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**Spur gear
FIG. 3. **

**pitch circle is shown in Fig. 4 in its (elation to the gear-teeth. **

**§ 17 GEAR CALCULATIONS.
8. circle.
5
The pitch diameter is the diameter of the pitch When the word " diameter " is applied to gears, it is always understood to mean the pitch diameter, unless otherwise specially stated, as outside diame-ter, or diameter at the root. **

**9. The distance from a point on one tooth to the corresponding point on the next tooth, meas-ured along the pitch circle, is called the circular pitch ; it is obtained by dividing the length of the circumference of the pitch circle by the num-ber of teeth in the gear. The circular pitch is shown in Fig. 4.
10. Diametral pitch is the number of teeth in a gear divided by the number of inches in the diameter of the pitch circle. It has also been defined as the num-ber of teeth in a gear 1 inch in diameter. It is obtained by dividing the number of teeth by the pitch diameter, and hence is equal to the number of teeth per inch of diameter. A gear, for example, has 60 teeth and is 10 inches in diameter ; its diametral pitch is -1-(v) = 6, and the **

**6
GEAR CALCULATIONS. § 17
gear is therefore called a 6-pitch gear. It therefore follows that teeth of any particular diametral pitch are of the same size and have the same width on the pitch line, whatever may be the diameter of the gear. Thus, if a 12-inch gear has 48 teeth, it will be 4-pitch. A 24-inch gear having teeth of the same size will have twice 48, or 96, teeth—since its cir-cumference is twice as long—and its diametral pitch is 96 24 = 4, the same as before. Fig. 5 shows the sizes of teeth of various diametral pitches. The diametral pitch multiplied by the circular pitch of the same gear equals 3.1416. Using for illustration, a wheel 10 inches in diameter with 60 teeth, we have circumference 10 x 3.1416. - Circular pitch = = = .5236 inch. number of teeth 60
number of teeth 60 Diametral pitch = — = 6. diameter — iu
The product of the two is 6 X .5236 = 3.1416. 11. The thickness of gear-teeth means the thickness measured on the pitch circle, as shown in Fig. 4. 12. The space in gear-teeth means the space between gear-teeth measured on the pitch circle. The thickness of a gear-tooth plus the space equals the circular pitch. The addendum is the part of a gear-tooth outside the pitch circle, as shown in Fig. 4. The addendum circle is a circle through the extreme outside of the gear-teeth ; its diam-eter is equal to twice the addendum plus the pitch diameter. 13. The root, or dedendum, is the part of the teeth inside the pitch circle, as shown in Fig. 4. The root circle is the circle that limits the bottom of the space between the teeth. The roots of the teeth are usually connected to the root circle by a short curve, so that there shall be no sharp corner. The curve that fills up the corner, as shown in Fig. 4, is called a fillet. 14. Backlash is the side clearance between two teeth in mesh ; it is equal to the difference between the space and the thickness of a tooth, measured on the pitch circle. **

**§17.
GEAR CALCULATIONS. page 7
15. The clearance is the space between the top of a tooth and the bottom of the space into which the tooth meshes when they are on the line connecting the centers of the gears. It is equal to the root minus the addendum. 16. The line of centers is the straight line drawn from the center of one gear to the center of another, with which it works when the pitch circles touch each other. The line of centers passes through the point of tangency of the two pitch circles.
17. The pitch point of a tooth curve is the point in which the outline of the tooth intersects the pitch circle. The term " pitch point " is defined in books on machine design as the point of tangency of the pitch circles of two gears working together. The first definition, however, is more in accordance with the use of the term in shops, and will be used here.
18. The face of a tooth is the working surface of the tooth from the pitch line to the top of the tooth ; the flank is the surface from the pitch line to the bottom of the tooth. 19. The length of a tooth is the distance from root circle to the addendum circle ; it is equal to one-half the difference between the root diameter and the outside diameter.
20. The breadth, or width, of a tooth is the distance from one flat surface or end to the other, or the distance from one top edge to the other top edge ; it is measured at right angles to the length of the tooth. 21. A gear blank is the cylindrical piece of metal or other material in the outer circumference of which gear-teeth are to be cut. The blank is turned up equal to the outside diameter of the gear, and the teeth are then cut about its periphery. 22. When two gears are so located that their teeth run together, the gears are said to be in mesh. **

