机械设计外文翻译-为卧式加工中心设计一种盘式自动换刀刀库【中文3663字】【PDF+中文WORD】

机械设计外文翻译-为卧式加工中心设计一种盘式自动换刀刀库【中文3663字】【PDF+中文WORD】,中文3663字,PDF+中文WORD,机械设计,外文,翻译,卧式,加工,中心,设计,一种,自动,刀刀,中文,3663,PDF,WORD 【中文3663字】 为卧式加工中心设计一种盘式自动换刀刀库 MUSTAFA ILHAN GOKLERt~ and MURAT BJLGJN KO~t (Received 12 April 1996) 摘要：自动换刀－是被用在数控加工机床的刀具更换中，就是用刀库的刀具更换主轴上的刀具。在这篇文章中介绍这种立式加工中心自动换刀的设计曾被认为是一种机床道具的制造者。在几经检验选择之后就决定实施这种磁盘式自动换刀装置的设计，这种刀库拥有24把刀，其中刀具最大直径可达150mm，刀具最大重量可达8 kg , 这种自动换刀的设计更换最近的刀具时间放在4s之内，在最远处的刀具的更换在6 s之内。 1.概述 每一个机床刀具生产者或者都必须专门的任何特殊的数控机床，刀具或者设计自己的刀具特殊加工制造数控机床刀具，选择新的技术是昂贵的，并且它并不总是适合修理和购买精密的系统。机床刀具制造者为避免由他们自己卖出新的计数进步，他们通常会去卖那些已经滞后的系统。 因此，一种特殊的数控机床刀具更新被寻找并且继续研究并设计和生产新的立式数控加工中心，就要求为这种新的自动换刀系统。 这种自动换刀装置可以设计成依据由机床控制单元的命令自动从刀库中选择并更换刀具，由不同的刀具加工者生产中，已经应用【1－17】。通常认为刀库或串连的或磁盘式的。尽管如此，今天有些别的刀库类型如球型，磁盘式等等。【17】 磁盘式的刀库可以分成以下几类。 （1） 在一种磁盘带更换手臂的刀库中，刀具通过手臂的方式更换，就是从刀库中抓起刀具更换主轴上的刀具。然后，磁盘有检索存储道具的功能。【1－5】。一些这些功能的例子，如途中所示。 （2） 一个没有更换道具手臂额磁盘刀库，这种类型的刀库实际上象一个刀具更换手臂。刀具由磁盘自己更换。【2，6－8】，这种例子如图中所示。 Fig.1. 几个有机械手自动换刀的例子 Fig.2.无机械手自动换刀的例子 在串连型自动换刀刀库中【9－16】，刀具机械手更换，依据刀具在刀库中定位的状态和位置，选择分类串连型刀库。 (1) 刀库中刀具坐标轴垂直于主轴的坐标轴，刀库在支柱左边或右边； 为了改变刀具的方向，一个机械手抓起刀具然后绕坐标旋转，并抓起新刀到正确的平行于坐标轴，如图所示，这种换手臂刀库中刀具与主轴上的刀具。 (2) 在串连刀库中刀具的坐标式平行的主轴坐标的平面内，如图所示，在刀具切削中刀具仅仅只有一个位置，为立式加工中心设计带磁盘式自动换刀装置，因为刀具坐标平行于主轴坐标，机械手臂在刀库与主轴之间换刀，并且，刀具在这个位置换刀。 (3) 刀库中刀具的坐标平行于主轴坐标，机械手臂在刀库与主轴之间换刀，并且，靠近主轴作线性移动，然后换刀如表【16】所示。 在表1中，不同的自动换刀装置和最短时间换刀时间（T/T）这是在交换基础上给出的最短换刀时间，在表1中，D+A,D,C+A指派磁盘式带机械手换刀装置的刀库，没有机械手换刀装置的磁盘式刀库，带机械手换刀装置的串连型刀库。 在磁盘式刀库中，随着数量的增加，磁盘式的半径增大，这样不符合节省空间的观点，尽管如此，在串连类型中，刀具的数目大约式同样空间盘式刀具的1.5倍，在串连自动换刀装置中能够存储更多的刀具，但是串连型刀库是比较贵的行对于盘式刀具俩说。 2 自动换刀装置的设计 在这一节中阐明数控加工中心刀具的具体设计，供应市场的要求此中意向的特殊的竞争产品，那样可以从相关的试验中，获得数控机床刀具的价格应该在考虑之内，根据cnc数控机床刀具的前景，设计特殊的数控机床的自动换刀装置。 把设计的标准降低看作为单元，相当低价位的，易于生产加工的一个没有机械手臂的盘式刀库型的自动换刀装置特殊的立式加工中心设计品。 表1 步骤1 去掉的刀具，刀库从用 A-A 方向看 2 转动刀具到正确的换刀位置，刀库沿着坐标顺时针旋转或者逆时针旋转。 3 刀库退回，而新刀安装到主轴上。 4．当控制单元收到主轴转刀具安装后，主轴开始转的加工周期，在自动换刀装置讨论会后，主要的工作是应用这种装置。 如下所示 1， 刀库中刀具数为24把 2， 刀具最大直径为100mm 3， 当刀库中没有刀时，可以容纳刀具的最大直径为150mm 4， 可以举起刀柄的重量最大为8kg 5， 刀库的类型为BT40 6， 刀具更换时间在4秒－6秒之间。 在如下的分支中，列举处特殊自动换刀装置的基本系数， 盘式刀库最小直径是由考虑存储具体刀具数量和最大直径，在刀库中存储的应与换下的距离，盘式刀库的直径是812mm每个刀具之间为6 mm。确定刀库直径后，刀库的盘式形状也确定了，因为刀库经常加速或减速，所以刀库的惯性为重要问题，这个问题可以用一种轻型材料解决，铝就是这种材料。