撥叉的加工工藝及其夾具設(shè)計(jì),加工,工藝,及其,夾具,設(shè)計(jì) 本科生畢業(yè)設(shè)計(jì) (論文) 外 文 翻 譯 原 文 標(biāo) 題 ?Gear manufacturing methods 譯 文 標(biāo) 題 齒輪的加工方法 作者所在系別 機(jī)械工程學(xué)院 作者所在專業(yè) 機(jī)械設(shè)計(jì)制造及其自動(dòng)化 作者所在班級(jí) B13113 作 者 姓 名 王瀟 作 者 學(xué) 號(hào) 200134011337 指導(dǎo)教師姓名 趙忠澤 指導(dǎo)教師職稱 副教授 完 成 時(shí) 間 2017 年 3 月 北華航天工業(yè)學(xué)院教務(wù)處制 譯文標(biāo)題 齒輪的加工方法 原文標(biāo)題 Gear manufacturing methods 作 者 Zhang Baozhu 譯 名 張寶珠 國(guó) 籍 中國(guó) 原文出處 百度文庫(kù) 譯文： 齒輪的加工方法 加工齒輪輪齒有兩種基本的方法：產(chǎn)生過(guò)程和形成過(guò)程。當(dāng)一個(gè)輪齒產(chǎn)生時(shí)，工件和切削或磨削工具，是不斷嚙合在一起的，輪齒的形式是由刀具決定的。換句話說(shuō)，工件和刀具是共軛的。滾齒機(jī)，成型切割機(jī)，剃齒機(jī)，磨床都使用這個(gè)原理。 當(dāng)一個(gè)輪齒形成時(shí)，該刀具是呈正被加工出來(lái)的空間的形狀的。一些磨床使用此原理，與一個(gè)指示裝置配套在一起使輪齒一個(gè)挨一個(gè)形成。刀就是同時(shí)加工所有輪齒形成刀具的例子。 成型 成型本質(zhì)上是與平面圖類似的，但采用了圓形的切削刀具替代了齒條，由此產(chǎn)生的往復(fù)慣性的減少，允許更高的行程速度：現(xiàn)代的成型切割汽車(chē)齒輪可以以每分鐘2000切割行程運(yùn)行。切削刀具的形狀大致是與漸開(kāi)線齒輪相同的，但輪齒的頂端是圓形的。 切削刀具和工件之間的發(fā)電驅(qū)動(dòng)器之間不涉及機(jī)架或連接螺釘 ，因?yàn)橹挥袌A周運(yùn)動(dòng)在涉及的范圍內(nèi)。切割機(jī)每走一個(gè)行程，工具和工件通常在切線方向移動(dòng)0.5毫米。在返回的行程中，刀具必須被縮進(jìn)約1毫米留有間隙，否則就會(huì)產(chǎn)生摩擦，馬上發(fā)生故障。這類型機(jī)床的速度被限制，保證大約50千克重的切割機(jī)和軸承可移動(dòng)1毫米的距離。加速度所涉及的扭矩可增加5000N的力，但必須保持高的精度。 成型機(jī)的優(yōu)點(diǎn)是生產(chǎn)效率相對(duì)較高，可能在齒頂上切出直角。不幸的是，對(duì)于斜齒輪，螺旋導(dǎo)向器需要在直線運(yùn)動(dòng)中施加旋轉(zhuǎn)運(yùn)動(dòng)，這種螺旋導(dǎo)向器不容易生產(chǎn)，也不便宜。所以該方法只適合在斜齒輪上的長(zhǎng)距離，因?yàn)閷?duì)每個(gè)不同的螺旋角就要生產(chǎn)特殊的刀具和導(dǎo)向器。成型機(jī)的一個(gè)很大的優(yōu)點(diǎn)，是它可以生產(chǎn)環(huán)形齒輪，例如那些需要大型epicyclie周轉(zhuǎn)圓的驅(qū)動(dòng)器。 非常高的精確度是十分重要的，而成型切割機(jī)的不準(zhǔn)確性也是相當(dāng)要緊的。因?yàn)樗鼈兛赡苻D(zhuǎn)移到削減齒輪。很明顯側(cè)面的錯(cuò)誤將轉(zhuǎn)移，但比起離心機(jī)或破碎機(jī)給予的特點(diǎn)， “掉落的輪齒” ，是相當(dāng)不明顯的。對(duì)于掉齒有幾個(gè)原因，但它發(fā)生最頻繁的是，當(dāng)工件的直徑大約是刀具直徑的一半，1.5倍或2.5倍時(shí)。如果刀具開(kāi)始在高點(diǎn)，在最后完成漸開(kāi)線齒輪期間結(jié)束在低點(diǎn)，在刀具上峰與峰的偏心誤差發(fā)生在最后的漸開(kāi)線切割齒輪的第一個(gè)和最后一個(gè)齒輪之間。當(dāng)?shù)毒叩睦鄯e螺距誤差可能剛好超過(guò)25微米時(shí)，切割輪齒時(shí)就會(huì)有一個(gè)突然的這個(gè)數(shù)量的螺距誤差。在機(jī)床上切割的下一個(gè)齒輪可能在鄰近的節(jié)圓上是好的，如果在切割機(jī)上最后的切割碰巧發(fā)生在一個(gè)有利的位置。 各種嘗試已經(jīng)作出，防止這種效應(yīng)，特別是通過(guò)連續(xù)旋轉(zhuǎn)，沒(méi)有任何進(jìn)一步的刀料，但如果成型機(jī)是不是很堅(jiān)固，刀具不是很尖銳，然后沒(méi)有進(jìn)一步的切割發(fā)生，誤差將不會(huì)被消除。 滾齒 滾齒是最常用的金屬切削方法，使用機(jī)架產(chǎn)生的原理，但避免了由在旋轉(zhuǎn)切削機(jī)上增加許多齒條引起的緩慢的往復(fù)運(yùn)動(dòng)。齒條在軸線方向上替換為切口蝸桿。 齒條不能為整個(gè)輪齒的工作長(zhǎng)度產(chǎn)生正確的漸開(kāi)線形狀，因?yàn)樗麄冊(cè)趫A弧軌跡上移動(dòng)，所以滾刀緩慢地沿輪齒走刀，在軸向或法向或傾斜的滾齒機(jī)螺旋線方向上。 金屬去除率高，因?yàn)槁菪姷痘蚬ぜ](méi)有做往復(fù)運(yùn)動(dòng)的需要，所以40m/min的切割速度可用于傳統(tǒng)的滾刀,切割速度高達(dá)150m/min的用于硬質(zhì)合金滾刀。通常一個(gè)直徑為100毫米的滾刀轉(zhuǎn)速達(dá)到100rpm ，所以20個(gè)齒的工件以每分鐘5轉(zhuǎn)的速度旋轉(zhuǎn)。工件的每個(gè)旋轉(zhuǎn)運(yùn)動(dòng)將對(duì)應(yīng)于0.75毫米的進(jìn)給量，所以滾刀會(huì)提前通過(guò)工件約每分鐘4毫米。