In Capable Hands

Putting NC programming and faster machine tools in the hands of machinists has helped this leading mold shop cut weeks out of the tool building cycle.

Article From: 4/15/1998 Modern Machine Shop, ,

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Updating controls with feed rate optimization capabilities enabled a dramatic increase in real machining rates.

Machinist

Machinists create programs with the block of metal right before them, taking the theoretical element out of NC programming.

It's hard for some people to believe that the shop floor is a fitting venue for any kind of NC programming, let alone the complex 3D contouring necessary for cutting mold cores and cavities. On another front, it's a stretch for some to see how you can make a serviceably good machine tool all that much better just by replacing the control. And hardly anyone would see a connection between the two. If anything, rendering a machine tool capable of faster machining rates with a high speed control would be all the more reason to carefully construct part programs in the quieter and more thoughtful environment of a CAD/CAM department. Wouldn't it?

Nick Buchok scoffs at the notion, and that's only because he's being polite. You suspect that if he said what he really thought about the contention that machinists aren't the best choice for creating their own tool paths, then you might hear a choice word or two, maybe three or four. Of course, Mr. Buchok is himself a master machinist who runs the machining department at Active/Burgess Mould & Design Custom Tooling Div. (Windsor, Ontario), so maybe you'd question his objectivity just a little about who's most capable of doing the best job of programming in the high stakes game of high speed mold making. But he's also done his turn as an off-line programmer too. He knows how CAD/CAM systems work. He knows how machine tools cut. And for his money, putting both in the hands of a skilled machinist is the best way to finish a job.

Mr. Buchok will concede, however, that the right tools for true 3D shop floor programming really weren't there until quite recently. But that's all changing quickly and, as a consequence, Active has dramatically changed its approach to core and cavity machining accordingly.

Active's master plan is not just about programming. It's about compressing the entire mold making cycle. By combining shop floor programming with a higher speed machining process, they are redefining the respective roles of process planning, machining, EDM and benching. And in this process, they are building more accurate molds in much less time.

Here's how all those "tools" fit together into an efficient system.

Better Than Good

If you know much about the Canadian tool making community, you've heard of Active. It's among the elite shops in Windsor, just across the river from Detroit, which unquestionably has one of the finest concentrations of tool shops to be found anywhere in the world.

Like many of those shops, Active mainly serves the North American automotive industry. Its focus is on molds of all sizes for interior and exterior parts for instrument pannels, consoles, door assemblies, under-the-hood parts and air duct systems. The broader company also includes a sister division in Windsor, Photometrics, and the recently acquired Burgess Machine & Tool in Wallaceburg, Ontario, that specialize in molds for automotive lighting. Combined, the three plants cover some 120,000 square feet of manufacturing space and employ over 400 people.

So this is no garage operation, and with a operational growth rate (not including acquisitions) roughly twice that of the mold making industry, a successful enterprise by any measure. Still, the company challenges its own methods on a regular basis, which is why two years ago they began to execute the shift away from a more conventional mold making scenario to a new system focused on better enabling the shop floor.

Active's methodology brings both more machining capability and the direct control of tool paths to the person who is executing the machining process. With these tools in hand, machinists can start jobs sooner, yet remain flexible to adapting to in-process variables along the way, and get higher quality tools finished in significantly less time. Cores and cavities are being cut much closer to the final finish, which is substantially reducing the need for EDM. And that saves on the cost and time of electrode fabrication and burning as well as time on the bench.

Quicker To The Floor

Active used to prepare jobs more or less the same way that most larger tool shops these days do. Designers examined the piece to be made, roughed out a mold plan, and then handed it over to the CAD/CAM department where all the detailed 2D drafting and 3D surfacing was done, and then the NC programs were created for the shop. The typical machining process was, and still is, to rough blocks on high-torque roughing mills, send the blocks out for stress relieving, and then to do semi- and final finishing on higher speed finishing machines.

