Unit3 Gear

39
J3103/3/1 GEAR General Objective : To understand the technology of gears manufacturing Specific Objectives : At the end of the unit you will be able to: Ø Know the methods of gear manufacturing Ø Know the methods of direct and simple indexing Ø Apply direct and simple indexing when cutting gears on a milling machine. Ø Apply various formula to calculate gear-tooth dimensions. UNIT 3 OBJECTIVES GEAR Click here to buy A B B Y Y P D F T r a n s f o r m e r 2 . 0 w w w . A B B Y Y . c o m Click here to buy A B B Y Y P D F T r a n s f o r m e r 2 . 0 w w w . A B B Y Y . c o m

Transcript of Unit3 Gear

Page 1: Unit3 Gear

J3103/3/1GEAR

General Objective : To understand the technology of gears manufacturing

Specific Objectives : At the end of the unit you will be able to:

Ø Know the methods of gear manufacturingØ Know the methods of direct and simple indexingØ Apply direct and simple indexing when cutting

gears on a milling machine.Ø Apply various formula to calculate gear-tooth

dimensions.

UNIT 3

OBJECTIVES

GEAR

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Page 2: Unit3 Gear

J3103/3/2GEAR

3.0. GEAR MANUFACTURING

Gears can be manufactured by casting, forging, extrusion, drawing,thread rolling, powder metallurgy, and blanking sheet metal (for making thingears such as those used in watches and small clocks). Nonmetallic gears canbe made by injection molding and casting.

Gears may be as small as those used in watches or a large as 9 m indiameter. The dimensional accuracy and surface finish required for gear

teeth depend on its intended use. Poor gear-tooth quality contributes toinefficient energy transmission and noise and adversely affects the gear’sfrictional and wear characteristics. Submarines gears, for examples, have tobe of extremely high quality so as to reduce noise levels, helping thesubmarine avoid detection.

There two basic gear manufacturing methods which involve themachining of a wrought or cast gear blank: form cutting and generating.

3.1. FORM CUTTING

In form cutting, the cutting tool is similar to a form-milling cuttermade in the shape of the space between the gear teeth (Fig. 3.1). The gear-

INPUT

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Page 3: Unit3 Gear

J3103/3/3GEAR

tooth shape is produced by cutting the gear blank around its periphery. Thecutter travels axially along the length of the gear tooth at the appropriatedepth to produce the gear tooth profile. After each tooth is cut, the cutter is

withdrawn, the gear blank is rotated (indexed), and the cutter proceeds to cutanother tooth. The process continues until all teeth are cut.

Each cutter is designed to cut a range of number of teeth. Theprecision of the form cut tooth profile depends on the accuracy of the cutterand on the machine and its stiffness. Although inefficient, form cutting canbe done on milling machines, with the cutter mounted on an arbor and the

gear blank mounted in a dividing head.

Because the cutter has a fixed geometry, form cutting can only be used

to produce gear teeth that have constant width, that is, on spur or helicalgears but not on bevel gears. Internal gears and gear teeth on straightsurfaces, such as in rack and pinion, are form cut with a shaped cutter, usinga machine similar to a shaper.

Broaching can also be used to produce gear teeth and is particularlyapplicable to internal teeth. The process is rapid and produces fine surface

finish with high dimensional accuracy. However, because broaches are

Figure 3.1. Producing gear teeth on a blank by form cutting

Gearblank

Formcutter

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Page 4: Unit3 Gear

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expensive and a separate broach is required for each gear size, this method issuitable almost exclusively for high-quantity production.

Gear teeth may be cut on special machines with a single-point cutting

tool that is guided by a template in the shape of the gear tooth profile. As thetemplate can be made much larger than the gear tooth, dimensional accuracyis improved.

Form cutting is relatively a simple process and can be used for cuttinggear teeth with various profiles, however, it is a slow operation, and sometypes of machines require skilled labor. Consequently, it is suitable only for

low-quantity production. Machines with semiautomatic features can be usedeconomically for form cutting on a limited production basis.

