part 3 gear manufacturing and indexing 2014

Islamic University of Technology (IUT) الجامعة اإلسالمية للتكنولوجيا

Université Islamique de Technologie

Dr. Mohammad Ahsan Habib Assistant Professor

E-mail: [email protected] Department of Mechanical and Chemical Engineering (MCE)

Room: 405, Level 4, First Academic Building

MCE 4627 Machine Tools

Part 3 Gear Manufacturing

Indexing or Dividing Head

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© Dr. Mohammad Ahsan Habib

Manufacture of Gears

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Manufacture of Gears Manufacture of gears needs several processing operations in

sequential stages depending upon the material and type of the gears and quality desired. Those stages generally are : Preforming the blank without or with teeth Annealing of the blank, if required, as in case of forged or cast steels Preparation of the gear blank to the required dimensions by machining Producing teeth or finishing the preformed teeth by machining Full or surface hardening of the machined gear (teeth), if required Finishing teeth, if required, by shaving, grinding etc. Inspection of the finished gears.

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Preforming Gear Blanks Casting: Gear blanks and even gears along with teeth

requiring substantial to little machining or finishing are produced by various casting processes. Sand casting : The blanks of large cast iron gears, if required to be made one or few pieces, are produced by sand casting. Then the blank is prepared to appropriate dimensions and the teeth are produced by machining that cast preform. Metal mold casting: Medium size steel gears with limited accuracy and finish are often made in single or few pieces by metal mould casting. Such unfinished gears are used in several agro-industries. Die casting: Large lot or mass production of small gears of low melting point alloys of Al, Zn, Cu, Mg etc. are done mainly by die casting. Such reasonably accurate gears are directly or after little further finishing are used under light load and moderate speeds, for example in instruments, camera, toys.

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Investment casting: This near-net-shape method is used for producing small to medium size gears of exotic materials with high accuracy and surface finish hardly requiring further finishing. These relatively costly gears are generally used under heavy loads and stresses. Shell mold casting: Small gears in batches are also often produced by this process. The quality provided by this process lies in between that of sand casting and investment casting. Centrifugal casting: The solid blanks or the outer rims (without teeth) of worm wheels made of cast iron, phosphor bronze or even steel are preferably preformed by centrifugal casting. The performs are machined to form the gear blank of proper size. Then the teeth are developed by machining.

Wed, 19 Jun, 2013 8:59:40 AM

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Manufacture of gears by rolling: The straight and helical teeth of disc or rod type external steel gears of small to medium diameter and module are generated by cold rolling by either flat dies or circular dies as shown in figure. Such rolling imparts high accuracy and surface integrity of the teeth which are formed by material flow unlike cutting. Gear rolling is reasonably employed for high productivity and high quality though initial machinery costs are relatively high. Larger size gears are formed by hot rolling and then finished by machining

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Powder metallurgy: Small size high quality external or internal spur, bevel or spiral gears are also produced by powder metallurgy process. Large size gears are rolled after briquetting and sintering for more strength and life. Powder metallurgically produced gears hardly require any further finishing work.

Extrusion process: High quality small metallic or non metallic external gears are often produced in large quantity by extrusion. Number of gears of desired width are obtained by parting from the extruded rod of gear – section.

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Wire EDM: Geometrically accurate but moderately finished straight toothed metallic spur gears, both external and internal type, can be produced by wire type Electro-discharge Machining (EDM) as shown in figure.

Production of teeth of external and internal spur gears by Wire-EDM



Production of Gear Teeth by Machining The most commonly practiced method is preforming the

blank by casting, forging etc. followed by pre-machining to prepare the gear blank to desired dimensions and then production of the teeth by machining and further finishing by grinding if necessary.

Gear teeth are produced by machining based on Forming – where the profile of the teeth are obtained as the replica of the form of the cutting tool (edge); e.g., milling, broaching etc. Generation – where the complicated tooth profile are provided by much simpler form cutting tool (edges) through rolling type, tool – work motions, e.g., hobbing, gear shaping etc.

Wed, 19 Jun, 2013 8:59:45 AM

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Methods of production of gear teeth by machining on Forming principle Shaping, planing and slotting: Figure schematically shows how teeth of straight toothed spur gear can be produced in shaping machine, if necessary. Both productivity and product quality are very low in this process which therefore, is used, if at all, for making one or few teeth on one or two pieces of gears as and when required for repair and maintenance purpose. In principle planning and slotting machines work on the same principle. Planing machine is used, if required at all, for making teeth of large gears whereas slotting, generally, for internal gears.

