The following points highlight the top five methods used for machining gears. The methods are: 1. Formed Tooth Process 2. Template Process 3. Generating Process 4. Bevel Gear Generators 5. Gear Finishing.
Method # 1. Formed Tooth Process:
The theory of formed tooth process is based upon forming the cutter tooth according to the shape of the tooth space to be removed. Theoretically, there should be different- shaped cutter for each size of gear of given pitch, as there is a slight change in the curvature of the involute.
However, one cutter can be used for several gears having different number of teeth without much sacrifice in their operating action. Commercially, each pitch cutter is made in eight slightly varying shapes to compensate for this change. These eight standard involute cutters are listed in Table 31.1.
Cutters for Helical Gears:
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The form cutter used for cutting spur gears on the milling machine is marked with the range of teeth for which it is suitable. For example, Cutter No. 4 is suitable for cutting gears with 26 to 34 teeth.
However, while cutting helical gear, then the size of cutter has to be modified due to the twist on its teeth and the size is:
The forming operation can be performed in three ways viz. milling, broaching and shaping.
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a. Milling:
The formed milling operations are usually employed for cutting spur gear, but these can be employed for cutting every type of gear by using a universal indexing mechanism. The cutter is mounted on the spindle in both horizontal and vertical milling machines and rotates while the work is mounted on the table and reciprocated under the cutter. Once the cutter finishes tooth profile, the work is indexed to the next position, and again the tooth profile is finished and so on.
Fig. 31.1 explains the gear milling process in brief. Gear milling process is employed for coarse pitch gears, racks of all pitches, segment gears, worms and toothed parts as sprockets, and ratchets. It is necessary to have a tool with a special profile for each gear with a different number of teeth and module as per Table 31.1.
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The advantages and disadvantage is cutting gears on milling machines are enumerated below:
Advantages:
(1) All types of gears, i.e. spur, helical, worm, bevel etc. can be cut.
(2) It can be employed both for roughening and finishing operations and fine surface finish can be obtained.
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(3) The initial cost of the cutters is low compared to other types of gears. Gear milling is a less costlier process but less accurate also.
(4) It can be used to machine almost any tooth form and usually confined to producing replacement gears or small lot production, roughing and finishing coarse pitch gears, and finish milling fine pitch gears having special tooth forms.
Disadvantages:
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It is not a production process. Since each tooth space is machined separately and time is lost in returning the job to its initial position and indexing for each tooth.
b. Broaching:
The formed tooth principle can be utilised in a broaching machine by making the broaching tool, conforming to the tooth space. In this process, full form finished gears are produced in one pass by a circular broach having inward-facing teeth. The broaching tool consists of a series of full-form finishing rings at the end of a series of generating ring. All the rings are keyed and assembled in octagon-shaped broach holder. It is important that adequate chip space be provided and provision be made of staggered chip breakers on successive broach teeth.
However, this process has limited application, because of high initial cost of tooling and is only used where mass production technique is to be applied. Gears can be broached accurately since a number of elements are controlled by the accuracy of the broach. It can also produce fine finish. It is usually used only for internal spur and helical gears though it can be used for external gears also.
c. Shear Cutting of Gears:
It utilises a cutter made from form-tool blades so shaped that the gap between these blades forms the desired gear teeth shape. It is an operation very similar to broaching. It cuts all teeth of external spur forms simultaneously at a high rate of production. It is possible to form-cut involute spur gear teeth with practically any required tooth modification. Even unsymmetrical or unequally spaced spur-tooth forms can be cut. The cutting tools are easily removed for sharpening and are sharpened all at one time on a surface grinder. The process is limited to only coarse-pitch gears having spur teeth.
Method # 2. Template Process:
In this method, the form of the tooth is controlled by a template which guides and reciprocates the tool. The tool is similar to a side cutting shaper tool and is given a reciprocating motion in the process of cutting. The machine used is called gear planer.
The frame of gear planer carries the reciprocating tool and is guided at one end by a roller acting against the template, while the other end is pivoted at a fixed point. Three sets of templates are used, one for roughening cut and one for finishing each side of tooth space. The gear blank is held stationary during the process and is moved only when indexed.
