By Gisbert Ledvon, Director of Business Development, Machine Tool, Heidenhain Corporation
In tool and moldmaking, dimensional and contour errors need to be so low as to be barely measurable and must certainly never be visible. These requirements are increasingly at odds with demands for higher productivity and lower costs. New control options offer possible solutions to the key questions that arise between the conflicting demands of a production process that is highly precise and at the same time highly efficient. The user can take advantage of these functions that bring out the best of a machine in any machining scenario while efficiently meeting workpiece accuracy requirements.
How can I optimally tune my machine to the given machining conditions?
Tool builders are finding ways to optimize speeds. As a result, either the cycle is under control or the increased speed causes gouging issues as the machine moves over the 3D surface. When these issues occur, there are basically two options: bring in the machine’s engineering team to fine-tune the machine for the application or – if the machine allows – change the dynamics using embedded cycles to which the operator has access.
If the machine supplier makes the adjustments, the supplier will send a service or application engineer who will look at the application and measure the behavior of the machine axes. Then the engineer can tune them electronically to optimize the jerk, the speed and the number of axes moving at the same time. This level of control typically is hidden in parameters most operators don’t have access to, which can mean additional downtime and costs while waiting for the service call.
With some systems, the operator can change the dynamics by using embedded cycles to optimize contour accuracy, which individually specifies the path width that is available to the control. The user can directly influence the maximum attainable contouring feed rate and also the machining time, in particular, for contour elements with numerous changes in direction, a common characteristic of free-form surfaces.
Optimizing the machining process in this scenario starts with the data quality of the numerical control (NC) program and enables optimized motion control for feed axes in three- and five-axis milling. An insufficient quality of data frequently causes poor motion control, leading to inferior surface quality of the milled workpieces. Some systems can dynamically calculate the contour in advance and adapt the axis speeds in time for contour transitions using acceleration-limited and jerk-smoothing motion control. As a result, clean surfaces can be milled in short machining times, even for contours with highly irregular point distributions in neighboring tool paths. This leads to advantages in the resulting symmetrical feed behavior on forward and reverse paths during bidirectional finish milling and in the form of particularly smooth feed-rate curves on parallel milling paths.
The problem to be solved is a desire for surface quality and consistency, while making sure the machine is running at optimum speeds. If customers have these problems, they should get involved with their machine tool builder or – if their machines have a control system – make sure the operators understand how to optimize the functionality and accuracy.
How can I take full advantage of a milling machine’s dynamics?
It comes back to what was discussed in the first question: The machine behavior and functionality are critical, which requires a dynamically responding machine tool with high accuracy, linear scales and a closed loop feedback system.
In a typical machine tool for average applications, there is an electronic device that measures the rotation of the ball spool. This measurement relies on the encoder to feed information back to the control – because as the machine is warmed up or runs for a while, the pitch begins to adjust. A closed loop feedback system compares the encoder information with the actual movement, measuring the position of the axes and then evaluating it to the position on the scale. This closed loop feedback ensures accurate positioning of the mechanics. Without that information, operators cannot compensate for any deviations.
With some systems, available controls provide functions to combine high accuracies with dynamic motions. These functions minimize not only forces that affect the mechanics of a machine tool during operation, but also the resulting deviations at the tool center point. For instance, these functions can compensate for forces that are introduced by dynamic acceleration processes and that briefly deform parts of the machine, leading to deviations at the tool center point.
Some controls also suppress dominant low-frequency vibrations and permit fast, vibration-free milling. This makes it possible to set high jerk values. Machining times can be reduced without compromising surface quality. In addition, these controls can continuously determine the current mass for linear axes, or the mass moment of inertia for rotary axes, and adapt the feed-rate control to the values measured at any given time. This improves the dynamic accuracy of the axis for every situation under load, enabling the use of optimized jerk values for the feed axes on the workpiece side. The result is a shorter machining time, since the feed axes reach the desired positions sooner.
How can I Efficiently implement the accuracy requirements of a workpiece?
Accuracy requirements are becoming ever more stringent, particularly in the realm of 5-axis machining. Complex parts must be manufactured with both precision and reproducible accuracy, including over extended periods of time. During machining, however, machine components are subjected to relatively high temperature fluctuations. The kinematic transformation chain should, therefore, be adapted correspondingly.
If the machine is not operating in a temperature-controlled room, when a machine heats up, the temperatures will change the kinematics of the machine – the axes, which then changes the pivot point. If that point needs to be held within a few microns of accuracy, the changes caused by temperature shifts – machines expand when it’s hot and contract when it’s cold – cause real problems.
In fact, one meter of steel can grow or shrink by as much as 11 microns when the temperature of the steel varies by 1° Celsius. That could be a big problem when machining parting lines, etc. All of this leads to the necessity for recalibration when there are significant temperature deviations, specifically in 5-axis applications.
Recalibration can be done manually by putting a block on the machine, machining the five sides and then measuring to see if the piece is still in sync. If not, operators can manually shift the datum point. There also are automatic functions that allow the operator to place a measuring board and a touch board on the table, and then the machine runs the cycle and automatically makes the corrections. No matter what, recalibration is necessary when completing high accuracy applications in 5-axis.
Also becoming more popular is 3D tool compensation. A triggering 3D touch probe is used to measure the position of a precise calibration sphere at various rotary axis settings. The result is a report providing the current actual accuracy during tilting. This 3D tool compensation also automatically can optimize the measured axes simultaneously, and requisite modifications to the machine data automatically can be implemented. The user needs no detailed knowledge about the kinematic configuration of the machine and can recalibrate the milling machine in just a few minutes. If the calibration sphere is permanently mounted on the machine table, then this procedure can even be performed as an automated step between the individual machining processes.
How can I plan and monitor automated production?
If the machine tool provides perfect machining results, then the associated processes should also run in an optimized manner. Intelligently networked systems for job planning, job management and job monitoring should provide a comprehensive view of job lists, running processes, work progress and any necessary interventions.
There are two components to this: The first is some type of management system to improve flexibility, so that when one program is set up, but another job suddenly becomes a higher priority, a switch can be made efficiently. This management system also provides important information prior to machining, such as when manual interventions will be necessary and how long the machine will be utilized. This allows for precise planning of the machining sequence and facilitates the smooth execution of pending jobs.
The second component is a monitoring software that provides a quick overview of what’s happening on each machine. It’s not necessary to invest huge amounts of money – instead, look for plug-and-play solutions that provide essential data that can be reviewed easily. Email and mobile device notifications allow operators to monitor processes from home during weekend or evening shifts.
When it comes to accuracy in 5-axis milling applications the motion control system is a critical part of the machine tool. Mold shop owners and decision makers should take a close look at CNC capabilities and operator training, rather than just relying on CAM data. At the end of the day, the cutting tool has to be moved at high speeds, assuring proper engagement in cutting process, to produce a perfect part the first time.
Gisbert Ledvon is the director of business development, Machine Tool, for HEIDENHAIN Corporation in North America. With over 25 years of extensive corporate industrial machine business experience, Ledvon is responsible for growing the CNC control side of the HEIDENHAIN business in this region. HEIDENHAIN’s CNC control models are referred to as TNCs and are growing in market share in North America. Ledvon is responsible for developing a strategy to drive technical value and commercial demand for the admired high-end TNC controls at the end user and OEM levels. In addition, Ledvon is charged with leading a team of 5-axis machining, programming and technology specialists at HEIDENHAIN’s North American subsidiary in Schaumburg, Illinois. For TNC control solutions (available for new machines and retrofits), training and application support,