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Pergamon Int. J. Mech. Tools Manufect. VoL 37. No. 3, pp. 277-286. 1997 1997 Elsevier Science Lid Pnnted in Great Britain. All ril0ts reserved 0890-6955/97517.00 + .00 PH: S00-6955(96)00066-1 DESIGN OF AN AUTOMATIC TOOL CHANGER WITH DISC MAGAZINE FOR A CNC HORIZONTAL MACHINING CENTER MUSTAFA ILHAN GOKLERt and MURAT BJLGJN KOt (Received 12 April 1996) Abstract-Automatic tool changers (ATCs) are devices used in CNC machine tools to exchange the tool in the spindle with the tool in the magazine. In this paper, the design of the ATC of a CNC horizontal machining center which was realized for a CNC machine tool manufacturer is introduced. After examination of several alternatives, it was decided to implement the disc-type ATC. A magazine was designed with 24 tools with a maximum tool diameter of 150 mm and a maximum tool weight of 8 kgf. The designed ATC can change the closest tool within 4 s and the tool farthest away within 6 s. 1997 Elvier Science Ltd. All fights reserved 1. INTRODUCTION Any machine tool manufacturer must either pay for the patent of any particular CNC machine tool, or design its own system to manufacture CNC machine tools. Buying a new technology is an expensive alternative, and it is not generally allowed to make any modifications and developments on the purchased system. The machine tool manufacturers refrain from selling new technologies developed by themselves, and they are generally willing to sell their abandoned technologies. Therefore, a particular CNC machine tool manufacturer has started research and devel- opment studies for the design and production of a new CNC horizontal machining center. It was required to design a new system of ATC for this particular machine tool. An ATC can be defined as a device which can select and change tools from a tool storage magazine according to the commands given by the machine control unit. Various types of ATC with quite different working principles have been used in horizon- tal machining centers produced by the different machine tool manufacturers 1-17. A tool storage magazine is generally either a chain type or a disc (carousel) type. However, nowadays, there are some other types of magazines such as spherical, cassette, etc. 17. Disc-type magazines may be categorized according to the functions of the discs. (1) In a disc with a changer arm-type magazine, the tools are changed by means of an arm which takes the tool from the magazine and exchanges it with the one in the spindle. Therefore, the disc has the functions of indexing and storing the tools only 1-5. Some examples of this type are shown in Fig. l(a) 1, Fig. l(b) 1 and Fig. l(c) 5. (2) A disc without a changer arm-type magazine acts like a tool changer arm. The tools are changed by the disc itself 2, 6-8. An example of this type is shown in Fig. 2 1 . As shown in the figure, two disc magazines and two spindles mounted on a turret exist in this application. In an ATC with chain-type magazines 9-16, the tool is changed by a changer arm. Chain-type magazines may be categorized according to the orientation of the tools in the magazine with respect to the spindle axis as follows. (1) The axes of the tools on the magazine are perpendicular to the spindle axis and the magazine is on either the right- or left-hand side of the column. Therefore, in order tDepattrnent of Mechanical Engineering, Middle East Technical University, 06531 Ankara, Turkey SAuthor to whom correspondence should be addressed. 277 278 Mustafa llhan GOkler and Murat Bilgin Koq (a) _ . , i J ih- - - 1 I I I ,J I I _ (c) - J I I II S ! I f I r / / Fig. 1. Some examples of di-type ATC with changer arm 1, 5. Fig. 2. An example of disc-type ATC without changer arm I. to change the direction of the tools, an arm takes the tool and makes a rotation about a vertical axis to bring the new tool to the position parallel to the spindle axis as shown in Fig. 3(a) 13. In another version of this type, the tool pots, where the tools are located, are rotated about their own axis to become parallel to the spindle axis, as shown in Fig. 3(b) 12. The changer arm exchanges the tools between the tool pot and the spindle axis. (2) The axes of tools on the chain magazine are in a plane parallel to the spindle axis. As shown in Fig. 3(c) 12, there is only one position of the tool at which the cutting Design of an Automatic Tool Changer with Disc Magazine for a CNC Horizontal Machining Center 