3.2. GEAR GENERATING

The cutting tool used in gear generating may be one of the following:

3.2.1. A pinion-shaped cutter3.2.2. A rack-shaped straight cutter3.2.3. A hob

3.2.1. The pinion-shaped cutter can be considered as one ofgears in a conjugate pair and the other as the gear blank (Fig 3.2);

it is used on machines called gear shapers (Fig 3.3). The cutter hasan axis parallel to that of the gear blank and rotates slowly withthe blank at the same pitch-circle velocity in an axial reciprocatingmotion. A train of gears provides the required relative motionbetween the cutter shaft and the gear-blank shaft.

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Page 5: Unit3 Gear

J3103/3/5GEAR

Cutting may take place at either the down stroke or theupstroke of the machine. Because the clearance required for cuttertravel is small, such as flanges (Fig 3.3). The process can be used

for low-quantity as well as high-quantity production.

3.2.2. On a rack shaper, the generating tool is a segment of arack (Fig.3.4) which reciprocates parallel to the axis of the gearblank. Because it is not practical to have more than 6 to 12 teethon a rack cutter, the cutter must be disengaged at suitable intervals

and returned to the starting point; the gear blank remain fixed.

Figure 3.2. Gear generating in a gear shaper using a pinion-shaped cutter

Figure 3.3.. Gear generating with a pinion-shaped gear cutter

Gear cutter Base circle Pitch circle

Base circle

Gear blank

Gear blank

Cutter spindle Gearteeth

Spacer

Pinion-shapecutter

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Page 6: Unit3 Gear

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3.2.3. A gear-cutting hob (Fig. 3.5) is basically a worm, or screw,

made into a gear-generating tool by machining a series of longitudinalslots or gashes into it to form the cutting teeth. When hobbing a spurgear, the angle between the hob and gear blank axes is 90o minus thelead angle at the hob threads. All motions in hobbing are rotary, thehob and gear blank rotate continuously, much as two gears meshinguntil all teeth are cut.

Figure 3.5. View of gear cutting with a hob

Figure 3.4. Gear generating with rack-shaped cutter

Hob

Top view

Gearblank

Gearblank

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Hobs are available with one, two, or three threads. If the hobhas a single thread and the gear is to have 40 teeth, for example, thehob and gear spindle must be geared together so that the hob makes 40

revolutions while the gear blank makes one revolution. Similarly, if adouble-threaded hob is used, the hob would make 20 revolutions to thegear blank’s one revolution.

In addition, the hob must be fed parallel to the gear axis for adistance greater than the face width of the gear tooth (Fig. 3.5) inorder to produce straight teeth on spur gears. The same hobs and

machines can be used to cut helical gears by tilting the axis of the hobspindle.

Because it produces a variety of gears rapidly and with gooddimensional accuracy, gear hobbing is used extensively in industry.Although the process is suitable for low-quantity production, it is mosteconomical for medium to high-quantity production.

Gear–generating machines can also produce spiral-bevel andhypoid gears. Like most other machine tools, modern gear-generatingmachines are computer controlled. Multi axes computer-controlledmachines are capable of generating many types and sizes of gearsusing indexable milling cutters.

3.3. CUTTING BEVEL GEARS

Straight bevel gears are generally roughed out in one cut with a formcutter on machines that index automatically. The gear is then finished to theproper shape on a gear generator. The generating method is analogous to therack-generating method already described. The cutters reciprocate across the

face of the bevel gear as does the tool on a shaper (Fig 3.6).

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The machines for spiral bevel gears operate on essentially thesame principle. The spiral cutter is basically a face-milling cutter thathas a number of straight-sided cutting blades protruding from itsperiphery ( Fig.3.7 ).

Gearblank

Cutter

Gear blank

Cutter

Figure 3.7. Cutting a spiral bevel gear with a single cutter

Figure 3.6. Cutting a straight bevel gear blank with two cutter

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Page 9: Unit3 Gear

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3.4 GEAR-FINISHING PROCESSES

As produced by any of the process described, the surface finish and

dimensional accuracy of gear teeth may not be sufficiently accurate forcertain applications. Moreover, the gears may be noisy or their mechanicalproperties, such as fatigue life, may not be sufficiently high.

Several finishing processes are available to improve the surface qualityof gears. The choice of process is dictated by the method of gear manufactureand whether the gears have been hardened by heat treatment. Heat treating

can cause distortion of parts. Consequently, for precise gear-tooth profile,heat-treated gears should be subjected to appropriate finishing operations.