**. **

**GEAR CALCULATIONS. § 17 23.**

**A rack is a gear with a pitch circle having an infinite radius, that is, the pitch cylinder has become a plane, so that all the teeth are arranged in a straight line. 24. A pinion is a small spur gear. The term is used especially for small gears that mesh with racks.
PROPORTIONS FOR GEAR-TEETH. 25. Gear-Teeth Based on Circular Pitch.-The relative proportions of gear-teeth are usually based on the circular pitch. It is customary to have the addendum, the whole depth, and the thickness of the tooth conform to some arbitrary part of the circular pitch. There is no uni-formly adopted standard, and gears made in different ways require different proportions. Cut gears require less back-lash and clearance than cast gears. This is because the teeth of cut gears are more uniform in size, regular in out-line, and truer in form than those of cast gears. Gears of large diameter require less backlash than those of small diameter. The proportions given in Table I are those in common use.
TABLE I. **

**GEAR-TOOTH PROPORTIONS.
I 2 3 Addendum 3o C .3o C .3o C Root 40 C .40 C. .35 C Working depth of tooth .6o C .6o C .6o C Total depth of tooth... .7o C .70. C .65 C Clearance 10 C .10 C .05 C Thickness of tooth 45 C .475 C .485 C Width of space 55 C .525 C .515 C Backlash. 10 C .05 C .03 C
4
1P. 1.157=-P to 1.125 P 2 2. 157 . 157 4- P to . 125 ± 1.51 P to 1.57 P 1.63 P to 1.57 P . 12 P to o
Recently some manufacturers have made gears with shorter teeth than those indicated in Table I. In some **

**§ 17 GEAR CALCULATIONS. page 9
*cases the total depth of teeth was not more than five-tenths the circular, or 11 divided by the diametral, pitch. The width of tooth and space remained the same except that the backlash might be slightly reduced. The Brown & Sharpe Company make the clearance one-tenth the thickness of the tooth on the pitch line, and The Pratt & Whitney Company make it one-eighth the addendum. The proportions given in the foregoing table have been used successfully and will serve as an aid in deciding upon suitable dimensions. Column 1 is for rough cast gears where the teeth are very irregular, and consequently a large amount of backlash and clearance is required ; column 2 is for the better class of cast gears ; column 3 is for cut gears ; and column 4 is for diametral pitch for cut gears. C stands for circular pitch, and P for diametral pitch. 26. Proportions of Gear-Teeth Based on Diam-etral PitCh.—As the gears most often met with are cut gears of diametral pitch, it would seem natural to propor-tion gear-teeth with the diametral pitch as a basis. It is much simpler to calculate gears using diametral pitch than when using circular pitch ; hence, this system is coming into very general use for all classes of gearing. Column 4 of Table I gives the proportions of gear-teeth based on this system as used by the leading manufacturers in this country. **

**RULES FOR SPUR-GEAR CALCULATIONS. 27. Relation Between Circular Pitch and Diam-etral Pitch.—The product of the circular pitch of a gear and the diametral pitch is always the constant number 3.1416. Hence the following rules : Rule.— To change circular pitch to diametral pitch divide 3.1416 by the circular pitch. EXAMPLE.-If the circular pitch is .3927 inch, what is the diametral pitch ?**

