Ch 8.1 gear manufacturing

production

1. Gear Manufacturing By S K Mondal

2. 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

3. Forming and Generation 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.

4. Gear Generation

5. Sunderland method using rack type cutter The rack type HSS cutter (having rake and clearance angles) reciprocates to accomplish the machining (cutting) action while rolling type interaction with the gear blank like a pair of rack and pinion. External gear teeth generation by rack type cutter (Sunderland method)

6. Sunderland method using rack type cutter Applications of this method (and machine) include: Moderate size straight and helical toothed external spur gears with high accuracy and finish Cutting the teeth of double helical or herringbone gears with a central recess (groove) Cutting teeth of straight or helical fluted cluster gears However this method needs, though automatic, few indexing operations.

7. Gear shaping Gear shaping is similar to the rack type cutting process, excepting that, the linear type rack cutter is replaced by a circular cutter where both the cutter and the blank rotate as a pair of spur gears in addition to the reciprocation of the cutter.

8. Gear shaping Generation method is characterised by automatic indexing and ability of a single cutter to cover the entire range of number of teeth for a given combination of module and pressure angle and hence provides high productivity and economy. The gear type cutter is made of HSS and possesses proper rake and clearance angles. The additional advantages of gear shaping over rack type cutting are:  Separate indexing is not required at all Straight or helical teeth of both external and internal spur gears can be produced with high accuracy and finish Productivity is also higher.

9. Gear Hobbing The HSS or carbide cutter having teeth like gear milling cutter and the gear blank apparently interact like a pair of worm and worm wheel. The hob (cutter) looks and behaves like a single or multiple start worms. (a) Straight (b) helical tooth and (c) worm wheel

10. Gear Hobbing Having lesser number (only three) of tool – work motions, hobbing machines are much more rigid, strong and productive than gear shaping machine. But hobbing provides lesser accuracy and finish and is used only for cutting straight or helical teeth (single) of external spur gears and worm wheels.

11. Advantages of Gear Hobbing (a) The method is versatile and can generate spur, helical, worm and worm wheels. (b) Since gear hobbing is a continuous process, it is rapid; economical and highly productive. (c) The method produces accurate gears and is suitable for medium and large batch production. (d) The cutter is universal, because it can cut all gears of same module, irrespective of number of teeth on the gear.

12. Disadvantages of gear Hobbing (a) Gear hobbing cannot generate internal gears and bevel gears. (b) Enough space has to be there in component configuration for hob approach. Applications of Hobbing The gears produced by gear hobbing are used in automobiles, machine tools, various instruments, clocks and other equipments.

13. Milling Gear teeth can be produced by both disc and end mill type form milling cutter. Fig. (a) disc type and end mill type for (b) single helical and (c) double helical teeth

14. Milling Production of gear teeth by form milling are characterised 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.

15. Shaping, Planning and Slotting Straight toothed spur gear can be produced in shaping machine. 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. Planning and slotting machines work on the same principle. Planning machine is used for making teeth of large gears whereas slotting for internal gears.

16. Fig- gear teeth cutting in ordinary shaping machine

17. Fast production of teeth of spur gears Parallel multiple teeth shaping It is similar to ordinary shaping but all the tooth gaps are made simultaneously, without requiring indexing, by a set of radially in feeding single point form tools. This old process was highly productive but became almost obsolete for very high initial and running costs.

18. Fast production of teeth of spur gears Broaching Teeth of small internal and external spur gears; straight or single helical, of relatively softer materials are produced in large quantity by broaching. This method leads to very high productivity and quality but cost of machine and broach are very high.

19. 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. 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.

20. 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.

21. 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).

22. Preforming Gear Blanks

23. Blanking in Press tool Plastic moulding Extrusion process

24. Casting Sand casting Metal mould casting Die casting Investment casting Shell mould casting Centrifugal casting

25. Gear finishing process

26. Gear finishing process One of the goals of the gear finishing process in gears is to obtain a certain level of toughness in the gear teeth to reduce and/or eliminate bending and contact fatigue failures. Reduction of index undulation errors associated with helical gear teeth caused by the grinding process during the manufacture of the gears without degrading other gear accuracies (e.g. profile, tooth spacing) below levels required for precision (AGMA16 or DIN1) gears. A mold of the space between several gear teeth is obtained, with the mold having a length equal to or greater than the wavelength of the undulation error to be reduced.

27. A micro finishing film is affixed to the mold and the mold is placed relative to a gear tooth so that the micro finishing film rests against a tooth surface having the undulation error. The grit size of the micro finishing film is such as to remove approximately 2 to 3 millionths of gear material with each pass through the teeth by the mold. Multiple passes are made by hand until the undulation error is reduced to an acceptable value. During the process the micro finishing film is replaced after approximately 3 or 4 passes and the process is repeated for each tooth of the gear.