對(duì)于汽車(chē)生產(chǎn)，近似多頭開(kāi)始的滾刀，可用于每轉(zhuǎn)3毫米的粗糙進(jìn)給量，以便在切割機(jī)上達(dá)到100rpm的速度，一個(gè)兩頭開(kāi)始的滾刀和20個(gè)齒的齒輪可提供每分鐘30毫米的進(jìn)給速率。 粗糙進(jìn)給速率的缺點(diǎn)是在工件上會(huì)留下明顯的標(biāo)志，尤其是在齒根，每轉(zhuǎn)在進(jìn)給速率的空間顯示一種圖案。齒側(cè)標(biāo)記的表面波紋比齒跟要少，當(dāng)有一個(gè)隨后的整理操作時(shí)，如剃齒或磨削，這一點(diǎn)就不重要了。當(dāng)沒(méi)有進(jìn)一步的操作時(shí)，每轉(zhuǎn)的進(jìn)給量必須加以限制，保證粗糙度在一個(gè)界限以下，通常這決定于潤(rùn)滑條件。齒根上波紋的高度指定乘以每轉(zhuǎn)的進(jìn)給量，然后除以滾刀直徑的4倍。1毫米的進(jìn)給量和100毫米的直徑可產(chǎn)生2.5微米高的波紋。對(duì)齒側(cè)波紋大約跟cos70一樣大，即約0.85微米。 滾齒機(jī)的精度對(duì)齒距和螺旋線來(lái)說(shuō)，通常很高，假設(shè)機(jī)床維持不變，漸開(kāi)線單單決定于滾刀齒廓的精度。漸開(kāi)線的形式隨著滾刀的切入產(chǎn)生，在滾刀上留有裂痕時(shí)，漸開(kāi)線是不真實(shí)的。但是，如果說(shuō)有14條切線產(chǎn)生在曲率半徑約20毫米的齒側(cè)，從真實(shí)的漸開(kāi)線分離，僅僅大約0.5微米。滾刀的制造和安裝誤差可以超過(guò)10微米。使用兩頭開(kāi)始的滾刀或斜滾齒機(jī)可增加誤差水平，因?yàn)闈L刀的齒距誤差的轉(zhuǎn)移到切割齒輪上。 拉削 ? 拉削不被用于斜齒輪，但對(duì)內(nèi)齒直齒輪時(shí)十分有用的。聯(lián)系全局來(lái)看，拉削的最重要的用途是用任何其他的方法都不容易加工的內(nèi)花鍵。跟所有的拉削方法一樣，這種方法對(duì)批量生產(chǎn)是經(jīng)濟(jì)的，因?yàn)榘惭b成本較高。 拉削技術(shù)對(duì)內(nèi)齒斜齒輪主要的應(yīng)用是由Gleasons在其G-TRAC機(jī)床上。這臺(tái)機(jī)器的運(yùn)作，增加滾齒切割機(jī)的有效半徑至無(wú)限遠(yuǎn)，使刀具的每一個(gè)齒都能在一條直線上轉(zhuǎn)動(dòng)，而不是對(duì)在一個(gè)半徑上。這使得切割行為延長(zhǎng)超過(guò)齒輪的整個(gè)端面寬度，替代了傳統(tǒng)的滾刀每轉(zhuǎn)0.75毫米的進(jìn)給量。由此產(chǎn)生的過(guò)程中提供了非常高的生產(chǎn)率，更適合于美國(guó)，美國(guó)的產(chǎn)量在整個(gè)歐洲來(lái)說(shuō)，相對(duì)較低，盡管初始成本高，但非常具有競(jìng)爭(zhēng)力。 拉削提供了較高的精確度和良好的表面光潔度，但象所有切削過(guò)程一樣，僅限于 “軟”材料，必須隨后進(jìn)行表面淬火或熱處理，使其變形。 剃齒 剃齒切割機(jī)看起來(lái)像一個(gè)在齒根有著額外間隙的齒輪，齒側(cè)有槽，提供切削邊緣。它是運(yùn)行在網(wǎng)格與粗糙齒輪軸交叉處，以便與做剩余運(yùn)動(dòng)的輪齒的相對(duì)速度有理論聯(lián)系點(diǎn)。該剃齒刀的輪齒相對(duì)靈活的彎曲，所以當(dāng)它們?cè)趦蓚€(gè)齒輪的輪齒間兩兩接觸時(shí)，只有有效地運(yùn)作。齒輪和刀具橫向在工作面以高轉(zhuǎn)速運(yùn)轉(zhuǎn)時(shí)，大約100毫米的材料被去除。周期時(shí)間可以少于半分鐘，機(jī)床并不昂貴，但刀具是精密的，很難制造。在剃齒機(jī)邊緣容易對(duì)齒廓作出調(diào)整，然后凸緣能被利用。剃齒可以使用刀具在凸肩處完成，向下到某一深度，無(wú)軸向運(yùn)動(dòng)。這種方法速度快，但需要更復(fù)雜的刀具設(shè)計(jì)。 磨削 磨削是非常重要的，因?yàn)樗怯不诩庸さ凝X輪的主要途徑。當(dāng)要求高精度時(shí)，熱處理不足以使其變形，那么，磨削是很必要的。 磨削的最簡(jiǎn)單的方法往往被稱為orcutt方法。車(chē)輪的輪廓使用單點(diǎn)鉆石精確的裝飾使其變形，被模板切割控制到所需要的真實(shí)形狀。一個(gè)縮放儀6:1的比例是常用的。車(chē)輪的輪廓然后沿齒輪作軸向往復(fù)運(yùn)動(dòng)，齒輪旋轉(zhuǎn)允許受螺旋角的影響。當(dāng)一個(gè)齒形已經(jīng)完成，通常包括100微米的金屬去除，齒輪被指引到下一個(gè)齒的空間。這種方法可清楚查看，但一貫有著高的精度要求。安裝尺寸過(guò)長(zhǎng)，因?yàn)槿绻?shù)，齒數(shù)，螺旋角或齒廓校正線改變時(shí)，需要不同的裝飾模板。 最快的磨削方法跟滾齒機(jī)使用相同的原理，但取代了憑借磨削輪增加切口和減輕的蝸桿，磨削輪是機(jī)架上的一節(jié)。由于高的表面速度的需求，砂輪的直徑被增大，使直徑為0.5米的砂輪可以超過(guò)2000 rpm的速度運(yùn)轉(zhuǎn)，給予必要的1000米/分鐘的速度。只有單頭蝸桿可在砂輪上切削，但齒輪轉(zhuǎn)速很高，通常情況下 為100rpm。因此很難設(shè)計(jì)驅(qū)動(dòng)系統(tǒng)提供精度和剛度。該過(guò)程的精度是在合理的高水平，雖然在磨削期間有砂輪和工件的轉(zhuǎn)向變化的一種傾向。所以砂輪的形式可能需要補(bǔ)償機(jī)床撓度的影響。磨削輪上一代蝸桿的形狀是一個(gè)緩慢的進(jìn)程，因?yàn)檠b飾的鉆石或滾子，不僅要形成機(jī)架上的輪廓，而且當(dāng)砂輪旋轉(zhuǎn)時(shí)必須做軸向運(yùn)動(dòng)。一旦砂輪已經(jīng)形成，齒輪必須被快速的磨削，直到要求再做調(diào)整。這就是用小齒輪創(chuàng)造高生產(chǎn)率的最流行的方法，通常被稱為reishauer方法。 大型齒輪通常由Maag方法產(chǎn)生，與其方法中的規(guī)劃相似，但使用大直徑的磨削輪，形成側(cè)面的理論嚙合齒條。