Linear encoders keep their accuracy when things start to heat up.
Dr. Jens Kummetz
Head of Application
Dr. Johannes Heidenhain,
GmbH, Traunreut, Germany
Schaumburg, IL 60173
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Edited by Robert Repas
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Machine tools today compete against each other on productivity and accuracy. But the rapidly changing conditions under which modern machine tools operate make it difficult to maintain productivity and accuracy. A growing number of parts made in small batch sizes need accurate yet economical production.
For example, the aerospace industry looks for maximum cutting capacity in roughing operations, yet maximum precision for finishing processes. The rapid production of high-quality molds also demands rapid material removal rates yet with excellent surface quality after finishing.
An area often overlooked is the thermal accuracy of the machine tool. Constant changes between drilling, roughing, and finishing operations contribute to thermal fluctuations. The constantly changing machine tasks of short runs make it impossible to reach a point of thermal stability in the machine. At the same time, the profitability of a job may depend on the accuracy of that first piece. Therefore, position measurement in the feed drives is central to stabilizing the thermal behavior of machine tools.
So it has become crucial to avoid thermally induced deviations in workpiece dimensions. Active cooling, symmetrically designed machine structures, and temperature measurements are already common practice.
The primary cause of thermal drift is in the recirculating ball screws of the axis feeds. Temperature distribution along the ball screw rapidly changes as a result of feed rates and moving forces. The change in length can reach 100 m/m within 20 min. This can cause significant flaws in the workpiece on machine tools without linear encoders.
A rotary encoder can measure the position of a ballscrew- powered carriage on a numerically controlled axis. Likewise, a linear encoder attached to the carriage can do the same. Ball screws must perform two tasks: As the drive system, it must transfer large forces to move the slide. But as the measuring device, it is expected to provide accurate position values and to reproduce the screw pitch. However, the position-control loop only contains the rotary encoder. There’s no compensation for any inaccuracies or play within the mechanical drive train.
It’s not possible to compensate for changes in the driving mechanics caused by wear or temperature with this type of control. Because of this, the process is classified as a semiclosed-loop operation. Drive positioning errors become unavoidable and can influence the quality of the work.
If a linear encoder measures the slide position, the position- control loop includes the complete feed mechanics for a closed-loop operation. Play and inaccuracies in the mechanical transfer of motion have no influence on position measurement. Measurement accuracy depends almost solely on the installation and precision of the linear encoder.
Position can be measured with a speed-reduction mechanism connected to a rotary encoder on the motor. Or it can be gaged with a highly accurate angle encoder on the machine axis. But higher accuracy and reproducibility come when angle encoders are used rather than standard rotary encoders.
To prevent the ball screw and the surrounding parts of the machine from heating, some ball screws feature hollow cores for coolant circulation. In semiclosed-loop operation, thermal expansion of the ball screw affects positioning accuracy. Thus positioning accuracy also depends on the temperature of the coolant. A temperature rise of only 1°K causes positioning errors up to 10 m over a traverse range of 1 m. Common cooling systems, however, are often unable to keep temperature variations within 1°K.
It’s possible for the control to model the thermal expansion of the ball screw for drives in semiclosed-loop operation. The model only offers an approximate correction, however, because the temperature profile is difficult to measure during operation. It’s also influenced by many factors such as the wear of the recirculating ball nut, the feed rate, the cutting forces, the traverse range used, etc. This method can incur residual errors up to 50 m/m.
Fixed bearings at both ends of the ball screw boost the rigidity of the drive mechanics. But even rigidly designed bearings cannot prevent expansion caused by generation of local heat. The resulting forces are considerable. They deform the most rigid bearing configurations and can even cause structural distortions in machine geometry. Mechanical tension also changes the friction behavior of the drive, thus degrading the contouring accuracy of the machine.
These restrictions limit the drive accuracy attained by taking the described additional measures. Accuracies cannot compare with those in closed-loop operation using linear encoders. Also, these additional measures for semiclosed- loop operation cannot compensate for changes in the bearing preload caused by wear, or elastic deformations of the drive mechanics.
Accuracy and parts manufacture
The machine building industry is seeing a growing demand for parts made in small production runs. The small quantities of these runs make it a necessity to minimize scrap. The accuracy of the first workpiece plays an important factor in profitability. Machine tools for high-accuracy production of small batches face a real challenge. Constant changes between setting up the workpiece, drilling, rough ing, and finishing cause constant changes in the thermal condition of a machine.
Typical feed rates for roughing a workpiece range from 3 to 4 m/min, whereas feed rates from 0.5 to 1 m/min are used for finishing. Rapid traverse movements during tool changes also raise average velocities. Feed rates during drilling and reaming generate negligible heat in recirculating ball screws. The strongly varying feed rates changes the temperature distribution along the ball screws during the individual process steps. In semiclosed-loop operation, the varying loads on the recirculating ball screw may cause workpiece accuracy to suffer, even if the workpieces are completely machined in just one setup. Machine tools therefore need linear encoders in closed-loop mode for high-accuracy production of small parts.
Let’s look at what happens during a typical machining operation. An aluminum blank 500 mm long is first drilled and then reamed on a machine tool. The functional dimension between the position of a hole and the bisecting line of the individual workpiece is 12 mm and must meet tolerance grade IT8 in the illustrated example. This results in a permissible deviation of ±13 m.