28. Gear shaving Gear shaving is a gear finishing operation with high efficiency and high precision. When a work gear has been shaved by a shaving cutter with a true involute profile , the ''mid-concave'' phenomena inevitably exist around the pitch points of the work gear tooth flanks. Aiming at this problem, a new-style shaving cutter with unequal depth gashes is designed and manufactured. This paper analyses the forming of the gash on the basis of the slotting principle, and proposes a gash-designing method. Experiment has proven that the shaved gear has a better surface finish that achieves the anticipated effect.

29. Gear burnishing It is designed to remove or reduce gear tooth nicks and burrs, along with improving the smoothness of the tooth's active profile finish. The action of the burnishing dies on the tooth surface allows the machine to accomplish these quality improvements without altering the tooth profile or lead. Both internal and external gears are possible to burnish.

30. Gear Lapping Gear lapping is used to finish hardened gears by correcting small errors in spacing, profile, helix angle, and eccentricity. The operation is performed with all forms of gears running together with mating gears, and cast iron toothed laps, under a flow of fine oil mixed with an abrasive compound.

31. IES - 1992 Gear lapping (a) An operation after heat treatment (b) An operation prior to heat treatment (c) An independent operation for gear reconditioning (d) None of the above

32. IES - 1999 Consider the following processes for the manufacture of gears: 1. Casting 2. Powder metallurgy 3. Machining from bar stock 4. Closed die forging The correct sequence in increasing order of bending strength of gear teeth is (a) 1, 2, 3, 4 (b) 1, 2, 4, 3 (c) 2, 1, 4, 3 (d) 2, 1, 3, 4

33. IES - 2006 Which of the following is/are used for cutting internal gears? 1. Gear hobber 2. Gear shaper 3. Rack cutter 4. Jig borer Select the correct answer using the codes given below: (a) Only 1 and 2 (b) Only 2 and 3 (c) Only 1 and 4 (d) Only 2

34. IES - 2005 In helical milling, the ratio of the circumference of the gear blank to the lead of the helix determines the: (a) Proper speed to use (b) Proper feed and depth of cut required (c) Angle setting of the machine table (d) Gear ratio for table screw and dividing head

35. IES 2010 Match List I with List II and select the correct answer using the code given below the lists: List I List II (Type of work) (Manufacturing) A. High rate production of worm Gears and 1. Gear shaving worm wheel B. Generating internal gears and Cluster gears 2. Gear milling C. Finishing of gear tooth profiles 3. Gear hobbing D. Repair and piece production of gears 4. Gear shaping A B C D A B C D (a) 2 1 4 3 (b) 3 1 4 2 (c) 2 4 1 3 (d) 3 4 1 2

36. IES - 1996 Gear cutting on a milling machine using an involute profile cutter is a (a) Gear forming process (b) Gear generating process. (c) Gear shaping process (d) Highly accurate gear producing process.

37. IES - 2000 Which one of the following processes of gear manufacture results in best accuracy of the involute gear tooth profile? (a) Milling (b) Hobbing (c) Rotary gear shaper (d) Rack type gear shaper

38. IES - 2009 Assertion (A): Gears produced by employing form-cutting principle using gear-milling cutter on a milling machine are not very accurate. Reason (R): Production of the correct gear tooth profile employing form-cutting principle would require a separate cutter for cutting different numbers of teeth even for the same module and also errors are associated with inaccurate operation of indexing mechanism. (a) Both A and R are true and R is the correct explanation of A (b) Both A and R are true but R is NOT the correct explanation of A (c) A is true but R is false (d) A is false but R is true

39. IES - 1996 Consider the following processes of gear manufacture: 1. Milling with form cutter 2. Rack type gear shaper (gear planer) 3. Rotary gear shaper (gear shaper) 4. Gear hobbing The correct sequence of these processes in increasing order of accuracy of involute profile of the gear (a) 3, 2, 4, 1 (b) 2, 3, 4, 1 (c) 3, 2, 1, 4 (d) 2, 3, 1, 4

40. IES - 2009 By which one of the following machines the teeth of an internal spur gear can be cut accurately? (a) Milling machine (b) Slotting machine (c) Hobbing machine (d) Gear-shaping machine

41. IES - 2004 Gear shaping is a process of manufacturing gears. Which one of the following principles is employed by it? (a) Form cutting with cutter (b) Generating tooth form with a reciprocating cutter (c) Generating tooth form by a rotating cutter (d) Generating form with a reciprocating and revolving cutter

