8 MP Notes Module 4 Gear Cutting Notes PDF

29-10-2024 Gear Cutting Dr. Shailendra Kumar Professor Dept. of Mechanical Engineering,...

29-10-2024 Gear Cutting Dr. Shailendra Kumar Professor Dept. of Mechanical Engineering, SVNIT, Surat Basic Purpose of Use Of Gears Gears are widely used in various mechanisms and devices to transmit power and motion positively (without slip) between parallel, intersecting (axis) or non- intersecting non parallel shafts, - without change in the direction of rotation - with change in the direction of rotation - without change of speed (of rotation) - with change in speed at any desired ratio Often some gearing system (rack – and – pinion) is also used to transform rotary motion into linear motion and vice-versa. Gears are basically wheels having, on its periphery, equi-spaced teeth which are so designed that those wheels transmit, without slip, rotary motion smoothly and uniformly with minimum friction and wear at the mating tooth – profiles. 1  29-10-2024 To achieve those favourable conditions, most of the gears have their tooth form based on involute curve, which can simply be defined as Locus of a point on a straight line which is rolled on the periphery of a circle or Locus of the end point of a stretched string while its unwinding over a cylinder. General Applications of Gears Gears of various type, size and material are widely used in several machines and systems requiring positive and stepped drive. The major applications are :  Speed gear box, feed gear box and some other kinematic units of machine tools  Speed drives in textile, jute and similar machineries  Gear boxes of automobiles  Speed and / or feed drives of several metal forming machines  Machineries for mining, tea processing etc.  Large and heavy duty gear boxes used in cement industries, sugar industries, cranes, conveyors etc.  Precision equipments, clocks and watches  Industrial robots and toys. 2  29-10-2024 Types of Gears And Their Characteristics 3 29-10-2024 4 29-10-2024 5 29-10-2024 6  29-10-2024 Module of a Gear Gear Manufacturing/Cutting Two principal methods of gear manufacturing include 1. Gear forming, and 2. Gear generation. Gear forming: In gear form cutting, the cutting edge of the cutting tool has a shape identical with the shape of the space between the gear teeth. Two machining operations, milling and broaching can be employed to form cut gear teeth. 7 29-10-2024 8 29-10-2024 9  29-10-2024 Broaching can also be used to produce gear teeth and is particularly applicable to internal teeth. The process is rapid and produces fine surface finish with high dimensional accuracy. However, because broaches are expensive-and a separate broach is required for each size of gear-this method is suitable mainly for high-quantity production. 10  29-10-2024 Gear Generation In gear generating, the tooth flanks are obtained (generated) as an outline of the subsequent positions of the cutter, which resembles in shape the mating gear in the gear pair. In gear generating, two machining processes are employed, shaping and milling. There are several modifications of these processes for different cutting tool used, - milling with a hob (gear hobbing), - gear shaping with a pinion-shaped cutter, or - gear shaping with a rack-shaped cutter. Cutters and blanks rotate in a timed relationship, a proportional feed rate between them is maintained. Gear generating is used for high production runs and for finishing cuts. 11  29-10-2024 Gear Hobbing:  It is a machining process in which gear teeth are progressively generated by a series of cuts with a helical cutting tool (hob).  All motions in hobbing are rotary, and the hob and gear blank rotate continuously as in two gears meshing until all teeth are cut. Schematic illustration of three views of gear cutting with a hob. 12  29-10-2024 The gear hob is a formed tooth milling cutter with helical teeth arranged like the thread on a screw. These teeth are fluted to produce the required cutting edges. When hobbing a spur gear, the angle between the hob and gear blank axes is 90° minus the lead angle at the hob threads. For helical gears, the hob is set so that the helix angle of the hob is parallel with the tooth direction of the gear being cut. Additional movement along the tooth length is necessary in order to cut the whole tooth length: 13  29-10-2024 The cutting of a gear by means of a hob is a continuous operation. The hob and the gear blank are connected