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Various scenarios generally lead to a retrofit of a machine's control and drive systems. The machine may have outdated networking technology without Ethernet connectivity; the drive system may not be giving the positional accuracy needed for today's jobs; and the list goes on. With new machine tools less expensive than ever, more control upgrades have gone to higher-end, specialized machines, including large gantry systems performing complex, five-axis work.
"We typically find that the cost and quality of the iron dictates the viability" of a retrofit, says Chris Weber, product manager, machine tools for heidenhain Corp., Schaumburg, Ill.
This month, Fabricating & Metalworking talks with Weber about five considerations when thinking about a retrofit. As the market continues to demand greater accuracy in the machining of larger parts, control packages are stepping up to the plate.
1. NETWORKING AND MEMORY
The impetus for a control upgrade can lie in CAM and the programs it generates. As surface-finish demands become more stringent, spaces between points generated by those programs becomes much finer, and G- and M-codes the post-processor generates defines all those points. Hence, today a program 50,000 lines long isn't as unusual as it once was. Apply these programs to older controls, and a host of problems move to the footlights.
Serial ports are too slow for many applications, Weber says. Drip-feeding programs through those ports can cause a serious data hang-up. This leads to a data-starved controller that processes code much faster than the RS-232 port can feed it—so the machine literally stops and waits for data. Here, new controllers have Ethernet ports to solve the problem.
More specifically, there also needs to be enough memory in the control to host the program, eliminating the requirements for drip-feeding. The Ethernet port gets the program into the control more quickly. Longer, more complex code has also pushed demand for more memory—in the gigabytes range—so entire program libraries can be stored at the control itself.
2. ACCOMMODATING FOR THERMAL GROWTH
With machining forces come heat, friction and thermal growth. Rapiding back and forth, the ball screw may be "growing" slightly, but "the motor encoder doesn't know the ball screw grew," says Weber. "So instead of moving 10 mm I move 10.1 mm." However, the legacy control does not take that into consideration. So for that reason many perform rapid moves for a period of time to reach thermal stability, accounting for that growth before cutting metal.
But what if you didn't have to spend that time "warming up" with rapid moves? Enter the linear scale, which makes the measuring system completely independent from the motor encoder and drive system. The scale allows the control to see how much the axis drive actually moved. "Putting the linear scale on the machine eliminates that thermal-warm-up requirement," Weber explains.
3. PROCESS SPEED AND PART QUALITY
CAM allows the writing of more complex code than ever, and all that code benefits from blazingly fast processing speed.
In determining speed, Weber says, it's best to determine how exactly a "block," or line, of code is defined. How complex is that line of code? For instance, a block could be defined as a 3D simultaneous move (three axes) without radius compensation. With this definition, says Weber, if a control is specified to process 0.5 ms, it means it can process 2,000 lines of code—to the complexity of a 3D simultaneous move without radius compensation—in one second.
The processing speed provides just that—speed—but not necessarily accuracy. Asks Weber, "If I've got a fast processor and I'm making parts fast, are those parts good?"
For quality surface finish, modern controls can accommodate through different levels of filtering of the servo loops. For instance, some may incorporate "jerk limitation" control. The transitions between hard accelerations and hard decelerations in the work envelope can occur so rapidly that an operator will hear a "bang" in the machine at the transition, which ultimately can mark the part. To eliminate that the "controls can smooth out the transition between acceleration and deceleration to eliminate that jerking," Weber explains.
Such filtering works in tandem with tolerance control. On most new control commissions a shop can specify a pre-determined tolerance limit, allowing no program to execute moves that would bring parts out of a specified tolerance range, say 10 microns. On top of this, newer controls allow operators to override this and specify limits for certain sections of the program. For a specified contour, for example, a programmer can specify a 2-micron tolerance limit, separate from the overall 10-micron limit.
Say "you want to cut to 2-micron tolerance at 300 inches a minute" over a certain contour, Weber explains. "The control may say, 'No, you can't do that.'" So the control automatically overrides the program's feed-rate settings and slows down for specific elements to maintain tolerance.
Another modern-control function offers adaptive feed rate as a function of spindle load, with control software monitoring current from the spindle. Explains Weber, "Let's say I'm cutting a part on a programmed feed rate and, suddenly, I hit an area where there's no part. With this, the spindle load is decreased, and the control will automatically rapid [instead of feed] until it detects the load again on the spindle and automatically decreases the speed to the program feed rate. Not only does this decrease the machining time, it also saves the spindle and increases the tool life."
Say the programmer makes a mistake, thinking he's only cutting 0.100 inch, but he's really cutting into a half inch. The control will catch this error by monitoring this spindle load and slowing down dramatically. This certainly beats slamming the spindle into a half inch of metal and, most likely, breaking the tool and causing spindle damage.
This is where the advantages of a digital system shine, according to Weber. Systems account for error through three different servo loops: current, velocity and position. In analog systems, the velocity and position loop are closed within the drives, while the current loop is closed within the control. Within a digital system, on the other hand, all three are accounted for in the control itself, "so there's no time lag time, or following error, of communication between the drive system and the control system," he says. "The closer you get following error to zero, the better your parts are going to be."
4. FASTER PROCESSING, FASTER DRIVES
Today's control processor speeds often exceed the machine's capability to move. So in order to take advantage of these fast processing speeds, direct-drive systems have stepped up, replacing the traditional mechanical ball-screw systems. "I see a direct-drive torque motor as much more responsive," Weber explains, particularly as more mass and distance come into play, as it does with larger machining centers. With more mass comes the possibility of more flex and play within mechanical systems. Direct drives help those issues immensely, he says.
5. ABSOLUTE VERSUS INCREMENTAL FEEDBACK
Traditional machine tools use incremental positioning, where the control integrator, through a series of limit switches, sets a home position, and the machine relates every other position it moves to that home position. This creates the need for referencing during setup.
Newer systems offer absolute positioning in which the control sees the work envelope in its entirety, just as the operator sees it. If a system, say, loses power in the middle of the operation, the machine can now start where it left off before the power outage.
Consider a five-axis scenario with the XYZ position, two rotary axes tilted with the tool in a pocket at an angle—and the power goes. An incremental system requires the operator to back the tool and rotating axes out and get everything back to a point where it can be referenced.
With an absolute system, the control "sees" the same work envelope the operator sees, doesn't base its intelligence off a single "home" position and so can start machining where it left off. Today's memory and processing power make this possible.
Editor's Note: Artwork courtesy of heidenhain, www.heidenhain.com.