Metal forming machines: a new market for laser interferometers O. Beltrami STANIMUC Ente Federate UNI, via A. Vespucci 8, Tbrmo, Abstract Laser interferometers have traditionally been a synonymous of very accurate metrology for very accurate machines, therefore being used only on "metal cutting" machine tools. In the early 90's most of the laser manufacturers developed dynamic software, able to measure displacements, speeds and accelerations versus time, which were considered as an enhancement of the laser performance, but without probably thinking of some possible new fields of application, which in effect have never been advertised so far. STANIMUC, besides assuring secretariat and chairmanship to both Technical Committees ISO/TC 39 "Machine tools" and CEN/TC 143 "Safety of machine tools", have always offered to industry a service of machine tool calibration, testing, noise measurements, safety consultancy, etc.. This allowed STANIMUC to become acquainted with a wide range of the machine tool industry needs on different subjects, as for instance the measurement of the stopping time of power presses, used for calculating the safety distance of protective devices. Since the beginning of 1995 we have been performing those tests, with a sampling frequency up to 10 khz (one reading every 0,1 millisecond). Methods for carrying out measurements are different from hydraulic presses or press brakes to mechanical eccentric presses, up to mechanical presses with link drive mechanism, which require a preliminary research of the point up the stroke where the maximum slide speed is reached. Another laser measurement for safety purposes is the assessment of the precise point up the stroke where the "muting" of the protective devices takes place, in order not to leave an unprotected part of the travel. Besides these measurements related to safety, many others are made possible by the dynamic software, as the instantaneous deflection of the table under the closing effort, which on mechanical presses can be measured only dynamically. Further measurements concern pitch and yaw angles of the slide during the closing movement, or angular deflections caused by an eccentric effort in the die. Other useful measurements can be carried out on mechanical presses equipped with the link drive mechanism and on screw presses.
126 Laser Metrology and Machine Performance 1. Introduction Laser interferometers have traditionally been a synonymous of very accurate metrology for very accurate machines, therefore being used only on "metal cutting" machine tools, as the "metal forming" machine tools are subject to less stringent requirements for both geometric and positioning accuracy. A power press has usually "large" clearances between slides and gibs, and during its normal work has "large" deflections of table, slide and other components, compared to metal cutting machine tools. The main features of laser metrology were therefore redundant for checking the accuracy of this kind of machines and not yet suitable for checking their dynamic performance. 2. One step forward in laser metrology In the early nineties most of the laser manufacturers developed dynamic software, able to measure displacements, speeds and accelerations versus time; this was considered as an enhancement of the laser performance, but without probably thinking of some possible new fields of application, as the one mentioned above, which in effect have never been advertised so far. The main area in which laser metrology seemed to have reached new goals was the dynamic high frequency data capture, to be used for 18 of the 21 geometric and positioning deviations of three axes of movement. This will also be in effect a new area for international standards on test methods, thanks to the larger amount of data which can be obtained in a shorter time with respect to the traditional measurements. 3. Measurement of stopping times STANIMUC, in its quality of association for standardizing and testing machine tools, besides assuring secretariat and chairmanship to both Technical Committees ISO/TC 39 "Machine tools" and CEN/TC 143 "Safety of machine tools", has always been providing industry with a service of machine tool calibration, testing, noise measurements, safety consultancy, etc.. This not only allowed STANIMUC to become acquainted with a wide range of machine tool industry needs on different subjects, but often required also our engineers to find solutions to specific cases with the available equipment. A successful example of this approach has been the measurement of the stopping time of power presses and press brakes, where specific equipment is on the market, but laser interferometers proved to be an excellent instrument.
