Gear manufacturing processes (cutting processes)

When gear teeth are cut on a gear blank, the manufacturing process consists of the following steps:

(a) Blank manufacture;
(b) Tooth cutting;
(c) Heat treatment;
(d) Tooth finish cutting.

All the steps listed above may not be used in the manufacture depending on the requirement. For example, non-hardened gears may not require heat treatment. If the accuracy required is not very high, special tooth finish cutting will not be carried out. The sequence of steps is generally as enumerated above, but it may differ. For example, in automobile gear manufacture the tooth finish cutting is carried out before heat treatment.

Gear blank manufacture

The quality of gear produced is as much dependent on the quality of gear lank used as on the accuracy of the machine used for teeth cutting. The locating surfaces should be selected and maintained throughout the manufacturing processes.

Gear blanks may be either of the following two types: (a) Shaft type having centers, and (b) Cylindrical blanks with round bores with or without keyway or with splined bores.

Shaft type blanks are usually mounted between centers during all the operations. Therefore, to get accurate gears, the centers should not be damaged during material movement, and they should be cleaned while mounting on the machine as well. A typical shaft like component is shown in second Figure.

In this case, the locating surfaces are the cone-surfaces C and D of the centers. The first requirement is that these centers should be in line so that you get proper contact on the locating surfaces. These cone surfaces should be smooth, and used for manufacturing both the mounting surfaces A and B as well as for tooth cutting. Gear outer surface E also should be produced in the same operation like the operation of producing surfaces A and B, and it (surface E) can be consequently used for dialing when mounted on the gear-cutting machine to check the accuracy of mounting.

Cylindrical blanks can be of two types, namely, (a) Blanks having round bore with or without keyway, and (b) Blanks having splined bore. Fig2 For this, normally the bore (Surface A) and the face (Surface B perpendicular to it are the locating surfaces, as shown in the first figure.
. The requirements of the blank are the following :
(a) Surface B should be at right angles to surface A. For very accurate gears, face runout of surface B required will vary from 5 (0.002”) to 25 m (0.001”) depending on the accuracy required. For very accurate gears it can be 5 m. for normal accuracy gears (such as DIN 7 Class) it can be 10-12 m.

(b) The bore A should also have close tolerance so that while it is mounted on the machine, there is minimal play between the bore and the mounting mandrel. This will reduce pitch errors generated due to the blank runout. It may, therefore, be necessary to green grind the cylindrical blanks for bore A and face B to achieve the necessary accuracy for process purposes though it may not be required for actual operation of the gear.

(c) Face C is kept concentric with respect to the locating surface A so that it can be used for initial dialing on the gear-cutting machine.

The tooth cutting processes can be divided into two types, namely, form machining and tooth generation process.

Form machining and Tooth-generation Processes

Form machining –
In this type of machining, the cutter is of the form of the tooth space to be produced and as this type of cutter cuts, it transfers its tooth form to the job. Some examples of this process are :

1. Milling of spur/helical gears on the milling machine;
2. Form grinding of gear tooth on Overcut or similar type of gear grinding machines, and
3. Cutting of hypoid gear wheel of non-generated type.



Tooth-generation Processes –
For this type, the involute form is produced by generation. The form may be generated in various ways which are described later in individual process descriptions. Some of the generation processes are :
(1) Milling
(2) Hobbing
(3) Shaping, and
(4) Rack planning, etc.

Manufacturing of spur and helical gears

Various methods used for tooth cutting of spur and helical gears are listed below :

(1) Tooth milling;
(2) Hobbing;
(3) Shaping;
(4) Rack planning;
(5) Gear shaving;
(6) Gear grinding;
(7) Gear lapping;
(8) Gear honing, and
(9) Gear burnishing.

The last five processes (5) to (10) are gear teeth finishing processes where the gears are rough cut using one of the four earlier mentioned processes.

