Indexing Devices

What Indexing Achieves

• Machine several faces or sides of a part in one set-up, keeping a single datum for every feature.
• Divide a circle into equal increments — 4, 6, 8, 12, 24 or more positions — for bolt circles, gear teeth, splines and drilled patterns.
• Bring every working position to the tool under the same locating and clamping geometry, so each index is a repeatable set-up.
• Process multi-station work such as drilling, tapping, reaming and milling without unloading and re-clamping between operations.
• Eliminate angular layout and manual line-up marks — the fixture decides the angle, not the operator.
• Support automation; a driven indexer gives a machine or robot a fixed, synchronised sequence of stations.

Production Benefits

• Better angular repeatability than hand layout, on every part of the batch.
• Shorter cycle times because handling and re-clamping happen only once.
• Fewer rejects from angular errors, mis-drilled holes and wrong clocking.
• One operator can load, machine and unload stations while the table steps.
• Simpler workholding — the same clamps and locators serve every station or face.

1Manual Index Plate

A hardened circular plate with a ring of index holes or notches around its periphery. The plate is bolted or keyed to the rotary member and stepped by hand between positions.

• Hole circles of 12, 24, 36 or 60 holes give quick divisions of 360°.
• Multiple concentric hole rings cover every division needed by the part family.
• A removable index pin or detent drops into the selected hole to lock the position.
• Stops or keys prevent over-travel once the pin is engaged.
• Hardened and ground hole bushes keep the pitch accurate for the life of the fixture.

2Spring-Loaded Detent Positioning

A spring-loaded plunger with a rounded or tapered tip rides over the plate and snaps into a V-notch or hole. The spring performs both locating and holding in a single motion.

• The tapered tip self-centres in the notch, so minor wear does not destroy accuracy.
• Detent force is set by the spring — heavy enough to hold, light enough to index by hand.
• A lever or cam lifts the detent clear for free rotation, then releases it to re-engage.
• Best for light to medium loads where indexing is frequent and speed matters more than absolute rigidity.
• Hardened V-notches outlast round holes under frequent indexing.

1How the Geneva Drive Works

A continuously rotating driver carries one or two pins; each pin enters a radial slot of the star wheel, turns it exactly one index, then leaves. Between pins, a locking shoulder holds the star stationary.

• One pin on the driver rotates the star by a fixed fraction — typically 1/4 or 1/6 of a turn.
• The star indexes and locks with no extra clutch or brake; the geometry does the work.
• Indexing is smooth and positive at constant driver speed.
• Shock loads occur at pin entry — harden the slots and control the drive speed to limit wear.
• The star starts and stops with a finite acceleration, so heavy parts must be balanced.

2Applying a Geneva Indexer

• Use for fixed, repeated division — a Geneva cannot be re-set to a different number of stations.
• Choose the driver speed so the pin enters and leaves the slot smoothly.
• Add a shot-bolt or taper lock if the star must be rigidly clamped during machining.
• Lubricate the slots and pin; a dry Geneva wears quickly under production indexing.
• For heavy workpieces, use the star as a locating element rather than a load-bearing one.

1How a Ratchet Steps

A spring pawl engages the teeth of a ratchet wheel. An oscillating lever advances the wheel one tooth at a time and the pawl prevents it from running backwards.

• Each stroke of the feed lever advances the wheel by one tooth pitch.
• The tooth pitch sets the index increment — change the wheel to change the division.
• The holding pawl locks the wheel during the return stroke.
• Ratchet teeth are one-way, so the direction of indexing is fixed.

2Where Ratchets Fit

• Coarse, low-accuracy indexing — the tooth pitch can be loose and the wheel inexpensive.
• Feed and step motions on automatic machines, spotters and rotary feeders.
• Single-direction stepping with an automatic or foot-operated feed lever.
• Backstop duties where a shaft must be allowed to move in one direction only.

1What an Index Head Does

A workshop index head carries the workpiece between centres or in a chuck and divides a full turn by an exact number, using a worm-and-wheel reduction with a hole plate for counting fractions of a turn.

• A 40:1 worm drive turns the spindle 9° for each full turn of the index crank.
• A hole circle on the crank plate lets the operator count any required fraction of a turn.
• Sector arms on the plate stop the crank at the correct hole without counting each time.
• The spindle can be locked rigidly for milling or drilling after indexing.
• A well-made head holds a few arc-minutes or better over a full revolution.