A characteristic feature of the template type of gear planer is the template or master former which serves to guide the planning tool, thus causing it to plane teeth having the correct shape or curvature. When the planer is at work, a slide or head which carries the tool is given a reciprocating motion, and as the tool feeds inward for each stroke, the path it follows is controlled by the template.
There are two general methods of machining the teeth by template method. In one, the teeth are roughened out with a single-pointed tool and then finished with a formed tool which removes the feed marks and gives the teeth a smooth finish.
In the other method, both roughening and finishing cuts are taken with single pointed tools. The use of the formed tool for finishing is impracticable for the larger pitches which are finished by a single pointed tool. The number of cuts required depends upon the size of the tooth, amount of stock to be removed, and the kind of material.
This methodis specially adopted to cutting large teeth which are difficult to cut by formed cutter, and also to cut bevel-gear teeth. It is not widely used at present.
Method # 3. Generating Process:
In cutting gears by using a generating type of machine, the gear teeth are formed as a result of certain relative motions between gear blank and cutter, instead of simply reproducing the shape of a formed cutter. A generating process is capable of enabling a cutter of a given pitch to cut gears having different numbers of teeth to the correct shape.
Gear cutter generating process is based on the fact that any two involute gears of same pitch will mesh together. Hence, if one gear is made to act as a cutter and is given a reciprocating motion, as in shaper, it will be capable of cutting into the gear blank and generating conjugate teeth forms.
Generally, this generating principle is applied in three ways, one with planing process using involute rack cutter, other with hob cutters and the third in gear shapers in which both reciprocating as well as rotating cutting principle are applied.
(a) Gear Planing:
In gear planing process, the cutter consists of true involute rack which reciprocates across the face of the blank and the blank rotates in the correct relationship to the longitudinal movement of the cutter as if both roll together as a rack and pinion (Refer Fig. 31.5). Initially the cutter is fed into full tooth depth with cutter reciprocating and blank stationary. Involute shape is generated as the blank rotates and involute rack cutter feeds longitudinally.
After completion of one or two teeth, the blank and cutter stop feeding and the cutter is withdrawn and indexed back to its starting position, thus enabling a short rack cutter of a practical length to be used. Cutter is again fed back to depth and cycle is repeated. Number of teeth is controlled by the machine gearing, and pitch and pressure angle by the rack cutter. This method is used for generation of external spur gears, being ideally suited for cutting large, double helical gears. For producing helical teeth, the cutter slides are inclined at the gear tooth helix angle.
(b) Gear Hobbing:
Hobbing is a process of generating a gear by means of a rotating cutter called a hob. It is a continuous indexing process in which both the cutting tool and work piece rotate in a constant relationship while the hob is being fed into work.
A hob resembles a worm, with gashes made parallel to its axis to provide cutting edges. For involute gears, the hob has essentially straight sides at a given pressure angle. The faces of the hob teeth are relieved radially to form clearance behind the cutting edge.
The hob is fed into the gear blank to the proper depth and the two are rotated together as if in mesh. The teeth of the hob cut into the work piece in successive order and each in a slightly different position. Each hob tooth cuts its own profile depending on the shape of cutter (which in case of involute hob is straight sided), but the accumulation of these straight cuts produces a curved form of the gear teeth, thus the name generating process. One rotation of the work completes the cutting upto certain depth upto which hob is fed unless the gear has a wide face.
Gear hobbing is faster than milling because several teeth are cut at a time and because of the continuous meshing process. Milling requires that the cutter and work disengage before indexing can occur. The milling machine can cut only one tooth at a time, while the hob operates on several teeth at a time.
The gear cutting with a hob involves three basic motions, all of them occurring at a time. The hob and the blank have a rotating motion and the third one is the radial advancement for the hob, thus causing the cutting and indexing simultaneously.
