Index Head

an attachment for machine tools (mainly milling machines) that makes possible the periodic rotation of the article being processed through equal or unequal parts of a revolution. A distinction is made between mechanical index heads (including simple and all-purpose types) and optical index heads. The most common all-purpose index head can be adjusted for simple and differential indexing and for cutting (milling) spiral grooves; it makes possible the milling of polyhedrons and rectilinear and spiral grooves (for example, on splined shafts, gears, drills, cutters, and reamers). Index heads are mounted on the table of the machine tool (see Figure 1), and the workpiece is held by a chuck in the centers between the tailstock and a spindle by which it is rotated.

Figure 1 Index head: (a) mounting on a milling machine, (b) kinematic diagram with adjustment for simple indexing; (1) machine-tool table, (2) tail-stock, (3) centers, (4) chuck, (5) index head, (6) sliding segment, (7) adjusting crank, (8) clamp, (9) dividing disk, (10) pin, (11) spindle

The adjustment of an index head for simple indexing is performed by means of a fixed dial or dividing disk (secured by pins) that has several circular sets of concentrically arranged openings from which the required angle of rotation of the workpiece is read off with a crank that is kinematically linked to the spindle. The number of turns of the crank is calculated from the formula n = l/(i_i,h z), where i_i,h. is the gear ratio of the index head (the main standard ratios are 1/30, 1/40, and 1/60) and z is the required number of divisions. Index heads are usually equipped with a set of dials with different numbers of holes. A sliding segment is used for convenience during indexing.

In adjusting for differential indexing, the pin that secures the dial is removed. A calculation is made for a hypothetical number of parts x close to the given z: n = l/(i_i,h•x). To compensate for the assumed approximation, the workpiece is given additional rotation by means of the interchangeable gears Z₁ Z₂, Z₃, and Z₄ (Figure 2)—that is, the crank is turned relative to the dial, which also rotates.

Figure 2 Kinematic diagram of an index head adjusted for differential indexing

When adjusting an index head for cutting (milling) spiral grooves (Figure 3), the rotation of the workpiece and its translational motion (from the table’s longitudinal lead screw) are linked kinematically through interchangeable gears; the crank and dial are connected by a clamp. The machine-tool table is turned to an angle equal to the angle of inclination of the spiral groove ω the axis of the machine tool’s spindle (the cutter’s axis) forms an angle θ = 90° - ω, where tan ω = πD/H, D is the diameter of the workpiece, and H is the pitch of the spiral groove. During one revolution the work-piece should move by H, and so i_ig = Z₁Z₃/Z₂Z₄=t₈/(i_ih. H)where ts is the pitch of the longitudinal lead screw.

Figure 3 Kinematic diagram of an index head adjusted for cutting spiral grooves

The operation of a simple index head is based on the direct (manual) rotation of the dial to a specified angle. The position of the dial is fixed according to the notch or hole provided on it.

An optical index head is used for accurate readings. It is equipped with a reading microscope having a 1’ scale division. Unlike the mechanical index head, the optical type requires no calculations.

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