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As per ASME Y14.5M-1994, paragraph 3.2 "Datum Feature Symbol“ Datum alphabet can use A to Z except I, O and Q.

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  • We have to arrest all the 6 DOF to stable the work piece

for accurate machining or accurate inspection

  • The reference frame is defined by three perpendicular

datum planes.

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  • The work piece is align with the datum planes of a

reference frame by using 3-2-1 principle of locating

called as Order of precedence

  • Depends up on functionality & assembly requirement

the Order of precedence will change.

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Datum types :

1) Planar datum

It will arrest 3 DOF

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(X, Y Rotation & Z Linear)

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2) Axis datum

It will arrest 4 DOF

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(Y, Z Rotation & Y, Z Linear)

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3) Spherical surface datum

It will arrest 3 DOF

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(X, Y, Z Linear)

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4) Conical surface datum

It will arrest 5 DOF

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(X, Y, Z Linear & Y, Z Rotation))

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Datum Targets

  • Used for cast, forged, molded surface.
  • Used for fixture or gauge
  • Not required to follow 3-2-1 principle
  1. Datum target points
  2. Datum target Line
  3. Datum target area

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  1. Datum target points

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2) Datum target Line

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3) Datum target area