3.4.1. Shaving

The gear shaving process involves a cutter, made in the exact

shape of the finished tooth profile, which removes small amounts ofmetal from the gear teeth. The cutter teeth are slotted or gashed atseveral points along its width, making the process similar to finebroaching. The motion of the cutter is reciprocating. Shaving andburnishing can only be performed on gears with a hardness of 40 HRCor lower.

Although the tools are expensive and special machines arenecessary, shaving is rapid and is the most commonly used process forgear finishing. It produces gear teeth with improved surface finish andimproved accuracy of tooth profile. Shaved gears may subsequently beheat treated and then ground for improved hardness, wear resistance,and accurate tooth profile.

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Page 10: Unit3 Gear

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3.4.2. Burnishing

The surface finish of gear teeth can also be improved by

burnishing. Introduced in the 1960s, burnishing is basically a surfaceplastic-deformation process using a special hardened gear-shapedburnishing die that subjects the tooth surfaces to a surface rollingaction (gear rolling). Cold working of tooth surfaces improves thesurface finish and induces surface compressive residual stresses on thegear teeth, thus their fatigue life. However, burnishing does not

significantly improve gear-tooth accuracy.

3.4.3. Grinding, honing and lapping

For the highest dimensional accuracy, tooth spacing and form,and surface finish, gear teeth may subsequently be ground, honed, and

lapped. Specially-dressed grinding wheels are used for either formingor generating gear-tooth surfaces. There are several types of grindersof gears, with the single index form grinder being the most commonlyavailable. In form grinding, the shape of the grinding wheel isidentical to that of the tooth spacing (Fig. 3.8)

The honing tool is plastic gear impregnated with fine abrasive

particles. The process is faster than grinding and is used to improvesurface finish. To further improve the surface finish, ground gearteeth are lapped using abrasive compounds with either a gear-shapedlapping tool (made of cast iron or bronze) or a pair of mating gears thatare run together. Although production rates are lower and costs arehigher, these finishing operations are particularly suitable for

producing hardened gears of very high quality, long life, and quietoperation.

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Page 11: Unit3 Gear

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3.5. METRIC GEARS AND GEAR CUTTING

Countries which have been using a metric system of measurement

usually use the module system of gearing. The module (M) of a gear equalsthe pitch diameter (PD) divided by the number of teeth (N), or M = only,

whereas the DP of a gear is the ratio of N to the PD, or DP =PDN . The DP of

a gear is the ratio of the number of teeth per inch diameter, whereas M is anactual dimension. Most of the term used in DP gears remains the same formodule gears; however, the method of calculating the dimensions has

changed in some instances. Table 3.2. gives necessary rules and formulas formetric spur gears.

Gear

Grinding wheel

Position: 15o or 0o Position: 0o

Figure 3.8. Grinding by generating with two wheels

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Page 12: Unit3 Gear

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3.6. METRIC MODUL GEAR CUTTERS

The most common metric gear cutters are available in moduls ranging

from 0.5 to 10 mm. However metric modul gear cutters are available in sizesup to 75 mm. Any metric modul size is available a set of eight cutters,number from #1 to #8. The range of each cutter is the reverse of that of a DPcutter. For instance, a #1 metric modul cutter will cut from 12 to 13 teeth; a#8 DP cutter will cut from 135 teeth to a rack. Table 3.1. shows the cuttersavailable and the range of each cutter in the set.

Milling Cutter NumbersModule size (mm)

Cutter No. For Cutting

0.50 3.50

0.75 3.75 1 12 – 13 teeth

1.00 4.00 2 14 – 16 teeth

1.25 4.50 3 17 – 20 teeth

1.50 5.00 4 21 – 25 teeth

1.75 5.50 5 26 – 34 teeth

2.00 6.00 6 35 – 54 teeth

2.25 6.50 7 55 – 134 teeth

2.50 7.00 8 135 teeth to rack

2.75 8.00

3.00 9.00

3.25 10.00

Table 3.1 Metric module gear cutter

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Page 13: Unit3 Gear

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To Obtain Knowing Rule FormulaAddendum (A) Normal Module Addendum equals module A = M

Module Multiply module by p CP = M x 3.1416Pitch diameterand number ofteeth

Multiply pitch diameter by pand divide by number of teeth CP = M x

N3.1416

Circular pitch (CP)Outside diameterand number ofteeth

Multiply outside diameter byp and divide by number ofteeth minus 2

CP =2-N

3.1416 xOD

Module andoutside diameter

Divide 90 by number of teeth.Find the sine of this angle andmultiply by the pitchdiameter.