by a proper gearing so that they rotate in mesh. To start cutting a gear, the rotating hob is fed inward until the proper setting for tooth depth is achieved, then cutting continues until the entire gear is finished. Machines for cutting precise gears are generally CNC-type Shaping with a pinion-shaped cutter Process for generating gear teeth by a rotating and reciprocating pinion-shaped cutter. Gear shaping is one of the most versatile of all gear cutting operations used to produce internal gears, external gears, and integral gear-pinion arrangements. Advantages of gear shaping with pinion-shaped cutter are the high dimensional accuracy achieved and the not too expensive tool. The process is applied for finishing operation in all types of production rates. 14  29-10-2024 Shaping with a rack-shaped cutter In the gear shaping with a rack-shaped cutter, gear teeth are generated by a cutting tool called a rack shaper. The rack shaper reciprocates parallel to the axis of the gear axis. It moves slowly linearly with the gear blank rotation at the same pitch-cycle velocity. Advantages of this method involve a very high dimensional accuracy and cheap cutting tool (the rack shaper’s teeth blanks are straight, which makes sharpening of the tool easy). The process can be used for low-quantity as well as high-quantity production of spur and helix external gears. Gear Finishing The surface finish and dimensional accuracy may not be accurate enough for certain applications. Several finishing operations are available, including – 1. Gear Burnishing 2. Gear Shaving 3. Gear Grinding 4. Gear Lapping. 15  29-10-2024 Gear Burnishing: In burnishing, the gear to be finished (called work gear) is rolled between three burnishing gears. The teeth of burnishing gears are very hard, smooth and accurate. Gears are arranged at 120 position around the work gear. Power drive is given to one of the burnishing gears. The other two gears idlers. Burnishing pressure is applied to the idlers. The gears are rotated in one direction for some period. Then they are rotated in the reverse direction for the same period. Gear Shaving: This is the most common method of gear finishing. In this method, a very hard gear shaving cutter is used to remove fine chips from the gear teeth. The shaving cutter may be in the form of a rack or a pinion. The rotary method using pinion cutter is used on all types of gears. The rotating cutter will have helical teeth of about 15 helix angle. The cutter has a number of serrations on its periphery. These act as cutting edges. Gear Grinding: Gear grinding is used for finishing gears after hardening. Gear grinding is done for - (i) increasing the accuracy of gears. (ii) improving the surface finish. (iii) removing the distortion due to heat treatment. There are two method of gear grinding - 1. Formed wheel grinding 2. Generation gear grinding 16  29-10-2024 Formed Wheel Grinding: In formed wheel grinding, the grinding wheel is dressed to the form of tooth space of the gear (involute profile). The grinding wheel is moved parallel to the work gear axis. After grinding one tooth space, the work gear is indexed to the next tooth space. About three passes are required to finish the tooth space. Proper coolant should be used while finishing by grinding. Generation Gear Grinding: The work gear is rolled along an imaginary rack. Rolling is done in both the directions to grind both sides of the tooth. The grinding wheel reciprocates along the length of the tooth. The grinding wheel is dressed in the form of rack teeth. The work gear rotates about its axis. The gear is also given a linear motion along its axis. The linear motion and rotary motion are opposite to each other. These two motions together give a generating motion. 17  29-10-2024 Gear Lapping: The hardened gears are finished accurately by lapping process. Lapping removes very small amount of metal (not more than 0.05 mm). The work gear is rolled between three lapping gears. The lapping gears are made of cast iron. The lapping gears are arranged at 120 position around the work gear. The axis of the two lapping gears is inclined at about 4 to the work gear axis. The axis of the other lapping gear is parallel to the work gear axis. The drive is given through this gear. When gears rotate, a lapping compound is applied between them. The compound is a mixture of a very fine abrasive powder and kerosene. The lapping process improves the tooth contact. 18