mechanical technology

Indexing is an operation of dividing a periphery of a cylindrical workpiece into equal number of divisions by the help of index crank and index plate. A manual indexing head includes a hand crank. Rotating the hand crank in turn rotates the spindle and therefore the workpiece. The hand crank uses a worm gear drive to provide precise control of the rotation of the work. The work may be rotated and then locked into place before the cutter is applied, or it may be rotated during cutting depending on the type of machining being done.

Most dividing heads operate at a 40:1 ratio; that is 40 turns of the hand crank generates 1 revolution of the spindle or workpiece. In other words, 1 turn of the hand crank rotates the spindle by 9 degrees. Because the operator of the machine may want to rotate the part to an arbitrary angle indexing plates are used to ensure the part is accurately positioned.

Direct indexing plate: Most dividing heads have an indexing plate permanently attached to the spindle. This plate is located at the end of the spindle, very close to where the work would be mounted. It is fixed to the spindle and rotates with it. This plate is usually equipped with a series of holes that enables rapid indexing to common angles, such as 30, 45, or 90 degrees. A pin in the base of the dividing head can be extended into the direct indexing plate to lock the head quickly into one of these angles.[3] The advantage of the direct indexing plate is that it is fast and simple and no calculations are required to use it. The disadvantage is that it can only be used for a limited number of angles.

Interchangeable indexing plates are used when the work must be rotated to an angle not available on the direct indexing plate. Because the hand crank is fixed to the spindle at a known ratio (commonly 40:1) the dividing plates mounted at the handwheel can be used to create finer divisions for precise orientation at arbitrary angles. These dividing plates are provided in sets of several plates. Each plate has rings of holes with different divisions. For example, an indexing plate might have three rows of holes with 24, 30, and 36 holes in each row. A pin on the hand crank engages these holes. Index plates with up to 400 holes are available.[2] Only one such plate can be mounted to the dividing head at a time. The plate is selected by the machinist based on exactly what angle he wishes to index to.

For example, if a machinist wanted to index (rotate) his workpiece by 22.5 degrees then he would turn the hand crank two full revolutions plus one-half of a turn. Since each full revolution is 9 degrees and a half-revolution is 4.5 degrees, the total is 22.5 (9 + 9 + 4.5 = 22.5). The one-half turn can easily be done precisely using any indexing plate with an even number of holes and rotating to the halfway point (Hole #8 on the 16-hole ring).

Brown and Sharpe indexing heads include a set of 3 indexing plates. The plates are marked #1, #2 and #3, or "A", "B" and "C". Each plate contains 6 rows of holes. Plate #1 or "A" has 15, 16, 17, 18, 19, and 20 holes. Plate #2 or "B" has 21, 23, 27, 29, 31, and 33 holes. Plate #3 or "C" has 37, 39, 41, 43, 47, and 49 holes.[citation needed]

Universal Dividing heads: some manual indexing heads are equipped with a power drive provision. This allows the rotation of the dividing head to be connected to the table feed of the milling machine instead of using a hand crank. A set of change gears is provided to select the ratio between the table feed and rotation. This setup allows the machining of spiral or helical features such as spiral gears, worms, or screw type parts because the part is simultaneously rotated at the same time it is moved in the horizontal direction. This setup is called a "PTO dividing head".