CT = PD x sinN

90

Module Multiply module by p anddivide by 2 CT =

23.1416 xMChordal thickness

(CT)

Circular pitch Divide circle pitch by 2 CT =2

CP

Clearance (CL) Module Multiply module 0.166 mm CL = M x 0.166Dedendum (D) Module Multiply module 1.166 mm D = M x 1.166

Pitch diameterand number ofteeth

Divide pitch diameter by themodule M =

NPD

Circular pitch Divide circular pitch by p M =3.1416

CPModule (M)

Outside diameterand number ofteeth

Divide outside diameter bynumber of teeth M =

2NOD+

Pitch diameterand module

Divide pitch diameter by themodule N =

MPD

Number of teeth (N)Pitch diameterand circular pitch

Multiply pitch diameter by pand divide product bycircular pitch

N =CP3.1416 xPD

Number of teethand module

Add 2 to the number of teethand multiply sum of module OD = (N + 2) x MOutside diameter

(OD) Pitch diameterand module

Add 2 modules to pitchdiameter OD = PD + 2M

Module andnumber of teeth

Multiply module by number ofteeth PD = M x N

Outside diameterand module

Subtract 2 modules fromoutside diameter PD = OD – 2M

Pitch diameter (PD)Number of teethand outsidediameter

Multiply number of teeth byoutside diameter and divideproduct by number of teethplus 2

PD =2N

OD xN+

Whole depth (WD) Module Multiply module by 2.166 mm WD = M x 2.166Centre-to-centredistance(CD) Pitch diameters Divide the sum of the pitch

diameters by 2 CD =2PDPD 21 +

Table 3.2. Formula for calculating metric gear

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Page 14: Unit3 Gear

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Example 1.1. A spur gear has PD of 60mm and 20 teeth. Calculate:

(a) Modul(b) Circular Pitch(c) Addendum(d) Outside diameter(e) Dedendum(f) Whole depth

(g). Cutter numberSolutions:

(a) Modul = PD/N= 60/20= 3 mm

(b) CP = M × p

= 3 × 3.1416

= 9.425 mm

(c) Addendum = Modul= 3 mm

(d). Outside diameter = ( N + 2 ) × M

= 22 × 3= 66 mm

(e). Dedendum = M × 1.666= 3 × 1.666= 4.998 mm

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Page 15: Unit3 Gear

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(f) Working depth = Modul × 2.166= 3 × 2.166= 6.498 mm

(g). Cutter number ( see Table 3.2 ) = 3

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Page 16: Unit3 Gear

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3.1. Two identical gears in mesh have a CD of 120 mm. Each gearhas 24 teeth. Calculate;

(a) Pitch diameter(b) Modul(c) Outside diameter(d) Whole depth(e) Circular pitch

(f) Chordal thickness

3.2. Name 3 methods of gear generating.

ACTIVITY 3A

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Page 17: Unit3 Gear

J3103/3/17GEAR

NPD

3.1.(a)PD =2

2xCD ( equal gears )

=21202x

=2

240

= 120 mm

(b) M =N

PD

=24

120

= 5

(c) OD = (N + 2 ) x M= 26 x 5= 130 mm

(d) WD = M x 2.166

= 5 x 2.166= 10.83 mm

FEEDBACK ON ACTIVITY 3A

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Page 18: Unit3 Gear

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(e) CP = M x P

= 5 x 3.1416= 15.708 mm

(f) CT =2ÕMx

=21416.35x

7.85 mm

3.2. 1. Pinion- shaped cutter1. Rack-shaped straight cutter2. A hob

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Page 19: Unit3 Gear

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3.7. THE INDEXING OR DIVIDING HEAD

The indexing or dividing head is one of the most important

attachments for the milling machine. It is used to divide the circumference ofa work piece into equally spaced divisions when milling gears, splines,squares and hexagons. It may also be used to rotate the work piece at apredetermined ratio to the table feed rate to produce cams and helicalgrooves on gears, drills, reamers, and other parts.