** pg 10--page 11**

**10 GEAR CALCULATIONS.
§ 17 SOLUTION.-Applying the rule just given, the diametral pitch is 3.1416 .3927 = 8. Ans.
28. Rule.— To change diametral pitch to circular pitch, divide 3.1416 by the diametral pitch. EXAMPLE.-If the diametral pitch is 4, what is the circular pitch ? SOLUTION.-Applying the above rule, the circular pitch is 3.1416 = .7854 in. Ans. 4 29. Relation Between Pitch Diameter, Number of Teeth, and Diametral Pitch. — The relation between the pitch diameter, number of teeth, and diametral pitch is expressed in the following rules : Rule.— To find the number of teeth when the pitch diam-eter and the diametral pitch are known, multiply the pitch diameter by the diametral pitch. EXAMPLE.-If a wheel is 30 inches in diameter and 3 pitch, how many teeth has it ? SOLUTION.-Applying the rule just given, the number of teeth is 30 x 3 = 90. Ans. 30. Rule.--To find the pitch diameter when the num-ber of teeth and the diametral pitch are known, divide the number of teeth by the diametral pitch. EXAMPLE.-What is the pitch diameter of a 21-pitch gear having 20 teeth ? SOLUTION.-By applying the rule just given, we find the diameter to be
20 = 8 in. Ans. 31. Rule.— To find the diametral Pitch when the number of teeth and the pitch diameter are given, divide the number of teeth by the pitch diameter. EXAMPLE.-If a gear contains 50 teeth and has a pitch diameter of 10 inches, what is its diametral pitch ?. SOLUTION.-Applying the rule,
50 = 5 diametral pitch. Ans. **

**17 GEAR CALCULATIONS.
page 11
32. Finding the Outside Diameter of a Gear-Blank.—The diameter to which the blank for a spur gear should be turned is equal to the outside diameter of the ' gear. By reference to Fig. 4 it is seen that the outside diameter, and, hence, the diameter of the blank, is equal to • the pitch diameter plus twice the addendum. With the diametral-pitch system, in which the addendum is equal to 1 divided by the pitch, the outside diameter may be calculated from the pitch diameter and the pitch by an application of the following rule :
Rule.— To find the outside diameter, or the diameter of the blank, when the pitch diameter and the itch, multiply the quotient by 2, and add the product to the pitch diameter.
EXAMPLE.-What should be the diameter of a gear-blank for a
the following rule :
Rule.— To find the outside diameter, or the diameter of the blank, when the diametral pitch and the number of teeth are known, add 2 to the number of teeth and divide the sum by the pitch.
EXAMPLE.-A wheel is to have 48 teeth, 6 pitch ; to what diameter ust the blank be turned ? SOLUTION.—By the rule just given, the outside diameter is 48 + 2 = 8.333 in. Ans. 6
34. Calculations Based on the Outside Diameter. -- The diameter of the blank and the pitch being **

**Gear-Calculations-Mach-1908-ICS.html**

**page 12 GEAR CALCULATIONS.
§ 17
given, the number of teeth may be calculated from the fol-lowing rule : Rule.—To find the number of teeth when the outside diameter of the blank and the diametral pitch are known, multiply the outside diameter by the pitch and subtract 2 from the product. EXAMPLE. —A gear-blank measures 10i inches in diameter and is to be cut 4 pitch. How many teeth should the gear-cutter be set to space ? SOLUTION.-By applying the rule just given, we find the number of teeth to be
10i x 4 — 2 = 42 — 2 = 40 teeth. Ans. **

**35. Rule.— To find the diametral pitch when the outside diameter and the number of teeth are known, add 2 to the number of teeth and divide the sum by the outside diameter. EXAMPLE.-It is required to select a cutter for a. gear having 54 teeth that is to mesh with the change gears of a lathe. One of the change gears, which has 64 teeth, measures 6.6 inches, outside diam-eter ; for what pitch should the cutter be selected ? SOLUTION.-Applying the rule given, the pitch of the change gear is found to be 64 ± 2 = 10 ; hence, this is the pitch of the cutter 6.6
required. Ans.
36. In applying the rule given in Art. 35, it will some-times be found that the result obtained does not correspond with any standard pitch number ; for example, a gear with 68 teeth measures 15196 inches in outside diameter. Apply-ing the rule to these values, the pitch would be 68+2 15.5625 = 4.4979+ ; this number is so near to 41, which is a stand-ard pitch, that it is evident that 44 is the pitch of the gear and that either the blank was not turned to the exact diameter called for by the pitch and number of teeth or that the exact diameter was not determined by the measurement. When a set of standard gear-cutters is available, the pitch of the gear can also be determined by trying different cutters until one is found that fits. **

