機械設(shè)計外文翻譯-為臥式加工中心設(shè)計一種盤式自動換刀刀庫【中文3663字】【PDF+中文WORD】,中文3663字,PDF+中文WORD,機械設(shè)計,外文,翻譯,臥式,加工,中心,設(shè)計,一種,自動,刀刀,中文,3663,PDF,WORD 【中文3663字】 為臥式加工中心設(shè)計一種盤式自動換刀刀庫 MUSTAFA ILHAN GOKLERt~ and MURAT BJLGJN KO~t (Received 12 April 1996) 摘要：自動換刀－是被用在數(shù)控加工機床的刀具更換中，就是用刀庫的刀具更換主軸上的刀具。在這篇文章中介紹這種立式加工中心自動換刀的設(shè)計曾被認為是一種機床道具的制造者。在幾經(jīng)檢驗選擇之后就決定實施這種磁盤式自動換刀裝置的設(shè)計，這種刀庫擁有24把刀，其中刀具最大直徑可達150mm，刀具最大重量可達8 kg , 這種自動換刀的設(shè)計更換最近的刀具時間放在4s之內(nèi)，在最遠處的刀具的更換在6 s之內(nèi)。 1.概述 每一個機床刀具生產(chǎn)者或者都必須專門的任何特殊的數(shù)控機床，刀具或者設(shè)計自己的刀具特殊加工制造數(shù)控機床刀具，選擇新的技術(shù)是昂貴的，并且它并不總是適合修理和購買精密的系統(tǒng)。機床刀具制造者為避免由他們自己賣出新的計數(shù)進步，他們通常會去賣那些已經(jīng)滯后的系統(tǒng)。 因此，一種特殊的數(shù)控機床刀具更新被尋找并且繼續(xù)研究并設(shè)計和生產(chǎn)新的立式數(shù)控加工中心，就要求為這種新的自動換刀系統(tǒng)。 這種自動換刀裝置可以設(shè)計成依據(jù)由機床控制單元的命令自動從刀庫中選擇并更換刀具，由不同的刀具加工者生產(chǎn)中，已經(jīng)應(yīng)用【1－17】。通常認為刀庫或串連的或磁盤式的。盡管如此，今天有些別的刀庫類型如球型，磁盤式等等?！?7】 磁盤式的刀庫可以分成以下幾類。 （1） 在一種磁盤帶更換手臂的刀庫中，刀具通過手臂的方式更換，就是從刀庫中抓起刀具更換主軸上的刀具。然后，磁盤有檢索存儲道具的功能?！?－5】。一些這些功能的例子，如途中所示。 （2） 一個沒有更換道具手臂額磁盤刀庫，這種類型的刀庫實際上象一個刀具更換手臂。刀具由磁盤自己更換。【2，6－8】，這種例子如圖中所示。 Fig.1. 幾個有機械手自動換刀的例子 Fig.2.無機械手自動換刀的例子 在串連型自動換刀刀庫中【9－16】，刀具機械手更換，依據(jù)刀具在刀庫中定位的狀態(tài)和位置，選擇分類串連型刀庫。 (1) 刀庫中刀具坐標軸垂直于主軸的坐標軸，刀庫在支柱左邊或右邊； 為了改變刀具的方向，一個機械手抓起刀具然后繞坐標旋轉(zhuǎn)，并抓起新刀到正確的平行于坐標軸，如圖所示，這種換手臂刀庫中刀具與主軸上的刀具。 (2) 在串連刀庫中刀具的坐標式平行的主軸坐標的平面內(nèi)，如圖所示，在刀具切削中刀具僅僅只有一個位置，為立式加工中心設(shè)計帶磁盤式自動換刀裝置，因為刀具坐標平行于主軸坐標，機械手臂在刀庫與主軸之間換刀，并且，刀具在這個位置換刀。 (3) 刀庫中刀具的坐標平行于主軸坐標，機械手臂在刀庫與主軸之間換刀，并且，靠近主軸作線性移動，然后換刀如表【16】所示。 在表1中，不同的自動換刀裝置和最短時間換刀時間（T/T）這是在交換基礎(chǔ)上給出的最短換刀時間，在表1中，D+A,D,C+A指派磁盤式帶機械手換刀裝置的刀庫，沒有機械手換刀裝置的磁盤式刀庫，帶機械手換刀裝置的串連型刀庫。 在磁盤式刀庫中，隨著數(shù)量的增加，磁盤式的半徑增大，這樣不符合節(jié)省空間的觀點，盡管如此，在串連類型中，刀具的數(shù)目大約式同樣空間盤式刀具的1.5倍，在串連自動換刀裝置中能夠存儲更多的刀具，但是串連型刀庫是比較貴的行對于盤式刀具倆說。 