What is Degree of Freedom (DOF), how many types are available, and what is the 3-2-1 rule?

Technical
Hard
TCS
136.2K views

A kinematics and fixture design question essential for manufacturing and robotics.

Why Interviewers Ask This

DOF determines how a part can move. The 3-2-1 rule is fundamental for locating parts. Interviewers check your grasp of fixture design principles.

How to Answer This Question

Define DOF as independent movements (6 in 3D space: 3 translation, 3 rotation). Explain the 3-2-1 principle: 3 points on primary plane, 2 on secondary, 1 on tertiary to fully constrain a rigid body.

Key Points to Cover

  • 6 Degrees of Freedom
  • 3-2-1 Locating Principle
  • Fixture stability
  • Constraint logic

Sample Answer

Degree of Freedom refers to the independent movements a rigid body can make, totaling six in 3D space (three translations and three rotations). The 3-2-1 rule locates a part by constraining 3 degrees of freedom on the primary datum, 2 on the secondary, and 1 on the tertiary, ensuring stable positioning for machining.

Common Mistakes to Avoid

  • Miscounting DOFs
  • Confusing 3-2-1 with 3-3-3
  • Not explaining datum planes

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