3.8. INDEX HEAD PARTS

The universal dividing head set consists of the headstock with indexplates, headstock change and quadrant, universal chuck, footstock, and thecentre rest ( Fig 3.9 ). A swiveling block mounted in the base enables theheadstock to be tilted from 5o below horizontal position to 10o beyond the

vertical position. The side of the base and the blocks are graduated toindicate the angle of the setting. Mounted in the swiveling block is a spindle,with 40-tooth worm wheel attached, which meshes with a worm ( Fig. 3.10 ).The worm , at right angles to the spindle, is connected to the index crank, thepin of which engages in the index plate. A direct indexing plate is attached tothe front of the spindle.

A 60o centre may be inserted into the front of the spindle, and auniversal chuck may be threaded onto the end of the spindle.

The footstock is used in conjunction with the headstock to supportwork held between centers or the end of work held in a chuck. The footstockcentre may be adjusted longitudinally to accommodate various lengths ofwork and may be raised or lowered off centre. It may also be tilted out of

parallel with the base when cuts are being made on tapered work.

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Page 20: Unit3 Gear

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Long, slender work held between centers is prevented from bending bythe adjustable centre rest.

Figure 3.10 Section through a dividing head, showingthe worm wheel and worm shaft

Figure 3.9. A universal dividing head set

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Page 21: Unit3 Gear

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3.9 METHODS OF INDEXING

The main purpose of the indexing or dividing head is to divide the

work piece circumference accurately into any number of divisions. This maybe accomplished by the following indexing methods: direct, simple, angular,and differential. However, this modul will only cover direct and simpleindexing.

3.9.1. Direct indexing

Direct indexing is the simplest form of indexing. It is performedby disengaging the worm shaft from the worm wheel by means of aneccentric device in the dividing head. Some direct dividing heads donot have a worm and worm wheel but rotate on bearings. The indexplates contain slots, which are numbered , and a spring-loaded tongue

lock is used to engage in the proper slot. Direct indexing is used forquick indexing of the work piece when cutting flutes, hexagons,squares, and other shapes.

The work is rotated the required amount and held in place by apin which engages in to a hole or slot in the direct indexing platemounted on the end of the dividing head spindle. The direct indexing

plate usually contains three sets of hole circles or slots: 24, 30, and 36.The number of divisions it is possible to index is limited to numberswhich are factors of either 24, 30, or 36. The common divisions thatcan be obtain by direct indexing are listed in Table 3.3

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Page 22: Unit3 Gear

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Plate

HoleNumber

24 2, 3, 4, -, 6, 8, - ----- 12 …………24

30 2, 3, -, 5, 6, -, -, -, 10, -, -, 15, ……….30

36 3, 4, -, 6, -, 9, -, 12, -, 18,…………… 36

Example:What direct indexing is necessary to mill eight flutes on a reamer blank?As the 24 hole circle is the only one divisible by eight (the required of

divisions), it is the only circle which can be used in this case.

Indexing =824 = 3 holes on a 24-hole circle.

Note: Never count the hole or slot in which the index pin is engaged.

To mill a square by direct indexing1. Disengage the worm and worm shaft by turning the worm

disengaging shaft lever if the dividing head is so equipped.2. Adjust the plunger behind the index plate into the 24-hole circle or

slot (Fig. 3.11. ).

Table 3.3. Direct Indexing Divisions

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Page 23: Unit3 Gear

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3. Mount the work piece in the dividing head chuck or between centers.4. Adjust the cutter height and cut the first side.5. Remove the plunger pin using the plunger pin lever ( Fig. 3.11.).6. Turn the plate, attached to the dividing head spindle, one-half turn

(12 holes or slots) and engage the plunger pin.7. Take the second cut.