**§ 17
GEAR CALCULATIONS.
page 13
A considerable difference between the value obtained by applying the rule and the nearest standard pitch will indi-cate that an uncommon pitch has been used. In general, however, it may be assumed that the pitch is the standard whose number agrees most nearly with the value obtained from an application of the rule. As far as practicable, the pitch and diameter should be so chosen that the number of teeth will correspond to a num-ber of divisions that can be readily obtained with the aid of the indexing mechanism of the machine in which the gear is to be cut. **

**37. Relation Between Pitch Diameter, Number of Teeth, and Circular Pitch.—If any two of the fac-tors named are given, the other can be found by applying one of the following rules: Rule.—To find the diameter of the pitch circle when the number of teeth and the circular pitch are known, take the continued product of the number of teeth, the circular pitch, and .3183.
EXAMPLE.-What is the pitch diameter of a gear-wheel that has '75 teeth and whose circular pitch is 1.625 inches ? SOLUTION.-Applying the rule, the diameter is found to be
1.625 x 75 x .3183 = 38.79 in. Ans.
38. Rule.—To find the circular pitch when the pitch diameter and the number of teeth are known, multiply the pitch diameter by 3.1416 and divide the product by the num-ber of teeth. EXAMPLE. —What is the circular pitch of a gear 32 inches in diam-eter and having 84 teeth ? SOLUTION.-Applying the rule just given, the circular pitch is found to be
32 x 3.1416 = 1.1968 in. Ans. 84
39. Rule.— To find the number of teeth when the pitch diameter and the circular pitch are known, multiply the pitch**

** 12 GEAR CALCULATIONS. page ****14 GEAR–CUTTING.
§ 18
slide d that is moved parallel to the axis of the spindle a, and is fed automatically to the work and returned. The cutter is adjusted for depth by lowering the gear blank. A limited side adjustment is usually provided for the cutter to allow cutters of different thicknesses to be set central. 30. Automatic gear-cutting engines are often arranged so that they can be used for cutting approximately correct bevel gears. The slide that carries the cutter is then ar-ranged in such a manner that it can be set at the required angle to the axis of the spindle. 31. Change Geafing.—The gearing that revolves the shaft carrying the worm is, as a general rule, actuated by a so-called stop-shaft, which is provided with a suitable clutching mechanism operated by the cutter slide. This clutching mechanism is so arranged that it allows the stop-shaft to make exactly one revolution whenever the return-ing cutter slide unlocks it. The change gears that will produce a certain number of divisions are selected in ac-teeth in worm-wheel cordance with the ratio In case of teeth to be cut simple gearing, this is the simple ratio that gears are to be selected for ; in case of compound gearing, it is the com-pound ratio, which is resolved into factors. The gears are selected in the same manner as is done in gearing a lathe for thread cutting or a milling machine for the cutting of helixes. In adjusting the gear-cutting engine, the tripping ar-rangement for the stop-shaft clutching mechanism must be set so that it will act only after the cutter on its return stroke is entirely clear of the gear.
CUTTING BEVEL GEARS WITH FORMED CUTTERS. 32. Selecting the Cutter.-While bevel gears cut with a cutter of fixed curve can be only approximately cor-. rect, the comparative cheapness of this method ha`s led to its being largely used. The ordinary cutters made for spur **