2 自動換刀裝置的設(shè)計 在這一節(jié)中闡明數(shù)控加工中心刀具的具體設(shè)計，供應(yīng)市場的要求此中意向的特殊的競爭產(chǎn)品，那樣可以從相關(guān)的試驗中，獲得數(shù)控機床刀具的價格應(yīng)該在考慮之內(nèi)，根據(jù)cnc數(shù)控機床刀具的前景，設(shè)計特殊的數(shù)控機床的自動換刀裝置。 把設(shè)計的標準降低看作為單元，相當?shù)蛢r位的，易于生產(chǎn)加工的一個沒有機械手臂的盤式刀庫型的自動換刀裝置特殊的立式加工中心設(shè)計品。 表1 步驟1 去掉的刀具，刀庫從用 A-A 方向看 2 轉(zhuǎn)動刀具到正確的換刀位置，刀庫沿著坐標順時針旋轉(zhuǎn)或者逆時針旋轉(zhuǎn)。 3 刀庫退回，而新刀安裝到主軸上。 4．當控制單元收到主軸轉(zhuǎn)刀具安裝后，主軸開始轉(zhuǎn)的加工周期，在自動換刀裝置討論會后，主要的工作是應(yīng)用這種裝置。 如下所示 1， 刀庫中刀具數(shù)為24把 2， 刀具最大直徑為100mm 3， 當?shù)稁熘袥]有刀時，可以容納刀具的最大直徑為150mm 4， 可以舉起刀柄的重量最大為8kg 5， 刀庫的類型為BT40 6， 刀具更換時間在4秒－6秒之間。 在如下的分支中，列舉處特殊自動換刀裝置的基本系數(shù)， 盤式刀庫最小直徑是由考慮存儲具體刀具數(shù)量和最大直徑，在刀庫中存儲的應(yīng)與換下的距離，盤式刀庫的直徑是812mm每個刀具之間為6 mm。確定刀庫直徑后，刀庫的盤式形狀也確定了，因為刀庫經(jīng)常加速或減速，所以刀庫的慣性為重要問題，這個問題可以用一種輕型材料解決，鋁就是這種材料。薄壁的盤庫適當?shù)木硗不驇缀涡螤畹?，這種形狀能夠保證必要的彎曲，而這些是應(yīng)該考慮的，應(yīng)盡可能降低這種結(jié)構(gòu)的降低成本，如簡圖所示 導(dǎo)軌。 在自動換刀裝置系統(tǒng)中，都必須有抽去主軸上的刀的轉(zhuǎn)動，在無機械手換刀裝置中，盤式刀庫有前后運動，這就有必要的導(dǎo)軌，這種運動由輸送形式氣壓活塞完成，實際上，有幾種別的線性運動方法，如用球滾動代替旋轉(zhuǎn)運動，然后，當有圓周運動時。這是非常昂貴的方式，如表6所示，導(dǎo)軌設(shè)計成空心形式以減輕重量，導(dǎo)軌由氣壓活塞推動，活塞或裝在導(dǎo)軌的最后面，導(dǎo)軌邊上設(shè)計了方形的邊緣，并不是u型的，用方形的邊是因為容易制造，價格極限空間都有優(yōu)勢。 2.3減震器 每一個刀具更換周期，刀庫都由導(dǎo)軌上前后移動，這占用的周期的大部時間，為了減少運整時間，優(yōu)先用了移動速度，但是坐標問題隨之產(chǎn)生，當動量較大時必須有相適應(yīng)的短距離停下來的系統(tǒng)，而這種系統(tǒng)又不會起過渡的振動，減震器就可解決此問題。 在設(shè)計中，用一個氣缸，和用壓縮空氣活塞缸開減增大壓力減震，減震器是必要的，在特殊在減震器前后壓力通過整個容器，隨著速度的降低為0，這樣持續(xù)的壓力仍舊比較大，因此線性就達到了。 2.4 刀具夾緊裝置 刀庫中的刀具用夾具夾在刀庫中，刀具可以夾持在鉗口之間。以這種方式設(shè)計的夾具，最大重量的刀具可以安全舉起。盤式刀庫繞其軸線旋轉(zhuǎn)，離心力增加刀具的重量。這些因素在夾具的設(shè)計中考慮到。刀庫和鉗口依據(jù)特定的刀柄類型而定。鉗口應(yīng)適合特殊的刀柄。 刀庫中的刀具由一個螺旋狀的彈簧和鐵片組合而夾在刀庫中的這種裝置設(shè)計的可以抓舉最大重量為8公斤的刀具。鉗口的形狀符合BT40刀柄的外形。 2.5 防屑保護刀柄的防護罩 設(shè)計ATC時，一些保護措施是有必要的。