非常大的直徑的齒輪不能被輕易移動(dòng)，所以齒輪基本上是平穩(wěn)的，而磨削輪的活動(dòng)部分在螺旋線方向上作往復(fù)運(yùn)動(dòng)。磨削輪只在斜齒輪的端面上有一小部分是接觸的，所以，當(dāng)在一年內(nèi)制造這個(gè)齒數(shù)的幾個(gè)齒輪時(shí)，這并不重要。與形成磨削相同，磨削后，一對(duì)側(cè)面的齒輪被指引到下一對(duì)。 類似的方法用于中等大小的齒輪，這種齒輪有固定的輪子，而粗糙的齒輪是走過(guò)了車(chē)輪下。齒輪相應(yīng)的旋轉(zhuǎn)運(yùn)動(dòng)由輪上的皮帶控制，這條皮帶從一圓柱體的節(jié)圓直徑上散開(kāi)，使齒輪相對(duì)齒條的運(yùn)動(dòng)是正確的。 另一種方法，尼羅河的做法，采用了車(chē)輪，它被形成提供了理論上的嚙合機(jī)架，而不是象Maag方法一樣用兩個(gè)杯輪子。這種做法最適合在小齒輪上中等精度的工作，速度介于reishauer方法和Maag方法之間。 所有磨削加工與切削加工相比，是緩慢和昂貴的，因此只用于精度要求至關(guān)重要的條件下。一個(gè)粗略的經(jīng)驗(yàn)法則是，磨削會(huì)增加齒輪的切削成本，這是10個(gè)因素之一，但輪齒的成本，往往只占變速箱總費(fèi)用的一小部分。令人驚訝地是，可獲得的精度不是非常取決于齒輪的大小，齒輪的直徑是5米或50米，可獲得的節(jié)圓漸開(kāi)線和螺旋線的精度是預(yù)想的5微米或更好，比起任何其他因素，更取決于操作工和檢查員的技術(shù)和耐心。 它往往是假定磨削將去除在粗的階段產(chǎn)生的所有誤差。不幸的是，磨床是較靈活的，所以砂輪有一個(gè)按照以往誤差的趨勢(shì)。誤差將因此減少，但并沒(méi)有完全消除，除非很多切削方法被使用。任何時(shí)候磨削過(guò)程給出的都一致的結(jié)果，這是可在粗切的階段檢測(cè)精度是可取的。唯一的例外是磨削的形成過(guò)程，將不跟隨漸開(kāi)線誤差，但仍允許螺旋線和節(jié)圓誤差。 原文： Gear manufacturing methods There are two basic methods of manufacturing gear teeth: the generating process and the forming process. when a gear tooth is generated, the workpiece and the cutting or grinding tool are in continuous mesh and the tooth form is generated by the tool. In other words, the work and the tool are conjugated to each other. hobbing :machines, shaper cutters, shaving machines, and grinders use this principle. When a gear tooth is formed, the tool is in the shape of the space that is being machined out. Some grinding machines use this principle with an indexing mechianism which allows the gear teeth to be formed tooth by tooth. Broaches are examples of form tools that machine all the gear teeth simultaneously. shaping Shaping is inherently similar to planning but uses a circular cuttrer instead of rack and the resulting reduction in the reciprocating inertia allows much higher stroking speeds: modern shapers cutting car gears can run at 2,000 cutting strokes per minmute. The shape of the cutter is roughly the same as an involute gear but the tips of the teeth are rounded. The generating drive between cutter and workpiece does not involve a rack or leadscrew since only circular motion in involved. The tool and workpiece move tangential typically 0.5 mm for each stroke of the cutter. On the return stroke the cutter must be retracted about 1 mm to give clearance otherwise tool rub occurs on the backstroke and failure is rapid. The speed on this type of machine is limited by the rate at which some 50kg of cutter and bearings can be moved a distance of 1 mm. the accelerations involved tequire forces of the order of 5000N yet high accuracy must be maintained. The advantages of shaping are that production rates are relatively high and that it is possible to cut