Indexing Mechanism is a mechanism that is used for dividing the periphery of a workpiece into any number of equal parts. The machine used for the indexing mechanism is called indexing head. Indexing head is also known as a dividing head or spiral head.
It is a specialized tool that allows a workpiece to be circularly indexed.
Indexing Mechanism is mainly used to cut gear teeth accurately. This mechanism is used for equal spacing of the teeth of the gear.
This mechanism is also used for machining flutes of a milling cutter, milling curved slots and drilling a bolt hole circle around the circumference of a part.
This indexing mechanism is mainly used on milling machines. This mechanism is also used in grinders, drill presses and boring machines.
Indexing mechanism can also be used for producing a square or hexagonal bolts, cutting splines on shafts, fluting drills, taps, reamers, etc.
This indexing mechanism can be achieved using a special attachment known as dividing head or indexing head.
Parts Used in Indexing Mechanism:-
Indexing Plate – Indexing plate is a circular plate and it has equally spaced holes. There are minimum 6 holes in an indexing plate. This indexing plate is connected to a crank which is connected to a handle. This indexing plate is stationary in Simple Indexing Mechanism and can move in Differential Indexing Mechanism.
Crank – This crank has a handle which is rotated manually and give the initial rotation to the worm shaft and worm which transferred to the worm wheel.
Crank Pin – Crank Pin is used to lock the rotation of the crank. Crank pin is inserted into the whole of indexing plate which is fixed and does not rotate and hence the rotation of crank is locked.
Worm Shaft – This shaft is connected with the crank and rotates with it. It connects the crank to the worm.
Worm – Worm is connected to the crank by worm shaft. This worm is like a threaded screw. It is a single-threaded worm. When the crank is rotated using handle this worm also rotates due to rotation of worm shaft and a single tooth in worm wheel which is connected to the worm passes through the worm.
Worm Wheel (W.W) – The worm wheel contains teeth like gears. When all the teeth of worm wheel passes through the worm. The worm wheel completes one rotation.
Spindle :- Spindle is connected with the worm wheel and rotates as the worm wheel rotates. When all the teeth of worm wheel passes through the worm, this worm wheel completes one rotation and the spindle connected to the worm wheel also completes one rotation. This spindle is connected to a plate known as face plate.
Face Plate – This face plate is connected to the spindle and rotates as the spindle rotates. This face plate is connected to the workpiece and this workpiece also rotates with this face plate.
Change Gears – These gears are used in differential indexing mechanism for rotating the indexing plate in forward or backward direction as required. These gears are connected to the worm wheels and takes worm wheel rotation as input and output its rotation to indexing plate.
The machine formed by combination of all these parts is called indexing head.
In this article we will study about two main types of indexing mechanism that are :-
1) Simple Indexing Mechanism
2) Differential Indexing Mechanism
1. Simple Indexing Mechanism:-
At first, the handle of the indexing plate is rotated manually. With the handle the crank also rotates. As the crank rotates, the worm shaft rotates attached to it rotates and hence the worm also rotates.
When the worm rotates, the teeth of the worm wheel pass through the worm and produce a partial rotation in the worm wheel. When the worm completes one complete rotation, one tooth of the worm wheel passes through the worm.
The spindle and the face plate connected to the worm wheel also rotates with the worm wheel. The workpiece is connected to the face plate and also rotates with the worm wheel.
Indexing mechanism is mainly used to cut teeth of the spur gear. It is also used for cutting splines, milling grooves in reamers and taps, and spacing holes on a circle
Let’s say there are 40 teeth in the worm wheel and we have to machine 8 teeth in the workpiece. So the number of rotation of crank handle that will be required to cut one tooth in the workpiece will be = 1/8 x 40 = 5. i.e 5 rotation of crank will be required to cut one tooth in the workpiece.
After every five rotation the crank pin will be inserted in the indexing plate and the rotation of the crank will be locked and the workpiece rotation will also be locked and the workpiece can be machined at this time.
In one rotation of crank one tooth or worm wheel passes by worm and in five rotation five worm teeth will pass by the worm.
When 5 teeth of worm wheel will pass by worm one tooth will be cut in the workpiece. One tooth will be cut in the workpiece for each 5th teeth of worm wheel that crosses worm.
