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Tool Setting, Guiding & Indexing Devices

Setting devices position the tool relative to the fixture, guiding devices lead the tool to the workpiece, and indexing devices rotate the part between successive operations with a repeatable lock.

1Tool-Setting Devices

A tool-setting (set-up) device fixes the tool position relative to the fixture so all parts are machined in the same place. A hardened set block is fitted to the fixture and a feeler gauge is passed between block and tool.

  • Set block ground to the required dimension; feeler gauge typically 0.5–3 mm.
  • Fast and repeatable — no trial cuts for each new setup.
  • Common on milling and boring fixtures where tool position must be reproduced.
Set block Feeler gauge Cutter Fixture base
Every part is cut in the same place: the feeler sets tool to datum; block dimension = tool-to-datum minus feeler thickness
Set block dimension = required tool-to-datum distance minus the feeler thickness.

2Guiding Devices

Guiding elements keep the tool on position and reduce deflection. The most common are drill jig bushings and boring-bar guides.

  • Drill bushings: press-fit (fixed), renewable (slip) and liner types.
  • Place the bushing close to the workpiece to reduce drill wander.
  • Boring bars are guided in the fixture or in a floating head aligned to the bore.
  • Extended bushing noses break and guide chips on through-holes.
Drill Slip bushing Liner Jig plate
Bushing types: press-fit fixed, renewable slip bushing with a head, and a wear-protecting liner
Drill Bushing Extended nose Workpiece
An extended nose leads the drill close to the through-hole and curls the chips away from the bore
Boring bar Bushing Workpiece Bore
Riding in support bushings keeps the overhang rigid so the toolbit holds the bore size

Indexing Devices

An indexing device rotates the workpiece (or the fixture) to a new position and locks it there. Used for drilling hole circles, machining flats on hexagons, and multi-station work without re-locating the part.

  • Indexing plate with precision holes or slots, one position per station.
  • Index pin or plunger engages the hole; a tapered pin gives highest accuracy.
  • Rotary indexing: for drilling/milling round patterns (3, 4, 6, 8, 12 stations).
  • Lock positively before cutting — never rely on friction to hold the index.
index plate with pin — 3, 4, 6, 8, 12 stations

Indexing Mechanisms

Simple pin & hole

Spring or lever pin into a ground hole. Simple and cheap; accuracy depends on hole spacing.

Tapered pin

Tapered pin into a taper seat eliminates clearance — repeatable to a few micrometres.

Notch & roller

V-notches engaged by a roller; used where the fixture must rotate freely between stations.

Power indexing

Pneumatic or hydraulic indexing for automatic cycles; the index is locked by a separate clamp.

Indexing accuracy is set by the pin/hole fit and the hole spacing, not by the plate diameter. Ream index holes H7 with hardened and ground pins.

Typical Indexing Accuracy

MethodRepeatability
Cylindrical pin in reamed hole±0.02–0.05 mm
Tapered pin in taper seat±0.005–0.01 mm
Ground notch & roller±0.01–0.02 mm
Precision Hirth coupling±0.002–0.005 mm

Design Notes

  • Match the indexing method to the angular tolerance on the drawing.
  • Always lock the index mechanically before the tool engages.
  • Locate the indexing axis on the machined datum of the part.
  • Keep index plates and pins hard (58–62 HRC) and ground to resist wear.
  • For combined setting + guiding work, verify the set block dimension and bushing fit against the drawing before production.