling the part. The change saves about 6 hours of setup time and 2 days of queue time, according to engineers at the plant.
Different manufacturing organizations view their costs differently, and the need for this process-wide outlook is part of why identifying the right applications for these machines is not necessarily cut and dried.
Another argument for consolidating operations relates to the part's size. A grinding wheel is continually reconditioned while it machines, but this is not the case with a turning insert. They both use a cutting edge to make a chip, and they both occur at roughly the same stage in a production process. This is particularly true for parts such as airfoils, in which the location of machined features may be defined with respect to ground surfaces that have complex geometries. The grinding cycle will vary. Stine says a different mindset focuses on a particular unit cost. A rotary table that can turn at 70 rpm can deliver 1,500 sfm of cutting speed to a turning tool that cuts at a diameter of 7 feet. Mr. Parts that used to be machined on a grinder, a vertical lathe and a boring mill now receive all of this machining on one Magerle machine tool.
These savings overcome the fact that the multitasking grinder is unlikely to be cost-efficient when it comes to milling or turning as isolated operations. This latter view, he says, is the one that is more likely to realize the value of consolidating the work of various diverse machine tools into one. Precise heavy milling can be performed on this machine, and milling and drilling tools can be stored in the tool magazine alongside the grinding wheels. A turning insert's dimensions slowly change while the insert cuts, affecting machining accuracy and finish. He says performing chip-making operations such as milling and drilling on a grinding machine is best suited to certain types of parts.