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Forming Gear Teeth

Introduction..... Milling..... Shaping..... Hobbing..... Shaving..... Burnishing..... Grinding..... Lapping..... Honing.....



Introduction

The notes below describe briefly how gear teeth are formed.    The performance, life , vibration and noise levels of a gear set is directly related to the machining process , heat treatment, and finishing of gear teeth.     The process involve include the following

Gear Blank Production

Each of the processes below has cost, material size, accuracy, strength, operating life advantages and selection of the primary forming process is very important.

  • Sand Casting
  • Fabrication
  • Forging
  • Shell Molding
  • Investment Casting
  • Permanent Mold Casting
  • Die Casting
  • Centrifugal Casting
  • Powder matallurgy
  • Extrusion
  • Injection Moulding
  • Pressing/ Stamping


Gear Teeth forming Processes
  • Shaping
  • Milling
  • Hobbing
  • Cold Forming/ rolling


Gear Teeth finishing Processes

Gears running a high speeds, transmitting large forces, and requiring quiet smooth running conditions are often finished using one of the following operations.

  • Shaving
  • Burnishing
  • Grinding
  • Lapping
  • Honing

The last three options are generally considered for finishing gear teeth which have been heat treated to increase the surface hardness of the teeth.



Heat Treatment

The life and strength of a gear can be greatly improved by heat treatmet. heat Treatment process include

  • Hardening and tempering
  • Surface Hardening



Milling

An involute form milling cutter with teeth shaped which conform to the tooth space are used to cut involute gear teeth.    This is generally completed on a horizontal milling machine    This process has a theoretical disadvantage that the tooth space profile is clearly different for gears having different modules and different number of teeth.    This limitation is such that, for each module, by allowing some innaccuracy of form, about eight cutters are needed for the whole range for gear teeth from 10 teeth to the rack form.     Milling is limited to making single gears for prototype or very small batches of gears as it is a very slow and uneconomical method of production.

The milling cutter form is is machined so that it will cut the desired gear tooth space. The cutter is then driven across across the gear blank repeatedly to cut out a space.    The the gear is rotated through one circular gear pitch and the next tooth space is cut.    this process is repeated until all of the teeth have been formed.    .    The depth of cut into the gear blank depends on the cutter strength, set-up rigidity and machineability of the gear blank material.




Shaping

Gear teeth msy be generated using a reciprocating cutter which is of a rack form or a rotating pinion form.     The process involving the rack shaped cutter is shown below.    This process is used for producing spur gears and helical gears.

The rack cutter is reciprocated across the blank-face . After each cut the rack cutter a is moved a small distance along the line of teeth and the blank is rotated through a small angle.    This is repeated until the end of the rack is reached, at which point the rack and blank have to be repositioned.

The machining process is often used for cutting very large gears



Gear shaping using a circular pinion-shaped cutter with the necessary rake angles to cut the teeth is generally faster than using the rack form of cutter.    Both the gear blank and cutter are set in a vertical plane and rotated such as that the two are like gears in mesh.    Gear cutter shaping is faster than using a rack because the cutting process is continuous and the cutter does not have to be stepped back.

Gear shaping using pinion shaped teeth can be used to produce external and internal gears and splines.    The disadvantage of this method is that the generated gear profile is directly dependent of the tooth profile any error on any tooth of the cutter is directly transferred to the gear being cut.




Hobbing

The hob is simply a rotating cutting tool that is shaped like a worm.    The teeth have straight sides . However the hob axis must be rotated through the lead angle in order to cut spur-gear teeth .    The hob is basically a straight cylindrical tool around which a thread with the same cross section as the rack tooth has been helically wound.    Both the hob and the blank are rotated at the correct angular velocity ratio.    The hob is then slowly fed across the face of the blank until all of the teeth have been producted.    The hob is then rotated with the gear blank fed onto the hob according to the depth of cut.    The helix pattern of hob as it rotates is the same as that of the rack moving laterally.

This is the most productive of all the gear production processes.    However, it can only be used for producing spur gears and worm gears.    It cannot be used to cut internal gears and it cannot work up a shoulder.





Finishing Processes





Gear Shaving

Shaving improves gear tooth finish where the cutting process has not provided the required standard. Shaving only cuts of minute amounts of metal bringing the a the tooth profile to within 6,5 μm.     However the preceding process must result in teeth which have small errors only in pitch, profile and concentricity.    This process cannot be completed if the gear teeth have been hardened.

The process is ideal for automative gearbox gears after hobbing and before hardening.   

The shaving process involves meshing the gear with a gear shaped cutter which has serrations across the flank of the teeth.     The serrations forming the cutting edges.    For a spur gear the cutter is a helical gear and for a helical gear the cutter is a spur gear.    The cutter axis is mounted at an angle to the axis of the gear.

There are various shaving actions : for traverse shaving - the shaving cutter drives the gear which reciprocates in the direction of its own axis.





Burnishing

This is cold working process accomplished by rolling the gear in contact and under pressure with a number of hardened burnishing gears.    The hardened burnishing gears have slightly oversize teeth.    The gear is run in mesh with the burnishing gears until the surface is smooth.    This process , as for shaving, cannot be carriered out if the gear is hardened.




Grinding

Heat-treated gears can be finished either by grinding when the high accuracy associated with profile grinding is required, it is the only process to be used.    By grinding, teeth can be finished either by generation or by forming.     With generation the work is made to roll in contact with a flat faced rotating grinding wheel, which corresponds to the face of the imaginary rack meshing with the gear.    One side of tooth is ground at a time.

With forming the grinding wheel is given the shape as formed by space between two adjacent teeth and both flanks are finished together.    ,Both give equally accurate results The second method (Forming) is quicker.  The process although producing accurate gears is time consuming and results in surface scatches or ridges which may have to be removed by lapping

This process of gear finishing is not often used as the shaving process is quite satisfactory and cheaper than gear-grinding.




Lapping

This process of gear finishing is accomplished by having the gear in contact with one or more cast iron lap gears of true shape.    The work is mounted between centers and is slowly driven by the rear lap.    The teeth of the gear and lap are made to slide (reciprocate) axially relative to each other so that the whole surface of the teeth is abraided equally. .    A fine abrasive is used with light oil to assist cutting action.    The process generally only takes a few minutes (10 to 15).

Automative gearbox gears finished before case hardening by shaving are usually finally lapped




Honing

Gear honing (Shave Grinding). is a finishing process that can be applied to external and internal spur or helical gears.   The process involves running the workpiece in mesh with a high quality gear honing tool/wheel with suitable abrasives .    The honing tool is like a gear driving the workpiece at high speed (upto 30 m/m) while oscillating such that the teeth slide against the workpiece teeth.    The honing tool is traversed back and forth across the gear face and the direction of rotation of the honing tool is reversed at the end of each stroke     General-purpose honing tools are made in variety of resin and abrasive mixes for gears that have been shaved and heat-treated.

With this process finishes of 0,75 μm. are easily achieved and 0,075μm. are possible.



Links to Gear Design
  1. Milling a gear Youtube ...Very clear video (5 minutes).
  2. manufacturing Gears ...Clear notes and diagrams
  3. GearsHub- Gear Cutting...Pictures of the various cutting tools used for cutting gear teeth
  4. Gear Honing -Gears hub...Detailed Notes and diagram
  5. Gear Technology Honing...Detailed information on gear Honing machines
  6. GEAR GENERATION AND FINISHING OPERATIONS...pdf download with some information on gear shaving


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Last Updated 20/01/2013