While that all may sound simple enough, not much about the real process ever was. It was difficult for programmers to anticipate all the ramifications of a complex 3D program, either in terms of how aggressively to machine or what the quality of the surface finish would be. Moreover, it was difficult for programmers to stay on top of the best machining practices, even though they all were familiar with the shop floor. As Mr. Buchok puts it, "Most of our off-line programmers started as machinists, but after they were away from the shop floor for a year or two, they just weren't as current. They were at an immediate disadvantage because they couldn't see what was going on. That's why we used to cut so slowly. We just didn't know we could safely do it faster."

So they were forced to err on the safe side, cutting slow and trusting most corners, slots and other details only to EDM.

Then there were the revisions. If there was a error in the original program, or if there was an engineering change after the job was released to the shop, the program would have to be returned to the CAD/CAM department for modifications. If the setup had already been made, the machine would either sit idle while the program revisions were made, or the block would be removed from the machine to be set up again later—not the most desirable option with a 20-ton block of tool steel.

Even if everything went right the first time, on an average job they might spend a week on surfacing, another two weeks creating the tool path, and then another week on the machine. Changes could string it out much longer.

Under the new system, however, virtually all the tool path creation has been moved to the shop floor—not to CAD/CAM stations located near the machines, but to auxiliary controls or computers located right at each machine. The programming software, PowerMILL from Delcam International (Windsor, Ontario), is designed specifically for a shop floor environment. Remarkably easy to use, the CAM system carries no surfacing capabilities of its own, only the ability to apply tool paths to an imported 3D surface or solid model. It allows Active's machinists to quickly create tool paths the way they want, and to visualize those cuts before committing them to metal. Moreover, it allows the machinists to program and machine concurrently. They can generate the first portions of program quickly, start the machine tool cutting, and then come back to the programming station to work on the rest of the program.

Having this capability on the shop floor greatly streamlines a job's path through the CAD/CAM department. "Today the engineers design a mold and hand it off to the CAD department to put surfaces on their work," says Mr. Buchok. "The surfaces go back to the mold designers for a final check, and then out to the machinists on the shop floor." And what used to take three or four weeks—moving from a finalized tool design to chips—is now done in one.

Flexibility And Control

If the NC programming time were simply transferred to the shop, there would be little gain. But once the surfaces file is released, they really do begin cutting almost immediately. Here's how it works.

First, the surfaces that represent the core or cavity are imported into the shop floor CAM station via network, and then the simple rectangular volume that represents the block of metal is constructed around it. Thereafter, the system models the material to be removed, automatically constructing tool paths based on the machinist's chosen cutting strategy and offset, or stock allowance, from the target surface model. The tool path can be applied to the entire core or cavity, or confined to any window, or boundary, which the machinist indicates by outlining the form with his mouse on a solid rendering of the surface model.

Internally, the system represents the 3D form not as true surfaces, but as a "tessellated" model, meaning that it is defined as a boundary representation solid model constructed entirely of triangular facets which are generated based on a user-specified tolerance to the original surface geometry. While that may sound inaccurate, it ultimately is no more so than most other contour programming techniques since (with the exception of circular or curve interpolation) the final point-to-point tool paths will be faceted and toleranced in any case.

A fundamental benefit of this modeling technique is that it is mathematically simple in comparison to surfaces. That allows the system to calculate tool paths and display shaded 3D images quickly—important factors in the impatient and visual world of the shop floor. It also allows a complex surfaces file to be represented as a single entity. As far as the machinist is concerned, the model is more conceptually akin to a physical pattern than the arcane math of NURBS surfaces. And the system, particularly in semi- and final-finish routines, can move a cutter in ways intuitively similar to a tracer mill.