8. Measure the work across the flats and adjust the work height ifrequired.

9. Cut the remaining sides by indexing every six holes until allsurfaces are cut.

Plunger pinlever

Figure 3.11. Adjusting the plunger pin to fit into theproper hole circle or slot

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Page 24: Unit3 Gear

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3.9.2. Simple Indexing

In simple indexing, the work is positioned by means of thecrank, index plate, and sector arms. The worm attached to the crankmust be engaged with the worm wheel on the dividing head spindle.Since there are 40 teeth on the worm wheel, one complete turn of theindex crank will cause the spindle and the work to rotate one-fortieth

of a turn. Similarly, 40 turns of the crank will revolve the spindle andwork one turn. Thus there is a ratio of 40:1 between the turns of theindex crank and the dividing head spindle.

To calculate the indexing or the number of turns of the crank formost divisions, it is necessary only to divide 40 by the number of

division (N) to be cut, or

Indexing =N40

Figure 3.12 The plunger pin and the direct indexing plate are used forindexing a limited number of divisions.

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Page 25: Unit3 Gear

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Example:

The indexing required to cut eight flutes would be:

840 = 5 full turns of the index crank

If, however, it was necessary to cut seven flutes, the indexing would be

740 = 5

75 turns

Five complete turns are easily made; however, the five seventh of a turninvolves the use of the index plate and sector arms.

Index plate and sector arms

The index plate is a circular plate provided with a series of equally spacedholes into which the index crank pin engages. The sector arms fit on thefront of this plate and may be set to any portions of a complete turn.

To get five-sevenths of a turn, choose any hole circle ( Table 3.4.) which isdivisible by the denominator 7, such as 21, then take five-sevenths of 21 = 15holes on a 21-hole circle. Therefore, the indexing for seven flutes would be

740 = 5

75 turns or 5 complete turns plus 15 holes on the 21-hole circle.

When extreme accuracy is required for indexing, choose the circle with themost holes.

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Page 26: Unit3 Gear

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Brown and Sharpe

Plate 1 15-16-17-18-19-20-Plate 2 21-23-27-29-31-33Plate 3 37-39-41-43-47-49

Cincinnati Standard Plate

One side 24-25-28-30-34-37-38-39-41-42-43

Other side 46-47-49-51-53-54-57-58-59-62-66

The procedures for cutting seven flutes would be as follows:

1. Mount the proper index plate on the dividing head.

2. Loosen the index crank nut and set the index pin into a hole on the21-hole circle.

3. Tighten the index crank nut and check to see that the pin enters thehole easily.

4. Loosen the set screw on the sector arms.5. Place the narrow edge of the left arms against the index pin.6. Count 15 holes the 21-hole circle. Do not include the hole in which the

index crank pin is engaged.7. Move the right sector arms slightly beyond the 15th hole and tighten

the sector arm setscrew.8. Align the cutter with the work piece.9. Start the machine and set the cutter to the top of the work by using a

paper feeler ( Fig. 3.15 ).

10. Move the table so that the cutter clears the ends of the work.11. Tighten the friction lock on the dividing head before making each cut

and loosen the lock when indexing for spaces.12. Set the depth of cut and take the first cut.

Table 3.4. Index plate hole circle

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Page 27: Unit3 Gear

J3103/3/27GEAR

13. After the first flute has been cut, return the table to the originalstarting position.

14. Withdraw the index pin and turn the crank clockwise five full turns

plus the 15 holes indicated by the right sector arm. Release the indexpin between the 14th and 15th holes, and gently tap it until it dropsinto the 15th hole.

15. Turn the sector arm farthest from the pin clockwise until it is againstthe index pin.

NOTE: It is important that the arm farthest from the pin be held andturned. If the arm next to the pin were held and turned, the spacing betweenboth sector arms could be increased when the other arm hit the pin. Thiscould result in an indexing error which would not be noticeable until thework was completed.

16. Lock the dividing head; then continue machining and indexing for theremaining flutes. Whenever the crank pin is moved past the requiredhole, remove the backlash between the worm and worm wheel byturning the crank counterclockwise approximately one-half turn andthen carefully clockwise until the pin engages the proper hole.

