There once was a time when a CNC was responsible for little more than relatively slow axis positioning and basic tool management. As machining centers have evolved, however, axis-positioning requirements have drastically increased while tool management and other functions have become vastly more complex, demanding far greater processing power to handle these and other functions. Faster processors, dual processors and even parallel processing are methods that have been used to increase processing speed.
But today's CNC is more than fast; it offers more functionality, canned cycles and automation features than ever. Tool offsets can be set with the push of one button. Servomotors can be tuned on the fly. Broken tools can be detected. And parts can be inspected on the machining center. Here are some of the important CNC issues and functions and what they mean to the cutting process.
CNC Speed And Feed Rates
Much is often made of a CNC's block processing speed, and rightfully so as it is one of the most fundamental performance measures. What is often missed in the discussion, however, is that different manufacturers don't necessarily mean the same thing when they refer to this metric. Some manufacturers define a block as a single CNC "word" such as X1 or Y1. Other manufacturers have defined a block as whatever will fit on a line up to the carriage return. This definition is important, because the block definition reveals the actual processing speed of the control. For example, a control with a 1,000 block-per-second speed that defines a block as a single CNC word, such as X1 or Y1, is only one-third as fast as a control with a 1,000 block-per-second speed that defines a block as containing as many CNC words as will fit on a line, such as X, Y, Z coordinates, and other program commands.
However you define a block of program information, the time it takes a CNC to process that block may not be as important as the time that it takes to actually execute the move, which is an issue that is affected by the density of points in a part program and by the specified tool path tolerance. The higher the accuracy of the tool path and density of points, the more important the sampling rate becomes.
What does that mean? The sampling rate is the interval at which the CNC receives position feedback from the servo loops. This is important because the CNC factors this information into both the position target and velocity of each new servo command. Usually, a more complex part has a higher point density that requires more axis moves, which increases the information the control must process. If the control can't process information fast enough, then accuracy can be compromised.
Whether talking about block processing or sampling, the critical issue is for the CNC to perform these tasks as quickly as necessary to keep up with a programmed feed rate. Keeping up with a programmed feed rate becomes all the more difficult when a CNC's CPU (central processing unit) must do several things at once.
Time-slicing occurs when a processor must perform multiple tasks and has to interrupt one function to initiate another. Because the processor moves from one task to another, it isn't possible to accomplish all the tasks when needed and some tasks have to wait. This is one cause of data starvation, which means that the CNC cannot process a program fast enough to keep up with the complexity of multi-axis moves. The result is poor surface finish, longer cycle times and even the inability to run a program.
Parallel processing technology allows the use of two or more microprocessors to be employed simultaneously. This is one way to alleviate the logjam of too many tasks on one processor. In the case of Fadal, for example, we have a dedicated processor for each axis to enhance processing speed. But it's not just a matter of more is better. The overall speed at which a CNC system performs is affected by a number of factors, which include speed of the processors, architecture of the CNC (how the microprocessors are connected), as well as the number of processors.
Looking At Loads
Supported by smarter software, some CNCs are making better use of sensor data that has actually been available for some time. For instance, there are many things that a load monitor can tell you about the machining process, if the CNC "knows" when to ask what kind of question. Some controls can monitor voltage fluctuation to determine if a tool is broken. At a relatively simple level, if there is a cutting load outside a pre-established set of parameters, the CNC knows there is a catastrophic problem with the tool, and shuts down the process. A more sophisticated method is to write a macro that monitors tool load over time, and that can determine when to change a tool based on more gradual patterns of wear.
Even more sophisticated software uses sensor feedback to achieve varying degrees of adaptive control. For example, tool load compensation is a feature that automatically changes the feed rates based on spindle load parameters set by the operator. It reduces cycle time by automatically changing the feed rates based on the tool load. Instead of setting a safe feed rate based on average tool load conditions, so the tool doesn't break, feed rates are set for the maximum tool load condition and the control automatically reduces the feed rate based on the actual tool load conditions while the part is being cut.
