Jigs and fixtures

Why it matters

A cylinder block passes through dozens of machining stations, and every bore, face and bolt hole must land in the same place relative to the others on every block. That repeatability comes from jigs and fixtures: they locate the part the same way each time, hold it rigidly against cutting forces, and (in the case of jigs) guide the tool. Good workholding cuts set-up time, removes marking-out, makes parts interchangeable and lets semi-skilled operators produce accurate work.

Key ideas

Definitions. A fixture locates and holds the workpiece in a fixed position relative to the machine (set to the cutter with setting blocks or feeler gauges, and to the table with tenons/keys); it does not guide the tool – milling, turning, grinding, welding, assembly and inspection fixtures. A jig locates and holds the workpiece and guides the tool, usually through hardened drill bushes – drilling, reaming, tapping and counterboring.

Degrees of freedom and the 3-2-1 principle. A free rigid body has 6 degrees of freedom: 3 translations and 3 rotations. Many workholding texts count each in both directions, giving 12. For a prismatic part:

• 3 locators on the largest, primary surface (non-collinear, spread as far apart as possible) define a plane;
• 2 on a secondary surface perpendicular to it;
• 1 on a tertiary surface perpendicular to both. In the 12-direction count these six locators restrain 9 directions (5 + 3 + 1) and the clamps restrain the remaining 3 by pressing the part onto the locators. In the 6-DOF count, each locator removes one freedom. Either way, six points of location fully locate a prismatic part; adding more fixed locators in the same direction is redundant location and causes rocking or distortion.

Locating principles.

• Locate from machined (or the most accurate) surfaces and, where possible, from the design datum, so tolerances do not stack up.
• Spread locators widely for stability; keep locator contact areas small and raised so chips and burrs do not upset location.
• Direct cutting forces towards fixed locators, not towards clamps.
• Provide foolproofing (poka-yoke) so the part cannot be loaded wrongly, plus clearance for chips and burrs and for hand loading.
• Cylindrical location: a round pin in one hole plus a diamond (relieved) pin in a second hole locates a part in the plane without jamming from hole-centre-distance variation.
• V-blocks locate round parts on their axis in one direction, with a predictable centre-height error as diameter varies.
• Nests and pads, conical locators and self-centring (equalising) devices are other options.

Supports. Fixed and adjustable (screw, spring or hydraulic) supports prevent deflection of thin or overhanging sections without over-constraining the part; adjustable supports are brought into contact after locating.

Clamping principles. Clamps hold the part against the locators; they must not move it off them. Clamp over a solid support or locator to avoid bending the part, use the minimum force that resists cutting forces with a margin, make clamping quick (cam, toggle, quarter-turn, quick-acting nuts, pneumatic or hydraulic power clamping for production), and keep clamps clear of the tool path. Common types: strap (lever) clamps, screw clamps, cam clamps, toggle clamps, wedge clamps, equalising clamps.

Drill bushes. Press-fit (fixed) bushes for small batches; renewable bushes in a liner bush for long runs; slip bushes when several operations (drill, then ream) use one hole location. The bush is set a small distance above the work (roughly half to one drill diameter is a common rule; check the handbook) so chips escape.

Types of jigs: template, plate, channel, leaf (hinged), box (tumble), indexing. Types of fixtures: vice-jaw, milling (string/gang), turning (faceplate, mandrel), grinding, welding (holding against distortion), assembly, modular fixtures built from standard grid-plate elements for short runs.

Design materials. Bodies of cast iron (good damping, stable) or welded/fabricated steel; locators, bushes and wear pads of hardened tool steel; aluminium for lightweight hand-held jigs.

Formulas

DOF = 6 (or 12 in the ± convention); 3 + 2 + 1 = 6 locators Minimum locators that fully locate a prismatic part.

F_cl ≥ FS·F_c / (μ₁ + μ₂) Minimum clamp force (N) when a cutting force F_c (N) parallel to the base is resisted by friction only at the base (μ₁) and at the clamp face (μ₂); FS = factor of safety; part weight neglected. With μ₁ = μ₂ = μ: F_cl ≥ FS·F_c / (2μ).

Δh = ΔD / (2·sin(θ / 2)) Change in the centre height (axis position) of a round part in a V-block (mm); ΔD = diameter tolerance band (mm), θ = included V angle. For a 90° V, Δh = 0.707·ΔD.

Worked examples

Example 1 (standard) – V-block locating error. Shafts of diameter 40 ± 0.05 mm are located in a 90° V-block for drilling a cross hole.

1. ΔD = 40.05 − 39.95 = 0.10 mm.
2. Δh = ΔD/(2 sin(θ/2)) = 0.10/(2 × sin 45°) = 0.10/1.414.
3. Δh = 0.071 mm variation in axis height. A 120° V-block would give 0.10/(2 sin 60°) = 0.058 mm, but locates less securely.
4. If the hole must be on the axis, a self-centring device eliminates this error.

Example 2 (GATE level) – clamping force. In a milling fixture the horizontal cutting force is 1200 N, and the design (poor practice) relies on friction alone at the base and under two clamps; μ = 0.2 at both surfaces, factor of safety 2. Neglect the part's weight.

1. Friction acts at the base and at the clamp faces, each carrying the total clamp force F_cl: resistance = (μ₁ + μ₂)·F_cl = 0.4·F_cl.
2. Required: 0.4·F_cl ≥ 2 × 1200 ⇒ F_cl ≥ 6000 N total, i.e. 3000 N per clamp.
3. Better design: put a fixed stop (locator) in the direction of the cutting force; the clamps then only need to keep the part seated, and the required force falls sharply.

Example 3 – counting restraints. A block sits on 3 base pins, against 2 side pins and 1 end pin, with one clamp pushing it towards each set. In the 12-direction count the pins remove 5 + 3 + 1 = 9 directions and the three clamp directions remove the remaining 3 – the block is fully constrained with no redundancy.

Common mistakes

• Saying a fixture guides the tool – only a jig does.
• Using more than three locators on a plane surface (redundant location makes the part rock).
• Pointing the cutting force towards a clamp; clamps should only hold the part against locators.
• Calculating clamp force from the part's weight instead of the cutting forces.
• Using two round pins in two holes; the second must be a diamond (relieved) pin.
• Clamping over an unsupported span, which bends thin parts so they spring back out of tolerance after unclamping.

For GATE ME

Questions are mostly conceptual: the 3-2-1 principle and degrees of freedom (watch which convention the question uses), jig vs fixture, types of locators, clamps, bushes, jigs and fixtures, and design principles such as redundant location and foolproofing. Occasionally numericals appear on V-block locating error or friction-based clamping force. Practise listing which freedoms each set of locators removes.

Quick check

1. How many locators fully locate a prismatic part?
2. Which device guides the drill in a jig?
3. Why is a diamond pin used with a round pin?
4. Centre-height variation for ΔD = 0.2 mm in a 90° V-block?
5. Towards what should the cutting force be directed?

Answers: 1. Six (3-2-1); 2. A drill bush; 3. To allow for centre-distance variation without jamming while still preventing rotation; 4. 0.141 mm; 5. Towards fixed locators.