The hob or cutter may be set with its teeth parallel to the axis of the gear blank when spur teeth are to be cut. If helical teeth are to be cut, the axis of the hob can be set over at an angle to produce the proper helix. The hob axis is set at angle, equal to helix angle of the thread in reference to the axis of the gear blank. This brings the blank teeth in the plane of the hob’s teeth.
This plane is termed as generating plane. The cutter finishes all the teeth in one pass over the work. Selection of proper gear ratio enables the hob to rotate in correct relation to the gear-blank, so as to cut the specified number of teeth. If several gears are to be made at one loading of the machine, the hob is caused to move along the blanks after each revolution of the work.
The principal motions required to produce a spur or helical gear are shown in Fig. 31.8. An accurate relationship is required between various elements of the machine in order to achieve the desired results. Work table and hob have to rotate in definite relationship obtained by index change gears.
Index change gear formula is:
For hobbing helical gears, the rotation of the work table is slightly advanced or retarded in relation to the rotation of the axial feed screw by means of another set of change gears called differential change gears. The differential change gears are calculated to satisfy the following basic relation between the axial feed screw and the work table rotation.
Revolutions of axial feed screw x lead of axial feed screw
= additional revolutions of work table x lead of work piece helix.
A typical hobbing machine can produce gears with an accumulated errors of tooth spacing not greater than 0.02 mm.
Hobbing machines can produce spur, helical, and herringbone gears, as well as splines, gear sprockets and other shapes. They cannot produce unsymmetrical shapes, such as interrupted tooth gears which a gear shaper can do. Because of the rotating cutter, they cannot cut close to a shoulder as a reciprocating tool. Hobbing is the most rapid of the generating processes.
Characteristics, Merits and Limitations of Hobbing Process:
(i) Any external tooth form which is uniformly spaced about the centre (not necessary involute form or a form symmetrical about an individual axis) so that all the teeth are identical, can be hobbed using suitable hob.
(ii) One hob of a particular module can be used to cut teeth of all involute spur and helical gears of any number of teeth of same module and pressure angle. It is thus a versatile process and can be used to cover a variety of work like spur, helical, worm, splines and a variety of special forms.
(iii) Accuracy of gears produced by hobbing is dependent upon the accuracy of machine, hob and the blank, care in mounting work and hob, and rigidity of the tooling.
(iv) The indexing is continuous and there is no intermittent motion to give rise to errors. There is no loss of time due to non-cutting on the return stroke.
(v) Finish is dependent on the amount of feed and upon the accuracy of hob and rigidity of tooling also.
(vi) It can’t be used to cut bevel and internal gears and for gears having adjacent shoulders larger than the root diameter of gear and close enough to restrict the approach or run-out of the hob.
(c) Gear Shaping:
A ‘generated’ surface, whether it is curved or straight, is produced by continuous motion of a point, a line or a surface. The direction in which the generating point travels determines the shape produced. The simplest example is the generation of a cylinder on a lathe. The cutting point of the tool generates a circle when the work piece is rotated. As the tool is led axially, cylindrical component is produced.
The geometrical accuracy of the circular component depends on:
(a) The trueness of the work piece rotation.
(b) Parallelism of the tool movement with the work- spindle axis.
(c) The effect of tool wear.
It does not depend on the form of tool profile. Besides, the process is continuous.
In gear shapers, the above generating principle is applied in the following way. Refer Figs. 31.9 (a) and (b).
The cutter is hardened disc shaped and slightly dished at the bottom (ground with top rake and clearance) to facilitate cutting. The teeth have desired tooth profile and pressure angle.
The cutter is reciprocated with the required cutting speed for stock removal along the face of the work piece, and is gradually fed radially to plunge if for correct teeth depth by means of a cam. A relieving mechanism enables the cutter to clear the work on noncutting or return stroke.
The continuous generation motion is obtained by feeding cutter to full depth and rotating the cutter and the work piece slowly in the exact ratio of their respective number of teeth by means of a chain of gears including change gears. The cutter is considered as the driving gear and the work piece as the driven member, exact transmission of motion being effected by an external link of gear train.