279 (a) (b) II- T (c) (d) Fig. 3. Some examples of chain-type ATC 12, 13, 16. tool becomes parallel to the spindle axis, and the tool is changed at that position. There is an arm which exchanges the tools between the spindle and the magazine. (3) The axes of tools on the magazine are parallel to the spindle axis. The arm which is in between the spindle and the magazine takes the tool and does a linear motion to approach the spindle and exchange the tools as shown in Fig. 3(d) 16. In Table l, different ATCs and their minimum tool to tool change times (T/T) are given based on the trade literature 2-5, 7-14. In Table 1, D+A, D and C+A desig- nate disc-type magazine with changer arm, disc-type magazine without changer arm, and chain-type magazine with changer arm, respectively. In disc magazines, as the number of tools increases, the diameter of the disc increases, which is an undesirable situation from the space point of view. However, in a chain type, the number of tools is approximately 1.5 times that of the tools in the disc type in the same space 18. The ATC of the chain type can store many tools in a very small space. However, the chain-type magazines are expensive compared with the disc-type magazines. 2. DESIGN OF ATC In the stage of clarifying the design specifications for a CNC machine tool, supply and demand in the particular market, potential customers preferences, specifications of competitors products which can be obtained from the related trade literature, price of CNC machine tools, etc. should be considered. In accordance with the desired features of the CNC machine tool, the design specifications of ATC are determined. By considering the basic design criteria as simplicity, relatively low cost, relative ease of manufacture, assembly, disassembly, maintenance, and availability of spare parts, a disc magazine without changer arm-type ATC was considered to be designed for the particular CNC horizontal machining center 19. 280 Mustafa llhan GOkler and Murat Bilgin Koq Table I. Tool mass and tool change time data for automatic tool changers of various machine tools available in the market Machine tool or ATC Tool mass T/T Type Ref. (kg) (s) Sidepalsa AHL-20/30 6 2.8 D+A 2 NTC/TNSM-2H 3 3 D+A 3 J Sidepalsa AH-30 TAMBOR 15 7 D+A 2 Sidepalsa AH-20 12 7 D+A 2 Olivetti/Horizon 40 8 D+A 4 Mandelli Quasar 25 10 D+A 5 White-Sundstrand Series 20 13.5 5 D 7 White-Sundstrand Series 30 18.1 7 D 17 Sidepalsa AH- 18 6 7 D 2 Leadwell HA400 8 8 D 8 MAZAK/800-H 20 2.5 C+A (91 Burkhard+Weber/MC60 30 3.5 C+A I 0 Matsuura MC-400 H 5 3.5 C+A 11 Mauser BZ 24 15 4 C+A 12 Mauser P5500T/630T 25 5 C+A 112 NTC TMC-63H 20 5 C+A 3 White-Sundstrand/OM 1 19 5 C+A 71 Matsuura/MC-600H 10 6 C+A I 1 Sidepalsa AH-60 18 8 C+A J 12 Matsuura/MC-900H 25 8.5 C+A I 1 l Hitachi Seiki/HC 500 20 10 C+A 13 Mechanicy/H-800 15 10 C+A 14 Sidepalsa AH-52 15 15 C+A 2 As shown in Fig. 4, the tool-changing cycle of the designed ATC includes the following basic steps. Step 1. The spindle approaches the magazine to insert the old tool into the tool pocket (Fig. 4(a). (a) / Fig. 4. Tool-changing cycle of designed ATC 19. Design of an Automatic Tool Changer with Disc Magazine for a CNC Horizontal Machining Center 281 Step 2. After the drawbar releases the old tool, the magazine goes forward along the A-A direction (Fig. 4(b). Step 3. To bring the new tool into the tool changing position as soon as possible, the magazine is rotated in a clockwise or counterclockwise direction about its axis, depending on the relative position of the tool on the magazine (Fig. 4(c). Step 4. The magazine is retracted back to insert the new tool into the spindle (Fig. 4(c). Step 5. When the control unit takes the message from the spindle that the drawbar has been operated to draw the new tool safely, the spindle is let go to start the new machining cycle (Fig. 4(d). After the decision concerning the type of ATC and the working principle to be used, the specifications for the disc magazine without changer arm-type ATC were determined as follows 20. 1. The number of tools stored (i.e. number of tool stations) in the magazine is 24. 2. The maximum tool diameter is 100 mm when the adjacent tool pockets are full. 3. The maximum tool diameter is 150 mm when the adjacent tool pockets are empty. 4. The maximum weight of the tool including that of the tool holder is 8 kgf. 5. The tool holder type is BT40 21. 