RENOVATION OF CONVENTIONAL LATHE, MILLING AND DRILLING MACHINES BY USING ENCODERS AND NUMERICAL DISPLAYS

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Vincent York
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1 2 nd International Conference From Scientific Computing to Computational Enguineering 2 nd IC-SCCE Athens, 5-8 July, 2006 IC- SCCE RENOVATION OF CONVENTIONAL LATHE, MILLING AND DRILLING MACHINES BY USING ENCODERS AND NUMERICAL DISPLAYS Despoina Papathanasopoulou, Theodore Costopoulos, Evaggelos Hatzicharalampous, and Antonios Patarias Machine Elements Laboratory, School of Mechanical Engineering National Technical University of Athens Iroon Polytexneiou 9, Zografou, Athens,Greece cost@central.ntua.gr, web page: Keywords: Renovation, Lathe, Milling Machine, Drilling Machine, Encoder, Numerical Display Abstract. In the context of automation of conventional machines that fulfil high requirements for positioning accuracy and machining speed, sealed linear encoders have been installed onto a lathe, a milling and a drilling machine.linear encoders measure the position of linear axes and they operate on the principle of photoelectrical scanning. Two scale gratings with equal grating periods move relative to each other and the light projected by a source of parallel light is modulated. Photocells convert the variations in light intensity into sinusoidal output signals. Once the encoder is mounted onto the machine, it becomes a component of the display and shows the exact position of the slides using the incremental measuring method. The scales of the encoders are built in extruded aluminium and on the edge, on which the head is mounted, there are flexible sealing lips which protect the inner parts from possible damage. Sealed linear encoders were mounted near each working plane and away from sources of heat or strong vibrations. Two encoders have been installed onto the lathe, one on its body to control the longitudinal motion and the other one by the tool carrier for the lateral motion. The milling machine was also equipped with three linear encoders, on the side of the plateau for the numerical display of the position of x axis and on the right side of the knee for the display of y axis. The position of z axis is indicated by a linear encoder placed on the knee. Concerning the drilling machine, a linear encoder that measures the position of the head of the drill has been mounted on its body and another one, which is responsible for the motion of the propulsion axis, has been mounted on the head of the drill. In most cases the rule of the linear encoder was first screwed onto a base to ensure a solid mounting surface.the potential of conventional machines has grown due to the development of sealed linear encoders which provide the means to a faster, more reliable and accurate machining. 1. INTRODUCTION The need of production of components with special features demands specialized machinery. The variety in shape, size and precision has led to the subsistence of various conventional machines which differentiate in power, structure and modulation capabilities. In the laboratory of machine elements at the National Technical University of Athens, among other conventional machines, there are three of the most popular ones in a machine shop. These are the lathe, the milling machine and the drilling machine. All three of them work on the principle of material dropout. It is commonly known that lathe and the milling machine are used in cases of modulation of materials either by the rotation of the piece (lathe) or by the rotation of the cutting tool of the milling machine. The drilling machine can generate holes of various shapes and sizes on any kind of material by using the right tool. Maintaining actions followed to ensure proper operation of these old machines. Suitable bases were constructed to absorb vibrations. Sufficient quantities of oil and cutting liquids were added. In addition to all that, the machines were levelled to ensure precision. Last but not least the cleaning of the machine bodies was essential so that former abatements of materials and oils would not influence the operation or even the temperature of elements of machines. 2. ENCODERS Encoders have been manufactured to satisfy all kinds of measurement of the motion of different components. There are three kinds of encoders. Linear encoders (measurement of the position of an axis) Rotary encoders (measurement of the angle of rotation of an axis) Length gauges (measurement of level of a surface or datum)

These linear encoders were chosen so as to reduce the time which is needed to place the tool carrier into the right coordinates and calculations during machining. 2.

2 These linear encoders were chosen so as to reduce the time which is needed to place the tool carrier into the right coordinates and calculations during machining. 2.1 Sealed linear encoders Linear encoders measure the position of linear axes without additional mechanical transfer elements. This eliminates a number of potential error sources such as the positioning error due to the thermal behavior of the recirculating ball screw or the backlash. The sealed linear encoders have slim line scale housing and therefore are designed for limited installation space. When they are installed with the mounting spar they are also available in greater measuring lengths and permit higher acceleration loading. They are designed primarily for use on machines and installations that operate in harsh environments such as milling machines, drilling and boring machines, machining centers, lathes, grinding machines, electrical discharge machines, welding machines and bending presses. 2.2 Mechanical design The scale, scanning unit and guideway of sealed linear encoders are protected against chips, swarf, dirt and splashwater by aluminum housing and flexible sealing lips (protection IP 53 at EN 60529). The scanning carriage travels on a low-friction guide within the scale unit. It is connected with the external mounting block by a coupling that compensates unavoidable misalignment between the scale and the machine guideways. With introduction of compressed air to the scale or mounting block, protection IP 64 (no entry of dust and water sprayed from every direction) is achieved. Other characteristics of the specific encoders which were installed are: Accuracy grades : to ± 2 µm Max traversing speed : 180 m / min Max acceleration in the address of measurement : 100 m / s 2 Graduation carrier made of glass, steel. 2.3 Thermal behavior The thermal behavior of the linear encoder is an essential criterion for the working accuracy of the machine. As a general rule, the thermal behavior of the linear encoder should match that of the workpiece or measured object. During temperature changes, the linear encoder should expand or retract in a defined, reproducible manner. There are graduation carriers which have differing coefficients of thermal expansion. This makes it possible to select the linear encoder with thermal behavior best suited to the application. 2.4 Measuring principles Encoders with optical scanning incorporate measuring standards made of periodic structures known as graduations. These graduations are applied to a carrier substrate of glass or steel. The scale substrate for large measuring lengths is a steel tape. The precision graduations are manufactured in different photolithographic processes. Graduations are fabricated from: Extremely hard chrome lines on glass Matte-etched lines on gold-plated steel tape Three dimensional structures on glass or steel substrates

