Indexing is the process of accurately spacing holes, gear teeth or other machined areas on the perimeter or face of a workpiece. There are several variations in the indexing operation such as direct indexing, simple indexing, angular indexing, differential indexing and rotary table indexing.
Direct indexing is also known as quick indexing. It is a operation that is done with a plate that has 3 holes circles containing 24, 30 and 36 holes. Any integer that divides evenly into 24, 30 or 36 is one of the numbers of divisions that can be indexed directly. A regular procedure that is normally used for direct indexing is as shown below:
1) Engage the plunger pin in one hole, lock the spindle and machine one side of the square.
2) Unlock the spindle and move the index plate 6 holes (1/4 of 24). Do not count the holes in which the pin was located.
3) Lock the spindle and machine the second side.
4) Repeat the process until all the sides are machined.
It is also known as plain milling and has a much wider range of divisions that can be indexed. The indexing head that uses only one plate has the following hole circles available:
Side 1: 24, 25, 28, 30, 34, 37, 38, 39, 41, 42, 43
Side 2: 46, 47, 49, 51, 53, 54, 57, 58, 59, 62, 66
The plates have several dividing heads:
Plate 1: 15, 16, 19, 23, 31, 37, 41, 43, 47
Plate 2: 17, 18, 20, 21, 27, 29, 33, 39, 47
For an indexing head with 40:1 ratio between the crank and the spindle, the number of turns and/or fractional parts of a turn necessary to cut a particular number of divisions is computed by using the following rules:
Number of turns of index crank (T) = 40/√number of divisions (N)
Angular indexing is an operation used when certain milling operation needs to establish the exact angular relationship between holes, surfaces and other machined areas. The procedure for determining the indexing head setup for spacing these holes is as follows:
Number of turns=Number of degrees to be indexed/9
This particular procedure is used for indexing divisions that are beyond the range of simple or angular indexing.
Rotary Table Indexing
The rotary table is used in many milling operations. The lower part of the rotating table is graduated in degrees and the dial near the headcrank is divided into smaller increments. To compute the indexing for a particular number or figure of divisions, the following formula is used:
Number of turns = number of teeth on worm wheel/number of divisions
Once the rotary table is set up properly, the following procedures are taken for machining operation:
1) The rotary table is placed at the centre of the vertical axis of the spindle
2) Move the table of the milling machine an amount equal to the radius of the hole circle to be drilled. Lock all slides.
3) Drill the 1st hole, making sure that the left sector arm is against the index pin.
4) Unlock the rotary table lock and turn the crank clockwise one full turn and 21 holes farther. Lock the rotary table lock and move the sector arms clockwise until the left sector arm touches the pin.
5) Drill the next hole and repeat the processes until all holes are drilled.
This kind of operation is necessary when making helical gears, cams, reamers, taps, cutters and other similar objects. A helix is a curve that moves around a cylindrical object and it advances at a uniform rate. Before the process for determining gears to be used can be started, the lead of the milling machine must be computed by using the following formula:
Lead of milling machine = lead of table feed × number of index crank turns to turn indexing the head spindle once
The angle on the other hand can be calculated by using the following formula:
Tan A = Circumference/Lead of milling machine