Filing to a Line: Marking Out, and Knowing When to Stop

Coat the surface with marking blue, let it dry, then cut the line with a scriber held against a square or a rule. The scriber removes the blue and cuts a fine groove into the metal, so the line is bright, thin and permanent. A pencil line is about 0.5 mm wide and rubs off; a marker line is wider still and dissolves in cutting oil. Neither can be filed to accurately, because half the width of the line is more than the tolerance you are working to. Scribe once, firmly. Going over a line repeatedly produces two or three parallel grooves and then you have to decide which one you meant. On soft metal like aluminium or brass a scriber cuts deeply, so a lighter hand is needed.

Work until the scribed groove is just being touched along its whole length, and stop there. The finished surface should still show the line, or half of it. Which half is a decision made before you start. For a part that must fit INTO something, leave the line on — file up to it and no further. For one that something must fit into, take the line off. Deciding halfway through is how a part ends up a millimetre undersize. Getting the line to be touched evenly along its whole length is the real skill. If it is being reached at one end first, that end is low: ease off there and concentrate the stroke on the high end, which is the cross-filing diagnosis applied to a line instead of a surface.

Cut close to the line with a hacksaw — about a millimetre outside it — and file only that last millimetre away. The removal-rate figures in the cross-filing rung make the case: three millimetres off a small face is over an hour of filing, and a hacksaw takes it in under a minute. Filing exists to make a surface true, not to remove stock, and using it for bulk removal is the commonest reason a beginner decides hand tools are hopeless. Leave enough to clean up the saw marks and the wander — a millimetre is comfortable, half is tight, and sawing exactly to the line means any wander cuts INTO the finished part with nothing left to correct it.