Gear Cutting Methods PDF

Gear cutting Gear cutting Methods Gears are manufactured by Casting Stamping Machining (Common method) Powder metallurgy Different methods of gear production by machining are Formed cutter methods Gear generating methods Form disc cutter...

Gear cutting Gear cutting Methods Gears are manufactured by Casting Stamping Machining (Common method) Powder metallurgy Different methods of gear production by machining are Formed cutter methods Gear generating methods Form disc cutter (milling Rack tooth cutter (gear cutting machine) machine) Form end mill cutter (milling Pinion cutter (gear cutting machine) machine) Formed single point cutting Hob cutter (gear cutting tool (shaper/planer machine) machine) Formed cutter (Gear shaper Bevel gear generator and broaching machine) Template method (Gear cutting machine) Formed cutter methods For small scale production Uses a single point cutting tool or mill cutter with cutting edges resembling the space between the tooth Can be performed in conventional machines with simple set up For cutting spur, helical or bevel gears with involute and cycloidal tooth Disadvantages Gear tooth accuracy is poor Low production rate due to wastage of time in indexing of workpiece Differently shaped cutter is required for each number of tooth in involute gears Spur Gear Terminology Name of the tooth Spur gear tooth element proportions (pressure angle 20 degree) Pitch Diameter Zm Addendum m Dedendum 1.25m Working depth 2m Tooth depth 2.25m Outside diameter/ blank m(z+2) dia Tooth thickness 1.5708m Clearance 0.25m Radius of fillet 0.4m to 0.45m * where m is module, Z is number of tooth 1. Spur gear milling by formed disc cutter Steps followed: Determination of important dimensions and proportions of gear tooth Selection of suitable indexing method to space the gear teeth accurately Selection of proper cutter for machining required number of teeth Selection of proper speed of cutter, feed of table and depth of cut in the machine Set the cutter and the work to perform actual machining operation Spur gear milling by formed disc cutter Procedure Check whether the tooth profile of formed cutter should correspond to tooth space (module) of the gear Mounting gear blank (work) at the end of a dividing head spindle in a milling machine Mounting formed cutter on horizontal arbor Feeding the gear blank past the rotating cutter Perform indexing operation after cutting each tooth space in the blank A set of 8 cutters are sufficient for cutting all involute gears having tooth ranging from 12 to rack A set of 24 cutters are sufficient for cutting all cycloidal gears having tooth ranging from 12 to rack 1. Calculate all machining particulars for milling an involute spur gear of module 3 and number of teeth 54 with proper indexing mechanism using formed disc cutter method. Determination of gear blank dia. and other particulars Blank diameter= m(Z+2)= 3(54+2) = 168 mm Tooth depth = 2.25m= 2.25x3= 6.75 mm Cutter pitch= 3 m = 3x 3= 9 mm Indexing Assuming simple indexing Index crank movement= 40/N= 40/54= 20/27 Therefore, index crank will be moved by 20 holes in 27 hole circle after cutting each teeth Selection of cutter Using table given above, choose cutter no.3 Selection of cutting speed, feed, depth of cut Depth of cut= tooth depth= 6.75 mm Cutting speed and feed are determined based on milling machine conditions Bevel gear milling by formed disc cutter TPS Steps followed 1. Determination of gear tooth proportions 2. Selecting proper indexing mechanism 3. Setting gear blank at cutting angle 4. Calculating offset 5. Selection of proper cutter 6. Determining machining parameters such as speed, feed, depth of cut 7. Set the cutter and the work to perform actual machining operation 1. Determination of gear tooth proportions TPS 2. Selecting proper indexing mechanism Simple indexing or direct indexing 3. Setting the gear blank Bevel gear cutting is performed only after swiveling the dividing head spindle to the cutting angle Cutting angle = Pitch cone angle + Adendum angle 4. Calculating offset Bevel gears are typically conical in shape, with their teeth cut along a slanting surface. This develops a variable cross-section of the tooth space in bevel gear To achieve the desired bevel tooth profile, the cutter must be positioned at a specific distance (offset) relative to the gear blank's rotational axis. In the context of bevel gear cutting, the term "offset" refers to the distance between the gear blank's rotational axis and the cutter's rotational axis This offset is a critical parameter in bevel gear manufacturing as it determines the proper alignment of the cutting tool with the gear blank to ensure that the bevel gear tooth are cut with the correct profile and dimensions.  To develop a variable cross-section of the tooth space, the blank is set over from radial position relative to the center line of the cutter, which is called offset (2). Offset is followed by angular movement of the blank (1) Involves machining of the first flank (a) followed by the second flank(b) R= Cone distance, b= face width, Z= No. of tooth 5. Selection of proper cutter A total of 8 cutters are available for bevel gear fabrication Range of tooth production for each cutter is similar to that of spur gear cutters The bevel gear cutter can be selected from the formula Disadvantages of bevel gear cutting by formed disc cutter The cross-section of the bevel gear tooth is not uniform throughout it length. Therefore, a cutter chosen for producing tooth space on one side of gear will not be correct for other side. The tapering cross-section of tooth profiles machined in a milling machine are theoretically incorrect.  