Quest for accuracy - Heidenhain explains

Heidenhain (Dr Johannes Heidenhain GmbH, in full) is well known to users in Europe as a manufacturer of CNC controls, having a particular following in the tool and die area, with German 5-axis machine tools also an area of strength for the company. Fundamentally, the accurate machining of 3D contours with high quality surfaces is one of its recognised central competences in the machine tool technology arena (it operates in other sectors, too). With Dr Johannes Heidenhain's roots in the production of accurate graduations on glass or other substrates to support accurate measurement on machine tools, the company developed digital readouts in 1974. Its first numerical control appeared in 1976 (3-axis positioning), with straight-line cutting offered in 1979. The first contouring control made its appearance in 1981 (3-axis); 4-axis technology followed, in 1984, and 5-axis contouring made its appearance in 1995. The company has supplied more than 240,000 CNCs. Dr Kummetz underlines Heidenhain CNC core strengths as user-friendliness, both from an end-user and machine tool builder perspective, and which also includes backwards compatibility for older programs and forwards compatibility, in terms of user knowledge; its path control concept, important for mould and die applications; and its ability to support identical path contours in raster (back and forth) contour machining. IMPORTANT DETAILS The second and third demand some further explanation, as they are important technical details. Path control relates to the means by which path transitions are smoothed and feedrate is adapted with respect to the curvature of the cutter path. You can't instantaneously change direction, so you have to build in a transition path between directions, with that transition related to speed and faithfulness to programmed path – higher speed means less faithfulness. So, there must be a tolerance for deviation from the programmed path, but in a controlled manner. This is achieved via Heidenhain's Cycle 32, which is "simple to use" and "reliable and effective", Dr Kummetz offers. As for the ability to produce identical paths in opposing directions, this is related to the fact that Heidenhain will receive programs from a CAM system as point data and then add more points between these to support position commands, according to geometric tolerances given by cycle 32 and speed, plus acceleration and jerk limitations given by OEM machine parameter values. This is instead of the use of spline interpolation to get a 'best fit' curve through a series of points. With that, the CNC specialist says, when traversing a contour in alternating directions (raster machining,) spline interpolation can give different results for each direction, if the point data employed by the spline function are not identical, which can be the case, and this would show as a witness between paths "in the single micron range", requiring more hand polishing, for example. The combination of Cycle 32 and the point data approach gives a "predictable output", Dr Kummetz underlines, adding: "Our customers [machine tool builders] rely on the performance of Heidenhain controls, particularly if it is required that the first part produced is to be of highest quality." With these core distinctions made, Heidenhain's CNC expert turns to the limits to machine tool accuracy and how the company's technology and services can help achieve highest performance. Fundamentally, differences between programmed geometry and actual geometry result from three areas: static errors that originate from a mismatch between machine kinematics and the kinematic model in the CNC (constant over time); dynamic errors that result from transient forces generated by feed drives, plus the interaction between structural modes and control loop dynamics (not constant over time and with a very short period - milliseconds); and thermal growth, which influences machine kinematics, although the effects are variable, depending on the power being drawn (not constant over time, but with a long period – minutes to hours). Image: The sources of errors "In many cases, thermal growth is the largest of these," advises Dr Kummetz, adding that for a machine of size 800 by 800 by 800 mm, rated power is typically 60 kW. This must be dissipated somewhere, but there will be major differences between roughing and finishing, he highlights. However, the actual effect of thermal growth depends on the method of position feedback. Using glass scales fitted to the structure of the machine will see thermal effects minimised, compared to indirect feedback via rotary encoders attached to ballscrews, however (see also later). But with all these error sources, the requirement for the machine is that the accuracy maintained between the tool and workpiece "should be within single microns". When looking to trace the root cause of errors in machine tools in order to understand how to develop products, the department that Dr Kummetz heads up employs a systematic approach to localise error sources (see diagram). In doing so, Heidenhain looks at all the process steps from CAD through to the geometry of the part workpiece. "We have to go step by step, looking at all the interfaces of the process chain. So, compare the CAM system cutter location file data to the CAD model geometry, analyse the tool centre point position, and review the commanded axis position versus actual position, for example." Image: Narrowing down the sources of errors takes a systematic approach, attending to all the various areas BUILT-IN ERROR SOURCE Commencing with data fed to a CNC, he says: "While the input to our CNC is the NC program, it may be that surface defects or accuracy problems are already within the NC program, because either the CAD system or the CAM system has introduced deviations." In the case of the CAD system, this might be due to the 'stitching' together of multiple sections, leading to local curvature deviation and to corresponding surface marks. "Or it may be within the CAM system, which has taken the splines from the CAD system and broken them