Influence of Position Measurement on Accuracy in 5-Axis Machining
- Friday, 01 June 2012
Productivity and accuracy are important attributes in the competition for machine tools. Five-axis machining provides considerable potential for increasing productivity. In many cases it permits higher metal removal rates than 3-axis machining. Production times can be significantly shortened thanks to a reduction of time required for resetting, for example, or through multi-operation machining in one setup. In any case, with increasingly complex workpiece geometry, 5-axis machining is becoming an indispensable part of the machining process.Since significantly larger traverse ranges of linear axes are usually required for 5-axis machining, machines need to provide high accuracy over the entire working space. Moreover, the two rotary axes in a 5-axis machine tool can greatly influence attainable workpiece accuracy.
Besides the rotary axes, in most cases the linear axes must also be moved in order to change the orientation of the cutter to the workpiece surface. This can cause visible flaws in the traverse range of up to five axes within a small area of the workpiece surface. In 5-axis machining, precision-limiting effects in the drives, such as screw-pitch and transmission error, reversal error, or thermally induced displacement can much more quickly lead to the production of scrap. The positioning accuracy of linear and rotary axes plays a decisive role here in the performance of a 5-axis machine tool.
By now, 5-axis machining has become indispensable in many areas of metalcutting machining. Clear economic advantages result from the capability to machine workpieces completely in one setup: the door-to-door time of a part can be dramatically reduced. At the same time, part accuracy can be significantly increased.
Beyond this, the additional rotary axes allow better access to complex workpiece contours; for example, cavities in dies or molds. Often, they permit shorter tools with less inclination to chattering so that even higher metal removal rates are achieved.
With 5-axis simultaneous machining, the cutting speed at the tool tooth can be held within narrow limits even on complex contours. This brings significant benefits with regard to the attainable surface quality. What is more, the use of highly productive tools (e.g. toroid cutters) when milling freeform contours would not be possible at all without 5-axis simultaneous machining.
Parts for Aeronautics and Space
High strength and low weight are essential for the aeronautics and space industry. Integral construction has established itself as the way to minimize the weight of “airborne” parts: components with complex structure are manufactured completely from a single blank. Metal removal levels can be as high as 95%. This high “buy-to-fly” rate leads to high costs for the raw material of the blanks.
In the area of structural components, 5-axis machining opens new opportunities for reducing weight without loss of component strength. First, a computeraided topology optimization is conducted that adapts the geometry of the component to the respective loads. The result — the material is brought specifically to where the mechanical load can be highest. In the other areas, material is specifically reduced. For example, the thickness of walls for stiffening can be easily adapted to the load distribution in the component. The wall thickness can decrease with increasing height, for example.
This workpiece geometry can be realized in a simple way through 5-axis pocket milling. Five-axis machining has long become the standard for the machining of jet engines. High efficiency requirements are driving continuous improvements in the flow characteristics of all jet engine components. The resulting component geometries are very complex and are therefore manufactured exclusively in 5-axis simultaneous milling movements.
Machining in Automobile Manufacturing
Uncounted molds and dies are needed in automobile manufacturing for sheet metal and plastic processing. Dies for sheet metal forming can be up to 6 m long, and need to be milled at the very high accuracy of ±0.02 mm so that the upper and bottom dies can work together with the correct gap. Moreover, a very high surface quality of all functional surfaces is necessary in order to ensure that the forming tools have a long service life.
When manufacturing the tool contour, the machine has to maintain a very small distance between cutting paths in order to fulfill the requirements for high surface quality. This automatically lengthens the run times of the NC programs. The required accuracy of the forming tools is a formidable challenge for machine tools: high accuracy during long program run times on large components necessitates high thermal stability of the machine structure and the feed drives.
Five-axis machining opens new perspectives for shortening machining times because even deeply curved contours of forming tools become more easily accessible. In addition, special tools such as toroid or radius cutters can be used that permit significantly larger path spacing and therefore reduce program run times.
Requirements of Position Measurement
In 3-axis milling, the feed axes move within the dimensions of the workpiece plus the tool diameter. Unlike with 3-axis machining, in 5-axis machining, the inclination of the tool can be adjusted with respect to the workpiece surface. If the position of the tool center point (TCP) remains unchanged, a change in the cutter orientation usually requires additional movement in the linear axes. These compensating movements necessarily increase the traverse range required by the linear axes. Because increasing traverse range also means increased positioning error, feed axes of 5-axis machines need significantly higher accuracy and reproducibility.