3 These processes permit very fine grating periods and are characterized by a high definition and homogeneity of the line edges. Together with the photoelectric scanning method, this high edge definition is a precondition for the high quality of the output signals. 2.5 Absolute measuring method With the absolute measuring method, the position value is available from the encoder immediately upon switch on and can be called at any time by the subsequent electronics. There is no need to move the axes to find the reference position. The absolute position information is read from the scale graduation, which is designed as a serial code structure or consists of several parallel graduation tracks with slightly different grating periods. A separate incremental track or the track with the finest grating period is interpolated for the position value and at the same time is used to generate an optional incremental signal. 2.6 Incremental measuring method With the incremental measuring method the graduation consists of regular rating structure. The position information is obtained by counting the individual increments (measuring steps) from some point of origin. Since an absolute reference is required to ascertain positions, the scales or scale tapes are provided with an additional track that bears a reference mark. The absolute position on the scale, established by the reference mark is gated with exactly one measuring step. The reference mark must therefore be scanned to establish an absolute reference or to find the last selected datum. In some cases this may necessitate machine movement over large lengths of the measuring range. To speed and simplify such reference runs, many encoders feature distance coded reference marks multiple reference marks that are individually spaces according to a mathematical algorithm. The subsequent electronics find the absolute reference after traversing two successive reference marksonly a few millimetres traverse. With distanced coded reference marks, the absolute reference is calculated by counting the signal periods between two reference marks using a specific formula. 2.7 Measuring accuracy The accuracy of linear measurement is mainly determined by: The quality of the graduation The quality of scanning The quality of the signal processing electronics The error from the scale guideway over the scanning unit A distinction is made between position error over relatively large paths of traverse for example the entire measuring range and that within one signal period. Position error over the measuring length: The extreme values of the total error F of a position lie with reference to their mean value over any max. One meter section of the measuring length within the accuracy grade (+a, -a). This tolerance band is also shown in the calibration chart and represents the position error within one signal period. With sealed linear encoders, this value applies to the complete encoder system including the scanning unit. It is then referred to as the system accuracy. Position error within one signal period: The position error within one signal period is determined by the quality scanning and the signal period of the encoder. At any position over the entire measuring length, it does not exceed approx. 2% of the signal period. The smaller the signal period, the smaller the position error within one signal period. 2.8 Photoelectric scanning The encoders operate on the principle of photoelectric scanning. The photoelectric scanning of a measuring standard is contact-free and therefore without wear. It detects even the finest graduation lines just a few micrometers wide, and generates output signals with very small signal periods. The finer the grating period of a measuring system is, the grater the effect of diffraction on photoelectric scanning To put it simply, the imaging scanning principle functions by means of projected light signal generation: two scale gratings with equal grating periods are moved relative to each other the scale and the scale reticle. The carrier material of the scanning reticle is transparent, where as the graduation on the measuring standard may be applied to a transparent or reflective surface. When parallel light passes through a grating, light and dark surfaces are projected at a certain distance. An index grating with the same grating period is located here. When the two gratings move relative to each other, the incident light is modulated. If the gaps are aligned light passes through. If the lines of a