**§ 18 GEAR–CUTTING. 15
wheels should never be used -for this purpose, as they will cut the teeth of the bevel gear entirely too thin at the small end. Special miter-gear and bevel-gear cutters are made for this purpose ; these cutters are of the involute form, but thinner than the standard cutters. They are numbered from 1 to 8, and cover the same range as the standard involute cutters. A bevel-gear cutter cannot be selected in the same manner as the ordinary spur-gear cut-ter, that is, directly in accordance with the number of teeth ,of the bevel gear. It is to be selected, instead, for a number of teeth that is calculated by one of the rules given below, the first of which is as follows: Rule. To find the number of teeth a bevel-gear cutter is to be selected for, divide the number of teeth of the bevel gear by the natural cosine of the center angle a d e, Fig. 6. EXAMPLE. — The center 4. angle of a bevel gear having 24 teeth is 53° 15'. What number of teeth should the cutter be selected for ? SOLUTION.—The cosine of 53° 15' is .59832. Applying the rule, we get 24 = 40 teeth. Referring to the Table of Standard Cutters, .59832 we find that for gears having between 35 and 54 teeth, a No. 3 cutter is to be used. Hence, use a No. 3 bevel-gear cutter. Ans. 33. When a drawing of the bevel gear is available, use the following rule: Rule. Measure the slant height of the back cone, as a b in Fig. 6; double: it and multiply by the diametral pitch. The product will be the number of teeth the cutter is to be selected for. EXAMPLE.—The slant height of the back cone being 5 inches, and the diametral pitch being 4, what number of bevel-gear cutter is to be used ? SOLUTION.—Applying the rule just given, we get 5 X 2 X 4= 40 teeth. Referring to the Table of Standard Cutters, it is seen that a No. 3 bevel-gear cutter is to be used. Ans.
34. Setting the Machine.-The cutter having been selected, place it on its arbor; put the gear blank into the 50 —36 **

**ppage 16 GEAR CALCULATIONS, section § 17**

**their respective velocity ratios are fixed, multiply twice the distance between centers by the velocity ratio of the smaller gear and divide the product by the sum of the velocity ratios of the two gears. EXAMPLE.-The smaller of two gears is to run five times as fast as the larger. The distance between centers being 12 inches, what must be the pitch diameter of the larger gear ? SOLUTION.-By an application of the rule just given, the pitch diam-eter of the larger gear is found to be 2 X 12 X 5 = 20 in. Ans. 5 ± 1
43. Rule.— To find the diameter of the smaller gear in a pair when the distance between centers of the two gears and their respective velocities are fixed, multiply twice the distance between centers by the velocity of the larger gear and divide the product by the sum of the velocities of the two gears. EXAMPLE.-Taking the last example again, what should be the diameter of the smaller gear ? SOLUTION.-Applying the rule just given, we have, as the diameter of the smaller gear,
2 x 12 x 1 5 ± 1 = 4 in. Ans.
44. Since the distance between centers is equal to the sum of the radii of the two gears, it follows that when the diameter of either gear has been calculated, the diameter of the other may be found as follows : Rule.—To find the diameter of one gear of a pair of gears, when the distance between centers is fixed and the diameter of the other gear is known, subtract the known diameter from twice the distance between centers. EXAMPLE.-The distance between centers being 8 inches, and the diameter of one gear being 4 inches, what is the diameter of the other gear ? SOLUTION.-Applying the rule just given, we obtain 2 8 — 4 = 12 in. Ans. **

**•
§ 17
GEAR CALCULATIONS. 17
LAYING OUT GEAR-TEETH.
FORMS OF GEAR-TOOTH OUTLINES. 45. Constant Velocity Ratio.—In order that a tooth of the driving gear shall press on a tooth of the driven gear in such a manner as to produce a constant speed of turning while they are in contact, it is necessary that the curved outline of the teeth shall be constructed according to a cer-tain law. The principle involved is that the line ik, Fig. 6,
N N **