But it can also do a lot more than that, and Active's machinists make the most of their control. For instance, rather than relying heavily on ball nose cutters, Mr. Buchok favors using flat bottom cutters wherever possible. It's much more efficient that way, and near flat horizontal surfaces can be cut very close to final finish. But it's also trickier from the standpoint of planning a tool path strategy. Giving control of the tool paths to someone who machines all day long, Mr. Buchok believes, gives Active the best chance of success.

The ability to create macros also helps. That is, once a routine has been created, the system has the ability to save it as a generic process. For example, a machinist may create a pocketing routine with a specific tool, pick increment, and tool path technique. That routine can be store as a macro, and then automatically applied to other pockets even though they are of entirely different dimensions. Macros save Active's machinists a lot of time, and help assure that best-practice techniques are more consistently applied across the shop.

Most roughing routines are now generated entirely with macros, which is the biggest reason why they can get new jobs cutting so fast. Because it's so simple to generate a roughing routine, Mr. Buchok will typically go ahead and do that right in his office and visually verify the tool path. It's a step that easily could be done at the machine, but Mr. Buchok uses this opportunity to quickly get a feel for each job going through the shop. It may take five minutes or so to download the surface file, depending on the size, and then he can immediately begin processing the roughing routine. On very complex cores or cavities, it may take as much as 20 minutes to process the whole job. But he can circumvent that time delay by processing just the first few levels of the roughing routine, stopping the processor, and sending the initial tool paths out to the machine. He then restarts the processing where it left off, and finishes the roughing program as the machine is cutting.

A similar tactic of concurrent programming and machining is used for semi- and final finish cuts, but all of those tool paths are generated in the shop by Active's machinists. So virtually no time is lost there either. But perhaps an even bigger benefit to having programming tools in the machinist's hands is to allow them to deviate from a plan when they see a better way. "The big advantage of at-the-machine programming is that the programmers, because they are the machinists, have the blocks of steel right in front of them," says Mr. Buchok. "Whenever a machinist sees the opportunity to program something faster and better, we don't have to go back to the CAM room every time. We program it right then and there."

Better Cutting

Active's system is not just about doing a better job getting jobs to the machine; it's also about doing a better job on the machine, and that's been enhanced by the shop's relatively newfound ability to achieve significantly higher machining rates on their existing equipment. In terms of pure technical capabilities, this has been accomplished by retrofitting each of the shop's finishing machines with feed rate optimization software from Omtronix Engineering (Mississauga, Ontario). Two of the machines have been directly fit with Omtronix CNCs; the others are linked to workstations that run the vendor's "Turbocut" software. (The keyboards and monitors for these Windows NT based CNCs and workstations are shared with the computers that run the PowerMILL software—though each is run on its own CPU—which allows machinists to quickly toggle between programming and CNC functions at the same station.)

Whether the optimization software is running onboard a CNC or on another computer, it still does essentially the same thing. It scans up to 60,000 blocks ahead in a part program looking for changes in the tool path direction, and then adjusts the feed rate accordingly. If it "sees" a tight curve coming, it will softly decelerate the feed rate beforehand in order to keep inertia from causing the machine to overshoot the intended cutter path and to make sure the CNC can process the program data points fast enough to keep up with the feed. Then when it sees the path straightening out again it will quickly accelerate the feed back to the maximum programmed rate. On the retrofit CNCs, this processing is done dynamically in real time. On the workstations, it is accomplished by inserting feed rate commands into the tool path file, and drip-feeding the modified program to the original machine control. The optimum feed rates are not universal, but instead tailored to the performance characteristics of each individual machine.

The difference in achievable feed rates has been dramatic. Before Active added this capability, programmers would have to use a feed rate that would be safe in the worst case scenario, and then be stuck with it in other portions of the program where they easily could machine much faster. "With most of these machines we'd start to cut off corners once we got over 30 or 35 inches per minute," says Mr. Buchok, and so that was the limit pretty much across the board on finishing cuts. With the feed rate optimization capability, however, they program finishing feeds for the best case scenario—typically between 200 and 400 ipm—and slow down only where the geometry of the cutter path demands it. On average, that has machinists cutting three or four times faster than before, and more accurately as well.

