SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING LESSON SIX GEAR MANUFACTURING METHODS There are two basic methods of generating process manufacturing gear teeth, the展成法 generating process and the forming forming process process. When a gear tooth is 成形法 generated, the workpiece and the grinding磨削 cutting or grinding tool are in continuous mesh and the tooth form ismesh啮合,网格 generated by the tool. In other word the work and the tool are conjugate to conjugate共轭的 each other. Hobbing machines, shaper cutters, shaving machines, and many‖ hobbing滚削,滚刀 grinders use this principle
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING LESSON SIX 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 word, the work and the tool are conjugate to each other. Hobbing machines, shaper cutters, shaving machines, and many grinders use this principle. generating process 展成法 forming process 成形法 grinding 磨削 mesh 啮合，网格 conjugate 共轭的 hobbing 滚削，滚刀
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING 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 mechanism 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成形,插齿 Shaping is inherently similar to planing but uses a circular cutter instead of a 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 minute. The shape of the cutter is roughly the same as an involute gear but the tips of the teeth are rounded
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING Shaping Shaping is inherently similar to planing but uses a circular cutter instead of a 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 minute. The shape of the cutter is roughly the same as an involute gear but the tips of the teeth are rounded. 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 mechanism, which allows the gear teeth to be formed tooth by tooth. Broaches are examples of form tools that machine all the gear teeth simultaneously. 成形，插齿
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING he generating drive between cutter and workpiece does not involve a rack or leadscrew since only circular motion is 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 I mm to give clearance otherwise tool rub occurs on the back stroke 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 l mm. The accelerations involved require forces of the order of 5000 N(0.5 Ton) yet high accuracy must be maintained leadscrew丝杆
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING The generating drive between cutter and workpiece does not involve a rack or leadscrew since only circular motion is 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 l 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 require forces of the order of 5000 N (0.5 Ton) yet high accuracy must be maintained. leadscrew 丝杆
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING 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 cut annular gears such as those required for large epicyclic drives annular gear内齿轮
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING 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 cut annular gears such as those required for large epicyclic drives. annular gear 内齿轮
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING 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 偏 obvious than an eccentrically mounted or ground cutter will C give a characteristic dropped tooth?. There are several causes 的 for " dropped tooth" but it occurs most commonly when the diameter of the workpiece is about half, one and a 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 error in the cutter occurs between the last and 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
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING 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 a 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 error in the cutter occurs between the last and 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. 偏 心 的
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING 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 tacks"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 hobbing or in the direction of the helix in"oblique" hobbing gash深砍,划开worm蜗杆helⅸx螺旋线 oblique倾斜的
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING 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 "tacks" 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 hobbing or in the direction of the helix in "oblique" hobbing. gash 深砍，划开 worm 蜗杆 helix 螺旋线 oblique 倾斜的
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING 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 150 m/ min for carbide hobs. Typically with a100 carbide硬质合金 mm diameter hob the rotation speed will be 100 pm and so a twenty tooth workpiece roughing粗加工 will rotate at 5 rpm. Each revolution of the workpiece will correspond to0.75 mm feedcoarse大的,粗的 so the hob will advance through the workpiece at about 4 mm per minute. For car production roughing multiple start hobs can be used with coarse feeds of 3 mm 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
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING Metal removal rates are high since no reciprocation of hob or workpiece is required and so cutting speeds 0f 40 m/min can be used for conventional hobs and up to 150 m/min for carbide hobs. Typically with a 100 mm diameter hob the rotation speed will be 100 rpm and so a twenty tooth workpiece will rotate at 5 rpm. Each revolution of the workpiece will correspond to 0.75 mm feed so the hob will advance through the workpiece at about 4 mm per minute. For car production roughing multiple start hobs can be used with coarse feeds of 3 mm 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. carbide 硬质合金 roughing 粗加工 coarse 大的，粗的
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING The disad vantage 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 波 fianks than in the root and is not important when there is a 'x subsequent finishing operation such as shaving or grinding. 剃 When there are no further operations the feed per revolution 齿 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 quaring the feed per revolution and dividing by four times he diameter of the hob.I mm feed and 100 mm diameter gives 2. 5 micron high undulations in the root. On the gear flank (at 70 on a large gear) the undulation is roughly cos 70o as large, 1. e, about 0.85 micron
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING 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, l mm feed and 100 mm diameter gives 2.5 micron high undulations in the root. On the gear flank (at 70ºon a large gear) the undulation is roughly cos 70ºas large, i.e., about 0.85 micron. 波 纹 剃 齿
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING Accuracy of hobbing is normally high for pitch and for helix provided machines are maintained, involute is dependent solely 节 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 of 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 tostart hobs or oblique hobbing gives increased error levels since hob errors of pitching transfer to the cut gear Broaching拉削 spline花键 Broaching is not usually 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 onl economic for large quantities since setup costs are high
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING 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 of 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 usually 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. 周 节 拉削 spline 花键
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING 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 etch 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 gives high accuracy and good sur face finish but like all cutting processes is limited to"" materials which must be subsequently casehardened, or heat treated giving distortion
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING 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 etch 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 gives 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
