Gear Manufacturing Methods

There are many different ways of manufacturing gears. Gear manufacturing methods are as follows:

The first three of these methods are the most commonly encountered production methods in gear manufacturing. Here we’ll explain the first three methods in a bit more detail. We’ll also touch briefly on the others.

Gear Manufacturing Methods Using the Generating (Rolling) Method

Nearly all gears produced today are made on machines that work by the generating (rolling) method. Since medium and large gears cannot be made any other way, these gear manufacturing methods carry great importance. Generally, the following three production methods are shown for manufacturing spur and helical gears:

1. Gear cutting with a rack-shaped cutter blade (MAAG system),
2. Gear cutting with a pinion-shaped cutter blade (FELLOWS system),
3. Gear cutting with a hob shaped like a worm screw (hobbing system).

Gear Cutting with the MAAG System

Using a Maag gear-cutting machine, parts producible by the generating and form-tool method are produced, such as spur and helical external cylindrical gears, sprockets, splined shafts, and similar profiles. With an additional tool, rack gears, spur and helical internal cylindrical gears are also produced.

How Does the Maag Method Work?

Schematic representation of the MAAG gear-cutting system

1) Start  2) Return    3) Repeat   4) Final and return

Since both surfaces of the rack-shaped cutter blade cut at the same time, the right and left tooth profiles are formed simultaneously.

After the rack-shaped cutter blade makes a planing motion from top to bottom and rises back up again — that is, after it separates from the gear it’s cutting — it makes a small rotation around the gear’s axis and also advances slightly parallel to the blade’s axis. Then the blade makes a second cutting motion.  These rotation and advancing motions represent the rolling motion of a wheel over a rack-shaped rail.

The smaller these rotation and advancing motions are, the higher the quality of the machined tooth surface. Since the rack-shaped cutter blade has a limited number of teeth, the machine periodically and automatically pauses the cutting operation and the gear returns to the blade’s starting point without rotating. Then these working motions repeat.

Gear Cutting with a Pinion-Shaped Cutter (FELLOWS System)

The difference between the “Fellows system” and the “MAAG system” is that the Fellows system uses round, pinion-shaped cutter blades instead of the rack-shaped flat blade used by the MAAG system.

The Fellows system, whose working principle is shown in the figures below, is very common in the United States.

In this system, while the cutter blade rises up without cutting, both it and the gear being cut make a small rotation, then the blade descends again to cut. The Fellows machine’s blade always cuts different teeth each time. For this reason, its cutting wheels last longer compared to rack-shaped blades, but they are also more expensive.

Since Fellows-type cutter blades are conical, the machine table is set at an angle while these blades are being made.

Gear Cutting with the Hobbing Method

Gear manufacturing with hob cutters is among the most preferred of the gear manufacturing methods. Both the “Maag” and “Fellows” machines, which cut teeth via the generating method, make a planing motion — meaning after a downward cutting motion, they make an empty upward return stroke. To turn this stop-and-go back-and-forth motion into a continuous cutting motion, the cutter blade is shaped into a worm screw form, creating the hob milling cutter.

Working principle of gear hobbing (above) As seen, the helical hob milling cutter, shaped like a worm screw, rotates while simultaneously slowly moving down and cutting the teeth.

The gear being cut rotates together with the helical hob, synchronized to its rotation. Generally, as with other machines, a rough cut is taken first, and then a fine cut is taken a second time with a sharp hob. This way, high-quality gears are obtained.

SEE ALSO AS A RELATED TOPIC: Gear Manufacturing with the Hobbing Method

Gear Production by Casting

For slowly rotating gears (at speeds below 2 m/s) and for rough work, cast gears are used entirely unmachined. For example: hand cranes, brick-making machines, etc.

A cannon mechanism dating from the 1800s. Since the gears don’t need to rotate at high speed, it’s quite natural to prefer casting over precision manufacturing. (above)

They have advantages in outdoor use, especially at sea or by the coast, because they resist rust. Full-size wooden models are used for small and medium-sized gears, while for larger ones, either sectional wooden gear patterns or forming machines are used.