如果任何鐵屑進入主軸前打擊到刀柄的外表面，那么將不會準確停下主軸并且引起錯位，不精確等相應(yīng)的問題。鐵屑可能損傷主軸，為了防止這樣，應(yīng)該防止鐵屑進入主軸，而用一個防護罩。防護罩應(yīng)與主軸一起移動。 2.6 電機的速度控制 用一電機和速度結(jié)合體控制刀盤的旋轉(zhuǎn)。選用電機類型根據(jù)動力和價格的要求選擇合適的電機。 考慮位置的精確性和重復(fù)性 ，選一種AC無刷式電機帶動盤式刀庫旋轉(zhuǎn)，選用一種可以提供4.5N/n的力矩和0.94KW動力，性價比好的 2.7 降速 在刀庫和電機之間降速，這樣可以通過蝸輪蝸桿實現(xiàn)的。一種無級變速對于刀具相對主軸中心軸精確位置更換刀具來說是必要的。 然而，在很小的面積內(nèi)獲得及時的降速是非常困難的，但用蝸輪就可以容易實現(xiàn)。但蝸輪也有問題。而對于降速來說，系統(tǒng)的效率明顯下降。如果用效率低的蝸輪實現(xiàn)降速，則電機的尺寸就相應(yīng)增加，進而增加成本和控制系統(tǒng)。 低價格的行星輪系統(tǒng)，通過減小齒輪中心輪到最小間隙，控制可以消除反沖力，這樣沒有太大的沖擊和噪音。這些自然引起的啟動力矩，因為相互干涉，較低的運動效率比那些同樣有反沖的情況。在世界上只有少數(shù)公司生產(chǎn)反沖的行星輪傳動。 諧波驅(qū)動是另一種無反沖的方法。這種方法的優(yōu)點如下述： （1） 位置精確度非常好。諧波驅(qū)動的設(shè)計能夠保證大約10%的齒在任何時刻相嚙合，這樣最小地降低齒輪齒的誤差。 （2） 因為諧波驅(qū)動的結(jié)構(gòu)簡單，且非常容易連在一個車間內(nèi)加工？？？？？ （3） 諧波驅(qū)動有一個非常緊湊的結(jié)構(gòu)和非常輕質(zhì)量，這樣拆和組裝較方便。 （4） 比別的無反沖齒輪都便宜。 總上優(yōu)點和必要的結(jié)論，選諧波驅(qū)動并應(yīng)用于ATC中，假如盤式刀庫旋轉(zhuǎn)一半，包括加速減速所需的時間，總共有2秒，旋轉(zhuǎn)速度16.67 rpm，這就要求有120：1的降速比。是從電機的2000rpm到輸出的16.67rpm. 2.8 解碼控制開關(guān) 對于刀具正確地插入到主軸來說，刀庫的控制開關(guān)非常重要。 用完全的或增值碼可以提供正確的操作。每一個碼代表用一個刀具位置，那樣可以檢索到刀具的位置。 編碼器是一個非常敏感的裝置，能夠給出正確的輸出。如果編碼器的電纜受到外界的干擾（如：干涉），送到控制單元的信號將會不同于實際的信號。因此，要求另一種計刀具數(shù)目的方法提供精確的安全防備?？梢杂靡环N計刀具數(shù)目的開關(guān)去解決。當檢索刀具時，由開關(guān)送去的信號和編碼器相互防備彼此的錯誤。 相對與增量編碼器來說完全編碼器是比較貴的。如Fig6中所示。用一個增量編碼器就可以使刀庫到達正確的位置。安裝一個直接與刀庫相連的增量編碼盤可以解決角位置反沖的問題。 有幾個控制目的就有幾個開關(guān)。安裝一個靠近刀庫的道軌上的近開關(guān)作安全防備。這個開關(guān)有24個關(guān)于刀庫刀具位置的裝置。因此，對每把刀有且僅有一個開關(guān)，這樣可以保證刀具位置與提供給控制單元的信號一致。 3．結(jié)論 在這篇文章中，已經(jīng)展現(xiàn)了沒有機械手的盤式自動換刀刀庫的設(shè)計標準和主要工作，當考慮到設(shè)計時期時，這種類型的自動換刀是適合一些特殊的數(shù)控機床刀具生產(chǎn)的，進而進入市場是非常有可能的。設(shè)計這種自動換刀的最短換刀時間是4秒，如表1所示?？梢钥吹竭@種自動換刀裝置在市場上能夠有很好的競爭力。 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