What is indexing in milling, and how is it performed?

Short Answer:

Indexing in milling is a method used to divide the workpiece into equal parts or angles for operations like cutting gears, slots, or hexagonal surfaces. It is done with the help of a special attachment called the indexing head or dividing head, which allows precise rotation and positioning of the workpiece on the milling machine.

This process helps the operator rotate the workpiece by a fixed amount between each cut. By doing this, multiple surfaces or teeth can be created with equal spacing. Indexing is important in manufacturing parts like gear wheels, bolt heads, fluted drills, and machine tool components.

Detailed Explanation:

Indexing in Milling

Indexing is a machining technique used in milling machines to accurately rotate the workpiece by a certain angle between operations. This allows the machine to cut equally spaced features such as teeth on a gear, grooves on a shaft, or faces on a hexagonal bolt. Indexing ensures that each cut is perfectly positioned and spaced, which is essential for symmetrical and rotating parts.

To perform indexing, a special device called an indexing head or dividing head is mounted on the machine table. The workpiece is attached to this device, and it can be rotated by turning a crank. The indexing head uses a gear mechanism and a dividing plate to control the rotation in precise steps.

Why Indexing is Important

  • To divide a circle into equal parts
  • To cut accurate gear teeth
  • To make slots and grooves at equal spacing
  • To machine square, hexagonal, or other multi-sided parts
  • To increase precision and productivity in complex jobs

Without indexing, such operations would be very difficult and time-consuming. Indexing helps make these tasks easier and highly accurate.

Main Methods of Indexing

There are several methods used for indexing, depending on the required number of divisions and the job type:

  1. Plain Indexing (Simple Indexing)

In plain indexing, the dividing head is used along with an indexing crank. Each full turn of the crank rotates the workpiece by a certain angle. The crank is rotated by a fixed number of holes in the index plate to get the required division.

Formula Used:
Number of turns of the crank = 40 ÷ Number of divisions required

Example: For 8 divisions → 40 ÷ 8 = 5 full turns

  1. Direct Indexing

In this method, the spindle of the indexing head can be rotated directly by hand using a plate with holes (usually 24 holes). This is useful for simple divisions like 2, 3, 4, 6, 8, 12, and 24.

Advantage:
Fast and easy for simple jobs
Limitation:
Can only be used for a limited number of divisions

  1. Differential Indexing

This is used when the required number of divisions is not possible using plain indexing. In this case, a set of gears is added to adjust the movement of the index plate while the crank rotates.

Use:
Helpful for cutting gears with odd numbers of teeth like 37, 41, etc.

  1. Angular Indexing

Angular indexing is used when the workpiece needs to be rotated by a specific angle instead of dividing into equal parts. The crank is rotated to achieve the required angle in degrees.

Formula Used:
One full crank = 9°
To rotate by 45°, crank is turned 5 times

Steps to Perform Indexing

  1. Mount the Dividing Head on the milling machine table.
  2. Fix the Workpiece on the chuck or between centers.
  3. Select the Index Plate and determine the number of holes based on the required division.
  4. Engage the Crank Pin in the selected hole circle.
  5. Rotate the Crank to the correct number of holes to get the required division.
  6. Lock the Spindle, perform the milling cut.
  7. Repeat the Process for the next divisions until the job is complete.

Applications of Indexing

  • Cutting gear teeth on spur gears
  • Machining square or hexagonal bolt heads
  • Milling flutes on drills or reamers
  • Cutting equally spaced slots, holes, or notches
  • Manufacturing parts in tool rooms and production shops
Conclusion

Indexing in milling is a vital technique used to divide a circular workpiece into equal parts or to rotate it by a specific angle for accurate machining. It is performed using a dividing head, which allows precise control over the position of the workpiece. Various methods like plain, direct, differential, and angular indexing help in achieving different types of jobs such as gear cutting, slotting, and forming polygonal shapes. Indexing ensures accuracy, repeatability, and smooth operation in many mechanical manufacturing processes.