The operator sets the target spindle load as a percentage of the spindle load to be maintained. Minimum feed rate reduction and maximum percentage feed rate increase must also be set. If the tool load is less than the target spindle load, the feed rate will be increased and if the tool load equals or exceeds the target spindle load, the feed rate will be reduced. The minimum feed rate reduction is the smallest percentage that the feed rate can be reduced. As the feed rate is reduced, so is chip load. If the feed rate is set too low, excessive tool wear may occur. Similarly, when the maximum percentage feed rate increases so does the chip load. If it increases too much, the tool could break.
A feed rate limit can also be used as a cue to determine when a tool has reached the end of its life. For example, when the tool becomes so dull that the feed rate reaches 80 percent of normal, the CNC will enter a slide-hold condition that allows the operator to change the tool.
Tool load compensation increases cutting efficiency and tool life, as well as saving time with customers reporting as much as 30 percent reduced cycle time. Not all tool load compensation features operate on the same principles, so results can vary.
Probing The Process
Your CNC can use a probe in a variety of ways to reduce setup time, inspection time and keep track of broken tools. For example, setting tool offsets by manually entering tool data can be very time-consuming, especially if there are a lot of tools. Many controls have a multiple tool setup feature that allows tool offsets to be captured using the axis-jog handle to manually move the tool to the probe. The operator pushes one button to enter the offset data. But a more efficient technique allows tool length and diameter offsets to be set automatically with a probe. The operator enters the starting tool number and the ending tool number and then pushes the start button after verifying each of the automatically set tool offsets. With an automated tool offset feature, offsets for twenty tools can be set in a few minutes.
Probes can be used to set the Z fixture offset automatically, which speeds up the process when setting offsets for multiple fixtures. Fixture offsets for the X-Y axes can be set automatically by writing an appropriate program. Automatically setting fixture offsets enables an automatic pallet changer to work fully unattended. When the pallet is changed, the fixture offsets are automatically set without intervention of an operator.
Before a casting can be machined, the datum point must be verified and a probe will save a lot of time. Especially when the casting (or the fixture) is not in the same position as the last run. A feature called rotation searching will probe the casting or fixture and automatically calculate the angle of difference and rotate the program to the appropriate degree.
Off-line part inspection to verify measurements can be very time-consuming, especially when the part doesn't pass and has to be re-machined. But many CNCs allow the use of a probe and macros to verify part measurements and correct the part if necessary. Through macros, a programmer can automatically account for tool wear by probing the part. Verifying measurements automatically on the machine tool can be faster than with a coordinate measuring machine (CMM) in part because the time it takes to re-mount and re-calibrate the part, if additional machining is required, is eliminated.
A table-mounted probe can also be used to detect a broken tool. Typically, the CNC is programmed to command the tool to touch the probe as part of the machining cycle. If the probe touches the tool, the next tool is loaded and machining continues. However, if a tool touch does not occur, the program is aborted and machining discontinued.
While tool breakage detection stops further damage, it does nothing to increase productivity. However, by loading back up tools in the toolchanger and writing a macro, the CNC can replace the broken tool with the back up and continue machining, without losing a whole night's production.
Fine "Tuning" Your Process
Because most servomotors typically are tuned at the factory to achieve balanced results over a wide range of machining operations, they are not necessarily tuned to achieve the optimum results with any one specific application. In the past, users had little choice but to accept the way the machine was set at the factory, as retuning a servo system required a high degree of specialized technical skills and equipment. With today's digital motion control systems and powerful CNC software, however, users are getting much more control over the fine adjustment of a servo system, which can be optimized for the individual process at hand.
One example is called Advanced Feed Forward. It features five on-the-fly adjustments to achieve optimum speeds, feeds, accuracy and surface integrity, which include detail, gain, acceleration, deceleration and feed rate. Advanced Feed Forward (AFF) is a patented feature developed by Fadal Engineering for high-speed contouring operations, which can reduce cycle time by up to 70 percent or more, increase accuracy and provide a smoother surface finish through on-the-fly tuning of axis servodrives. AFF provides interactive control of machining speed and accuracy parameters, which allow the operator to change speed and accuracy parameters and customize control parameters for a specific tool or application based on observation of the cut.
