Fixture and Jig Design with FEA: Stiffness, Clamping and Vibration for Accurate Machining

• It locates the part in exactly the same place every time.
• It clamps firmly without distorting the part.
• It stays rigid against cutting, drilling and clamping forces.
• It does not vibrate at the frequencies the cutting excites.
• It loads quickly, clears chips and is light enough to handle.

A rigid part has six ways to move: three translations and three rotations. The 3-2-1 principle removes all six with the minimum number of locators: three points on the primary datum face, two on the secondary and one on the tertiary. Clamps then push the part against the locators, ideally directly opposite them, so clamping force does not bend the part.

• Fixture deflection at the locators under cutting and clamping forces, compared with the part tolerance. It should use only a small share of the tolerance budget.
• Part distortion from clamping: a thin part clamped out of shape is machined flat and springs back out of tolerance once released.
• Stress in fixture plates, locators, clamps and bolts.
• Contact pressure at locators and clamp pads, which can mark soft or finished surfaces.
• Natural frequencies, which should stay away from the tooth-passing frequency of the cutter (spindle speed times the number of teeth).

Thin walls, castings and large plates are especially sensitive. FEA models the clamping sequence as separate load steps, with contact between the part, locators and clamps, and shows how much the part moves when clamped and how much springs back afterwards. Moving a clamp, spreading the force over a larger pad or adding a support under a thin wall is often enough to bring a part back into tolerance.

A fixture that resonates near the cutting frequency amplifies vibration, leaving chatter marks, poor finish and short tool life. Modal analysis finds the natural frequencies and mode shapes of the fixture with the part in it, so stiffening can go exactly where the fixture moves most. Our guide to vibration analysis for industrial machinery explains modal analysis in more detail.

In our FEA of an industrial fixture in SolidWorks Simulation, the maximum stress was 146.8 MPa, well below the yield strength of the steel (typically above 250 MPa), and the deflection was minimal. That margin also showed room to remove material and make the fixture lighter.

Fixtures and tooling often work alongside automation. In our structural analysis of a 6-DOF robotic drilling arm, the whole arm and its bolted joints were checked under a 200 N·m drilling torque, the same kind of process load a fixture has to react.

• Material: aluminium weighs about a third as much as steel but is also about a third as stiff, so it needs deeper sections to match.
• Pockets and ribs: FEA shows which material carries load and which can go. Our guide to topology optimization and design for manufacturing explains how to find the best shape.
• 3D printed fixtures suit light-duty work, nests and inspection, once their stiffness has been checked.
• Welded bases need a fatigue check at the welds, covered in our guide to weld fatigue analysis.

• 3D CAD of the fixture and the part, or drawings for our 3D CAD team to model.
• The machining operations: tools, speeds and feeds, or the cutting forces if you know them.
• Clamp types, clamping forces and the clamping sequence.
• Part tolerances and materials.