Hence 5 rotation of crank handle is required to cut one teeth in workpiece when there are 40 teeth in worm wheel and 8 teeth has to be cut in the workpiece.
Case 2 :-
In this case, we have to machine 7 teeth in the workpiece which is not a factor of 40. Now the number of rotation of crank handle rotation that will be required to cut one tooth in workpiece will be = 1/7 x 40 = 5 whole 5/7 rotations i.e 5 complete rotations and 5/7 of rotation of the crank handle.
Since a fraction of rotation of crank handle is required. Hence, the holes of indexing plates will be used to obtain this fractional rotation of crank.
5/7 can also be written as 15/21. So a 21 hole indexing plate will be used to obtain this result. i.e 5 whole rotation of the crank handle and after that crank handle should rotate till the 15th hole of the 21 hole indexing plate is reached.
This is the main function of indexing plate with holes, its main use is seen when we need a mixed fraction of rotation of the crank handle.
2. Differential Indexing Mechanism:-
Differential indexing is used when the fraction of rotation of crank handle can not be achieved using holes of indexing plates.
This situation occurs when the workpiece has to be divided into a number of divisons which is greater than the maximum number of holes an indexing plates can have.
Maximum holes that can be in indexing plate is 66. For more than 66 divisions Differential Indexing Mechanism is used.
In differential indexing mechanism along with parts of simple indexing mechanism some change gears are used. These change gears are used to rotate the indexing plate in forward or backward direction as required so that the required division of workpiece is obtained.
Let’s say we have to cut 67 holes in the job or the workpiece. For that, the job should rotate 1/67 of the total rotation after each cut.
So the number of rotation required to cut one tooth in workpiece = 1/67 x 40
Since we do not have an indexing plate with 67 holes, so we can not do this with simple indexing mechanism.
So now we will see how we can achieve this using a differential indexing mechanism.
In differential indexing mechanism, the change gears are connected to the change gears through bevel gears and transmit rotation in 90 degrees angle.
These change gears are connected to the indexing plate and rotate the indexing plate to some extent so that we can cut certain numbers of cuts in the job.
We can rotate the job to 1/66 of total rotation using simple indexing but we need to rotate it to 1/67 of total rotation.
If there are two points A and B int the job covering which the job rotates 1/66 of total rotation then a point C lies in between these two points such AC covers 1/67 of total rotation.
If we reach AB or 1/66 by rotating crank then to reach 1/67 we have to come back some distance so that it becomes 1/67 or we reduce rotation to some amount so that we reach 1/67 instead of 1/66.
We can reduce the amount of rotation by some amount by rotating the indexing plate by some amount in opposite direction of rotation of crank.
The amount that the indexing plate needs to rotate so that 1/67 of rotation is obtained instead of 1/66 = 1/66 – 1/67.
i.e Index Plate Rotation = 1/66 – 1/67 …… (i)
Now we will have to find the gear ratio of change gears that is required to rotate index plate so that we can rotate job to 1/67 of total rotation.
We know that Gear Ratio (G) = Output Rotation ( No ) / Input Rotation ( Ni )
We also know that Input rotation for change gear is the rotation of worm wheel.
Since we have to divide the job into 67 parts. So let’s take crank rotation = 1/67.
Now rotation of worm wheel = 1/67 x 1/40 (Since gear ratio of worm wheel is 1/40)
Now Input rotation for change gear ( Ni ) = 1/67 x 1/40
Now the output rotation i.e rotation of index plate = G x Ni
= 1/67 x 1/40 x G …. (ii)
Now equating eq (i) and eq (ii),we get
G x 1/67x 1/40 = 1/66 – 1/67
G = 40/66 ( 67 – 66) = 40 / N (available) ( N (required) – N (available) )
This is the generalized formula calculating the gear ratio of change gears in differential indexing mechanism.
Using this formula if the gear ratio has a negative sign then the indexing plate needs to be rotated in the opposite direction of the crank handle and if it has a positive sign then it needs to be rotated in the same direction of the crank handle.
In this way, we can divide the job into a number of divisions more than the maximum number of holes available indexing plate using differential indexing mechanism.