Laser Metrology and Machine Performance 127 The stopping time is used for calculating the safety distance of protective devices, such as light curtains or two-hand control devices. The safety distance at which they shall be placed is obtained multiplying the conventional speed of the hand/arm entering the danger zone by the stopping time of the slide, plus some uncertainty factors. The idea was very simple. If speed versus time can be measured, the time in which the slide speed decreases from its current value to zero is the quantity to be measured. We have been successfully performing those tests since the beginning of 1995, and industry showed to appreciate this simple and accurate method. Laser optics are easily mounted on the machine: the interferometer is fixed on the table, bending the beam 90 upwards, and the retro-reflector is fixed to the slide; the laser head is standing on its tripod in front of the machine. This simple setup of the equipment not only takes few minutes, but also allows to measure the pure relative speed between slide and table, neglecting all movements of the machine with respect to the floor. 3.1 Stopping time of hydraulic machines Figure 1 shows one graph from measurements made on a hydraulic press brake, where the initial speed proved to be fairly constant and very repeatable. The first horizontal part of the graph shows the delays in the control system, for approximately 55 milliseconds, before the stopping performance actually takes place. Then the speed decreases rapidly down to zero and it becomes even negative, when the elastic potential energy is released by the machine frame. The speed returns then back to zero with some vibration. Measurements can be easily repeated a number of times in order to assess the repeatability, for each adjustment of the hydraulic system and closing speed. For hydraulic machines the slide speed can be quite constant during a part of the closing stroke, so the stopping signal can be given in any point with repeatable results. The only arrangement required is to manufacture a simple interface between the machine and the laser interferometer in order to transmit the stopping signal to the machine and the start signal to the laser simultaneously. A sampling frequency of 5 to 10 khz proved to give a very good resolution (one reading every 0,2 or 0,1 milliseconds). 3.2 Stopping time of mechanical presses Methods for carrying out measurements are different from hydraulic presses or press brakes to mechanical eccentric presses, up to mechanical presses with link drive mechanism, which require a preliminary research of the point up the stroke where the maximum slide speed is reached. In this case, with the same setup described above, the actual speed during the complete cycle is measured, and an example of these
128 Laser Metrology and Machine Performance measurements is given in figure 2, in continuous running. When these data are available, the slide distance from the table is identified for exactly the same point in which the speed is maximum (as an average value of several measurements, if necessary, although the repeatability is quite good). The slide is then brought to that position by microinching, and a mechanical fixture holding a switch is placed on the table, adjusted at that height, and then taken off, if required. After completing the cycle up to the top dead centre the fixture is placed again on the table, or simply reset. When the slide comes down and touches the switch in the point of maximum closing speed, the stopping signal to the machine and the start signal to the laser are launched simultaneously. 4. Muting of protective devices Another laser measurement for safety purposes is the assessment of the precise point up the stroke where the "muting" of the protective devices takes place. The "muting" is the temporary automatic suspension of a safety function by safety related parts of the control system during the normal operation of a machine. Muting may be provided for protective devices such as light curtains or two-hand control devices. They shall only be muted when the dangerous phase of the closing stroke is passed and there is no risk of injury at the tools. The muting point is to be placed as high as to prevent the die and the material from being jammed by a possible late stopping signal, and also in order to anticipate the possibility of entering the danger zone, as soon as it is no longer dangerous, for unloading the workpiece, but it shall not be so high as to leave an unprotected part of the stroke. With the same laser setup described above, the muting switch shall also transmit the start signal to the laser, and the precise position of the slide in that point can be read, thus providing the distance from the bottom dead centre. 5. Table deflection on straight-sided power presses The table deflection under load has traditionally been measured as the variation of distance between the centre of the front side (or back side) of the table and the centre of a steel beam resting at both ends of the table side. On hydraulic machines the effort can be applied statically, keeping the pressure constant for the duration of the reading, and the measurement can be carried out even by means of a simple dial gauge, but on mechanical power presses this measurement can be made only dynamically, during an actual cycle.
Laser Metrology and Machine Performance 129 Figure 3 refers to the table deflection of a double action mechanical press in continuous running, clearly showing first the effect of the blank holder effort and then the one, shorter in time, applied by the punch during the drawing operation. This figure shows the rough graph, which of course can be smoothed, both by computer and mechanically, since the vibration belongs mostly to the steel beam. This measurement, when carried out in several points along the beam on both sides from the centre, can also check the symmetry of the applied effort, due to the punch and die design or to other reasons. 6. Total relative slide/table deflection and eccentric closing efforts During the normal working operations both slide and table are deflected under load, mostly if the tools are smaller than the slide/table surface. If the die is perfectly symmetric and everything works properly, the deflection of both slide and table should be symmetric. By means of the optics for angular measurements the total relative deflection between slide and table under load can be checked in several points along the table side, thus identifying possible eccentric efforts due to the die design or to an abnormal die closure, as already mentioned above. These measurements can be carried out on hydraulic power presses by bringing first slowly the punch in contact with the die without efforts other than gravity, zeroing the instrument, and then applying the hydraulic pressure at the required value. On mechanical presses measurements are to be performed dynamically, in order to check the deflections under the actual load. 7. Pitch and yaw of the slide movement Further measurements with the angular optics concern pitch and yaw angles of the slide during the closing stroke, before any effort is applied. This information could not be obtained by the traditional method, using dial gauge and square, where the reading included both straightness and angular deviations, and the measurement could not be easily made dynamically. On mechanical presses these angular measurements can supply information on the geometric accuracy of the whole system shaft/rods/slide when the connecting rods are more than one, whereas on hydraulic presses they can supply information on possible different performances of the cylinders, when more than one are used.