Advantage and Limitations of Gear Milling using formed disc cutter : Advantage : Spur, Helical and bevel gears are cut on commonly available machines. Low tooling cost Method is economical for one type of gear production at a time. Tooth profile that can not be produced by gear generation method can be produced by formed disc cutter method. Both roughing and finishing operation can be carried out. Limitations : Internal teeth can not be produced. Pitch accuracy depends on accuracy of the indexing mechanism. Processes is very slow. Mass production not applicable. 2. Gear cutting by formed end mill cutter Using end mill cutters having cutting edges formed to correspond to the tooth space in a vertical milling machine For cutting spur, helical or herringbone gears of large module, from 20 mm or larger Used where ordinary formed disc type cutters are unsuitable Operation similar to formed disc type cutter 3. Gear cutting by formed single point tool Using single point cutting tool having cutting edges formed to correspond to the tooth space For cutting spur or bevel gear in a shaper or planer machine or CNC lathe Gear blank is indexed by shaper center 4. Gear cutting by shear speed process The quickest method of producing external/internal spur gear Production of all teeth on a gear simultaneously by a ring of formed blades Blades are arranged on the periphery of the gear blank Depth of cut can be adjusted by radial movement of the blades Number of blades on the cutter equals number of tooth spaces Gear blank is reciprocated against fixed blades of the cutter Cutting edges of each blade cuts one tooth space in the cutting stroke Blades are retracted during return stroke of the gear blank Old and obsolete method 5. Gear cutting by broaching Using broaching tool having formed cutting edges For producing small internal/external gears of accurate shape on soft materials Number of cutting edges equal to number of teeth space on the gear High productivity since the whole gear is produced in single pass For cutting large gears, broach tool is made as sector of a cylinder and an indexing mechanism is used to rotate the gear blank by one tooth after each stroke TPS Gear generating methods Consist of a rotating cutter gear reciprocates over the entire width of the gear blank. Working principle: During cutting, if the pitch surface of the cutter gear rolls without slipping on the pitch surface of the gear blank, accurate tooth profiles can be generated. Only applicable for involute gears The cutter employed may be either rack type or pinion type or a hob. 1. Gear generating with rack type cutter Also called gear shaping process or Sunderland method Cutting action similar to a shaper machine with a rack cutter Cutter reciprocates and remove material only during cutting stroke Blank is indexed slowly and uniformly After each cutting stroke, the blank rolls along the length of the cutter to a distance equal to pitch of the gear By swivelling the cutter to the required helix angle, the helical gears can be cut TPS  Advantages Most accurate tooth profiles can be generated Gears of any number of teeth having same pitch (or gear blank of different diameters) can be cut with a cutter of same pitch High production rate than any formed cutter method TPS Gear generating with pinion type cutter Similar to rack type cutting process, but with a pinion cutter Pinion cutter may be reciprocated either in horizontal or vertical direction The cutter and the blank rolls about its axes till all the tooth are cut The cutter is fed radially into the gear blank to a distance equal to the depth of cut of the tooth Advantages The gear cutting mechanism is simple than rack cutter process Internal gears can be generated Gears of any number of tooth having same pitch can be cut with a cutter of same pitch High production rate than any formed cutter method TPS Gear Hobbing Gear generation by means of a cutter called hob. Hob rotates and cut like a milling cutter Hob are steel worms with 1,2 or more right hand/ left hand threads Double thread hob cuts two teeth simultaneously Straight/helical flutes are cut across threads Rough cuts are taken with multi-thread hobs and finish cuts are taken with single thread hobs TPS  Hobbing is performed in gear hobbing machines Machines are provided with horizontal or vertical spindles for hobs Gear blank is first moved radially towards rotating hob until proper depth is achieved. As soon as proper depth is achieved, hob is fed across the face of the gear Can produce spur, helical and worm gears Axis of the hob is set to proper helix angle in cutting helical gears Any number of tooth can be hobbed with the given cutter, but will generate only single pitch. TPS Bevel gear generating process Accurate bevel gears are manufactured by gear generating process Manufactured by rolling of bevel gear blank on a crown wheel, which resemble a rack type cutter. Crown wheel is a bevel gear having pitch cone angle 180 degree. The tooth on the crown wheel are straight, radial and are provided on the flat face The crown wheel and the gear blank traces a rolling motion, while only one tooth of the crown wheel act as the cutting tool at any instant. Tooth of spiral and hypoid bevel gears can be generated by the same process, but the cutter resembles a face milling cutter TPS