6. Tool changing times are 4 s and 6 s for the closest and farthest tool on the maga- zine, respectively. In the following subsections, basic components of the particular ATC are presented. 2.1. Disc magazine The minimum diameter of the disc-type magazine is calculated by considering the speci- fied number of tools to be stored and the maximum tool diameter. A distance between successive tools to be stored in the magazine should be left. The diameter of the disc magazine was calculated as 812 mm by leaving 6 mm between successive tools 19. After the diameter of the disc magazine has been calculated, the shape of the disc magazine is determined. Low inertia has primary importance because of the frequency of acceleration and deceleration of the disc magazine. This can be obtained by using a light material. Aluminium is selected as the material for the disc magazine. A thin walled disc geometry with appropriate webbing which ensures necessary stiffness should be considered. The structure of the disc magazine was made as simple as possible to reduce the cost of the pattern to be used for casting the magazine. A simple sketch is shown in Fig. 5 19. 2.2. Sliding body In any of the ATC systems, there have to be extraction and retraction motions to take the tool holder out of the spindle or to insert a new one. In the disc without changer arm-type ATC, the magazine should make a back and forth motion. This necessitates a sliding body. These motions can easily be achieved by either a hydraulic or a pneumatic piston. In fact, there are some other methods of having a linear motion such as using a / .-= Fig. 5. Disc magazine of designed ATC 19. 282 Mustafa llhan Grkler and Murat Bilgin Ko ball screw to convert the rotary motion into linear motion. However, this method is extremely expensive when only a simple repetitive motion cycle is required. As seen in Fig. 6, the sliding body was designed hollow to reduce the weight 19. At the rear, there is a space for the encoder and the coupling inside the body. A pneumatic piston was selected and implemented to push and pull the sliding body. The very back of the body was closed with a lid to which the pneumatic piston was assembled. The slides of the sliding body were designed as square edge but not of dovetail type. The reason for using the square edge type of slide is the ease of manufacturing, price and limited space advantages. 2.3. Shock absorbers For each tool-change cycle, the magazine performs forward-backward motion by the sliding body, which takes up most of the cycle time. To reduce this time. higher traverse speeds are preferred. However, a problem arises concerning the deceleration, where an extremely high level of kinetic energy must be absorbed over a very short distance to stop the system without causing excessive shock loads or vibration. Shock absorbers can be used for this purpose. In the design, a pneumatic piston was used. Owing to the compressibility of the air, pneumatic pistons have sharply rising force characteristics towards the end of the stroke during the cushioning operation. Most of the energy is absorbed near the end of the stroke 22. Therefore, shock absorbers are required. The pressure generated in front of the parti- cular shock absorber piston remains constant throughout the entire stroke, as the velocity is reduced to zero owing to the specific spacing of the orifices. Consequently, the resisting force remains constant and uniform, and therefore linear deceleration is achieved 22. 2.4. Clamping of the tools The tools on the disc magazine are kept in the pockets with clamps. The tool may be held between clamping jaws. The clamp is designed in such a way that a tool with a maximum weight can be safely held. Since the disc magazine rotates about its axis, the centrifugal force adds up to the weight of the tool. This is considered during the design of the clamping system. The tool pocket and clamping jaws are designed according to the tool-holder type specified. Clamping jaws should have appropriate mating surfaces which fit into the specified tool holder. The tools are kept in the pockets by a combination of leaf and helical springs which have been designed to hold a tool with a maximum weight of 8 kgf safely 19. The jaws have tapered surfaces which fit the tool holder of the BT40 21. S l i d i n g L i d Fig. 6. Some components of designed ATC 19. Design of an Automatic Tool Changer with Disc Magazine for a CNC Horizontal Machining Center 283 2.5. A shield to protect the tool holders from chips In design of an ATC, some protective measures are necessary. If any chip sticks on to the tapered surface of the tool holder before it is inserted into the spindle shaft, it will not be accurately seated in the spindle shaft and will cause run-out, inaccuracy and repeat- ability problems, and the chip may damage the spindle taper. To prevent these, the tool holders should be protected from the chips by a shield which opens just before the spindle reaches the tool change position (Fig. 7) 19. The shield is kept closed during the machin- ing processes. 