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1、英文原文Gear manufacturing methodsThere 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, th
2、e 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
3、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 mu
4、ch 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 sinc
5、e 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 machin
6、e 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
7、 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 m
8、ust 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 ob
9、vious 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
10、 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 cum
11、ulative 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
12、 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. hobbinghobbing, the most used metal cutting method, uses t
13、he 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
14、 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
15、 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 multip
16、le 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 patte
17、rn 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 k
18、eep 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
19、 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 m
20、any 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 tw
21、ostart 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 mad
22、e 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
23、 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
24、 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 subseq
25、uently 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 co
26、ntact 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
27、 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 ca
28、rried 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 suffic
29、ient 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 scal
30、ing 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 s
31、how 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 grind
32、ing 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
33、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 wor
34、m 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 produ
35、ction 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 cann
36、ot 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 w
37、ith 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
38、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 wor
39、k 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
40、 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
41、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 previ
42、ous 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
43、involute errors though it will still allow helix and pitch errors.中文译文齿轮的加工方法加工齿轮轮齿有两种基本的方法：产生过程和形成过程。当一个轮齿产生时，工件和切削或磨削工具，是不断啮合在一起的，轮齿的形式是由刀具决定的。换句话说，工件和刀具是共轭的。滚齿机，成型切割机，剃齿机，磨床都使用这个原理。当一个轮齿形成时，该刀具是呈正被加工出来的空间的形状的。一些磨床使用此原理，与一个指示装置配套在一起使轮齿一个挨一个形成。刀就是同时加工所有轮齿形成刀具的例子。成型 成型本质上是与平面图类似的，但采用了圆形的切削刀具替代了齿条，由此
44、产生的往复惯性的减少，允许更高的行程速度：现代的成型切割汽车齿轮可以以每分钟2000切割行程运行。切削刀具的形状大致是与渐开线齿轮相同的，但轮齿的顶端是圆形的。切削刀具和工件之间的发电驱动器之间不涉及机架或连接螺钉 ，因为只有圆周运动在涉及的范围内。切割机每走一个行程，工具和工件通常在切线方向移动0.5毫米。在返回的行程中，刀具必须被缩进约1毫米留有间隙，否则就会产生摩擦，马上发生故障。这类型机床的速度被限制，保证大约50千克重的切割机和轴承可移动1毫米的距离。加速度所涉及的扭矩可增加5000N的力，但必须保持高的精度。成型机的优点是生产效率相对较高，可能在齿顶上切出直角。不幸的是，对于斜齿轮
45、，螺旋导向器需要在直线运动中施加旋转运动，这种螺旋导向器不容易生产，也不便宜。所以该方法只适合在斜齿轮上的长距离，因为对每个不同的螺旋角就要生产特殊的刀具和导向器。成型机的一个很大的优点，是它可以生产环形齿轮，例如那些需要大型epicyclie周转圆的驱动器。非常高的精确度是十分重要的，而成型切割机的不准确性也是相当要紧的。因为它们可能转移到削减齿轮。很明显侧面的错误将转移，但比起离心机或破碎机给予的特点， “掉落的轮齿” ，是相当不明显的。对于掉齿有几个原因，但它发生最频繁的是，当工件的直径大约是刀具直径的一半，1.5倍或2.5倍时。如果刀具开始在高点，在最后完成渐开线齿轮期间结束在低点，在
46、刀具上峰与峰的偏心误差发生在最后的渐开线切割齿轮的第一个和最后一个齿轮之间。当刀具的累积螺距误差可能刚好超过25微米时，切割轮齿时就会有一个突然的这个数量的螺距误差。在机床上切割的下一个齿轮可能在邻近的节圆上是好的，如果在切割机上最后的切割碰巧发生在一个有利的位置。各种尝试已经作出，防止这种效应，特别是通过连续旋转，没有任何进一步的刀料，但如果成型机是不是很坚固，刀具不是很尖锐，然后没有进一步的切割发生，误差将不会被消除。滚齿滚齿是最常用的金属切削方法，使用机架产生的原理，但避免了由在旋转切削机上增加许多齿条引起的缓慢的往复运动。齿条在轴线方向上替换为切口蜗杆。 齿条不能为整个轮齿的工作长度产
47、生正确的渐开线形状，因为他们在圆弧轨迹上移动，所以滚刀缓慢地沿轮齿走刀，在轴向或法向或倾斜的滚齿机螺旋线方向上。金属去除率高，因为螺旋铣刀或工件没有做往复运动的需要，所以40m/min的切割速度可用于传统的滚刀,切割速度高达150m/min的用于硬质合金滚刀。通常一个直径为100毫米的滚刀转速达到100rpm ，所以20个齿的工件以每分钟5转的速度旋转。工件的每个旋转运动将对应于0.75毫米的进给量，所以滚刀会提前通过工件约每分钟4毫米。对于汽车生产，近似多头开始的滚刀，可用于每转3毫米的粗糙进给量，以便在切割机上达到100rpm的速度，一个两头开始的滚刀和20个齿的齿轮可提供每分钟30毫米的
48、进给速率。粗糙进给速率的缺点是在工件上会留下明显的标志，尤其是在齿根，每转在进给速率的空间显示一种图案。齿侧标记的表面波纹比齿跟要少，当有一个随后的整理操作时，如剃齿或磨削，这一点就不重要了。当没有进一步的操作时，每转的进给量必须加以限制，保证粗糙度在一个界限以下，通常这决定于润滑条件。齿根上波纹的高度指定乘以每转的进给量，然后除以滚刀直径的4倍。1毫米的进给量和100毫米的直径可产生2.5微米高的波纹。对齿侧波纹大约跟cos70一样大，即约0.85微米。滚齿机的精度对齿距和螺旋线来说，通常很高，假设机床维持不变，渐开线单单决定于滚刀齿廓的精度。渐开线的形式随着滚刀的切入产生，在滚刀上留有裂痕
49、时，渐开线是不真实的。但是，如果说有14条切线产生在曲率半径约20毫米的齿侧，从真实的渐开线分离，仅仅大约0.5微米。滚刀的制造和安装误差可以超过10微米。使用两头开始的滚刀或斜滚齿机可增加误差水平，因为滚刀的齿距误差的转移到切割齿轮上。拉削 拉削不被用于斜齿轮，但对内齿直齿轮时十分有用的。联系全局来看，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内花键。跟所有的拉削方法一样，这种方法对批量生产是经济的，因为安装成本较高。 拉削技术对内齿斜齿轮主要的应用是由Gleasons在其G-TRAC机床上。这台机器的运作，增加滚齿切割机的有效半径至无限远，使刀具的每一个齿都能在一条直线上转动，而不是对在一个半径上。这使得切割行为延长超过齿轮的整个端面宽度，替代了传统的滚刀每转0.75毫米的进给量。由此产生的过程中提供了非常高的生产率，更适合于美国，美国的产量在整个欧洲来说，相对较低，尽管初始成本高，但非常具有竞争力。拉削提供了较高的精确度和良好的表面光洁度，但象所有切削过程一样，仅限于 “软”材料，必须随后进行表面淬火或热处