The work piece gear is fully generated when the cutter has plunged to correct tooth depth (set previously) into the work piece and the teeth are cut fully and uniformly over the entire periphery of the work piece gear. The machine comes to rest automatically after the work piece gear is fully cut.
The cutting action of a pinion type of cutter to generate a gear is shown clearly in Fig. 31.10. The principle of cutting gear by the generating method has the advantage that with a particular module of DP cutter it is possible to cut accurately gears having identical module or DP, but different number of teeth. This makes the generating process functionally superior and economical for the manufacture of gears.
Thus in brief, it could be seen that since an involute gear will roll with any other involute gear of same normal base pitch and hence this principle has been utilised in the gear shaper (a method of gear manufacture). In this cutter is made in the form of pinion provided with relieved cutting edges, both cutter and work are given rotary motions about their respective axes to simulate the conditions obtaining, had both members been complete gears rolling together at the correct centre distance.
This is achieved by gearing the cutter to the work in the same ratio as the numbers of teeth in the cutter and the finished work. Cutting is effected by imparting axial reciprocating motion to the cutter coincident with its rolling action, the length of stroke of the ram being slightly more than the width of the blank.
Fig. 31.13 shown a general duty machine for cutting external spur and helical gears.
For spur gear the reciprocating motion is made parallel to the work axis, but for single helical gear the direction of the cutting stroke follows a helical path and the cutter itself is provided with helical teeth. The cutter is fed radially into the blank in succeeding cuts until full depth is attained. The teeth formed have same pressure angle as that of the cutter on the pitch circle of the generation.
The provision of two opposed cutter guides, with their axes horizontal, along which one cutter advances in the working stroke in turn while the other is being withdrawn, enables double helical gears to be cut. To prevent the cutter rubbing during the withdrawal, the cutter ram is rocked slightly away from the work as soon as the working stroke is completed.
In rack planing of double helical gears, the blanks are often provided with a turned groove to separate the helix, but continuous teeth can be produced without a central flash by over running the helix width by 0.25 mm.
Helical gears upto helix angle of 23° can be produced using helical cutter of opposite hand than the helix hand required on work piece.
Advantages:
(1) The gears produced by this method are of very high accuracy.
(2) Both internal and external gears can be cut by this process.
(3) Non-Conventional types of gears can also be cut by this method.
Disadvantages:
The production rate with gear shaper is lower than hobbing. There is no cutting on the return stroke in a gear shaper. Worm and worm wheels can’t be generated on a gear shaper.
Method # 4. Bevel Gear Generators:
Since the teeth of bevel gears constantly change in form from the large to the small end, it is impossible to form the bevel teeth but these have to be generated.
There are two common types of bevel-gear generators, one cuts straight teeth and other cuts spiral teeth.
(a) Straight Bevel-Gear Generator:
This machine has two reciprocating tools which work on top and bottom sides of a tooth and are carried on the machine cradle. The cradle and work roll up together like a crown gear (a bevel gear in which tooth faces lie in one plane), rolling with the gear blank.
At the top of roll, when a tooth has been completely generated, the work is withdrawn from the tool, and the machine indexed while the cradle is rolled down to the starting position. The operating cycle is repeated automatically until all the teeth in the gear have been cut.
The advantages of this process are that a previous roughening cut is not necessary, thus saving one handling of the blank, longer cutter life, improved quality of gear and less set up time.
(b) Spiral Bevel-Gear Generator:
In this machine, a rotating circular cutter generates spiral teeth that are curved and oblique. Proper tooth profile shapes are obtained as a result of relative motion in the machine between work and cutter. The machine has adjustments by which both spiral-bevel gears and hypoid gears can be generated.
Spiral bevel gears have an advantage over straight bevel gears in that teeth engage with one another gradually, eliminating any noise and shock in their operation.
Method # 5. Gear Finishing:
Although many gears ‘as cut’, either hobbed, planed or shaped are entirely satisfactory for their intended application, for others an additional finishing process is either necessary or desirable. All the mechanical finishing processes in common use are intended to amend tooth shape by making the flanks conform more nearly to the true or modified involute desired and to improve surface finish and tooth spacing.
