Gears are mechanical components that enable mechanical power transmission between driving and driven shafts and are used in various machinery and automobiles. Alongside power transmission between two shafts, the advantage of using gears is the ability to multiply either speed or torque on the output shaft. For any of the gear manufacturing processes, the first step is to prepare and machine the circular blank to a suitable diameter on a lathe which will be used as the gear blank for cutting the teeth. In this article we will look at some of the prominent methods to machine the gear teeth profiles. Gear manufacturing can be classified into 2 main categories. Gear Generating and Gear Form-cutting.
Gear Generating Methods
In Gear Generating Methods, the cutting tool makes a series of cuts resembling that of a mating gear till the profile of the teeth is obtained.
1. Gear Hobbing
Hobbing is one of the efficient ways to cut gears. The gear is cut progressively by a rotating cutting tool called the hob while the gear blank also rotates in proper synchronization. For each rotation of the hob, the teeth of the hob traverse by an amount equal to the pitch of the gear teeth. Feed is given along the axis of the gear till the entire width of the blank is cut. After one complete pass, the blank is fed horizontally into the hob for the next increment of the tooth depth. This process is repeated till the complete tooth depth of the gear is machined. Multiple blanks can be stacked and cut if multiple gears of same size and teeth are needed. Hobbing is used to cut both spur gears and helical gears. Only external teeth can be cut with hobbing.
2. Gear Shaping
In shaping, the cutting tool can be either in the shape of a gear pinion, a multi-point rack, or a single point tool. The tool reciprocates back and forth in the shaping operation while the gear blank is fed. Shaping can be used to cut both internal and external teeth.
In Form-Cutting methods, the cutter has the shape that is same as the gap or spacing between two teeth, thus producing the tooth profile in one pass.
1. Gear Broaching
In broaching, the gear teeth are cut in a single pass using a tool called a broach. The broaching tool has cutting teeth of the profile that is same as that of the spacing between the gear teeth to be formed in increasing depths one behind the other so that as the broach passes, the depth of cut increases till the entire depth of the gear teeth is achieved. Broaching can be used to cut internal spur gears and internal helical gears. External spur gears can also be made using a process called pot broaching. Pot broaching has cutters assembled like a stack called the pot and the pot is then pressed onto the blank in special pot broaching machines to form the teeth.
2. Gear Milling
Gear milling is the process of cutting gear teeth with a milling cutter that has the profile of the space between gear teeth. Only one tooth is cut at a time and the blank is indexed along the circumference to cut all the groves that make the teeth. Only external teeth can be cut with milling.
Table for Gear Types and Methods suitable
|Internal Gears||External Gears|
|Spur Teeth||Shaping (Pinion), Broaching||Hobbing, Shaping, Broaching|
|Helical Teeth||Shaping, Broaching||Hobbing, Shaping|
Furthermore, there are separate methods for finishing the gear tooth to required accuracy and surface quality that work in similar way as Gear Hobbing and Gear Shaping. They are Gear Shaving, Gear Honing, Gear Lapping and Gear Grinding.