2.6. Motor and speed reduction The rotation of the disc can be provided by using a motor and speed reducer combi- nation. After the decision has been made for the type of motor, the power and torque requirements are calculated to select an appropriate motor. By considering the positional accuracy and the repeatability, an AC brushless servo motor was used for the rotation of the disc magazine 19. The motor was selected which can give 4.5 Nm and 0.94 kW rated torque and rated power values, respectively, for At=60 K and 5.5 Nm and 1.2 kW for At=-100 K. The motor has a rotational speed of 2000 rpm 23. 2.7. Speed reduction Speed reduction between the magazine and the motor may be provided through worm gear, planetary gear train, harmonic drive, etc. A backlash-free reduction is necessary for accurate positioning of the tools relative to the spindle bore center while changing the tools. A timing belt is another method which can ensure a zero backlash power trans- mission. However, high reduction is very difficult to obtain with timing belts in a very small area. Very high reduction ratios can be obtained with worm-gear sets in a very small area. However, worm gears have backlash problems. Moreover, for high reductions, the efficiency of the system decreases abruptly. If the reduction is done with a very low efficiency worm gear, the required capacity of the motor size gets bigger, which in turn increases the cost of the motor and its control system. In zero backlash worm gears, the worms are of split type and their tooth profiles are special 24. In planetary-gear systems of low prices, the backlash elimination is obtained by just decreasing the center distance between the gears until a minimum clearance condition is reached without too much noise or heating up. This normally causes high starting torque because of interference, and much lower running efficiency than the same with a normal backlash condition. There are only a few companies in the world which manufacture zero backlash planetary gear trains 25. The harmonic drive is another way of having a zero backlash reduction. The advantages of harmonic drive can be stated as follows 26. (!) The positional accuracy is quite good. The harmonic drive gearings design ensures that approximately 10% of the total teeth are engaged at any point in time, which minimizes the effect of tooth-to-tooth error. (2) Because of the simple structure of the harmonic drive, it is very easy to incorporate into a housing which is easy to manufacture, since the machining required is simple in-line boring and turning operations. (3) Since the harmonic drive has a very compact structure and very light weight, the assembly and disassembly are very easy. (4) It is cheaper than any other zero backlash gearing. Because of the above advantages and after the necessary calculations, the harmonic drive was selected and implemented in the ATC 26. Assuming that half rotation of the disc magazine, including the time elapsed for acceleration and deceleration of it, is com- pleted in 2 s, the rotational speed of the magazine is calculated as 16.67 rpm. This requires a 120:1 reduction to reduce the 2000 rpm motor output to 16.67 rpm magazine rotation. The harmonic drive was selected to obtain 50:1 reduction together with a spur gear pair 284 Mustafa Ilhan Gtikler and Murat Bilgin Koq providing 2.4:1 reduction. The output shaft of the harmonic drive is connected to a pinion with the pitch circle diameter of 125 mm which meshes to a gear with a diameter of 300 ram. The gear is bolted on to the disc magazine 19. 