A scribe line, punch mark or paint line made before disassembly so parts go back in the same orientation, or a faint trace of an original surface deliberately left after machining to prove only the minimum was removed. Inspectors and repair machinists also read accidental witness marks, such as rub marks and fretting, to work out how a failed part was moving.
Put a witness mark across the flange and hub before you pull it, I want the keyway going back exactly where it was.
Also heard as
- match mark
- witness line
Related terms
Dykem
Shop name, from a trade name, for blue layout fluid painted on metal so scribed lines show up bright.
Skim cut
A very light cut that just cleans up a surface or establishes size without removing much material.
Kiss
To touch a tool or grinder to a surface as lightly as possible, removing almost nothing.