2Direct, Simple & Differential Indexing

Simple indexing divides the worm ratio by the required number. Direct indexing uses a fast plate on the spindle. Differential indexing uses a geared train to reach divisions that are otherwise impossible.

• n — crank turns per index (40:1 head)
• N — required number of divisions

• Simple: n = 40 / N turns of the crank — the workhorse for most jobs.
• Direct: a 24-hole plate on the spindle gives quick divisions of 2, 3, 4, 6, 8, 12 and 24.
• Differential: change gears between spindle and plate extend the range to primes and awkward numbers.
• Always disengage the worm before using the direct plate, or the drive will bind.

1Manual Rotary Table

A horizontal or vertical table that rotates about its centre axis, graduated in degrees with a vernier for fine setting, and locked by a clamp ring after each position.

• A 360° graduated scale with a vernier reading to a minute of arc or finer.
• Used for circular milling, radial drilling, bolt circles and dividing work too large for an index head.
• The table locks rigidly while cutting; the worm drive is disengaged for quick free rotation.
• The worm-and-wheel drive gives smooth fine motion and holds position against cutting torque.
• The T-slot face accepts the same clamps and fixtures as a machine table.

2NC / CNC Rotary Tables

A servo- or stepper-driven table that indexes on a programmed angle and can rotate continuously for 4-axis work.

• Indexing on a programmed angle with sub-arc-second positioning resolution.
• Index or contour simultaneously with the machine axes for 4-axis machining.
• A brake or clamp locks the table during heavy cutting, then releases for the next index.
• Settling and clamp time are part of the cycle — program them to hold the true cycle time.
• Indexing error is small but repeatable; calibrate the table into the machine compensation.

1Rotating Turret / Indexing Table Fixture

A rotating carrier plate with several identical work nests. Each station is loaded, machined or unloaded while the others sit at their own stations — machining overlaps handling and the machine never waits for the operator.

• Four-station layouts are common: load, machine, idle and unload in one cycle.
• The fixture indexes 90° per step so the same tool works on every station in turn.
• A precision shot-bolt or taper pin locks each station to a hard stop before cutting.
• Balance the mass around the centre to keep indexing smooth and the plate true.
• Add part-present and clamp-verified sensors for automatic or interlocked use.

2Designing the Indexing Cycle

Build the sequence so that the longest operation sets the pitch time and idle time is used for loading.

1. Define the number of stations and the index angle (90°, 60° or 45°).
2. Place the longest machining operation at the station where the operator is least busy.
3. Time the index, clamp, machining and unclamp steps; the slowest one sets the cycle.
4. Index and clamp before the tool approaches; never move the table under the cut.
5. Interlock the index drive with spindle and slide motion so a mid-cycle stop is safe.
6. Verify the first-off angular position and repeatability before releasing the batch.

How Indexing Accuracy Is Defined

The two figures that matter are the accumulated index error — where the last position lands after a full revolution — and the repeatability, or how close each return to a position is.

• E — angular index error (arc-min)
• e — linear error measured at radius R (mm)
• R — radius at which the error is measured (mm)

• Accumulated error: the worst deviation of any index position from its true angle, quoted in arc-seconds or arc-minutes.
• Repeatability: the scatter between successive returns to the same index position.
• Resolution: the smallest angular step the device can command — it is not the same as accuracy.
• Measure error with an autocollimator, a precision polygon and an electronic level.

Locking the Index Position

• Spring detents and pawls locate but do not clamp — they must not carry cutting force.
• A shot-bolt with a tapered pin seats in a hardened bush for a rigid, repeatable lock.
• Clamp rings and brake packs on rotary tables take the load so the worm gear is not overloaded.
• Lock before the tool enters the cut and release only after the tool has retracted.
• Keep the lock concentric with the index axis — a clamp that shifts the plate destroys the accuracy it guards.

Where Indexing Errors Come From

• Worn, dirty or chipped index holes and V-notches.
• Backlash in the worm drive or clearance in the shot-bolt mechanism.
• Chips and swarf between the plate and its locating face.
• Distortion from clamping — a clamp that pulls the plate off-axis moves every station.
• Run-out and eccentricity between the index axis and the workpiece centre.
• Thermal growth during long production runs changes the fixture geometry.

Reducing Indexing Error

1. Index always in the same rotational direction to take up backlash consistently.
2. Clean and protect the locating faces; cover the holes against swarf.
3. Clamp the plate against its datum, not away from it.
4. Harden the index holes and replace worn bushes before the error drifts out of tolerance.
5. Check the first-off with a dial indicator or autocollimator and record the trend across the batch.