Gear Manufacturing Method with Module Milling Cutters

This type of tooth cutting is done on ordinary milling machines, and after one tooth is cut, the gear is advanced one tooth pitch with the help of a dividing head, and the second tooth is cut.

Since it’s impossible to make a separate module milling cutter for every different tooth count, module milling cutters have only been made for gears with certain tooth counts.

Most gears produced this way will have some error. For this reason, they cannot be used for fast-rotating gears. Because of this, this tooth-cutting method is not preferred in the machine industry.

SEE ALSO AS A RELATED TOPIC: Information About Module Milling Cutters

Other Gear Manufacturing Methods

Gears that transmit large amounts of power are manufactured using the methods we just described. However, there are many other methods used to manufacture gears that transmit small, even negligible amounts of power.

Tooth Cutting Using a Template

Since manufacturing with this system is simple and can be done locally on universal machines, it’s used by small and medium-sized manufacturers who encounter bevel gear manufacturing challenges. Their working system is the copy-milling system.

Tooth Cutting by Rolling (Thread-Rolling Type Forming)

The good results achieved in manufacturing various screws and bolts from solid material by the rolling method led to gears being manufactured with the same method. Rolling manufacturing was first started with automobile gears produced in large series. The system is the same as bolt manufacturing.

Tooth Cutting by Stamping (Punching) Method

Gear manufacturing by stamping is applied to gears transmitting very small amounts of power, such as those in watches, meters, and small mechanical devices. These gears, cut from fairly thin sheet metal, can be made quite precisely depending on the accuracy of the cutting die. Since manufacturing the cutting die is very expensive, they are only made in large production runs.

Gear Manufacturing by Die Casting

Gear manufacturing by die casting is applied for meters, small devices, and similar items that transmit small amounts of power and are manufactured in large series. These are heated and sprayed under pressure into metal molds, and cool in the mold in a very short time. Shrinkage allowances must be calculated during cooling, and the molds must be sized accordingly. For gears manufactured by die casting, the shaft is placed in the mold during casting so that the shaft and gear are cast joined together.

Plastic gears are also generally manufactured by die casting/injection molding. In series smaller than 1000 where a ready-made mold isn’t available, gear blanks are cut from solid plastic rods and manufactured using the generating method. They are cut with milling cutters prepared for light metals like aluminum. Plastic gears manufactured in large series are made by injection into a mold.

See related topic: Injection Molding

Gear Manufacturing by Sintering

For tooth manufacturing by sintering, powdered steel is pressed into a gear mold using special presses and sintered to produce the gear. The gears obtained are somewhat porous, but sturdy. These gears are used in precision mechanics.

Gear Manufacturing by Broaching

In gear manufacturing by broaching, generally a long rod (broach) is used. This rod has blades arranged one after another along it. The blades are machined progressively, and a single pull of the rod produces the gear.

Gears with a surface quality of 5…8 μm can be produced with the broaching method, and gears with diameters of normally 6…80 mm are quite easily manufactured. Some of these machines can manufacture gears up to 200 mm in diameter. Since machines that manufacture gears by broaching are made in small numbers, and the cutting broach tool is very expensive, these haven’t become very widespread despite their high manufacturing efficiency.

SEE ALSO AS A RELATED TOPIC: What Is Broaching? How Is It Done?

Gear Manufacturing by Wire EDM

Wire EDM (electrical discharge machining) is based on the principle of moving an electrically charged wire via a CNC program and, wherever it passes, creating an arc that generates high heat (the material being machined is also given the opposite electrical polarity), melting and vaporizing the material. This method allows extremely precise manufacturing, and materials that are normally difficult to machine, being hard, can be easily machined. Gear manufacturing by wire EDM is just one of the application areas of wire EDM.
For detailed information, see: What is wire EDM, how does it work?

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