**FIG. 6. **

**called the normal to the tooth curves at their point of tan-gency, that is, the point where they touch, must pass through the point of tangency of the pitch circles for every position in which the two teeth are in contact. When this condition is satisfied there will be a constant velocity ratio for the gears. In Fig. 6, c and d are two pitch circles with their respect-ive centers at a and b. The curve outlines e and f represent the teeth of two gears with these pitch circles. These curves are tangent to each other at g, and hj is a straight line tan-gent to both curves at the point g. A line perpendicular to this tangent line at g is the normal to both curves. Such a normal k i passes through the point of tangency g of the tooth curves and the point of tangency 1 of the pitch circles. The curves e and f are so designed that for every position in which they can be in contact their common normal passes 50-33**

**page 18 GEAR CALCULATIONS.
§ 17
through the point of tangency of the pitch circles. These curves therefore satisfy the condition for constant velocity ratio.
46. Devices for Drawing Gear-Teeth.—An odon-tograph is an arrangement to facilitate the laying out of the curved outlines of gear-teeth. The term has been applied in a number of different forms, but its application to the templets for laying out the tooth curves seems to be ' the best, hence many persons define an odontograph as a templet for laying out gear-teeth. The term, however, has also been applied to tables that give the radii for the tooth-curve outlines of gears, though it would seem better prac-tice 'to call these odontograph tables. In some cases the templet has to be used in connection with a table, the tem-plet being simply a means for finding the centers from which to draw the tooth curves. 47. Tooth Curves in General Use.--As stated in Art. 45, the motion transmitted by one gear to another will be smooth and uniform only when the teeth of the gears are given definite forms. Theoretically, the number of forms that meet these conditions is large practically, how-ever, owing to the necessity of simplicity and ease of con-struction, this number is restricted to a few simple types, while the importance of uniformity has still further restricted the types in common use to two general systems, known as the involute, or single-curve, system and the cycloidal, or double-curve, system. Of these two systems, the involute has a number of important advantages, especially when used for cut gears, that are constantly bringing it into more extensive use,. and many of the best authorities on gearing urge its universal adoption to the exclusion of the cycloidal system.
INVOLUTE SYSTEM.
48.. Definition of an Involute.—Mathematically, an involute is the curve that would be drawn by a pencil point at the end of a thine band, that will not stretch, and **

**§ 17 GEAR CALCULATIONS.
page 19
that is drawn tight while being unwound from a cylinder. For example, suppose such a band to be unwound from the cylinder in Fig. 7, beginning with the pencil point at a on the circumference. As the band is unwound, the pencil point traces the curved line a-1-2-3-4, etc., which line is a part of the involute of the circle that represents the circumference of the cylinder. With true in-volute teeth, the path followed by the point of cont act of the tooth curves of the teeth on the two gears is a straight line, called the line of action, that is tangent to the base circles of both gears. The faces of involute rack teeth are straight lines perpendicular to this line of action. In practice, the base circles are usually so chosen that the line of action makes an angle of 15° with a line that is tangent to both pitch circles at their point of tangency. As the faces of the rack teeth are perpendicular to the line of action, they make an angle of 75° with their pitch plane. In the approximate tooth curves frequently used in practice, the faces of the rack teeth are composed wholly or partially of curves. **

**49. Base Circle for Involute Teeth.—The base circle in the involute system of gearing is the circle to which the involute that forms the outline of the tooth is drawn. The radius of the base circle is smaller than that of the pitch circle, the difference between the two being gen-erally found by multiplying the pitch diameter by a number that is constant in any given system, but varies somewhat with different systems. For most purposes, a difference **

** **

**GEAR-CUTTING. . § 18
TEMPLET-PLANING PROCESS. 42. The Machine.—The principle of operation of a templet-planing machine intended for planing the teeth of bevel gears is shown in diagrammatic form in Fig. 10. The gear blank a is attached to the index spindle b, which carries an indexing wheel c at its other end. An arm d supports a longitudinally movable slide e which carries the pointed cutting tool f. The arm d is mounted on a univer-sal joint in such a manner that its center of rotation g coin-cides with the axis of rotation of the gear blank. The **