right up to a shoulder. Unfortunately, for helical gears, a helical guide is required to impose a rotational motion on the stroking motion; such helical guides cannot be produced easily or cheaply so the method is only suitable for long runs with helical gears since special cutters and guides must be manufactured for each different helix angle. A great advantage of shaping is its ability to annular gears such as those required for large epicyclie drives. When very high accuracy is of importance the inaccuracies in the shaping cutter matter since they may transfer to the cut gear. It is obvious that profile errors will transfer but it is less obvious than an eccentrically mounted or ground cutter will give a characteristic “dropped tooth”. There are several causes for “dropped tooth” but it occurs most commonly when the diameter of the workpiece is about half, one and half, two and a half, etc, times the cutter diameter. If the cutter starts on a high point and finishes on a low point during the final finishing revolution of the gear the peak to peak eccentricity errors in the cutter occurs between the last and the first tooth of the final revolution of the cut gear; as the cumulative pitch error of the cutter may well be over 25 microns there is a sudden pitch error of this amount on the cut gear. The next gear cut on the machine may however be very good on adjacent pitch if the final cut happened to start in a favorable position on the cutter. Various attempts have been made to prevent this effect, in particular by continuing rotation without any further cutter infeed but if the shaping machine is not very rigid and the cutter very sharp then no further cutting will occur and the error will not be removed. hobbing hobbing, the most used metal cutting method, uses the rack generating principle but avoids slow reciprocation by mounting many “racks ” on a rotating cutter. The “racks” are displaced axially to form a gashed worm. The “racks” do not generate the correct involute shape for the whole length of the teeth since they are moving on a circular path and so the hob is fed slowly along the teeth either axially in normal or in the direction of the helix in “oblique” hobbing. Metal removal rates are high since no reciprocation of hob or workpiece is required and so cutting speeds of 40 m/min can be used for conventional hobs and up to 150m/min for carbide hobs. Typically with a 100mm diameter hob the rotation speed will be 100rpm and so a twenty tooth workpiece will rotate at 5 rpm. Each revolution of the workpiece will correspond to 0.75mm feed so the hob will advance through the workpiece at about 4mm per minute. For car production roughing multiple start hobs can be used with coarse