Jigs and fixtures: location and clamping

Why it matters

In batch and mass production every part must sit in exactly the same place relative to the cutter without marking out or trial cuts. Jigs and fixtures make that possible: they cut setting time, make parts interchangeable and let semi-skilled operators hold tight tolerances. Poor location or clamping shows up as scrapped parts, chatter and unsafe set-ups.

Key ideas

Jig vs fixture

• A jig locates and holds the work and guides the tool, usually through hardened drill bushes; used for drilling, reaming and tapping. Types: template, plate, channel, leaf, box (tumble), indexing and post jigs.
• A fixture locates and holds the work but does not guide the tool; the tool is set relative to the fixture with setting blocks or feeler gauges. Used in milling, turning, grinding, welding and inspection. A fixture is usually bolted to the machine table; a jig often is not.

Degrees of freedom and the 3-2-1 principle

• A free rigid body has 6 degrees of freedom: translation along and rotation about x, y and z. Older Indian textbooks count 12 (each translation and rotation in the + and − sense).
• 3-2-1 location of a prismatic part: 3 locators under the primary (largest) face, 2 on the secondary face, 1 on the tertiary face. Locators act only in one sense each: in the 12-movement count, the 6 locators remove 9 movements and the clamps remove the remaining 3; in the 6-DOF view, the six point contacts fix all 6 DOF once the clamps hold the part against them.
• A long V-block (or long pin in a bore) constrains 4 DOF of a cylinder (2 translations and 2 rotations); a short pin constrains 2 translations; a flat face 3 DOF.
• Redundant location (more locators than needed in a direction, e.g. two full-length pins in two holes) causes jamming or loading; use one round pin and one diamond (relieved) pin.

Principles of location

• Locate from machined, functional surfaces and, where possible, from the design datum, so tolerance stack-up is least.
• Locators should be spaced as far apart as possible, small in contact area, hardened and replaceable, and placed so chips and burrs cannot sit on them (relief grooves, raised pads).
• Foolproofing: a pin or block that prevents a part being loaded the wrong way round.
• Use adjustable or spring-loaded supports (not locators) for rough or cast surfaces, and equalising locators for uneven ones.

Principles of clamping

• Clamp against fixed locators, never against the cutting force: cutting forces should push the part onto solid locators; clamps only hold it there.
• Clamp over a support so the part is not bent; use the minimum force that prevents movement (excess force distorts thin parts).
• Clamps should be quick acting (cam, toggle, quarter-turn, pneumatic, hydraulic) in production and must not obstruct loading.
• Common clamps: strap (heel) clamps, swing clamps, cam clamps, toggle clamps, wedge, equalising and hydraulic or pneumatic clamps; vacuum and magnetic chucks for thin plates.

Drill bushes: fixed (press-fit), renewable and slip (quick-change for drilling then reaming); the bush is typically 1–2 diameters long, with a gap below it for chip clearance (rules of thumb; take actual values from a design data book).

Formulas

• F_r = μ·n·F_c — maximum force the clamp can resist by friction, N; μ coefficient of friction, n number of friction interfaces (2 when the work is gripped between clamp and base), F_c clamping force, N.
• F_c ≥ SF·F_cut / (μ·n) — required clamping force when friction alone must resist the cutting force F_cut; SF safety factor (often 1.5–2.5).
• W = Q·b / (a + b) — strap (heel) clamp: force on the work W, N, for bolt (or screw) force Q, N; a = distance from bolt to work contact, b = distance from bolt to heel support, mm.
• Δh = Δd / (2·sin α) — vertical shift of the axis of a cylinder of diameter variation Δd located in a V-block of included angle 2α, mm. For a 90° V: Δh = 0.707·Δd.
• Top generator of the cylinder shifts by Δh + Δd/2; bottom generator by Δh − Δd/2.

Worked examples

Example 1 (standard) — strap clamp and friction. A strap clamp has its bolt 40 mm from the work contact and 60 mm from the heel. The bolt is tightened to 6 kN. The work is gripped between the strap and the fixture base, with μ = 0.2 at both contacts. A milling force of 1.2 kN acts parallel to the base. Find the clamping force and the factor of safety against slip.

1. W = Q·b/(a + b) = 6 × 60/(40 + 60) = 3.6 kN.
2. F_r = μ·n·W = 0.2 × 2 × 3.6 = 1.44 kN.
3. Factor of safety = 1.44/1.2 = 1.2 — too low for milling; better to add a solid stop so the cutting force acts onto a locator instead of relying on friction.

Example 2 (GATE level) — location error in a V-block. Shafts of diameter 40 ± 0.05 mm are located in a 90° V-block, and a keyway is milled with the cutter at a fixed height. Find (a) the variation in the shaft axis height, (b) the variation in the keyway depth measured from the top of the shaft, (c) the variation of the keyway bottom measured from the lowest point of the shaft. (d) Repeat (a) for a 120° V-block.

1. Δd = 40.05 − 39.95 = 0.10 mm; α = 45°.
2. (a) Δh = Δd/(2·sin α) = 0.10/(2 × 0.7071) = 0.0707 mm.
3. (b) Top of shaft shifts by Δh + Δd/2 = 0.0707 + 0.05 = 0.121 mm — this is the variation in keyway depth from the top.
4. (c) Bottom of shaft shifts by Δh − Δd/2 = 0.0707 − 0.05 = 0.0207 mm — dimensioning the keyway from the bottom gives far less variation.
5. (d) α = 60°: Δh = 0.10/(2 × 0.8660) = 0.0577 mm. A wider V reduces the axis shift but grips less securely.

Common mistakes

• Calling any work-holding device a jig — a jig must guide the tool.
• Clamping so that the cutting force acts on the clamp instead of on a fixed locator.
• Using two round pins in two holes (redundant location) — one should be a diamond pin.
• Counting friction at only one face when the work is gripped between two surfaces (or at two faces when it is not).
• Confusing supports (which only prevent deflection) with locators (which define position).
• Locating from rough or unmachined surfaces when a machined datum is available.

For GATE PI

• MCQs on the 3-2-1 principle and the number of degrees of freedom (6 or 12 convention) removed by locators and clamps; DOF removed by a V-block, pins and plane faces.
• Jig vs fixture, types of jigs, bushes, diamond pins, foolproofing.
• NAT on clamping force with friction and strap clamps, and on location error in a V-block.

Quick check

1. How many locators does the 3-2-1 principle use, and on which faces?
2. Why is a diamond pin used with a round pin?
3. Strap clamp: bolt force 4 kN, a = 30 mm, b = 50 mm. Force on the work?
4. Axis shift in a 90° V-block for a diameter variation of 0.04 mm?
5. Should the cutting force be directed towards a clamp or a locator?

Answers: 1. Six: three on the primary face, two on the secondary, one on the tertiary. 2. To avoid redundant location when the hole spacing varies; it constrains rotation only. 3. 4 × 50/80 = 2.5 kN. 4. 0.04/(2 × 0.7071) = 0.0283 mm. 5. Towards a locator.