4. Marking Out from a Datum Edge

Metal should be marked out and cut slightly over-size. This will allow the metal to be accurately marked to size from a datum edge.

A datum edge is a straight edge, running the length of the piece of metal, from which the project is marked to size.

Preparing a Datum Edge

A datum edge, in thin sheet metal, may be prepared as follows:

Rule a straight line with a rule and scriber.
Carefully cut along the line with guillotine.
Test for straightness with a rule.
Mark the edge with a datum edge mark (essentially a 'V' pointing to the datum edge).

When marking to size all marking should be done from the datum edge.

Place the stock of the try square firmly against the datum edge and scribe a line close to the end of the metal.

2. Using a rule and scriber, mark off the length of the project.

3. With stock of try square firmly against the datum edge, scribe a line through the mark and square to the datum edge.

4. Set an odd-leg caliper to the width of the project and with the butt of the odd-leg caliper against the datum edge, mark to width.

Once the project is marked to size, the waste metal may be removed.

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