By

V. THULASIKANTHAssistant Professor

Mechanical Engineering Department

vtkvsk@gmail.com

GEAR MANUFACTURING AND SURFACE FINISHING PROCESS

GearsIt is an important machine element which is used for transmission of power.

SPUR GEAR

Helical Gear

BEVAL GEAR

RACK AND PINIONWORM GEARHERRINGBONE GEARS

Materials for Gear

METALS NON-METALS

Cast Iron Synthetic Plastic Fibers

Cast Steel Laminated Wood

Structural Steel Nylon, etc.

Gun Metal

Brass

Bronze

Aluminium, etc.

Gear ManufacturingManufacturing of gears require special tools and equipment, therefore it is costlier

than other drives.

The error in cutting teeth may cause vibrations and noise during operation.

It requires suitable lubricant and reliable method of applying it, for the proper

operation of gear drives.

Gear cutting machines are single purpose machines.

Milling machine is a multipurpose machine, which in addition perform other

operations but not fit for mass production.

Commercially produced by other methods like sand casting, die casting, stamping,

extrusion, and powder metallurgy.

Above processes are used for gears of low wear resistance, low power

transmission, and relatively low accuracy of transmitted motion.

When the application involves higher values for one or more of these

characteristics, cut or machined gears are used.

Gear production by cutting involves two principal methods—forming and

generating processes.

Gear finishing involves four operations—shaving, grinding, lapping, and burnishing

Based on Syllabus highlighted process should be concentrated

Powder Metallurgy (Sintering)

1. Formation of metallic powders.

2. Mixing or blending of the metallic powders in required proportions.

3. Compressing & compacting the powders into desired shapes & sizes of gear

4. Sintering the compacted articles in a controlled furnace atmosphere.

5. Subjecting the sintered articles to secondary processing if needed so.

Formation of Metal Powders

In the majority of powders, the size of the particle varies from microns to 0.5 mm.

The most common particle size of powders falls into a range of 10 to 40 microns.

The chemical and physical properties of metals depend upon the size and shape of

the powder particles.

The commonly used powder making processes are given as under.

1. Atomization

2. Chemical reduction

3. Electrolytic process

4. Crushing

5. Milling

6. Condensation of metal vapors

7. Hydride and carbonyl processes.

Mixing or Blending of Metallic Powders

After the formation of metallic powders, proper mixing or blending of powders is

the first step in the forming of powder metal parts.

The mixing is being carried out either wet or dry using an efficient mixer to

produce a homogeneous mixture.

Compacting of PowderCompacting is the technique of converting loose powder in to compact accurately

defined shape and size of a gear.

The die consists of a cavity of the shape of the desired part.

Metal powder is poured in the die cavity (gear) and pressure is applied using

punches, which usually work from the top and bottom of the die.

Sintering

The metal parts obtained after compacting are not strong and dense.

To improve these properties, the parts should be sintered.

Sintering is the process of heating of compacted products in a furnace to below

the melting point of at least one of the major constituents under a controlled

atmosphere.

Secondary Operations

Some powder metal parts may be used in the sintered condition while in some other

cases additional secondary operations have to be performed to get the desired

surface finish, close tolerance etc..

1. Annealing.

2. Repressing for greater density or closer dimensional control.

3. Machining.

4. Polishing.

5. Rolling, forging or drawing.

6. Surface treatments to protect against corrosion.

7. In some cases infiltration is needed to provide increased strength, hardness,

density obtainable by straight sintering.

Gears manufactured by P/M technique, secondary machining is not required.

Gears manufactured will be very small but high accurate.

Process chart for P/M gear manufacture

Characteristics:

Accuracy similar to die-cast gears

Material properties can be Tailor

made

Typically suited for small sized gears

Economical for large lot size only

Automotive engine components manufactured by sintering

ExtrudingExtruding is used to form teeth on long rods, which are then cut into usable

lengths and machined for bores and keyways etc.

Nonferrous materials such as aluminum and copper alloys are commonly

extruded rather than steels.

This result in good surface finishes with clean edges and pore free dense

structure with higher strength.

Materials:

Aluminum, copper, naval brass, architect-ural bronze and phosphor bronze are the

materials that are commonly extruded.

Applications:

Splined hollow & solid shafts, sector gears are extruded

Helical gear made by extrusion

Stamping

Used for mass production of small and thin gears out of metal sheets at a thickness

of 1.5mm to 12.5mm maximum.

Sheet metal can be stamped with tooth shapes to form low precision gears at low

cost in high quantities.

The surface finish and accuracy of these gears are poor.

Applications:

Stamped gears are used as toy gears, hand operated machine gears for slow speed

mechanism.

Precision stamping:

In precision stamping, the dies are made of higher precision with close tolerances

wherein the stamped gears will not have burrs.

Applications:

Clock gears, watch gears etc.

Machining

The bulk of power transmitting metal gears of machinery are produced by

machining process from cast, forged, or hot rolled blanks.

The initial operations that produce a semifinishing part ready for gear machining

as referred to as blanking operations.

The starting product in gear machining is called a gear blank.

Roughing processes:

Roughing process consists of forming, generation, shaping and hobbing

processes.

By this method gears are made to an accuracy which is more than adequate for

the slow speed operations.

Roughing processes actually produce a smooth and accurate gear tooth.

Only for high precision and quiet running, the secondary finishing operation is

justified at added cost.

Roughing processes include milling the tooth shape with formed cutters or

generating the shape with a rack cutter, a shaping cutter or a hob cutter

which are shown below

Form MillingIn form milling, the cutter called a form cutter travels axially along the length of

the gear tooth at the appropriate depth to produce the gear tooth.

After each tooth is cut, the cutter is withdrawn, the gear blank is rotated

(indexed), and the cutter proceeds to cut another tooth.

The process continues until all teeth are cut.

Each cutter is designed to cut a range of tooth numbers.

The precision of the form-cut tooth profile depends on the accuracy of the cutter

and the machine and its stiffness.

Indexing of the gear blank is required to cut all the teeth.

Indexing is the process of evenly dividing the circumference of a gear blank into

equally spaced divisions.

Index fixture consists of an index head (also dividing head, gear cutting

attachment) and footstock, which is similar to the tailstock of a lathe.

The index head and footstock attach to the worktable of the milling machine.

An index plate containing graduations is used to control the rotation of the index

head spindle.

Gear blanks are held between centers by the index head spindle and footstock.

Form Milling

Forming is sub-divided into milling by disc cutters and milling by end mill cutter

which are having the shape of tooth space.

The usual practice in gear milling is to mill one tooth space at a time, after which

the blank is indexed to the next cutting position.

Form milling by disc cutter: The disc cutter shape conforms to the gear tooth space.

Each gear needs a separate cutter.

Tooth is cut one by one by plunging the rotating cutter into the blank as shown

below

Form milling by end mill cutter: The end mill cutter shape conforms to tooth spacing.

Each tooth is cut at a time and then indexed for next tooth space for cutting.

A set of 10 cutters will do for 12 to 120 teeth gears. It is suited for a small volume

production of low precision gears.

To reduce costs, the same cutter is often used for the multiple-sized gears

resulting in profile errors for all but one number of teeth.

Form milling method is the least accurate of all the roughing methods.