feeds of 3mm per revolution so that 100 rpm on the cutter, a two-start hob and a 20 tooth gear will give a feed rate of 30mm/minute. The disadvantage of a coarse feed rate is that a clear marking is left on the workpiece, particularly in the root, showing a pattern at a spacing of the feed rate per revolution. This surface undulation is less marked on the flanks than in the root and is not important when there is a subsequent finishing operation such as shaving or grinding. When there are no further operations the feed per revolution must be restricted to keep the undulations below a limit which is usually dictated by lubrication conditions. The height of the undulations in the root of the gear is given by squaring the feed per revolution and dividing by four times the diameter of the hob; 1 mm feed and 100mm diameter gives 2.5 micron high undulations in the root. On the gear flank the undulation is roughly cos70 as large, i.e., about 0.85 micron. Accuracy of hobbing is normally high for pitch and for helix, provided machines are maintained; involute is dependent solely on the accuracy of the hob profile. As the involute form is generated by as many cuts as there are gashes on the hob the involute is not exact, but if there are, say, 14 tangents generating a flank of 20 mm radius curvature about 4 mm high the divergence from a true involute is only about half a micron; hob manufacturing and mounting errors can be above 10 microns. Use of twostart hobs or oblique hobbing gives increased error levels since hob errors of pitching transfer to the cut gear. broaching Broaching is not used for helical gears but is useful for internal spur gears; the principal use of broaching in this context is for internal splines which cannot easily be made by any other method. As with all broaching the method is only economic for large quantities since setup costs are high. The major application of broaching techniques to helical external gears is that used by Gleasons in their G-TRAC machine .this machine operates by increasing the effective radius of a hobbing cutter to infinity so that each tooth of the cutter is traveling in a straight line instead of on a radius. This allows the cutting action to extend over the whole facewidth of a gear instead of the typical 0.75 mm feed per revolution of hobbing. The resulting process gives a very high production rate , more suitable for U.S.A. production volumes than for the relatively low European volumes and so, despite a high initial cost ,is very competitive. Broaching give high accuracy and good surface finish but like all cutting processes is limited to “soft” materials which must be subsequently casehardened or heat treated, giving distortion. Shaving A