up into discrete short lengths, to subsequently create cutter location information point data fed to the CNC." The first element that is under the control of Heidenhain is path control, where the control interprets the program and then interpolates positions, considering the various limitations of the machine tool, such as maximum acceleration, maximum jerk (rate of change of acceleration) and maximum feed, so as to arrive at figures that the machine can faithfully follow – these are governed by parameters set for the machine by the machine tool builder. This interpolation process also takes into consideration the path tolerance (Cycle 32). Of course, the cutting tool centre point (TCP) is at some distance from where the axis position is measured, offers Dr Kummetz, with the machine structure responsible for physically restraining cutter position outside of programmed movement. Many problems of surface finish or accuracy are related to oscillations, he says. "There will be some oscillation in the machine structure and this led us to undertake some basic research on machine tool oscillations. Since 2005, we have been able to provide finite element analysis of machine tools, including the control loops. By that, we support machine tool builders to localise the critical oscillation modes that limit the dynamic accuracy." A way to gain a deeper understanding is to attach sensors to various parts of a machine tool and to 'excite' the structure via tapping it with a special hammer or moving axes. Based on this approach and the methods of modal analysis, Heidenhain can support machine tool builders in analysing oscillation modes of existing machines. IN SEARCH OF ERROR SOURCES Heidenhain applies a number of tools to gain information, some of these are PC tools that allow the reading of information from the CNC, drives or encoders, for example. It also has some additional tools, such as its cross grid encoder or KGM, which supports measurement of TCP in two axes in a contactless manner and which in its latest version KGM 282 is accurate to ±1 micron. Versus an alternative approach using a ballbar, Dr Kummetz highlights that even a circle of radius 1 micron can be programmed and measured, while it will also support analysis of freeform movement, such as a rectangle, or any two dimensional movement of an NC program. There is also no speed limitation, since it is non-contact. "The KGM is widely used to classify machines," he offers. Image: KGM can show what's really happening at the tool Another piece of equipment available is VM 182, an alternative to laser interferometer technology and is used to check the accuracy of linear travel (longitudinal accuracy is ±1 micron; transverse is ±1.5 micron). It is easier to use and, unlike a laser source, independent of air conditions, it is underlined. Having discovered an error between programmed and actual path using KGM, for example, and discovering a repeating static error, the next step is to decide what is causing the error. Does the error relate to the kinematics of the machine, for example? For any one axis, there are six degrees of freedom. For a linear axis, there are position errors, two straightness errors as well as roll, pitch and yaw errors - component errors. So, for a 3-axis machining centre, there are 18 error sources, plus three perpendicularity errors (one between each axis and another) making 21. In fact, Dr Kummetz suggests these perpendicularity errors can be considered as part of component errors, but there is some confusion sometimes. As axes move, all such errors affect TCP. Image: Six errors per linear axis Today, volumetric error compensation, as opposed to simple linear error correction (employed for many years), is an increasingly common technology (see http://is.gd/8lYvtS) to account for kinematic error sources, especially on large machine tools. But here again, Heidenhain draws attention to its approach. Component errors of each axis are contained in look-up tables with corrections values for each axis converted via maths to each component error - X, Y and Z, A, B C figures - and applied in real time to the programmed axis positions. Heidenhain uses a closed mathematical description of machine kinematics offering a consideration of all component errors of linear and rotary axis (others employ simple X,Y,Z input and output only, not six components per axis). In this way, variable tool length and rotary axis movements will be automatically considered in the compensation approach offering the opportunity to reach enhanced tool centre point accuracy in 3-, 3+2- and 5-axis simultaneous operation. REDUCED ERRORS The Heidenhain CNC capablity to support this approach is called KinematicsComp. A reduction in TCP position error of 5.5 times is claimed as typical in a volume of 800 by 800 by 800 mm, although lack of thermal stability will have a major impact on this benefit. Heidenhain will only apply its system to machines fitted with linear measurement scales, as without this, thermal drift will cause the compensation values to become inaccurate. (Thermal drift is primarily caused by recirculating ball screws, with machine tools without dedicated position encoders seeing changes in length typically of up to 100 micron/m within a 20 min period; and a 20 °C change in temperature over an hour results in a change in length of 200 micron/m.) Volumetric error compensation is particularly important in 5-axis machines. These use two additional rotary axes to manipulate the spindle and which clearly have errors, too – 41 error sources in a 5-axis machine, in fact. Heidenhain's KineamticComp technology can consider all 41 kinematic errors sources to correct the TCP position, assuming that the errors have been measured with suitable measurement equipment, like laser tracer, laser tracker, cross grid encoder, or other. "Our approach allows a systematic way to compensate for errors in 3- and 5-axes machines." Dr Kummetz underlines. "The KinematicsComp approach also makes it much easier to consider thermal deviation in a systematic way, by measuring temperature at points along an axis and compensating