130 Laser Metrology and Machine Performance 8. Straightness of the slide movement and clearances Dynamic Straightness measurements can also be carried out, by means of the proper optics. As mentioned in the above clause, the traditional method using dial gauge and square could not separate Straightness deviations from angular deviations, whereas the laser interferometer can do that by means of two separate measurements with specific optics. Such Straightness measurements can give information on the clearances between slide and columns in the "front to back" and "left to right" directions, clearances which of course vary with temperature. 9. Column deflection Besides the "long term" thermal variation of the clearances mentioned above, another variation takes place during every cycle. In effect the table and slide deflections under load impose corresponding deflections in the columns, which under the closing effort reduce the clearances between themselves and the slide up to "squeeze" the slide horizontally. This is known as the "sand-glass effect" and can be easily measured dynamically as variation of distance between two columns. It can be measured even on the four sides, in order to check the symmetry of performance of the structure under load in the "front to back" and "left to right" directions. 10. Deflection of open front power presses As straight-sided power presses have in principle a symmetric structure and behaviour, open front power presses are asymmetric at least in the "front to back" direction and are therefore affected by both linear and angular distortions under load, which can be dynamically measured respectively using distance optics and angular optics. This second measurement is the more important because an angular distortion can affect more the performance of the die during a drawing operation. 11. Speed (and acceleration) monitoring on link drive mechanical presses The "link drive" mechanism (see figure 4) is installed on some mechanical presses in order to reduce speeds and mostly accelerations of the moving elements in the lower part of the stroke, where the drawing takes place, with the purpose of obtaining a smoother drawing of the metal sheet. As a result the closing stroke is slower and takes more than 180 of the shaft rotation, and the following opening stroke is faster (see figure 5).
Laser Metrology and Machine Performance 131 The reduction of speeds and accelerations is accurately sought by means of the special design of the link drive, but the actual values reached are of course not identical to those calculated and plotted by a computer, due to elasticity, clearances, inertia of masses. Their measurement is immediate using the laser interferometer, allowing the slide speed and acceleration to be plotted versus time or listed in tables of data captured with high sampling frequencies. Figure 6 shows a graph of the actual slide speed of a mechanical press equipped with link drive: this is one of a set of measurements made on the same machine at different numbers of strokes per minute. 12. Actual speed and energy in screw presses In screw presses the energy available at the die depends upon the closing speed of the slide. The screw rotation is caused by the contact between the flywheel controlling the closing stroke and the central wheel, and the eventual slide speed (axial speed of the screw) can be accurately proportioned through the length of time of this contact. The accurate measurement of the instantaneous speed and acceleration of the slide allows an accurate check of the available energy and at the same time the calibration of the control system, in order to apply the required energy with the necessary accuracy. 13. Advantage from the laser measurements All measurements described in this paper can be carried out both on machines equipped with dummy dies, for testing or research purposes, and on machines in use, equipped with the actual die during normal production, with a view to checking the effective performance of machines and tools during their working life. Advantage from these measurements can be taken by: - power press manufacturers, who can check the assembled machines and obtain data for improving the design; - die manufacturers, who can reach fine adjustments, and a better understanding and improvement of the dynamic performance of dies; - users, who can regularly check the stopping times (thus monitoring the wear of the brake), properly adjust the muting point, and carry out all the other measurements for diagnostic and maintenance purposes. These three categories of persons could daily make a profitable use of a laser interferometer with simple and rapid setups of the optics for a variety of measurements, thus adding another large industrial sector to the market of this instrument. Note: No reference section.
132 Laser Metrology and Machine Performance Figure 1 - Stopping time of a hydraulic press brake -400-600 1.20 2.40 6 7.20 8.40 9.60 10.8 Figure 2 - Slide speed of a mechanical press in continuous running
Laser Metrology and Machine Performance 133 150 3 4.50 6 7 JO 9 10 J 12 13.5 Figure 3 - Table deflection of a double action mechanical press Figure 4 - Example of link drive mechanism
134 Laser Metrology and Machine Performance Figure 5 - Calculated slide speed with a link drive mechanism -300- -450 0 1.50 3 4.50 6 7.50 9 1O5 12 13.5 Figure 6 - Measured slide speed with a link drive mechanism