In the case of Fadal's feed forward system, some of the parameters you can control "on the fly" include:
Detail, orcorner tolerance, specifies how closely corner portions of the workpiece are to be machined. By dynamically calculating optimum detail constraints for each axis according to distance and direction changes, it insures that a specified level of accuracy is maintained even at high programmed feed rates. When the detail parameter is set at a large value, the speed of the cut is increased. So, you might want to set the parameter high for roughing or semi-finishing cuts, but then reset it to a smaller value for finishing in order to improve accuracy.
Gain represents the "stiffness" of the servo system, or in other words, the responsiveness of the drive assembly to the difference between the desired and current positions. An increase in gain speeds up the cutting process through a reduction in the buffer between the desired and current positions. That's good up to a point, but will result in excessive vibration if set too high, so you want to achieve a proper balance with the desired speed and smoothness of the cut.
Acceleration and deceleration (acc/dec) time specifies the amount of time that the cutter has to accelerate or decelerate to the specified feed rate. Here the tradeoff is mainly between speed and accuracy. For example, if the default deceleration time is 30 and the part is being gouged, an increase in the deceleration time to 40 will eliminate the gouges by allowing the cutter more time to slow down, so it doesn't overcut. A decrease in acc/dec time speeds up the cutting process, but an acc/dec that is too short makes the machine susceptible to jerky motion.
Better Axis Synchronization
New CNCs' ability to do many things at once certainly applies to how today's machines offer better control and synchronization of the Z axis with the rest of the system, including the spindle. That results in such capabilities as rigid tapping, which eliminates the cost of spring-loaded tap holders. Rigid tapping delivers higher accuracy and repeatability because side forces (from springs) on the flanks of the threads are eliminated.
Fine synchronization between the Z and X-Y axes also makes helical interpolation possible, the biggest benefit of which is thread milling, a very cost-effective method for threading large-diameter (over 1 inch) holes, holes in odd-shaped parts, blind holes, pipe threads and tapered pipe threads. Not only do thread mill inserts last longer than taps, but thread milling is generally the only way to thread odd-shaped parts that can't easily be fit into a lathe chuck.
And it's not just about the X-Y and Z axes. Some CNCs have fourth-axis mapping capability, also called A-axis mapping or cam wrapping. That saves a lot of programming time when an X-Y program is to be wrapped around the circumference of a part. Basically, A-axis mapping converts Y-axis motion into A-axis motion. Conversion of X-A axes is used when the rotary table axis is A; and Y-B conversion is used when the rotary table axis is B. An example of A-axis mapping is a pocket machined or text engraved around the circumference of a part.
Many CNCs also offer enhanced capability to communicate with other computers. One example is Fadal's "Assist" software that, when combined with communications software and a multi-position serial switch, permits DNC file downloads and uploads directly from the PC or the CNC. With it, the operator saves time by eliminating the need to run around to manually set up the communications parameters or move disks from a PC to the machining center.
And increasingly, CNCs offer the ability to connect to the Internet. More and more shops will use this capability to access tool databases, interface CNCs with shop control networks, as well as to transfer part programs from customer to job shop or from manufacturing facility to manufacturing facility. Companies with national or global customers and/or operations will continue to increase their use of the Internet in order to transfer programs.
One particular advantage of the more "open" environment of many of these CNCs is that they permit software upgrades. These permit CNCs to offer continually enhanced functionality throughout their life. The CNC of the future is going to provide greater control over processes. For example, the CNC will monitor temperatures of machine components, as well as the workpiece, and activate coolant chillers to maintain constant temperatures. When a part is completed the CNC will utilize a modem to call or page the operator with information that it's ready for the next job. Controls will also perform self-diagnostic accuracy tests and automatically adjust to maintain tolerances, as well as maintain historic records that can be used for preventive and predictive maintenance programs. Audio and video diagnostic tools will be utilized by the service technician to observe the machine tool, to make sure that he brings the parts with him to make the repair.
Today's controls already provide a very high level of automation with a very short payback time. It's just a matter of learning how to apply all of the features that are currently available and determining which are the ones that can best increase your shop's productivity.
About the author. Glenn de Caussin is director of control technology at Fadal Engineering in Chatsworth, California.