The medium feed-rates during the two machining operations are low, so there is negligible heat generation in the recirculating ball screws. The contour is milled in the next production step and the medium-feed rate rises significantly. The milling action generates considerable heat in the ball screws.
If the milling machine operates in semiclosed-loop mode, thermal expansion of the recirculating ball screws creates deviations between the drilling and milling patterns. Maximum deviations of 135 m were measured near the loose bearings of the ball screw. Closed-loop operation completely avoids these errors.
The 135-m deviation means the workpiece only complies with tolerance grade IT13 instead of meeting the required tolerance grade IT8. When a linear encoder replaced the rotary encoder, all machined workpieces fell within the ±13-m tolerance.
Use of integral components in the aerospace industry benefits from combining optimum utilization of material characteristics with minimum weight. Typical integral components feature stock removal of 95% and more. High-performance HSC machine tools in conjunction with high feed rates and cutting speeds are used in the manufacturing processes.
The material removal rates made possible by these new machines produce great economic significance in manufacturing. But the resulting feed rates and machining forces also generate considerable frictional heat in the recirculating ball screws. Also, friction losses and the resulting thermal expansion of the ball screws vary during a machining process. For example, as the machine switches speeds between roughing and finishing, the ball screws undergo different degrees of heat generation for each step in the process.
If the feed drives operate in semiclosed-loop mode without linear encoders, small production runs with short cycle times can produce parts with dimensions that differ for each part in the run. Thermal expansion might prevent the mill from reaching specified manufacturing tolerances. Such sources of errors are reduced through the use of linear encoders, since any thermal expansion of the ball screws is fully compensated by the closed-loop operation.
Another example describes the manufacturing of a coupling lever. Two holes are machined at a distance of 350 mm from each other with a tolerance grade of IT7. The integral component is manufactured twice from the same blank form to permit evaluation of the accuracy possible in semiclosed-loop mode. The second workpiece is simply machined 10 mm below the first. Between the two machining operations, 20 machining cycles for the same part execute above the blank to simulate a short-run production of the part.
In semiclosed-loop operation, the contour of the second workpiece deviates from the contour of the first workpiece, as shown by an edge. The thermal expansion of the ball screw becomes more obvious the farther the drives move away from the fixed bearings of the recirculating ball screw.
The dimension of 350 mm with a tolerance grade of IT7 corresponds to a permissible deviation of ±28 m. The second workpiece machined in semiclosed-loop mode cannot meet this requirement with a deviation of 44 m. However, a closed-loop mode process using linear encoders met the requirements easily at 10 m.
The residual deviations of 10 m that did arise in closed-loop operation came from thermally induced, structural distortions of the machine geometry. The specified dimensions for the two bore holes can be improved to IT5. Closed-loop operations thus guarantee a reproducible accuracy from the very first part.
The manufacturing of molds or dies for injection molded or die-cast components is a time-consuming task because it requires a superior surface finish. Many times fine structures within the mold need that finish. Many molds today are directly milled to avoid the costly and time consuming eroding process. Surface textures are applied using ever smaller milling cutters with diameters as small as 0.12 mm. Mold and die making for milling is therefore characterized not only by high demands upon dimensional accuracy, but also by high feed rates on what may be hardened materials to reduce machining times.
Typical machining times for molds and dies range from 10 min to several days. Dimensional accuracy, however, must not be sacrificed to fast execution. The first and last machining paths must be identical to ensure that time previously gained is not lost to complex rework.
The heating of the recirculating ball screws in the feed axes substantially depends on the velocity profile specified for the individual axes by the NC program. Changes in recirculating-ball-screw length caused by heat expansion up to 150 m/m make it impossible to guarantee dimensional accuracy in semiclosed-loop operation. Typical heating of the recirculating ball screw would cause an edge deviation of more than 20 m in a mold with a length of only 150 mm. Expansion of the recirculating ball screw might result in errors so large that the flaw can no longer be corrected by rework.
This example illustrates the machining of a mold with the classic profile of the Watzmann a legendary mountain in the German Alps. A 500-mm-long workpiece is machined with multipass, climb, and upcut milling cycles in the X direction, using a ball-nose cutter with a diameter of 12 mm and a maximum feed rate of 4.5 m/min. It takes around 60 min to machine the contour with an infeed of 0.2 mm in the Z and Y directions.
The high feed rate of 4.5 mm/min, together with the constant accelerations and decelerations, generate heat in the recirculating ball screw and cause thermally induced linear deviations of 130 m in semiclosed-loop operation. The linear deviation with this mold component is difficult to visualize, so machining was deliberately begun in the middle of the workpiece. Start and end paths therefore lie side by side and clearly show the thermal drift. The farther the workpiece position is away from the fixed bearing, the higher the thermal drift.
To fulfill the high requirements of mold and die making, it is necessary to compensate the expansion of ball screws by using accurate linear encoders.
All in all, it takes machine tools with high thermal stability to successfully fulfill manufacturing orders. Machine accuracy must be maintained even under strongly varying load conditions. As a consequence, feed axes must stay accurate over the entire traverse range even with strongly varying speeds and machining forces. Thermal expansion in the recirculating ball screws of linear feed axes hampers accuracy and varies depending on the velocity and load with rotary encoders. Those that use linear encoders take the expansion of the ball screw into consideration as part of the measurement process.