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Page 28: Unit3 Gear

J3103/3/28GEAR

3.10. TO CUT A SPUR GEARThe procedure for machining a spur gear is outlined in the following:

Procedure:

3.1.1. Calculate the necessary gear data, refer to Table 3.1Two identical gears in mesh have a CD of 120 mm. Each gear has24 teeth. Calculate;

i. Pitch diameterii. Modul

iii. Outside diameteriv. Whole depthv. Circular pitch

vi. Chordal thickness

3.1.1.i. PD =2

2xCD ( equal gears )

=21202x

=2

240

= 120 mm

ii M =N

PD

=24

120

= 5

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Page 29: Unit3 Gear

J3103/3/29GEAR

iii. OD = (N + 2 ) x M= 26 x 5= 130 mm

iv WD = M x 2.166= 5 x 2.166= 10.83 mm

v. CP = M x P

= 5 x 3.1416= 15.708 mm

vi. CT =2ÕMx

=21416.35x

=7.85 mm

2. Turn the gear blank to proper outside diameter3. Press the gear blank firmly onto the mandrel.

NOTE: If the blank was turned on a mandrel, be sure that it is tight becausethe heat caused by turning might have expanded the blank slightly.

4. Mount the index head and footstock, and check the alignment ofthe index centers ( Fig. 3.13. ).

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Page 30: Unit3 Gear

J3103/3/30GEAR

5. Set the dividing head so that the index pin fits into a hole on the39- hole circle and the sector arms are set for 30 holes.

NOTE: Do not count the hole in which the pin is engaged.

6. Mount the mandrel (and work piece), with the large end towardthe indexing head, between the index centers.

NOTE:a. The footstock centre should be adjusted up tightly into

the mandrel and lock in position.b. The dog should be tightened properly on the mandrel

and the tail of the dog should not blind in the slot.c. The tail of the dog should then be locked in the driving

fork of the dividing head by means of the sets screws.

Figure 3.13. Checking the alignment of index centers with a dial indicator

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Page 31: Unit3 Gear

J3103/3/31GEAR

d. This will ensure that there will be no play between thedividing head and the mandrel.

e. The dog should be far enough from the gear blank toensure that the cutter will not hit the dog when the

gear is being cut.

7. Move the table close to the column to keep the setup as rigid aspossible.

8. Mount a cutter on the milling machine arbor over the approximatecentre of the gear. Be sure to have the cutter rotating in the

direction of the indexing head.9. Centre the gear blank with the cutter by either of the following

methods:a. Place a square against the outside diameter of the gear

(Fig 2.23). With a pair of inside calipers or a rule,check the distance between the square and the side of

the cutter. Adjust the table until the distances fromboth sides of the gear blank to the sides of the cutterare the same.

Figure 3.10.. Centering a gear cutter and the work piece

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Page 32: Unit3 Gear

J3103/3/32GEAR

b. A more accurate method of centralizing the cutter is touse gauge blocks instead of the inside calipers or rule.

10. LOCK THE CROSSLIDE.

11. Start the milling cutter and run the work under the cutter.12. Raise the table until the cutter just touches the work. This can be

done by using a chalk mark on the gear blank or a piece of paperbetween the gear blank and the cutter to indicate when thecutter just touching the work ( Fig. 3.15 ).

13. Set the graduated feed collar on the vertical feed to zero (0).Move the work clear of the cutter by means of the longitudinalfeed handle and raise the table to about two-thirds the depth ofthe tooth (4.572 mm); then lock the knee clamp.

NOTE: A special stocking cutter is sometimes used to rough out the teeth.

14. Slightly notch all gear teeth on the end of the work to check forcorrect indexing ( Fig. 3.16 ).

Figure 3.15. Setting a gear cutter to the diameter of the work piece

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Page 33: Unit3 Gear

J3103/3/33GEAR

15. Rough out the first tooth and set the automatic feed trip dog afterthe cutter is clear of the work.

16. Return the table to starting position.

NOTE: Clear the end of the work with a cutter.

17. Cut the remaining teeth and return the table to the startingposition.

18. Loosen the knee clamp, raise the table to the full depth of 0.270in., and lock the knee clamp.

NOTE: It is advisable to remove the crank from the knee elevating shaftso that it will not be moved accidentally and change the setting.

19. Finish-cut all teeth.

NOTE: After each tooth has been cut, the cutter should be stopped before the

table is returned to prevent marring the finish on the gear teeth.