The following processes are generally used for finishing of gears:
(i) Gear shaving or burnishing.
(ii) Gear grinding.
(iii) Gear lapping.
(iv) Shot blasting.
(v) Phosphate coating.
(vi) Gear Tooth Honing for Precision.
(i) Gear Shaving or Burnishing:
It is the newest method of gear finishing. It is cold working process accomplished by rolling the gear in contact and under pressure with three hardened burnishing gears. In this case, a cutter harder than the work and in the form of conjugate gear which meshes with it in such a way that when rotated together, relative sliding between the cutter and work teeth obtains, is used. The teeth of the cutter are serrated normal to the tooth profiles and in operation, the cutter and work are meshed together as helical gears with the planes of their respective axes crossing.
In action one member of the pair is driven and makes the other rotate, the new parallelism of the axes causes sliding action between the teeth. The value of the crossed axis angle controls the finish produced to some extent since smaller the angle, finer the finish. Angles ranging between 8° to 45° are generally found most satisfactory.
Shaving improves gear tooth finish where the cutting process has not provided the required standard, e.g., that with high speed hobbing; but the cut gear must have small errors only in pitch, profile and concentricity.
The process is ideal for automotive gear box gears after hobbing and before hardening. The disadvantage of this process is that the surface of the tooth is covered with amorphous or smear metal rather than metal having true crystalline structure.
(ii) Gear-Grinding:
Heat treated gears can be finished either by grinding or by lapping. This process of gear finishing is becoming obsolete these days as the shaving process is quite satisfactory and cheaper than gear-grinding. But when the high accuracy associated with profile grinding is required, it is the only process to be used.
By grinding, teeth can be finished either by generation or forming. In the former the work is made to roll in contact with a flat faced rotating grinding wheel which corresponds to the face of the imaginary rack meshing with the gear.
One side of the tooth is ground at a time. In the latter, the grinding wheel is given the shape as formed by space between two adjacent teeth and both flanks are finished together. The second method tends to be rather quicker, but both give equally accurate results and which of the methods is to be used depends upon the availability of the type of grinding machine.
The disadvantage of gear grinding is that considerable time is consumed in the process. Also the surfaces of the teeth have small scratches or ridges which increase both wear and noise. To eliminate these defects ground gears are frequently lapped.
(iii) Gear-Lapping:
It is another extensively used process of gear finishing and is accomplished by having the gear in contact with one or more cast iron lap gear of true shape. The work is mounted between centre and is slowly driven by rear lap. It in turn drives the front lap, and at the same time both laps are rapidly reciprocated across the gear face. Each lap has individual adjustment and pressure control.
A fine abrasive is used with kerosene or light oil to assist the cutting action. The largest time of gear lapping is about 15 minutes; prolonged lapping damages the profiles but in exceptional cases the time may be increased, e.g., for profile and pitch correction. Automotive gear box gears finished before case hardening by shaving are usually finally lapped.
(iv) Shot Blasting:
It provides a finishing process resembling that produced by lapping although it has other functions, such as removing slight burrs, reducing stress concentration in tooth fillets and sometimes providing slight tip and root relief to teeth.
(v) Phosphate Coating:
It is a chemical process which attacks the treated ferrous surface and leaves a deposit on it of about 0.01 mm in thickness. It prevents or retards scuffing, particularly in hypoid gears, apparently by permitting the engaging tooth surfaces to embed more readily under the prevailing boundary lubrication conditions.
(vi) Gear Tooth Honing for Precision:
Gear honing is a finishing process that can be applied to external and internal spur or helical gears. Honing can improve the sound quality of shaved, hardened gears by removing nicks and burrs. Also, the process can improve involute, lead, tooth spacing, pitch diameter run-out and surface finish. Typically, shaved gears have a tooth surface finish ranging from 0.6 to 1 micron while honed gears are in the 0.2 to 0.35 micron range. Honing always improves the characteristics of shaved, hardened gears.