shaving cutting cutter looks like a gear which has extra clearance at the root and whose tooth flanks have been grooved to give cutting edges. It is run in mesh with the rough gear with crossed axes so that there is in theory point contact with a relative velocity along the teeth giving scraping action. The shaving cutter teeth are relatively flexible in bending and so will only operate effectively when they are in double contact between two gear teeth. The gear and cutter operate at high rotational speeds with traversing of the workface and about 100 mm micron of material is removed. Cycle times can be less than half a minute and the machines are not expensive but cutters are delicate and difficult to manufacture. It is easy to make adjustments of profile at the shaving stage and crowning can be applied. Shaving can be carried out near a shoulder by using a cutter which is plunged in to depth without axial movement; this method is fast but requires more complex cutter design. grinding Grinding is extremely important because it is the main way hardened gear are machined. When high accuracy is required it is not sufficient to pre-correct for heat treatment distortion and grinding is then necessary. The simplest approach to grinding, often termed the Orcutt method. The wheel profile is dressed accurately to shape using single point diamonds which are controlled by templates cut to the exact shape required; 6:1 scaling with a pantograph is often used. The profile wheel is then reciprocated axially along the gear which rotates to allow for helix angle effects; when one tooth shape has been finished, involving typically 100 micron metal removal the gear is indexed to the next tooth space. This method is fairly show but gives high accuracy consistently. Setting up is lengthy because different dressing templates are needed if module, number of teeth, helix angle, or profile correction are changed. The fastest grinding method uses the same principle as hobbing but replaces a gashed and relieved worm by a grinding wheel which is a rack in section. Since high surface speeds are needed the wheel diameter is increased so that wheels of 0.5 m diameter can run at over 2000 rpm to give the necessary 1000 m/min. only single start worms are cut on the wheel but gear rotation speeds are high,100 rpm typically, so it is difficult to design the drive system to give accuracy and rigidity. Accuracy of the process is reasonably high although there is a tendency for wheel and workpiece to deflect variably during grinding so the wheel form may require compensation for machine deflection effects. Generation of a worm shape on the grinding wheel is a slow process since a dressing diamond or roller must not only form