14.11 Principles of Work Holding, Jigs & Fixtures

Jig Versus Fixture

The distinction is the single most examined point in this topic.

The memory hook: a jig guides the tool; a fixture merely holds the work. A drill jig has hardened drill bushes that steer the drill to the exact position, so no marking out or centre punching is needed. A milling fixture has no such guidance, because the milling cutter's path is set by the machine's slides.

Why they exist

Both devices exist to transfer accuracy from a once-made tool to every part produced. The benefits are:

• Interchangeability — parts are identical without individual measurement.
• Reduced cycle time — no marking out, no individual setting.
• Lower operator skill required, and therefore lower cost.
• Improved safety, since the work is securely held.
• Consistent quality independent of operator judgement.

The cost is the tooling itself, so jigs and fixtures are justified only by production volume.

Degrees of Freedom and the 3-2-1 Principle

A free rigid body in space has twelve degrees of freedom: along each of three axes it can translate in two directions, and about each axis it can rotate in two senses. That is $3\times2 + 3\times2 = 12$.

The 3-2-1 principle

The standard scheme for locating a prismatic workpiece uses six locating points arranged on three mutually perpendicular planes:

These six points restrain nine degrees of freedom. The remaining three — motion away from each of the three locating planes — are restrained by the clamping forces, not by the locators. This division between locating and clamping is the essential idea, and it is why clamps must always press the work against the locators.

Three points define a plane uniquely, which is why the primary plane uses exactly three: a fourth would be redundant and would rock.

Over-location

Locating must be complete but never redundant. If two features attempt to control the same degree of freedom, the workpiece position becomes indeterminate — it will seat against one or the other depending on tolerances — and clamping introduces distortion. Common over-location errors include using two full-round locating pins, or locating on both a face and a shoulder that control the same direction.

The standard remedy for two-pin location is to make the first pin round and the second a diamond pin, relieved on two sides. The round pin fixes position; the diamond pin fixes rotation only, without fighting the round pin over the centre distance tolerance.

Locating Devices

Design rules for locators

1. Locate from a machined surface wherever possible, and always from the same datum used in design and inspection. Mismatched datums between design, manufacture and inspection produce tolerance stack-up.
2. Make locators hardened, ground and replaceable, since they wear.
3. Position locators so that swarf cannot accumulate on them; provide relief grooves and chip clearance.
4. Make locating errors visible — the part should not fit at all if loaded incorrectly. This is foolproofing or poka-yoke, commonly achieved by adding an asymmetric pin that prevents loading the part the wrong way round.
5. Space locators as widely apart as the part allows, to minimise angular error.

Clamping Devices

Design rules for clamping

1. Clamping force must act towards the locators, never away from them, so the work is pressed onto its location.
2. Clamp over a solid, supported part of the workpiece, ideally directly above a locator, so the force is not carried by an unsupported span.
3. Use the minimum force that resists the cutting forces — excess clamping distorts the part, and the distortion springs back after machining, leaving the finished dimension wrong.
4. Clamping must not obstruct loading, unloading, tool access, or chip and coolant flow.
5. Clamps should be quick-acting where cycle time matters, and should be captive so they cannot be lost.
6. The cutting forces should ideally act into the locators, so that the tool helps hold the work rather than lifting it.

Types of Jigs

Drill bushes

The guiding element of a jig:

Bush length is normally 1.5 to 2 times the hole diameter, and the clearance between bush and work is typically about one drill diameter, providing chip escape while retaining accurate guidance.

Types of Fixtures

Economic Justification

The decision to build a jig or fixture is an economic one. Given tooling cost $C_t$, saving per part $s$ and quantity $N$, tooling is justified when

$N\,s > C_t$

so the break-even quantity is $N = C_t/s$. For a small batch, modular fixturing — reusable standard elements assembled on a gridded base — is often the right answer, because the elements are recovered and reused rather than scrapped with the job. In a maintenance workshop of the kind found at a coal subsidiary, where batch sizes are small but repeat work is common, modular fixturing and simple plate jigs usually make more sense than dedicated tooling.