薄壁的盘库适当的卷筒或几何形状的，这种形状能够保证必要的弯曲，而这些是应该考虑的，应尽可能降低这种结构的降低成本，如简图所示 导轨。 在自动换刀装置系统中，都必须有抽去主轴上的刀的转动，在无机械手换刀装置中，盘式刀库有前后运动，这就有必要的导轨，这种运动由输送形式气压活塞完成，实际上，有几种别的线性运动方法，如用球滚动代替旋转运动，然后，当有圆周运动时。这是非常昂贵的方式，如表6所示，导轨设计成空心形式以减轻重量，导轨由气压活塞推动，活塞或装在导轨的最后面，导轨边上设计了方形的边缘，并不是u型的，用方形的边是因为容易制造，价格极限空间都有优势。 2.3减震器 每一个刀具更换周期，刀库都由导轨上前后移动，这占用的周期的大部时间，为了减少运整时间，优先用了移动速度，但是坐标问题随之产生，当动量较大时必须有相适应的短距离停下来的系统，而这种系统又不会起过渡的振动，减震器就可解决此问题。 在设计中，用一个气缸，和用压缩空气活塞缸开减增大压力减震，减震器是必要的，在特殊在减震器前后压力通过整个容器，随着速度的降低为0，这样持续的压力仍旧比较大，因此线性就达到了。 2.4 刀具夹紧装置 刀库中的刀具用夹具夹在刀库中，刀具可以夹持在钳口之间。以这种方式设计的夹具，最大重量的刀具可以安全举起。盘式刀库绕其轴线旋转，离心力增加刀具的重量。这些因素在夹具的设计中考虑到。刀库和钳口依据特定的刀柄类型而定。钳口应适合特殊的刀柄。 刀库中的刀具由一个螺旋状的弹簧和铁片组合而夹在刀库中的这种装置设计的可以抓举最大重量为8公斤的刀具。钳口的形状符合BT40刀柄的外形。 2.5 防屑保护刀柄的防护罩 设计ATC时，一些保护措施是有必要的。如果任何铁屑进入主轴前打击到刀柄的外表面，那么将不会准确停下主轴并且引起错位，不精确等相应的问题。铁屑可能损伤主轴，为了防止这样，应该防止铁屑进入主轴，而用一个防护罩。防护罩应与主轴一起移动。 2.6 电机的速度控制 用一电机和速度结合体控制刀盘的旋转。选用电机类型根据动力和价格的要求选择合适的电机。 考虑位置的精确性和重复性 ，选一种AC无刷式电机带动盘式刀库旋转，选用一种可以提供4.5N/n的力矩和0.94KW动力，性价比好的 2.7 降速 在刀库和电机之间降速，这样可以通过蜗轮蜗杆实现的。一种无级变速对于刀具相对主轴中心轴精确位置更换刀具来说是必要的。 然而，在很小的面积内获得及时的降速是非常困难的，但用蜗轮就可以容易实现。但蜗轮也有问题。而对于降速来说，系统的效率明显下降。如果用效率低的蜗轮实现降速，则电机的尺寸就相应增加，进而增加成本和控制系统。 低价格的行星轮系统，通过减小齿轮中心轮到最小间隙，控制可以消除反冲力，这样没有太大的冲击和噪音。这些自然引起的启动力矩，因为相互干涉，较低的运动效率比那些同样有反冲的情况。在世界上只有少数公司生产反冲的行星轮传动。 谐波驱动是另一种无反冲的方法。这种方法的优点如下述： （1） 位置精确度非常好。谐波驱动的设计能够保证大约10%的齿在任何时刻相啮合，这样最小地降低齿轮齿的误差。 （2） 因为谐波驱动的结构简单，且非常容易连在一个车间内加工？？？？？ （3） 谐波驱动有一个非常紧凑的结构和非常轻质量，这样拆和组装较方便。 （4） 比别的无反冲齿轮都便宜。 总上优点和必要的结论，选谐波驱动并应用于ATC中，假如盘式刀库旋转一半，包括加速减速所需的时间，总共有2秒，旋转速度16.67 rpm，这就要求有120：1的降速比。是从电机的2000rpm到输出的16.67rpm. 2.8 解码控制开关 对于刀具正确地插入到主轴来说，刀库的控制开关非常重要。 用完全的或增值码可以提供正确的操作。每一个码代表用一个刀具位置，那样可以检索到刀具的位置。 编码器是一个非常敏感的装置，能够给出正确的输出。如果编码器的电缆受到外界的干扰（如：干涉），送到控制单元的信号将会不同于实际的信号。因此，要求另一种计刀具数目的方法提供精确的安全防备。可以用一种计刀具数目的开关去解决。当检索刀具时，由开关送去的信号和编码器相互防备彼此的错误。 相对与增量编码器来说完全编码器是比较贵的。如Fig6中所示。用一个增量编码器就可以使刀库到达正确的位置。安装一个直接与刀库相连的增量编码盘可以解决角位置反冲的问题。 有几个控制目的就有几个开关。安装一个靠近刀库的道轨上的近开关作安全防备。这个开关有24个关于刀库刀具位置的装置。因此，对每把刀有且仅有一个开关，这样可以保证刀具位置与提供给控制单元的信号一致。 3．结论 在这篇文章中，已经展现了没有机械手的盘式自动换刀刀库的设计标准和主要工作，当考虑到设计时期时，这种类型的自动换刀是适合一些特殊的数控机床刀具生产的，进而进入市场是非常有可能的。设计这种自动换刀的最短换刀时间是4秒，如表1所示。可以看到这种自动换刀装置在市场上能够有很好的竞争力。 9 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 app