There are still some spindle limitations to deal with, since the older machines top out at 2,000 rpm. Active boosts that capability with mechanical speed increasers that get them cutting between 8,000 and 10,000 rpm with most tools, and pneumatic spindle units go as high as 40,000 rpm for very small detailing tools.

Active employs all of this additional machining capability to extend the functional range of the machining process relative to the entire mold making cycle. Or put more simply, it allows them to do more work on the machine tool in less time, leaving less to be done in EDM and hand finishing.

Without increasing overall machining time, they now can reduce stepovers to about one third of what they once ran. For instance, with a 2-inch cutter, they might use a stepover in the range of 40 to 60 thousandths of an inch. And remember that we are talking mainly about flat-bottom cutters with relatively small edge radii. That means they are machining virtually scallop-free surfaces on near-horizontal planes, and are effectively machining with a very large radius on sloping planes. Many bottom corners in cavities can be finished with flat-bottom cutters as well.

To further enhance the machining process, Active has also made changes in the selection of cutters, frequently moving, for instance, from 4-flute to 7-flute tools for finish cuts. "We get the same amount of cutting with much lower chip loads," says Mr. Buchok, resulting is cooler cutting, less defection, better tool life and better surface finishes. All these factors allow the shop to machine right to final dimensions in many cases, and to about half the material allowance they used to require everywhere else.

On-The-Fly Decisions

The shop floor programming and high speed machining capabilities, combined, have enabled Active to shift when and where key process decisions are made. One of the best examples is how to decide what features need to be EDMed. Before, they had to err on the conservative side. Now they often can delay that decision until they actually see the machined surface. If a bit more clean-up on an area will get the job done, they can generate the tool paths right on the spot.

They can also deal with 11th hour engineering changes, executing many modifications without having to stop or move the job. Says Mr. Buchok, "We want to do all the machining, or as much as possible, while the mold is still on the machine. We don't want to pick it up, move it and go through the entire setup process again." If there's an area of the mold they know is going to change, they can keep the job moving and work around it: "We block up and cover over the areas being changed and rough right around them. We have even sent molds out for stress-relieving while waiting for customers to make final changes."

There's no getting around EDMing some features, but those decisions can be made and executed quickly. "When we do have to burn some detail, we can easily and immediately program the electrode. We just rotate the same surface CAD model 180 degrees and calculate a negative stock allowance," says Mr. Buchok. In cases where they know they are going to have to EDM features, the electrodes are all created while the rest of the mold is still being machined.

The ability to make the right process call at the right time really pays off in reduced benching. "After finish machining, all we have to do for most molds is just clean up some of the cutter marks," says Mr. Buchok. That clearly gets tools out the back door faster.

Indeed, Active's mold manufacturing system allows the shop to make better use of all their resources. Where NC programming was once a bottleneck, and frankly a pain in the side of the CAD/CAM department, now the task has been removed from that department without the need to hire a single additional person. The CAD department is freed to concentrate on what they do best, and the shop floor is freed to create tool paths that they know work best. Moreover, now Active has 30 programmers versus the handful they had before, programmers who are always there when the machinists need them . . . because they are the machinists. That's better utilization of people and equipment, particularly on second shift when good tool path wasn't always readily available.

It's also resulting in better machining processes because they are determined by people who cut metal all day long, people who know what works and what doesn't. As Mr. Buchok puts it: "What is important is that the machinists are now involved in the process. These are their programs. They made them. They know what the job should look like as it runs, what the cutters should sound like, whether it's running correctly or not."

And then there's the human angle. Giving machinists responsibility for the process naturally results in a greater sense of ownership. "It really comes down to people taking pride in their work," says Mr. Buchok. "This is most definitely the case at Active."

These are most capable hands to which Active has entrusted its mold making process. We suspect they'll do more than hold up their end of the bargain.

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