What is a dividing head?

A dividing head (also called an indexing head) is a precision workholding attachment used primarily on milling machines, not lathes, though it can occasionally be adapted for use on a lathe in specialized setups. Its purpose is to allow controlled rotation of a workpiece through precise angular increments, enabling accurate spacing for operations like cutting gear teeth, flutes, bolt hole circles, and other equally spaced features.

Context and Background

In machining, especially in gear cutting or making components with regularly spaced features, it's often necessary to rotate a workpiece by a known, exact fraction of a circle between cuts. The dividing head provides this functionality. While CNC machines do this numerically, a dividing head allows manual or semi-manual machines to perform indexing operations.

Though typically used on milling machines, on a lathe a dividing head may be used for:

• Cutting radial slots or flats with a milling attachment.

• Gear hobbing or worm cutting (with special gear setups).

• Drilling bolt circles on faceplate-mounted parts.

• Splining or fluting with a toolpost-mounted mill.

Components of a Dividing Head

1. Spindle – Holds the workpiece or a chuck. Can be rotated by hand or indirectly via gearing.

2. Index plate – A circular disc with several hole patterns; used for manual indexing by counting holes.

3. Sector arms – Adjustable pointers used to quickly count a given number of holes.

4. Worm and worm gear – Allow for high-precision angular movement. Usually 40:1 ratio (40 turns of the crank = 1 spindle revolution).

5. Tailstock – Supports long workpieces via a center.

Types of Indexing

1. Simple Indexing – Uses the index plate directly. For example, to divide a circle into 6 parts, you rotate the crank 40 ÷ 6 = 6 2/3 turns.

2. Compound Indexing – Combines hole plates to achieve divisions not possible with simple indexing.

3. Differential Indexing – Uses a gear train between the spindle and index plate to get unusual divisions.

4. Direct Indexing – Some heads have a plate with fixed holes (e.g., 24) allowing direct rotation without the worm gear.

Usage on a Lathe

Lathes are not the natural environment for a dividing head, but in setups where a milling spindle is mounted on the carriage or cross-slide, or where the lathe is being used in a more general-purpose machining role (as in some toolrooms), the dividing head can provide:

• Indexing capabilities for off-center or radial features.

• Setup flexibility for hybrid operations, like gear cutting, without a full milling machine.