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The condition of some workpiece surfaces does not permit easily visible scribed lines.
Such surfaces must be coated with special paint.
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Workpiece surface |
Paint coat |
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Rough, big-pore surfaces of castings and forgings |
To be coated with whiting mixed in water adding a bit of linseed oil. |
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Hard and scaled steel parts |
To be coated with copper sulphate solution (CuSO4) -danger - toxical! |
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Big pre-machined surfaces and light metal |
To be coated with shellac or scribing varnish. |
The workpiece must have pre-scribed lines, pre-machined edges and faces from where further layout out of dimensions can be done with various possibilities of datum:
Datum faces and datum edges
Usually pre-machined faces or edges which are flat and partly perpendicular to other faces or edges. They are used as location for measuring tools or as supporting face on surface plates. When datum faces or edges are used, scribing is mostly done by means of scribing blocks.
Figure 18 Scribing based on two datum
faces or edges
1 datum faces or edges
Datum lines
On symmetrical parts or parts with indefinite shape, lines or centre lines are marked which are used as datum for further scribing.
Figure 19 Scribing based on one datum
line (centre line) and one datum edge
1 datum line
2 datum edge
Why must certain workpiece surfaces be coated prior to scribing?
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Which possibilities of datum for scribing are there?
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