Jigs and fixtures

Why it matters

In batch production every part must sit in exactly the same place relative to the cutter, so holes line up and parts interchange without fitting. Jigs and fixtures make that repeatable, cut setting and marking-out time, and let semi-skilled operators or robots load parts quickly. In automated cells the fixture often also carries sensors (part-present, clamp-closed) and powered clamps, making it a small mechatronic system in its own right.

Key ideas

Jig versus fixture. Both locate and clamp the workpiece. A jig additionally guides the tool, usually through hardened drill bushes, and is used for drilling, reaming and tapping. A fixture only locates and holds the work; the tool is positioned by the machine itself or set with setting blocks and feeler gauges (milling, turning, grinding, welding, assembly, inspection). Jigs are often light and are moved on the machine table; fixtures are bolted to the table.

Degrees of freedom and the 3-2-1 principle. A free rigid body has six degrees of freedom: three translations and three rotations. Many Indian textbooks count them as twelve directional movements (± along and about each axis). The 3-2-1 (six-point) principle locates a prismatic part:

• 3 locators on the primary (largest) face define a plane and arrest translation normal to it and the two rotations about axes in that plane.
• 2 locators on the secondary face arrest one translation and the remaining rotation.
• 1 locator on the tertiary face arrests the last translation. The six locators arrest six degrees of freedom (nine of the twelve directional movements); the clamp force pushes the part against the locators and takes care of the remaining three movements. Adding more locators on the same face over-constrains the part and makes location depend on the accidental surface errors.

Locating practice.

• Locate on machined (or the most accurate) surfaces and on the same datum the drawing uses, so locating errors do not add to the tolerance chain.
• Keep locators as far apart as possible on a face to reduce angular error, and make them small (pins, buttons) so chips and burrs do not disturb seating.
• For a part with two holes, use one full cylindrical pin and one diamond (relieved) pin: the diamond pin stops rotation without fighting the cylindrical pin over the hole-centre-distance tolerance.
• V-blocks centre cylindrical work; conical and centre locators suit turned parts.
• Provide chip clearance and fool-proofing (a pin that stops the part being loaded the wrong way round).

Clamping practice. Clamps should press the part on to the locators, act directly over a support (so the part does not bend), and the main cutting force should be directed into fixed locators, not resisted by clamp friction alone. Clamps must be quick-acting (cam, toggle, quarter-turn, pneumatic or hydraulic) and must not mark finished surfaces.

Types of jigs. Template, plate, angle-plate, channel, leaf (latch), box (closed, for drilling from several sides) and indexing jigs.

Drill bushes. Press-fit (permanent) bushes for low volumes, renewable bushes in a liner for long runs, and slip bushes when one hole is drilled, then reamed through different bushes. The bush is kept a short distance (about one drill diameter, less for accurate work) above the work to let chips escape.

Types of fixtures. Milling (with tenon blocks and a setting block), turning (faceplate and mandrel-type), grinding, broaching, welding (resist distortion and hold the joint gap), assembly and inspection fixtures. Modular fixtures built from standard grid plates and elements suit low-volume CNC work.

Materials. Bodies of grey cast iron (rigid, damps vibration), welded or bolted steel, or aluminium for light hand-loaded jigs; locators and bushes of hardened tool steel or case-hardened steel for wear resistance; polymer or 3D-printed nests for light assembly work.

Formulas

W ≥ n·F / (k·μ)

• Clamping force to resist a force acting along the clamped face by friction alone. W = clamp (normal) force (N); F = force tending to slide the part (N); μ = coefficient of friction (dimensionless); k = number of friction interfaces carrying W (1 or 2: clamp face and base); n = factor of safety (typically 1.5–3, take from company practice). Use only when the force cannot be directed into a positive stop.

h = D / (2·sin(α/2))

• Height of the centre of a cylinder above the V-block vertex. D = cylinder diameter (mm); α = included angle of the V (degrees).

Δh = ΔD / (2·sin(α/2))

• Location error of the cylinder axis caused by diameter variation ΔD (mm); vertical shift, horizontal position unchanged for a symmetric V.

DOF arrested by 3-2-1 location = 3 + 2 + 1 = 6

Worked examples

Example 1 (clamping force, standard). A milling fixture holds a block with a top clamp. The horizontal cutting force along the face is F = 1200 N and there is no end stop on that side. The coefficient of friction is μ = 0.2 at both the clamp–work and work–base interfaces. With a factor of safety of 2, find the clamp force.

1. Friction acts at both faces, so total friction resistance = 2·μ·W.
2. Condition: 2·μ·W ≥ n·F
3. W = n·F/(2·μ) = 2 × 1200 / (2 × 0.2) = 2400/0.4 = 6000 N Answer: W = 6000 N (6 kN). A better design adds an end stop so the 1200 N goes into the locator and the clamp only has to keep the part seated.

Example 2 (V-block location error, GATE level). Shafts of diameter 50 ± 0.025 mm are located in a 90° V-block for drilling a cross hole through the axis. Find the variation in the axis height and compare with a 120° V-block.

1. Diameter variation ΔD = 50.025 − 49.975 = 0.05 mm
2. For α = 90°: Δh = ΔD/(2·sin 45°) = 0.05 / (2 × 0.7071) = 0.0354 mm
3. For α = 120°: Δh = 0.05/(2·sin 60°) = 0.05 / (2 × 0.8660) = 0.0289 mm Answer: the axis height varies by 0.0354 mm with a 90° V and 0.0289 mm with a 120° V. The horizontal position of the axis does not change, so a vertical hole drilled through a bush stays on the centre line for every shaft – the reason V-blocks are favoured for cross-drilling.

Example 3 (degrees of freedom). A rectangular block rests on three pins on its base and two on one side. How many degrees of freedom remain, and what is the least needed to complete location?

1. Base: 3 pins arrest translation along z and rotations about x and y (3 DOF).
2. Side: 2 pins arrest translation along y and rotation about z (2 DOF).
3. Remaining: translation along x (1 DOF). Answer: one DOF remains; a single end locator completes the 3-2-1 scheme, and clamps then hold the part against all six locators.

Common mistakes

• Saying a fixture "guides the tool"; only a jig does.
• Putting four supports under a flat face; the part rocks on any three, and location is no longer defined.
• Using two full cylindrical pins in two holes – the part jams when the centre distance varies; one pin must be a diamond pin.
• Letting the cutting force push against the clamp instead of a fixed locator.
• Locating on rough or unmachined surfaces when a machined datum is available.
• Clamping away from a support so the part springs and returns out of tolerance after release.
• Forgetting chip clearance and coolant drainage, which spoil location after a few parts.

For GATE ME

• The 3-2-1 principle: number of locators per face and degrees of freedom arrested.
• Difference between jigs and fixtures, types of drill bushes, diamond pin, and fool-proofing.
• Simple numericals on clamping force with friction and on V-block location error. Practise drawing a block with six locators and naming the freedom each one removes.

Quick check

1. What is the essential difference between a jig and a fixture?
2. How many locators does the 3-2-1 principle use, and how many degrees of freedom do they arrest?
3. Why is one of the two locating pins made diamond-shaped?
4. Which type of drill bush is used when a hole is drilled and then reamed in the same set-up?
5. Why should the cutting force be directed towards a fixed locator rather than a clamp?

Answers: 1. A jig guides the tool as well as holding the work; a fixture only locates and holds it. 2. Six locators, arresting six degrees of freedom. 3. To stop rotation without over-constraining the part when the hole spacing varies. 4. A slip bush. 5. Locators are rigid and positive, whereas clamps rely on friction and can slip or deflect.