Honing can also be used to prolong the wear life and increase the load carrying capacity of hardened ground gears by improving surface finish to a point where up to 80 per cent surface contact can be achieved.
Honing Tools:
Honing tools are usually throwaway types that are discarded at the end of their useful life. They are abrasive-impregnated tools in the form of a helical gear.
The honing tool teeth are thinned as the tool wears. This tooth thickness reduction can continue until the hone outside diameter causes root or fillet interference with the work gear. Then the honing tool outside diameter must be reduced by a dressing tool. It is not unusual for a tool to hone from 5,000 to 8,000 pinion gears in a production operation.
Eventually, the thinning of the hone teeth results in interference at the root of the hone teeth with the work gear outside diameter, at which time the hone is considered to have ended its useful life. In some cases the hone root can be recut (gummed out) with a grinding wheel in the tool-room to provide additional life.
General purpose honing tools are made in a variety of resin and abrasive mixes for gears that have been shaved and heat treated. They are available in eight grit sizes ranging from 50 to 600 grit.
Moreover, special resin-abrasive mixes are available to suit the following production applications:
(i) Honing hardened ground gears to improve surface finish and make minor tooth corrections.
(ii) General purpose long wear honing that provides relatively fast, heavy stock removal from shaved, hardened gears.
(iii) High impact strength and fast cutting action for high production nick removal on shaved, hardened gears.
(iv) Removing heavy nicks in high production shaved, hardened gears.
Precision honing tools, similar to the general purpose type, have tooth forms (all critical dimensions within 5 micron) that do not require break in. They are used on precision hardened ground gears to improve spacing and run-out, and provide surface finishes ranging from 0.2 to 0.25 micron.
In order to obtain extra critical surface finishes around 0.15 micron and maximise the total tooth contact area on ground or shaved gear teeth that have been honed, use a Poli Tool. This tool is a flexible, porous polyurethane polishing tool that uses an abrasive liquid compound (about 500 grit) during the finishing process. Honing tools are made in diameters from 85 to 320 mm with 25, 40, 50 mm face widths. Most gear hones are made with approximately 225 mm diameter.
Honing Machines:
Honing tools arc mounted on special machines in a crossed axes, controlled mesh relationship with the gear to the honed. During the honing cycle, the work gear is run with the honing tool at about 180 mpm. The honing tool is traversed back and forth across the gear face and the direction of rotation of the honing tool is reversed at the end of each stroke. Particles removed by the honing process are flushed away with conventional honing oil.
There are four methods used to apply pressure between the work piece and the honing tool (not to exceed 10 kg). Honing machines are equipped with a tilting table and air feed arrangement that can be either locked in position to provide pressure control or advanced with an air cylinder to provide constant pressure.
Methods used for honing hardened shaved or ground gears for sound quality and accuracy improvement are:
(1) Zero backlash with constant pressure or
(2) Loose backlash, which is often used for internal gears with braking on one axis.
For salvage operations where run-out corrections are required, use zero backlash with loose locked overload relief or zero backlash with heavy lock. In salvaging operations, the amount of honing should be limited and stock removal should be done slowly.
Gears with crowned teeth can be honed by all of these methods. The worktable on the honing machine is rocked during the honing cycle. The amount of crown can be selected and controlled by a precision cam mechanism. Taper honing can also be performed by adjusting the angular table setting.
The production rate of honing operations depends on pitch diameter and face width. A gear 25 mm dia x 25 mm wide can be honed in about 15 sec. A gear 600 mm x 75 mm wide will require about 10 min to hone. Of course, honing of salvage gears requires longer cycles. Stock removal with honing ranges from 10 to 50 micron measured over pins.
Although modern gear-cutting and gear-generating machines are capable of producing correctly formed, but it is advisable to test gears before assembly particularly those required for high speed and accurate machining.
The tests may vary from condition to condition but include factors such as concentricity, size, noise, tooth bearing, spacing etc. All tests should be made with accurate and rigid equipment and in shortest possible time. The best method of testing and inspection is one which most nearly stimulates the actual working conditions of the gears.