Cross-Filing: Taking Metal Down Fast and Keeping It Flat

Hold the handle in your dominant hand with the thumb along the top, and the tip of the file between the thumb and fingers of the other. Push the file forward with the weight of your upper body, not by pressing down with your arms. The pressure balance shifts through the stroke. At the start most of the file hangs off the far side of the work, so the leading hand carries the weight; at the end the handle hangs off the near side, so the pressure shifts back. Getting that transfer right is the whole skill, and it is what keeps the stroke level. Lift on the return. The teeth cut only forward, so dragging back wears them and drags chips across the surface. The one exception is soft metal like lead where a light return helps clear the teeth.

File at roughly 30 degrees across the work for a dozen strokes, then swing to 30 degrees the other way and file until the first set of scratches has gone. The moment they have all gone, the surface is level to the depth of those scratches. If a patch of the old scratches survives, that patch is LOW and everything around it is high — so work the high ground, not the hollow. This is the single most useful thing about cross-filing and it costs nothing. Chalk rubbed on the file face makes the pattern easier to read and slows clogging on soft metal. Marking blue on the work does the same job more clearly on steel.

Lay a straightedge across the work and hold it up to a light. A gap anywhere shows as a line of light, and the eye can see a gap of a few micrometres this way — far finer than any rule you can read. Check in three directions: along, across, and diagonally. A face can be straight along its length and still be twisted, and the diagonal is what finds that. For square, stand a try square on the reference face and look for light under the blade the same way. Check every dozen strokes near the end, not every hundred. Metal comes off far faster than it goes back on, and the published surface-plate-and-marking-blue blueprint is the next step up in precision when light-gap checking is no longer good enough.