BY LAWRENCE HAFTL
ADAPTIVE CONTROL SYSTEMS have been in use since the early 1960s, but in the last few years, faster microprocessors and lower cost computer memory have dramatically increased the benefits that come from using these systems. Typically, those benefits include: reducing cycle times by optimizing tool loads, extending tool life by preventing tool breakage, reducing maintenance and downtime by eliminating unnecessary tool changes, decreasing part temperatures, reducing the need for manual part program optimization and reduced scrap and rework.
An adaptive machining control system uses a sensor to measure the horsepower load on a spindle motor and adjusts the feedrate to achieve a continuous load. It is an automated equivalent of having a machine operator constantly adjusting the feedrates to maintain a steady horsepower load. While the adaptive control system can respond to load changes in microseconds, human operators are usually a bit slower.
But how do adaptive control systems stack up against toolpath-optimization software?
Some of today's CAM and CAM-verification software have toolpath-optimization functions built into them that define toolpath segments in a cutting pattern to keep a constant load on the cutter and to adjust feedrates automatically based on the specific cutting conditions for each segment. The software assigns the toolpath and feedrates based on the specific cutting conditions of each segment, including depth, width and angle of cut. Verification software has the added advantage of checking the integrity of the toolpaths and identifying potential costly mistakes before beginning to cut metal.
Unfortunately, none of these software products and their resulting codes have any way of automatically adjusting to a number of common conditions that lead to tool breakage. Those conditions include variations in material hardness, uneven dimensions and surfaces of the workpiece, tool wear, temperature variations during cutting and fixture instability. To avoid breaking a tool because of any of these conditions, programmers must use worst-case feedrates for a safer, but more inefficient, cutting process. Even so, it is still possible to break tools, especially when machining many of the tough metals used in today's products.
Adaptive control on the other hand is designed specifically to handle those possible conditions safely while maximizing metal removal. It is not a replacement for optimization and verification software, it is an extension of a machine's control that enables the machine to cope with the variables that software cannot tackle.
How Adaptive Controls Work
Adaptive controls are an extension available on some tool-monitoring systems. A typical adaptive control system will measure the horsepower load on a spindle motor either by tapping into a machining center's spindle-load sensor data or by getting data from an added transducer. The user programs an ideal horsepower for a particular cut, and the system takes control of the feedrate override to maintain that constant horsepower. Until recently, adaptive control systems were cumbersome and difficult to program. Now, these systems come with graphic displays that show the feedrate that is being applied and the horsepower being used. The operator can increase or decrease the amount of feedrate override with the push of a button. Additionally, the machine's controller can be programmed to turn the adaptive control function on and off at any point, and the function can also be assigned to each tool in the machine.
A tool-monitoring system allows the machine operator to set load thresholds and other parameters to trigger an alarm and to stop the machine if the thresholds are exceeded. However, an adaptive control system adjusts the feedrate to prevent tools from reaching those thresholds. While there are a number of companies making tool-monitoring controls, only a handful also provide the adaptive control option.
Caron Engineering (www.caron-eng.com) and its distribution partner CNC Engineering Inc. (www.cnc1.com), Maintenance Technologies (www.maint-tech.com), GE Fanuc (www.gefanuc.com) and Heidenhain Corp. (www.heidenhain.com) offer adaptive control in addition to tool monitoring.
"Adaptive control is especially useful when machining forgings and castings," said Rob Caron, president of Caron Engineering. "We've had customers find chunks of harder metals in their castings — steel nuts and bolts that had fallen into the molten cast iron. When a drill or milling insert hits something like that, it often breaks, but the adaptive control system immediately senses the difference in hardness and just slows the feed rate down until the bit is through that spot. Even if there are no unexpected chunks in the metal, the hardness of castings can easily vary as much as 300 percent, and the adaptive control system will adjust feedrates to handle those variations. We sell a lot of our adaptive control systems to aerospace — General Electric and Pratt & Whitney. They machine metals so hard that tools can get dull on a single cut. When that happens, the tool might break in the cut causing scrap or rework, but with adaptive control, the feedrate slows down and allows the dulled tool to finish the cut."