the temperature effects on kinematic errors via temperature dependant look-up tables." (This additional thermal compensation corrects for structural movement that the linear encoder cannot 'see', due to its distance from the centre of action of the axis.) In addition, the Heidenhain software option KinematicsOpt allows on-machine measurement/compensation technology to support automatic error correction for rotary axes location errors. KinematicsOpt identifies this subset of kinematic errors through a user friendly cycle controlling the probing process of a precision sphere in different orientations of the rotary axis. This fast calibration process (usually less than 10 minutes) is typically used right before a precise part geometry is manufactured, so that thermal effects on the rotary axis kinematics are considered appropriately in the kinematic machine model of Heidenhain's TNC control. "Our approach is the only systematic way to compensate for errors in five axes. And this is confirmed by independent companies involved in the measurement of machine tool errors," Dr Kummetz underlines. "Our approach also makes it much easier to consider thermal deviation in a systematic way, by measuring temperature at points along an axis and compensating via look-up tables." Returning to the KGM 182 cross grid encoder and turning to dynamic errors, this device can highlight contouring accuracy at any given feedrate. The KGM tool can also be employed to analyse reversal errors (caused as direction slows, then reverses at quadrant points around a circle) that are due to backlash and friction, and analyse how much compensation has been employed, and where (the output path, for example) to offset these. Under-compensation is, Dr Kummetz offers, always the rule by machine tool OEMs, as overcompensation would result in problems in the field by having potentially unwanted oscillations. Image: KGM can highlight this over-compensation within path control, which sees diameter variation at different feedrates for the same programmed path The cross grid encoder can also provide data for another Heidenhain capability that compensates for dynamic errors - Cross Talk Compensation (CTC). It counteracts the elastic deformation that may be noted in a contouring feedrate test. Say a 3-axis VMC accelerates in Y-axis towards the front of the machine, the likelihood is that the machine column will react by tilting backwards, both lifting the tool centre point and pushing it forward: CTC provides a way to negate the effects by moving axes in order to maintain a constant tool centre point, to within ±1 micron. This will improve surface finish. Image: Crosstalk compensation is drawing interest SLOWER IS NO ANSWER If the rate of change of acceleration were reduced by a factor of five, CTC would not be necessary, says Dr Kummetz, but then cycle time would suffer. "This capability [CTC] is generating a lot of interest in the market, with many machine tool OEMs undertaking tests," he offers, claiming this as a unique capability in the market. KGM 182 also supports the measurement of machine vibration during contouring, with this information used to support CNC input to minimise vibration by means of a mechatronic model for the relevant machine dynamics. This will reduce oscillations, helping to produce highest quality. The ease with which such parameters can be set to avoid oscillations in the machine structure and so gain a good surface finish is a claimed Heidenhain strength. "We are proud of our simple concept, which machine tool builders like, because they can easily work with different machines." Moving to errors generated by tooling, these can be introduced via imperfections in tool geometry, especially ballnose cutters that are used at varying contact angles across a surface. Radius errors of between 20 and 30 microns are not unusual, Dr Kummetz highlights. Such errors, which can be measured by using a cycle in the Heidenhain control and a touch probe, or via a presetter, can again be compensated for using a look up table (option 3D Tool Comp, available for a couple of years). The CAM system must be able to output relevant information of normal surface vector and tool angle vector (LN programming) to support this, however. Finally, thermal effects, as already stated, are the main source of errors in many situations. Spindle growth affects Z and Y axes on a 3-axis VMC, with 40 microns displacement in Z and 20 microns in Y-axis typical figures, even with some sort of temperature compensation, while movement of 500 micron in Z-axis has been measured by Heidenhain elsewhere. With spindle cooling, Z-axis deviation of 10 micron is achievable, Dr Kummetz offers. As already highlighted, direct position measurement (linear scale encoder) versus indirect position measurement (rotary encoder) is superior in avoiding thermal effects for machine axes - 200 micron/m of travel growth is a typical figure for a ballscrew. However, in the case of spindle growth (not an axis), this must be compensated for by controlling temperature or reacting to temperature change. And to underline the importance of linear scale use in combating thermal effects on 5-axis machine tools, where like-for-like component axis travels are typically larger than for 3-axis machines, Heidenhain has evolved the Telstar Football test. This solid aluminium football has hexagonal seams machined with a ballnose cutter on a machine with and one without linear scales. Mismatch is evident on the latter. In conclusion, there are many errors sources, but Heidenhain has a systematic method of measuring, analysing and compensating for them, using a variety of tools. More than that, its controls, claims the company, have easy-to-use facilities to support compensation, together with key technological features, such as the use of point data, path tolerance control, and its special volumetric compensation approach for 5-axis machine tools. In all of this, however, accurate axis position measurement and feedback via linear encoders is a critical element in supporting accurate high quality machining – Heidenhain's roots, of course.