42. IES - 1992 In gear hobbing (a) Only hob rotates (b) Only gear blank rotates (c) Both hob and gear blank rotate (d) Neither hob nor gear blank rotates

43. IES - 2003 A spur gear of 40 teeth is machined in a gear hobbing machine using a double start hob cutter. The speed ratio between the hob and the blank is (a) 1:20 (b) 1:40 (c) 40: 1 (d) 20: 1

44. IES - 2008 Which machining processes are used for gear manufacture? 1. Form milling 2. Broaching 3. Roll forming 4. Hobbing Select the correct answer using the code given below: (a) 1, 2 and 3 (b) 1, 3 and 4 (c) 1, 2 and 4 (d) 2, 3 and 4

45. IES - 1999 A 60-teeth gear when hobbed on a differential hobber with a two-start hob, the index change gear ratio is governed by which one of the following kinematic balance equations? (a) 1 revolution of gear blank = 1/60 of hob revolutions (b) 1 revolution of gear blank = 2/60 of hob revolutions (c) 1 revolution of hob = 2/60 of blank revolutions (d) 1 revolution of hob = 1/60 of blank revolutions

46. IES - 1997 Which of the following motions are not needed for spur gear cutting with a hob? 1. Rotary motion of hob 2. Linear axial reciprocator motion of hob 3. Rotary motion of gear blank 4. Radial advancement of hob. Select the correct answer using the codes given below: (a) 1, 2 and 3 (b) 1, 3 and 4 (c) 1, 2 and 4 (d) 2, 3 and 4

47. IES - 2007 Which of the following methods are gear generating processes? 1. Gear shaping 2. Gear hobbing 3. Gear milling Select the correct answer using the code given below: (a) 1, 2 and 3 (b) 1 and 2 only (c) 2 and 3 only (d) 1 and 3 only

48. GATE – 2007 (PI) Which one of the following gear manufacturing processes is NOT based on generation principle? (a) Gear Hobbing (b) Gear Shaping (c) Gear Milling (d) Gear Shaving

49. IES - 1993 Internal gear cutting operation can be performed by (a) Milling (b) Shaping with rack cutter (c) Shaping with pinion cutter (d) Hobbing

50. GATE-2016 Internal gears are manufactured by (a) Hobbing (b) Shaping with pinion cutter (c) Shaping with rack cutter (d) Milling

51. IAS - 1998 Assertion (A): Internal gears are cut on a gear shaper. Reason (R): Hobbing is not suitable for cutting internal gear. (a) Both A and R are individually true and R is the correct explanation of A (b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false (d) A is false but R is true

52. IES - 2006 Which of the following cannot be cut by hobbing process? (a) Helical gears (b) Bevel gears (c) Worm gears (d) Spur gears

53. IES - 1996 For the manufacture of full depth spur gear by hobbing process, the number of teeth to be cut = 30, module = 3 mm and pressure angle = 20°. The radial depth of cut to be employed should be equal to (a) 3.75 mm (b) 4.50 mm (c) 6.00 mm (d) 6.75 mm

54. IES - 1995 While cutting helical gears on a non-differential gear hobber , the feed change gear ratio is (a) Independent of index change gear ratio (b) dependent on speed change gear ratio (c) Interrelated to index change gear ratio (d) Independent of speed and index change gear ratio.

55. IES - 1992 Gear burnishing process for (a) Removing residual stresses from teeth roots (b) Surface finishing (c) Under-cut gears (d) Cycloidal gears

56. IAS - 2003 Which one of the following is not a feature of gear hobbing process? (a) High rate of production (b) Generation of helical gears (c) Very accurate tooth profile (d) Generation of internal gears

57. IAS - 2001 Consider the following motions and setting in a hobbing machine: 1. Hob rotation 2. Job rotation 3. Axial reciprocating hob rotation 4. Tilting of hob to its helix angle Which of these motions and setting in a hobbing machine are required to machine a spur gear? (a) 1, 2 and 3 (b) 2, 3 and 4 (c) 1, 2 and 4 (d) 1, 3 and 4

58. IES - 1994 Consider the following machine tools: 1. Hobbing machine 2. Gear shaping machine 3. Broaching machine. The teeth of internal spur gears can be cut in (a) 1, 2 and 3 (b) 1 and 2 (c) 1and 3 (d) 2 and 3

59. GATE-2016 (PI) A helical gear with involute tooth profile has been machined with a disc-type form gear milling cutter. The helical gear has 30 teeth and a helix angle of 30 o . The module of the gear milling cutter is 2. The pitch circle diameter (in mm) of the helical gear is_______________.