"There are several companies that make tool-monitoring systems, but they don't get into adaptive control," said Caron. "Everyone that has it loves our system, and they buy more of them and put them on more machines. But why more people are not using them is beyond me. There is some work being done in university labs, but they are trying to take it to an extravagant level where they want to incorporate acoustics and modify the spindle speed at the same time as the feedrate. By trying to do both, you compound the complexity, and the control may not be responsive enough to override the feed rate quickly enough. Of all adaptive cutting, 80 percent to 90 percent can be done simply by overriding the feedrate based on horsepower."
"Adjusting feed rate based on horsepower can give you a lot of value without a lot of cost," said Mark Brownhill, product manager for productivity solutions at GE Fanuc. "But we are now exploring high-speed machine learning that will look at what the servos are doing on a particular part, and if it sees resonances appearing, it can compensate or predictably adjust feedrates to avoid those conditions. These systems need a lot of memory, and it is only recently that memory in the control has been cheap enough to make it cost effective."
Smith & Wesson Firearms (www.smith-wesson.com) uses Caron's adaptive control systems on 28 of its machining centers. "It really helps us control our tooling costs," said Richard Picard, manager of new technology at Smith & Wesson. "We've been using tool life management across the board for the last 15 years, but if you get a bad grind or your tool supplier gives you something a bit different, what happens sometimes is you break a tool before the pre-established cut time expires. Adaptive control prevents that. Now we virtually never break tools. We're able to get ten or more regrinds on all our carbide drills and end mills. We measure tool life in minutes. With tool monitoring, when the tool gets really dull it will trip an alarm, and the machine will stop. With one operator working five or six machines, that means the operators are going to be constantly changing tools and those machines are going to sit idle waiting for the tool change. To make matters worse, a broken tool can cause machine damage, disrupting machine uptime and throughput while running up a repair bill. But with adaptive control, it will adjust the feedrate so that it doesn't hit the alarm rate but alerts the operator to the need for a tool change coming up. It makes the operators more efficient."
Adaptive controls are used primarily in rough-cut applications where finish is not a concern, but Smith & Wesson also uses them for finishing. "We use adaptive control on the finish tools as well as rough cuts," said Picard. "We are a production shop and make custom tools to do finishing and rough cuts quickly."
While preventing tool breakage is a major benefit to these systems, all of the companies making them also claim they reduce cycle times.
"Your typical part program will quickly bring the tool close to the material to be machined and then pause for a moment before beginning the cut," said Steve Blum, applications engineer at Maintenance Technologies. "With the microsecond response of the new adaptive control systems, that pause is not necessary as the feedrate is instantly reduced to a suitable speed as soon as the edge of the material is encountered. One potential customer who made small medical parts figured out that by eliminating that pause his time savings would add up to a lot of time per part." In addition to reducing or eliminating unnecessary machining pauses, the fact that these systems can increase feedrates if conditions allow it also reduces cycle times.
The systems offered by Caron, GE Fanuc and Maintenance Technologies are added on to a machine and interface with the machine's existing controller. Heidenhain has built its adaptive control function directly into its new iTNC 530 controller family. "Adaptive control is an inhouse developed solution and an integral part of our iTNC 530 controllers," said Chris Weber, Heidenhain marketing manager. "You can turn adaptive control on and off with M-code instructions or you can assign adaptive feed control to specific tools with loads and feeds tied to that tool. You can set parameters to select a new tool when the load gets too high on the current tool. All the process data is displayed on the controller's screen, so you don't need a personal computer sitting there to get all the information." Customers can order an iTNC 530 controller with or without the adaptive control function enabled. If they want to later activate that function, it can be done by purchasing and entering a code word.
The per-machine list price of the Maintenance Technologies system is $9,000 to $14,000, depending on the machine that you are going to install it on. Caron Engineering's system lists for about $12,000. Heidenhain's adaptive control option is $5,400, but you also have to buy one of their iTNC 530 controllers, which cost between $4,000 and $30,000 depending on the model you need.
"The surprising thing for me is that the machine tool builders all have their tool management and load control stuff, but they are not user friendly. They don't have adaptive control, and it's almost an afterthought," said Smith & Wesson's Picard. "I keep thinking some machine tool builder is going to take over Caron Engineering and start equipping all of their machines with it. I get to visit a lot of shops, and it is amazing to me how few people use machine control to help their operators and to make their operators' lives easier and more efficient."