An example using the concept of restraining the six degrees of freedom, can also be described as a 3-2-1 alignment. The following gives a generic explanation of that process.

Numbers 3, 2, 1 refer to the minimum number of contact points.

Carry out three steps to the co-ordinate system in the correct order. In this instance we:

1. Level to the part -

• The drawing will identify the Primary datum
• Ensures co-ordinate system is perpendicular to the component and not to the axes of the of the co-ordinate measuring equipment.
• Involves contacting the (Primary) surface at three or more points and fitting a surface used to define an axis normal to the surface, which is mathematically aligned with the z-axis. This axis is then used as a rotation axis for further alignment.
• It is wrong to assume that the part is sitting flat or perpendicular to the vertical axis of the co-ordinate measuring equipment, there could be dirt under the part or the face that the part is sitting on is not parallel to the datum face.

 

2. Rotate to the axis -

• Now the part is levelled the next step is to rotate to the axis.
• The drawing will identify the secondary datum.
• Involves contacting the (Secondary) surface or edge with a minimum of 2 points and calculating the orientation of the edge to the Primary datum surface.
• It is not necessary to physically align the part to an axis of the machine, as the software will then mathematically adjust the part.

 

3. Set the origins -

• The drawing will identify the Tertiary datum
• Involves contacting the (Tertiary) surface or edge with a minimum of 1 point to establish the datum with the software
• The origin is a position or a combination of features which is at x = 0, y = 0 and z = 0.

 

 

 

3-2-1 alignment is generally applicable to parts that are cuboid in nature. Aligning a cylindrical shape requires the axis to be defined as the primary datum.

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Good practice online modules

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• Dimensional
• Overview of basic design and interpretation of an engineering drawing
• An introduction to datum
• An introduction to geometric tolerancing
• An introduction to the designer's role when designing a component
• An introduction to the manufacturer's role when designing a component
• An introduction to the inspector's role when designing a component