Dimensioning a Block with a Notch or Cutout

When dimensioning a block with a notch or cutout, it is usually best to dimension from a common line or surface. This can be called the datum line or surface. This eliminates the addition of measurement or machining inaccuracies that would come from “chain” or “series” dimensioning.

Questions

• What is a datum line or surface?
• Why is it best to dimension from a datum line or surface?
• What are the disadvantages of chain or series dimensioning?

Answers

• A datum line or surface is a common line or surface that all dimensions originate from.
• Dimensioning from a datum line or surface eliminates the addition of measurement or machining inaccuracies.
• Chain or series dimensioning can lead to inaccuracies because the dimensions are not independent of each other.