Figure 3.16. Notching all gear teeth eliminates errors

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Page 34: Unit3 Gear

J3103/3/34GEAR

3.3. What direct indexing is necessary to mill 8 flutes on a reamer blank ?

3.4. Explain how the ratio 40:1 is determined on a standard dividing head.3.5. What procedure should you follow in order to set the sector arms for 12

holes on an 18-hole circle ?3.6. With the aid of a labelled diagram, list down the procedures to align

the index centers.

3.7. How would you center the gear blank with the cutter ?

ACTIVITY 3B

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Page 35: Unit3 Gear

J3103/3/35GEAR

3.3. Indexing =824

= 3 holes in a 24-holes circleNOTE: Never count the hole/slot in which the index pin is engaged.

3.4. The index head spindle carries a 40-tooth worm wheel, which mesheswith a worm. The worm ( at right angle to the spindle ) is connected to theindex crank, the pin of is engaged in the index plate. Since there are 40 teethon the worm wheel, one complete turn of the index crank will cause thespindle and the work to rotate 1/40th of a turn. Similarly 40 turns willrevolve the spindle and the work one turn. Thus there is a ratio of 40:1between the turns of the index crank and the dividing head spindle.

3.5. Index plate and sector armsThe index plate is a circular plate provided with a series of equally spacedholes into which the index crank pin engages. The sector arms fit on thefront of this plate and may be set to any portions of a complete turn.To get 12 holes in an 18-hole circle choose plate no. 1 of Brown and Sharpe

index plate. Follow these procedures:1. Mount the index plate no. 1 on the dividing head.2. Loosen the index crank nut and set the index pin into a hole on

the 18-hole circle.3. Tighten the index crank nut and check to see that the pin enters

the hole easily.

4. Loosen the set screw on the sector arms.

FEEDBACK ON ACTIVITY 3B

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Page 36: Unit3 Gear

J3103/3/36GEAR

5. Place the narrow edge of the left arms against the index pin.6. Count 12 holes the 18-hole circle. Do not include the hole in

which the index crank pin is engaged.7. Move the right sector arms slightly beyond the 12th hole and

tighten the sector arm setscrew.

3.6.1. Mount the index head and footstock, and check the alignment of

the index centers.

2. Set the dividing head so that the index pin fits into a hole on the39- hole circle and the sector arms are set for 30 holes.

NOTE: Do not count the hole in which the pin is engaged.

3. Mount the mandrel (and work piece), with the large end towardthe indexing head, between the index centers.

Checking the alignment of index centers with a dial indicator

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Page 37: Unit3 Gear

J3103/3/37GEAR

NOTE:a. The footstock centre should be adjusted up tightly into the

mandrel and lock in position.b. The dog should be tightened properly on the mandrel and the

tail of the dog should not blind in the slot.c. The tail of the dog should then be locked in the driving fork of

the dividing head by means of the sets screws.d. This will ensure that there will be no play between the dividing

head and the mandrel.e. The dog should be far enough from the gear blank to ensure that

the cutter will not hit the dog when the gear is being cut.

3.7. Centre the gear blank with the cutter by either of the following methods:a. Place a square against the outside diameter of the gear. With a

pair of inside calipers or a rule, check the distance between thesquare and the side of the cutter. Adjust the table until thedistances from both sides of the gear blank to the sides of thecutter are the same.

.. Centering a gear cutter and the work piece

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Page 38: Unit3 Gear

J3103/3/38GEAR

1. Calculate the tooth caliper settings for measuring the following gears.(a) 37T, 6 mm module; (b) 40T 20 mm circular pitch.

2. The figure below shows two (2) gears in mesh. Gear A has 66 teeth andmodul 2.5. The gear ratio is 3:2. Calculate the following:

(a). Outside diameter of gear and pinion(b). Number of teeth of pinion(c). Centre distance of gear and pinion(d). Whole depth of gear

66T

SELF-ASSESSMENT 3

B

A

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Page 39: Unit3 Gear

J3103/3/39GEAR

1. (a) 9.4128 mm, 6.111 mm(b) 9.995 mm, 6.4923 mm.

2. (a) 170 mm, 115 mm.(b) 44, 165 mm, 110 mm(c) 137.50 mm(d) 5.415 mm

FEEDBACK OF SELF-ASSESSMENT 3

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