4 grating coincide with the gaps of the other, no light passes through. Photocells convert these variations in light intensity into nearly sinusoidal electrical signals. The specially structured grating of the scanning reticle filters the light current to generate nearly sinusoidal output signals. The smaller the grating period of the grating structure is the closer and more tightly toleranced the gap must be between the scanning reticle and scale. Practical mounting tolerances for encoders with the imaging scanning principle are achieved with grating periods of 10µm and larger. 2.9 Mounting on machines Sealed linear encoders are fastened to a machined surface either directly or over a mounting spar. They are mounted so that the sealing lips are directed downwards or away from splashing water. Because they are rigidly fastened, the linear encoders largely adapt themselves to the thermal behavior of the mounting surface. When fastened on the mounting spar, the encoder is fixed to the midpoint of the mounting surface. The flexible fastening elements ensure reproducible thermal behavior.it is quite simple to mount the sealed linear encoders on to the machines. The shipping brace already sets the proper gap between the scale unit and the scanning unit. It is only needed to align the scale unit at several points to the machine guideway. Stop surfaces or stop pins also can be used to align the scale. The use of a mounting spar can be of great benefit when mounting slim line linear encoders. A mounting spar can be fastened as part of the machine assembly process, so that the encoder can be easily clamped later as a final step. Easy exchange also facilitates servicing. To simplify cable routing, the mounting block of the scanning unit is usually screwed onto a stationary machine part. The mounting location for the linear encoders should be carefully considered in order to ensure both optimum accuracy and the longest possible service life. The encoder should be mounted as closely as possible to the working plane. To function properly, linear encoders must not be continuously subjected to strong vibration. The more solid parts of the machine tool provide the best mounting surface in this respect. Encoders should not be mounted on hollow parts or with adapters. A mounting spar is recommended for the sealed linear encoders with small cross section. To avoid temperature influences, install the encoder away from sources of heat. 3. THREE CASE STUDIES Before installation: Cut of aluminium mounting spars with suitable dimensions Generation of holes by the drill Modulation of threads Surfaces were dressed to avoid causing any defaults to the equipment or causing accident to the technical staff. The distance that the head should be able to measure must be shorter than the length of the rule. 3.1 Lathe Two sealed linear encoders of length 385 mm and 900 mm where chosen to measure the motion of x axis and z axis. Their distance of measurement is 200 mm and 596 mm correspondingly. The encoder (x axis lateral motion) was installed next to the tool carrier. A stock was used to transfer the motion to the head of the encoder. A second encoder has been placed on the body of the lathe to control the longitudinal motion (moving upon z axis). In both cases the head of the encoder is movable and the rule is fixed. The installation of the encoders onto the lathe increased the precision of machining from 0.02mm to 0.005mm. Special care was given to the alignment between the rules and the motion of the tool carrier to ensure the prevention of defaults during measurement. The receptible force of astriction onto the mounting spars was also determined on the purpose of regular operation. In both cases the head of the encoder is movable and the rule is fixed. The installation of the encoders onto the lathe increased the precision of machining from 0.02mm to 0.005mm. Special care was given to the alignment between the rules and the motion of the tool carrier to ensure the prevention of defaults during measurement. The receptible force of astriction onto the mounting spars was also determined on the purpose of regular operation.

3.2 Milling machine Three sealed linear encoders where mounted onto the milling machine. Their length is 1185 mm, 385mm and 585mm for the x, y and z axis correspondingly.

5 3.2 Milling machine Three sealed linear encoders where mounted onto the milling machine. Their length is 1185 mm, 385mm and 585mm for the x, y and z axis correspondingly. Maximum displacement is 955mm, 234mm and 485mm for each case. The encoder of the numerical display of x axis was installed onto the side of the plateau of the milling machine (movable rule fixed head) and the encoder of the y axis was mounted onto the right side of the knee (fixed rule movable head mounted on the lowest surface of the lateral carrier). The position of z axis is indicated by a linear encoder placed on the knee (movable rule fixed head on the body). The installation of the encoders onto the milling machine increased the precision of machining from 50µm to 5µm (x and y axis) or from 20µm to 5µm (z axis). 3.3 Drilling machine Axis x was set to be the axis of propulsion and y the axis of the perpendicular movement of the drill s head. Two linear encoders were used, one for each axis (movable head fixed rule). Mounting spars were essential only for the encoder responsible for the propulsion axis ( Figure 12). Relative movement was achieved with the use of a plastic block (elaborated on CNC) which connects the rule s head with the moving part of the drill. 4. DISPLAYS The screens were screwed upon metal bases and cables were fixed so as not to prevent movement of machine levers and the heads of the encoders. Further possibilities offered by such display units: 1) Tool compensation 2) Error messages 3) Saving co-ordinate systems in case of power loss 4) Scale factor which offers possibility for manufacturing similar pieces of different sizes 5) Compensation of systematic fault REFERENCES [1] D.Johnson, Why cutting tools fail, CNC Machining, τεύχος 18, 2001 [2] TOS KURIM FA3A, milling machine operatng handbook [3] ELBO Series EV, Operating Instructions [4] Heidenhain Catalog for Sealed Linear Encoders [5] Heidenhain Catalog for Length Gauges [6] Heidenhain Catalog for Rotary Encoders [7] Heidenhain-Sealed Linear Encoders March 2003 [8] Heidenhain Montageanleitung LS 303/LS 303 C Figure 1 Mechanical Design

6 Figure 1. Sealed linear encoder Figure 5. Encoder ( x axis) Figure 6. Encoder (z axis) Figure 3.Incremental signals Figure 4. The principle of photoelectric scanning Figure 7. Detail of mounting spars

7 Figure 8. Encoder (x axis) Figure 12 Figure 9. Alignment Figure 13 Figure 10. Encoder (y axis) Figure 14 Figure 11

Exposed Linear Encoders

Exposed Linear Encoders May 2011 Exposed Linear Encoders Linear encoders measure the position of linear axes without additional mechanical transfer elements. This eliminates a number of potential error