the rack profile but has to move axially as the wheel rotates. Once the wheel has been trued, gears can be ground rapidly until redressing is required. This is the most popular method for high production rates with small gear and is usually called the Reishauer method. Large gears are usually generated by the Maag method which is similar to planning in its approach but uses cup grinding wheels of large diameter to form the flanks of the theoretical mating rack. Gears of very large diameter cannot easily be moved so the gear is essentially stationary while the grinding wheel carriage reciprocates in the direction of the helix. The wheel is only in contact over a small part of the facewidth in helical gears so this is not important when only a few gears of this size are made in a year. As with form grinding, after grinding a pair of flanks the gear is indexed to the next pair. A similar method used for medium size gears has stationary wheels, while the rough gear is traversed under the wheels. Corresponding rotational movement of the gear is controlled by steel bands unwrapping from a cylinder of pitch circle diameter so that the motion of gear relative to “rack” is correct. Another method, the Nile approach, uses a wheel which is formed to give the “theoretical mating rack” instead of using two cup wheels as in the Maag method. This approach is best suited to medium precision work on smaller gears and is intermediate in speed between the Reishauer and Maag methods. All grinding processes are slow and costly compared with cutting processed and so are only used when accuracy is essential. A rough rule of thumb is that grinding will increase gear cutting costs by a factor of 10 but the cost of the teeth is often only a small part of the total cost of a gearbox. The accuracies attainable are surprisingly not very dependent on size of gear ; whether a gear is 5 m or 50 m diameter the pitch involute and helix accuracies attainable are of the order of 5 microns or better and more dependent on the skill and patience of the operator and inspectors than on any other factors. It is often assumed that grinding will remove all error generated at the roughing stage. Unfortunately, grinding machines are relatively flexible and so the grinding wheel has a tendency to follow previous errors. The errors will thus be reduced but not completely eliminated unless very many cuts are used; whenever a grinding process is giving in consistent results it is advisable to check the accuracies at the rough-cut stage. The only exception is the form grinding process which will not follow involute errors though it will still allow helix and pitch errors. 39 指 導(dǎo) 教 師 評(píng) 語(yǔ) 外文翻譯成績(jī)： 指導(dǎo)教師簽字： 年 月 日 40