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Locating principle and design of a drill jig

Short answer

The 3-2-1 principle uses six locators to arrest all six degrees of freedom: three on the base, two on a side, one on an end. Drill jig design points: locate on machined surfaces, clamp firmly without distortion, use hardened bushes close to work, provide chip clearance and foolproofing. Two types: template jig and box jig.

Explain the 3-2-1 principle of location with a figure. State the points to be considered in the design of a drill jig, and name any two types of drill jigs.

3-2-1 principle

A rigid body in space has six degrees of freedom (three translations and three rotations). It is fully located by six points of contact arranged as follows:

  • 3 points on the base (primary plane) stop vertical movement and two rotations.
  • 2 points on a side (secondary plane) stop one horizontal movement and one rotation.
  • 1 point on an end (tertiary plane) stops the remaining movement.
        top clamp
           |
       +--------+
   2   |  work  |
  pts->|        | <- 1 pt (end stop)
       +--------+
        o  o  o   <- 3 pts (base)

Locators are hardened, ground pins or buttons that touch the work. Clamps then hold the work, and the clamping force must push the work towards the locators, not away from them.

Design points for a drill jig

  • Locate on machined surfaces and follow the 3-2-1 principle, with no over-constraint.
  • Clamping must be quick and firm and must not distort or damage the work; the cutting force should act towards the locators, not against the clamps.
  • Hardened drill bushes (fixed, renewable or slip) guide the drill and are placed close to the work.
  • Provide chip clearance, easy loading and unloading, and foolproofing (poka-yoke) so that the work can be loaded only one way.
  • Use light but rigid construction (mild steel, with case-hardened wearing parts); provide feet so the jig rests steadily on the table.
  • Make it safe to use and economical for the production quantity.

Types of drill jigs

Template jig (a plate with bushes laid on the work), plate jig, box (tumble) jig, leaf jig, channel jig, indexing jig.

✓ Answer checked by Claude Opus 5.5 (high effort)

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