Rasps and Files for Woodworking

Both are hardened steel covered in teeth, and both cut on the push stroke. The difference is the teeth. Tools for Working Wood, which imports Auriou’s rasps, puts it simply: a rasp has little individual teeth cut all over its surface, while a file has long teeth running across its width. Nicholson’s guide describes the same distinction from the file side, and notes that the rasp cut is used primarily on wood. In practice a rasp shapes, removing wood quickly, and a file refines, removing the rasp’s marks more slowly and leaving a finer surface.

A rasp’s teeth are raised one at a time with a pointed tool, and the pattern they form decides the surface left behind (see the plate above).

• Machine-cut rasps have their teeth in regular rows. Tools for Working Wood explains that rows of teeth that follow one another cut little troughs, and do not leave a smooth surface.
• Hand-stitched rasps have their teeth raised by hand. Even the most skilled maker varies the placement just enough that each tooth cuts a slightly different path, so the surface is not furrowed.

Makers name their grades differently, and there is no conversion between systems. Auriou numbers its rasp grains from #1, the coarsest, to #15, the finest; Tools for Working Wood notes that woodworkers mostly use #9 to #15, and that a finer grain has more, smaller teeth rather than fewer. Files are graded by cut. Nicholson names three coarsenesses — bastard, second cut and smooth — and two tooth patterns: single-cut, one set of parallel diagonal rows, for a smooth finish with light pressure; and double-cut, a second set crossing the first, for faster removal with more pressure. Nicholson also notes that size and coarseness go together: a larger file removes more stock, and a smaller one leaves a finer finish.

Keep the jobs separate. Shaping is about getting the form right, and a coarse rasp does it fastest. Smoothing is about removing the marks of the tool before, and each finer tool only needs to remove the scratches of the one before it. Trying to smooth with a coarse rasp, or shape with a fine file, wastes time. Tools for Working Wood describes the rasp as a shaping tool that leaves a surface ready for scraping and finish.

Match the rasp’s section to the curve. A flat face shapes convex curves and flat areas; a half-round rasp’s curved face reaches into concave curves, with a radius smaller than the curve; a round rasp fits tight holes and hollows. On a convex curve, rock the rasp over the shape as you push, so it follows the curve rather than making flats. Work across the grain or diagonally to it where you can: a rasp tears less than a blade, but long strokes along an edge can still splinter the far side, so work in toward the corner from both sides.

• Fit a handle. Nicholson’s guide is emphatic that a file should never be used without a secure handle; the tang is pointed.
• Cut on the push. Tools for Working Wood advises a long, smooth forward stroke, and not pulling the rasp back against the work; Nicholson advises lifting a file clear on the return stroke.
• Hold the work low and firm, in a vice or clamped, so both hands can guide the tool and the work does not flex.
• Keep the teeth clean. Tools for Working Wood recommends a short, stiff natural-bristle brush for rasps, rather than a wire file card, and a light coat of oil against rust.

A rasp or file leaves scratches that a finish will show. Once the shape is right, move to a scraper or sandpaper; the card scraper guide explains when each fits. For curves that a blade can follow, a spokeshave leaves a cleaner, cut surface in less time, and a rasp is better kept for the tight curves, carved forms and reversing grain where a blade would tear.