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Gear teeth cutting in ordinary shaping machine



Production of gear teeth by form milling are characterized by : use of HSS form milling cutters use of ordinary milling machines low production rate for need of indexing after machining each tooth gap slow speed and feed

low accuracy and surface finish inventory problem due to need of a set of eight cutters for each module pressure angle combination.

End mill type cutters are used for teeth of large gears and / or module.

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Producing external teeth by form milling cutters (a) disc type and end mill type for (b) single helical and (c) double helical teeth

Milling: Gear teeth can be produced by both disc and end mill type form milling cutter as shown in figure.



Indexing or Dividing Head

Wed, 19 Jun, 2013 8:59:49 AM

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Indexing (Dividing) Head Once one of the more important attachments for milling machine Used to divide circumference of workpiece into equally spaced

divisions when milling gear teeth, squares, hexagons, and octagons Also used to rotate workpiece at predetermined ratio to table feed

rate

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Index Head Parts Headstock with index plates Headstock change gears Quadrant Universal chuck Footstock Center rest

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Index Head Parts Swiveling block Mounted in base enables headstock to be tilted from 5º below horizontal to 10º beyond vertical

Spindle Mounted in swiveling block with 40-tooth worm wheel, meshes with worm

Worm Right angle to spindle, connected to index crank

Direct indexing plate Engaged by pin and attached to front of spindle

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Index Head Parts Wed, 19 Jun, 2013

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Section view of a dividing head Wed, 19 Jun, 2013

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Index Head Parts Universal chuck Threaded onto end of spindle

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Index Head Parts Footstock Used in conjunction with headstock to support work held between centers or in chuck May be adjusted longitudinally, raised or lowered off center, and tilted out of parallel

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Index Head Parts Adjustable center rest Holds long, slender work between centers

Wed, 19 Jun, 2013 9:00:00 AM

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Methods of Indexing

1. Direct 2. Simple 3. Angular 4. Differential

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Direct Indexing Simplest form of indexing Performed by disengaging worm shaft from worm wheel by means of

eccentric device in dividing head Spring-loaded tongue lock engages numbered slots in index plate Used for quick indexing of workpiece when cutting flutes, hexagons,

squares, etc. Direct indexing plate usually contains three sets of hole circles or slots:

24, 30, and 36 Number of divisions possible to index limited to numbers that are factors of 24, 30, 36

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Slots Direct indexing divisions 24 2 3 4 _ 6 8 _ __ 12 __ __ 24 __ __ 30 2 3 _ 5 6 _ _ 10 __ 15 __ __ 30 __ 36 2 3 4 _ 6 _ 9 __ 12 __ 18 __ __ 36



Example: Direct Indexing What direct indexing is necessary to mill eight flutes on a reamer

blank? Since the 24-hole circle is the only one divisible by 8 (the required number of divisions), it is the only circle that can be used in this case.

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Slots Direct indexing divisions 24 2 3 4 _ 6 8 _ __ 12 __ __ 24 __ __ 30 2 3 _ 5 6 _ _ 10 __ 15 __ __ 30 __ 36 2 3 4 _ 6 _ 9 __ 12 __ 18 __ __ 36

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



Milling a Square with Direct Indexing 1. Disengage worm and worm shaft by turning worm disengaging shaft

lever if dividing head is so equipped 2. Adjust plunger behind index plate into the 24-hole circle or slot 3. Mount workpiece in dividing head chuck or between centers 4. Adjust cutter height and cut first side 5. Remove plunger pin using plunger pin lever 6. Turn plate attached to dividing head spindle one-half turn and engage

plunger pin 7. Take second cut 8. Measure work across flats and adjust work height if required 9. Cut remaining sides by indexing every six holes until all surfaces cut 10. Check for finish size

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Simple Indexing Work positioned by means of crank, index plate, and sector arms Worm attached to crank must be engaged with worm wheel on

dividing head spindle 40 teeth on worm wheel One complete turn on index crank cause spindle and work to rotate one-fortieth of a turn (ratio of 40:1)

Calculating the indexing or number of turns of crank for most divisions, simply divide 40 by number of divisions to be cut or,

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Simple Indexing The indexing required to cut eight flutes:

The indexing required to cut seven flutes:

The five-sevenths turn involves use of an index plate and sector arms.

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Index Plate and Sector Arms Index plate Circular plate provided with series of equally spaced holes into which index crank pin engages

Sector arms Fit on front of plate and may be set to any portion of a complete turn

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Finishing Indexing for Seven Flutes Wed, 19 Jun, 2013

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Index-plate hole circles Brown & Sharpe Plate 1 15-16-17-18-19-20 Plate 2 21-23-27-29-31-33 Plate 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

Choose any hole circle that is divisible

by denominator 7

5/7 = /21

So, 5 full turns plus 15 holes on 21 hole circle!

