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
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.
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.
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.
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.
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.
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.
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.
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).
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.