2.8. Encoder and control switches The position of the magazine is very important to insert the right tool into the spindle. This can be provided by using an absolute or incremental encoder. An encoder can also be used for counting the tool stations, which is necessary to index the required tool. Enco- ders are very sensitive devices which are capable of giving very precise output. If the cables of the encoder are affected by some means from the environment (i.e. interference), the message that is sent to the control unit will be different from its real value. Therefore, another method of counting the tools is recommended to support the encoder as a safety precaution. A switch to count the tools can be used for this purpose. The signals sent by the switch and the encoder are compared to prevent any mistake while indexing the tools. Absolute encoders are expensive when compared to the incremental encoders. As shown in Fig. 6, the positioning of the magazine was achieved by using an incremental encoder directly assembled on the axis of the disc magazine and not on the motor as is normal with servo drive. The angular positioning backlash problem was overcome by placing an incremental encoder directly connected to the disc magazine and situated within the sliding body. The incremental encoder was selected with 3600 lines per revolution. A proximity switch was placed on the sliding body near the disc magazine as a safety precaution. The switch is actuated by 24 dogs mounted on the disc magazine. So, for each tool, there is one dog which actuates the switch, thus enabling both counting of the tool stations and providing a signal to the control unit. Several switches are also required for control purposes. Since the ATC has a rotation axis, the control unit should be capable of recognizing the reference point of the rotation. Otherwise, the control unit does not know the positions of the reference tool and the other tools. Since an incremental encoder was used, a proximity switch was placed on the sliding body to set the home position. If an absolute encoder were used, there would be no need for a home switch since the absolute encoders know at which absolute position they are. A drawbar is a drawing unit, which is used in spindles to pull the tool holder. The ATC and drawbar should work sequentially, otherwise serious problems may occur. To synchronize the operation of the drawbar, the sliding body and rotation of the magazine, two limit switches were used for the front and rear positions of the sliding body to give reliability while changing tools. Another proximity switch was placed within the spindle which senses whether the drawbar unit is in its clamp position or not. If the switch is not actuated, it means that either the tool holder is not clamped or it is improperly clamped. The tool pocket which will pick the tool in the spindle must be unoccupied before the approach of the spindle, otherwise a crash occurs, which causes damage to the tools, spindle and magazine. To prevent such damage, a proximity switch was used to check whether the particular tool pocket was occupied (Fig. 7) 19. 3. IMPLEMENTATION AND TEST OF AUTOMATIC TOOL CHANGER The particular CNC horizontal machining center was manufactured and assembled. Each system was tested and run separately or as a whole of the machine tool by the CNC machine tool manufacturer. At the end of the tests, it was reported that everything within the ATC was running without any problems. The designed ATC was tested for which the T/T for the closest tool is 4 s, whereas it is 6 s when the farthest tool is of concem. The particular machine tool was used in the production line in the particular CNC machine tool manufacturer without any reported problems relating to the designed ATC system. 4. CONCLUSION In this paper, the working principle and the design criteria of an ATC with a disc magazine without a changer arm has been presented. This type of ATC has been found Design of an Automatic Tool Changer with Disc Magazine for a CNC Horizontal Machining Center 285 / Proximitt Switch 1 Fig. 7. Shield protecting tool holders and proximity switch checking tool pocket 19. tO be suitable for the particular CNC machine tool manufacturer when considering the design period, the compulsion being to enter the market as quickly as possible. The mini- mum T/T for the designed ATC, which is 4 s, is quite reasonable when compared with those given in Table 1. Therefore, the realized ATC system has provided a competitive position for the machining center in the market. Acknowledgements-The authors wish to express their appreciation to TAKSAN Machine Tool Industry and Trade Co., Kayseri, Turkey, for support in the design and manufacturing of the particular ATC. The authors also thank M
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