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9:00:16 AM

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by denominator 7

5/7 = /49


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9:00:17 AM

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by denominator 7

5/7 = /28


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9:00:19 AM

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by denominator 7

5/7 = /42


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9:00:21 AM

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by denominator 7

5/7 = /49


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Cutting Seven Flutes 1. Mount B&S Plate 2 index plate on dividing head 2. Loosen index crank nut and set index pin into hole on 21-hole

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

easily 4. Loosen setscrew on sector arm 5. Place narrow edge of left arm against index pin 6. Count 15 holes on 21-hole circle Do not include hole in which index crank pin is engaged. 7. Move right sector arm slightly beyond fifteenth hole and tighten

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

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Cutting Seven Flutes 10. Move table so cutter clears end of work 11. Tighten friction lock on dividing head before making each cut

and loosen lock when indexing for spaces 12. Set depth of cut and take first cut 13. After first flute has been cut, return table to original starting

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

the 15 holes indicated right sector arm Release index pin between 14th and 15th holes and gently tap until it

drops into 15th hole 15. Turn sector arm farthest from pin clockwise until it is against

index pin

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Cutting Seven Flutes The arm farthest from the pin is held and turned. If the arm next to

the pin were held and turned, the spacing between both sector arms could be increased when the other arm hits the pin. This could result in an indexing error not noticeable until the work was completed

16. Lock dividing head; continue machining and indexing for remaining flutes

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Angular Indexing Setup for simple indexing may be used Must calculate indexing with angular distance between divisions instead number of divisions

One complete turn of index crank turns work 1/40 of a turn 1/40 of 360º equals 9 degrees

Calculate indexing for 45º Calculate indexing for 60º

5 complete turns 6 full turns plus 12 holes on 18 hole circle

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Angular Indexing Calculate indexing for 24' Divide 24'/540' = 4/90 4/90 = 1/22.5 1 hole on a 22.5 hole circle The nearest is a 23 hole circle. Indexing would be 1 hole on a 23

hole circle with a slight error (approximately 1/2 minute). A need for higher accuracy requires differential indexing.

Calculate indexing for 24º30' First, convert angle into minutes (24 x 60') = 1440' now add 30' = 1470' Convert 9° to minutes 9°x60' = 540' Divide 1470'/540' = 2 13/18 2 full turns and 13 holes on 18 hole circle

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Differential Indexing

Used when 40/N cannot be reduced to a factor of one of the available hole circles Index plate must be revolved either forward or backward part of a turn while

index crank turned to attain proper spacing (indexing) Change of rotation effected by idler gear or gears in gear train.

Number chosen close to required divisions that can be indexed by simple indexing Example: Assume index crank has to be rotated 1/9th of a turn and only 8-hole

circle Crank moved 1/9th, index pin contacts plate at spot before first hole Exact position would be the difference between 1/8th and 1/9th of a revolution of the crank

one-seventy-second of a turn short of first hole

Since there is no hole at this point, it is necessary to cause plate to rotate backward by means of change gears one-seventy-second of a turn of pin will engage in hole.

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Method of Calculating the Change Gears

A = approximate number of divisions N = required number of divisions

If A is greater than N, resulting fraction is positive and the index plate must move in same direction as crank (clockwise). This positive rotation uses an idler gear.

If N is greater than A, resulting fraction is negative and index plate must move counterclockwise. This negative rotation required use of two idler gears.

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Gearing Simple One idler for positive rotation of index plate and two idlers for negative rotation

Compound One idler for negative rotation of index plate and two idlers for positive rotation

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Example: Wed, 19 Jun, 2013

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Calculate the indexing and change gears required for 57 divisions. The change gears supplied with the dividing head are as follows: 24, 28, 32, 40, 44, 48, 56, 64, 72, 86 The available index plate hole circles are as follows: Plate 1: 15, 16, 17, 18, 19, 20 Plate 2: 21, 23, 27, 29, 31, 33 Plate 3: 37, 39, 41, 43, 47, 49

No 57 hole circle so select number close to 57

5/7 would be 15 holes on 21-hole circle

Choose plate 2: 21 holes



Example: continued

The fraction is negative and simple gearing is to be used, the index plate rotation is counterclockwise and two idlers must be used.

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Example: continued For indexing 57 divisions, a 40-tooth gear is mounted on the

dividing head spindle and a 56-tooth gear is mounted on the worm shaft.

Index idlers must be used. plate rotation is negative and two idlers must be used

After proper gears installed, the simple indexing for 56 divisions should be followed

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part 3 gear manufacturing and indexing 2014

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