**FIG. 10.
cutting tool is adjusted in the slide e in such a manner that the line of motion of its cutting point passes exactly through the center of rotation of the arm. A pin h is fastened to the free end of the arm, and is in contact with a templet 1, which is shaped to conform to a correct tooth curve for the number of teeth contained in the bevel gear. **

**I
§ 18 GEAR-CUTTING. 21
The arm d remains stationary while the cutting tool is traversed through the gear blank. One side of a tooth is finished at a time by successive cuts converging toward the apex of the pitch cone, which point, by reason of the con-struction of the machine, is also the center of rotation g of the arm. After the tool f has cleared the blank on its re-turn stroke, the arm is moved slightly along the templet i, keeping the pin /bin contact with the templet ; the position of the arm and, hence, the formation of the tooth curves, is thus determined by the templet. The form of the templet is reproduced on a smaller scale by the planing tool on the tooth operated on, and any errors existing in the templet are reduced.
43. It is obvious that a different templet will be re-quired for each number of teeth, at least theoretically. Owing to the small divergence in the shape of the tooth curves, one templet can be made to serve for several gears, however, just as is done with formed gear-cutters. One templet will answer for all pitches within the range of the machine ; different sizes of bevel gears are cut by varying the distance from the gear to the center of rotation of the arm d. In an actual machine, the templet is movably mounted on a quadrant having its center of curvature at g; this adapts the machine for different gears, since it allows the angle between the axis of the spindle and the line of motion of the tool to be changed to suit the number of teeth of the gear.
44. Templet-Grinding Process. — A modification of the templet-planing process has recently been perfected by the Leland & Faulconer Company, Detroit, Michigan, who have substituted a corundum wheel for the planing tool and thus are enabled to finish the teeth of hardened-steel bevel gears to a correct shape. The fundamental principle under-lying this templet-grinding process does not differ in any essential particular from that explained in connection with Fig. 10. **

**page**** Page ****22 GEAR–CUTTING. § 18 GENERATION SYSTEM. CONJUGATE-TOOTH METHOD.
MOLDING PROCESS.
45. The different processes employed in the conjugate-tooth method of generating gear-teeth are all based on the so-called molding process, which has not been put into practical use, however, at least not to any extent. This process may be explained as follows : Let a correctly formed rack made of some hard material, as steel, be passed over a gear blank made of a plastic material, as beeswax, while the blank is given a positive rotation that imparts to it at the pitch circle a velocity equal to that of the rack. Then, the pitch line of the rack and the pitch circle of the blank being tangent, the teeth of the rack will mold teeth in the blank that are conjugate to its own.
MOLDING-PLANING PROCESS. ■61) 46. Principle of Operation.-Since the materials of which gear-wheels are constructed are not plastic, the mold-ing process cannot be employed very readily, but the same effect can be produced by transforming the generating rack into a cutting tool that reciprocates across the blank in the direction of the axis of the latter. The cutting tool does not advance during its cutting stroke in a line tangent to the pitch circle of the blank and at right angles to the axis of the latter but, after the tool has cleared the blank on its return stroke, the tool and the gear blank are given a slight motion equivalent to that of a meshing rack and pinion and the tool is reciprocated through the blank again. This cycle of operations is repeated until the gear blank has been trans-formed into a gear. The molding process thus becomes the
§ 18
•
GEAR–CUTTING. Page 23
molding-planing process in execution, however, this proc-ess is modified for practical reasons, the chief of which are the great length of rack required and the difficulty of making it.
47. Fellows Gear-Shaper.—In the Fellows gear-shaper, in which machine the molding-planing process is employed, the cutter a, Fig. 11, is made in the form of a gear. The process by which the cutter is generated is equivalent to its generation by an involute rack, and it is **

**FIG. 11.
given teeth conjugate to those of the generating rack. In consequence of the method by which the cutter .is formed, the teeth of all gears cut by it are conjugate to the rack and hence to one another that is, the different gears will run together correctly. **