Milling Spur Gear on Milling Machine

The gear blank is mounted on a mandrel which is supported between the center of the dividing head and one more center at the other end, as shown in fig. At a time one tooth space is cut by the milling cutter, and a dividing head is used to index the job to the next required tooth space. The cutter is chosen according to the module (or DP) and number of teeth of the gear to the cut. This cutter is mounted on the milling arbor. Before the gear can be cut, it is necessary to have the cutter centred accurately relative to the gear holding mandrel. One way is to adjust the machine table vertically and horizontally until one corner of the cutter just touches the mandrel on one side. Both the dials (of the table and the knee) are then set to zero. The table is then adjusted for the cutter to just touch on the other side of the mandrel with vertical dial showing zero. The reading of the horizontal feed screw is read. This reading divided by two gives the central position of the mandrel relative to the cutter. When the table is set centrally in this manner it should be locked in that position. The table is then fed vertically so that the blank just touches the cutter. The vertical dial is then set to zero. This is required to give the depth of cut on the job.

With these settings the machine can be started and traversed along the axis of the job to cut the tooth over the whole width of the gear. Depth is increased slowly until it reaches the full depth of the tooth. With the depth setting the backlash of the gear can be controlled suitably. After one tooth space is cut, the blank is indexed through 1/z revolution by means of the dividing head, and the process is repeated until all the teeth are cut.

Dividing Head Settings

There are four methods of indexing;
(a) Rapid indexing;
(b) Plain indexing;
(c) Compound indexing, and
(d) Differential indexing.

Rapid Indexing –
This is the simplest method and is suitable for dividing into 2,3,4,6,8,12 and 24 divisions.. For this rapid indexing, the worm is first dropped from engagement with the worm wheel by means of a knob by the side of the index head. Then the pin P in the hole of the index plate I1 is disengaged by the lever L. The job is then rotated through the number of holes required to get the necessary division. Then the pin P is reengaged. Due to dropping of the worm out of engagement, it is easy to rotate the job from the front.

Plain indexing –
This will be explained with the help of an example. Suppose we want to cut a gear with 23 teeth, i.e., we need to index the job through 1/23 of a revolution. For this the index plate handle J will have to be rotated through 40/23=117/23 revolution. To get 17/23 of a revolution, we will use the circle with 23 equi-spaced holes on the index plate I2. To aid this counting of 17 holes, the sector on the index plate is used. With the nut N loosened, the arm carrying the crank-lkever handle is moved to position the pin in one of the holes of the circle with 23 holes. Then one arm is positioned to touch one side of the latch pin of the handle. Then the other arm of the sector is positioned 18 holes away., on the further side of the hole. With the sector thus positioned, the nut N is locked. Then the pin P is withdrawn and one revolution is carried out and the hand is further rotated so that the pin will rest in the hole touching the second arm of the sector. The sector is then loosened and rotated for the other arm to touch the latch-pin in new position for the next indexing.

Compound indexing –
Many a time the number of divisions required cannot be obtained by plain indexing. Then compound indexing is used in which in one motion the crank handle is rotated through a certain rotation and its latch pin engaged. Afterwards the index plate itself is rotated through a certain rotation either in the same direction or in the opposite direction depending on the requirement. Normally index plate is prevented from rotating by a stationary pin at the rear which engages one of the holes of the same index plate I2. After this stop pin is removed, the index plate can be rotated. Let us consider milling a 93- teeth gear. The job will have to be rotated through 1/93 revolution or the crank lever through 40/93 revolutions, which will not be possible with the standard index plate. For this purpose, then the crank lever will be rotated through 11/33 revolution and the index plate through 3/31 revolution in the same direction.


Differential indexing –
This case is similar to the compound indexing. Only difference is that in this case the index plate is rotated through gearing connected to the dividing head spindle.

Form Milling of Helical Gear

The procedure is similar to form-milling of spur gear. In this case a universal milling machine is used. The module of cutter chosen is equal to the normal module of helical gear to be cut. With the cutter mounted on the arbor of horizontal milling machine, the table is set to the helix angle of the gear. To carry out the helical milling, the dividing head is connected to the lead screw of the table. The gears are chosen so as to give the necessary lead on the gear. Lead of helical gear is given by:

L = d. cot
where L = Lead of the gear,
d = reference diameter of the gear, and
 = helix angle of the gear.

The gear is then cut similar to cutting a spur gear. If the helix angle of the gear is large, it may not be possible to set the table of the universal milling machine to the required helix angle. In this case a vertical milling attachment is used and is set in the horizontal position, setting the cutter at 90o from the normal position. The table is then set to the lead angle of the helical gear.