The compensating movements of the linear axes are superimposed on the movements of the tool center commanded by the NC program in X, Y, and Z. Due to this superimposition, the axis feed rates for the tool center can significantly exceed the programmed feed rate. The increase in feed velocity results in increased heat generation in the motors, transmission, and recirculating ball screw. Depending on the principle of position measurement, the generation of heat can cause significant position error. To prevent faulty workpieces, precise position measurement in the feed axes directly at the moving machine elements is imperative.
Position Acquisition on Linear Axes
The position of a feed axis can be measured either through the recirculating ball screw in combination with a rotary encoder, or through a linear encoder. If the axis position is determined from the pitch of the feed screw and a rotary encoder, then the ball screw must perform two tasks. As the drive system, it must transfer large forces, but as the measuring device, it is expected to provide a highly accurate screw pitch. However, the position control loop only includes the rotary encoder. Because changes in the driving mechanics due to wear or temperature cannot be compensated in this way, this is called operating in a semi-closed loop. Positioning errors of the drives become unavoidable and can have a considerable influence on the quality of workpieces.
If a linear encoder is used for measurement of the slide position, the position control loop includes the complete feed mechanics. This is referred to as closed-loop operation. Play and inaccuracies in the transfer elements of the machine have no influence on position measurement. This means that the accuracy of the measurement depends almost solely on the precision and location of the linear encoder.
Various conditions of application, together with growing feed rates and forces, lead to constant changes in the ball screw’s thermal condition. Local temperature zones develop on a recirculating ball screw, which change with every change in position and decisively reduce accuracy in the semi-closed loop.
High reproducibility and accuracy over the entire traverse range of linear axes can therefore be achieved only through operation in a closed loop. The results are precise workpieces and a drastically reduced rejection rate.
As an alternative to using a recirculating ball screw, linear feed axes can also be driven by linear motors. In this case, the position of the machine axis is also measured directly by a linear encoder mounted on the axis slide. For highly dynamic, and at the same time quiet operation, linear motors already depend on high-resolution, accurate linear encoders. For this type of drive, the advantages of closed-loop operation apply unrestrictedly.
The basic principle for linear axes also applies to rotary axes. Here, too, the position can be measured with a rotary encoder on the motor, or with a highaccuracy angle encoder on the machine axis. If the axis position is measured using a rotary encoder on the feed motor it, too, is referred to as a semiclosed loop (Figure 2), because the transmission error of the gear mechanisms cannot be compensated through a closed position loop.
Errors of the mechanical transmission in rotary axes are caused by eccentricity of the gear wheels, play, or friction and elastic deformations in the tooth contacts and the bearings of the transmission shafts. Beyond this, most prestressed transmissions are subject to significant friction that heats the rotary axes and can therefore — depending on the mechanical design — result in positioning error.
In a semi-closed loop, the error in the transmission of rotary axes leads to substantial positioning error and to significantly reduced repeatability. The errors of the rotary axes are transferred to the geometry of the workpiece, which can greatly increase the number of rejected parts.
The positioning accuracy and repeatability of rotary axes can be decidedly improved with the use of precise angle encoders. Since the axis positions are no longer measured on the motor, but rather directly on the rotary axes of the machine, this is referred to as closedloop operation (Figure 3). Errors of rotary axis transmissions have no influence here on positioning accuracy. The accuracy with which a rotary axis can move to a certain axis position over a long period is also decidedly increased. Economic manufacturing with minimal scrap is the result.
The rotary axes that are driven directly by a torque motor (Figure 4) play a special role. The torque motors’ special design permits very high torque without additional mechanical transmission. Rotary axes with torque motors require a high-resolution angle encoder immediately on the machine axis. They are always operated in a closed loop.
Five-axis machining places particularly demanding requirements on the accuracy of feed drives because the traverse ranges and axis feed rate grow in comparison with 3-axis machining. The generation of heat and mechanical transmission error in the feed drives turn position measurement of the feed drives into a decisive factor for machining precision. Scrap and costs are minimized with the correct position acquisition. Therefore, linear encoders for linear axes as well as angle encoders for rotary and tilting axes are indispensable for machine tools on which high positioning accuracy and a high machining speed are essential.
This article was contributed by DR. JOHANNES HEIDENHAIN GmbH. For more information, visit http://info.hotims.com/40434-320.