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Programming efficient piece part production: multi-task machining requires advanced CNC tools. (Software).

Today's manufacturers in the US are faced with numerous challenges from a global market place. Offshore suppliers have various advantages ranging from government subsidies to vast pools of lower cost labor. To many American OEMs, producing parts overseas is often done to just stay competitive as others source parts offshore.

Certainly when it comes to production variables, American manufacturers do not have the option of producing goods locally at the same labor rate of overseas facilities. But there is another option, which is coming into play, which minimizes the advantage that cheap offshore labor offers. American companies are adopting new technologies and leveraging them for a competitive advantage. Multi-tasking machine tools, a new breed of CNC machine tools being developed, minimize the amount of manual operator involvement to produce the part. As a result, multi-task machining is becoming one of the fastest growing segments of CNC machine tools.

The number of machine tools and the number of setups required to produce a part significantly increase the cost, as well as the potential for introducing errors, to produce it.

Multi-task machining CNC machine tools minimize the human factor in production by completely machining a part from start to finish. This is accomplished by an extremely versatile configuration consisting of multiple spindles and turrets or tool groups. The multi-task machining process transfers the stock from spindle to spindle allowing the machine to gain access to all areas of the part. Since the transfer between spindles is handled totally by the machine itself, operator intervention is unnecessary. Inaccuracies introduced through manual setups done by operators are eliminated. Overall part accuracy is maintained to the accuracy of the machine tool.

Multi-task machining gets it name because at any given time, multiple machining operations can be occurring in parallel. These can be either on a single spindle, or across multiple spindles. Some examples of multiple cutting operations occurring on a single spindle at the same time are pinch turning, when two turning tools are applied to opposite sides of the part, or OD/ID turning to minimize deflection and improve surface finish, lead/lag turning, when one turning tool follows after another to remove material quickly, or turning/drilling combinations, when an on-axis center drill can be applied while OD turning. Multiple machining operations across multiple spindles essentially treat each spindle! tool pairing as separate machine tools (though some special operations can be applied to the two spindles at the same time grouping them together).

Unconventional CAM

The process complexity of multi-task machining introduces an entirely new level of requirements for computer-aided manufacturing (CAM) systems. Conventional CAM systems have been developed around the concept of a single flow, or single cutting event, at a time. These cutting events are sequentially ordered to take the stock from its initial raw state through various stages during its production. Managing a single flow of sequential operations is relatively straightforward, since the element of time is really only a single, linear dimension - transitioning from one operation to the next. In a multitask machining process, though, multiple operation flows occur at the same time. Each operation flow will have its own sequence going from one operation to the next, but the flows themselves may require synchronization between them. This additional level of control transforms a multi-tasking process from a single dimension to an n-dimension problem, depending on the number of concurrent flows.

In order to support multi-task processes, CAM systems have had to significantly extend these process capabilities. CAM systems now need not only to address all the complexities involved in multiple flows, but also all the subtleties entailed in the machine tool architectures themselves, such as which moving tool is the dominant player when determining speed and feed rates. The time aspect of the machining process, which was important in a single flow system as it determines overall job duration, becomes extremely important in multi-flow processes. Process synchronization relies upon an accurate notion of time in order to efficiently coordinate the processes.

One way that CAM systems can support multiple flows is through some sort of synchronization manager interface. This interface displays the multiple flows with each operation in their order of sequence. The overall length of the block displayed can be used to graphically depict the operation's duration. This allows the overall timing of the flows to be visualized and synchronization strategies to be planned. Links, which correspond to specific synchronization conditions, such as start-start, start-finish, finish-finish, can then be defined between the flows. Synchronization links can also include dwell amounts, which allow the programmer to specify a time delay offsetting two flows. Once inserted, the flows are then adjusted to reflect the synchronizations and the overall timing of the process is updated.

The simple graphical display of the synchronization manager hides a wealth of detail and complexity behind its interface. The process parameters, such as speeds and feeds of the dominant tool on a spindle not only determine the operation's duration, but they also affect the duration of the subordinate tool's operation. The synchronization manager's interface hides this detail and presents the relevant information to the user in a way that is intuitive and informative. Switching the dominant tool causes the synchronization manager to refresh the relationships and operation timing and display an updated view of the process. Since the synchronization manager masks the underlying details of the machine tool, only the relevant information is presented to the programmer allowing them to concentrate on optimizing the program.

Multi-spindle, multi-turret

Multi-tasking machine tools also introduce a level of configurability not normally experienced with conventional CNC machine tools. This configurability goes beyond modeling relatively simple mill/turn devices. These machine tools can have an extremely broad range of number of spindles, turrets and/or tool groups. The turrets can have either conventional fixed tooling, such as OD/ID turning, threading or grooving tools, or live tooling, such as end mills or drills. Also, the turrets can be either fixed in orientation supporting motion along one of more axes or articulated allowing it to manipulate the tools over a wide range of motion. Turrets can potentially be tied to just one spindle or can work across many spindles.

It is not uncommon for multi-task machines to have in excess of a dozen axes. Machines with twenty or more axes exist. Additional features, such as bar stock feeders and parts catchers, can also be added to the base configuration to facilitate producing great volumes of parts. Though they may be built off of the same base configuration, multi-task machine tools can quickly become one-off variants.

This additional degree of configurability with multi-task machine tools further complicates programming them. In order to accommodate the various configurations of this class of machine tool, CNC programming systems not only have to be able to model the physical characteristics (such as degrees of motion and extent of travel) of the various options, but they also have to be able to accurately model the time aspect of their operation. Without this comprehensive representation of these machine components, the programming system would be unable to maintain an accurate representation of the machine tool performing the multi-task process. Timing calculations would be thrown off, causing cycle times of the various operations to be inaccurate.

Of course, once the multi-task program has been developed there is still the final step of creating the machine specific M- and G-code output to actually communicate the program to the machine tool's controller. Given that there is no one universal language for programming CNC machine tools, this final stage of creating a multi-task CNC program can be extremely challenging for a CAM system. The complexity of the program itself, the variety of the machine tools dialects and how the multiple process flows need to be handled in the machine tool program greatly complicate post processing the M- and G-code output. Generating a production-grade, multi-task CNC program requires that the post processor is capable of outputting robust and efficient code.

It is easy to program a multi-task machine tool as if it were a single flow device. But to realize the full capability of these advanced devices requires a CAM programming solution that is capable of supporting all the multi-task machine tool's complexities and subtle nuances.

Gibbs and Associates, or circle 210
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Author:Callen, John
Publication:Tooling & Production
Date:Apr 1, 2003
Previous Article:Contour, roughness measurement. (Product Spotlight).
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