By Robert W. Lang
Rasps are simple tools, yet incredibly versatile. With a good rasp, you can remove band saw marks from a curved surface, shape a cabriole leg, round over an edge, tweak a tenon and modify a mortise. Powered by hand, they are quite efficient, and unlike the dozens of power-tool alternatives for these tasks, rasps offer a tremendous degree of control.
As with many seemingly simple tools, rasps come in a range of shapes, styles and sizes and the differences among these variations can be subtle. The good news is that you don’t need them all. Depending on the type and scale of work that you do, two or three will likely serve your needs.
For quality tools, individual rasps aren’t cheap. But what these tools allow you to accomplish makes them a real value. You probably have your share of flap sanders, sanding discs and other devices that promised to make life easier, but these sit gathering dust in some corner of your shop. A few good rasps cost less, and rasps deliver on the promise of control while shaping. And they get you close to a finished surface.
Sink Your Teeth in It
Rasps start out as a shaped blank of steel that is soft enough in which to raise a sharp tooth if hit just right with a harder steel punch. Teeth are punched by machine or “stitched” by hand. After the teeth are in, the tool is hardened. The machine process is fast and inexpensive. There was a time when decent machine-cut rasps could be found, but those days are gone (see “Stay Away From These Pretenders” page 53).
Hand-stitched rasps are often referred to as having the teeth punched at random. That’s a poor choice of words because the teeth are arranged in rows and are as neatly spaced as possible. The small variations in this intricate work result in teeth that are not directly in line with one another.
Almost every tooth in a hand-stitched rasp is working, which leaves behind a scored but uniform surface. With a machine-cut rasp, the first tooth makes a rut and the following teeth deepen that rut. You can remove a lot of wood in a hurry with a machine-cut rasp, but there is a great deal of work left to do to achieve a smooth surface.
The size and spacing of the teeth in a hand-cut rasp are referred to as the “grain” of the tool – the lower the number, the coarser the cut. If you’re using a rasp on what will eventually be a finished surface, you’ll get the best results with a series of two or three grains. Remove most of the material with a 9- or 10-grain, then follow that with a few strokes from an 11- or 12-grain.
The surface can be brought closer to smooth with an even finer rasp, a couple of grains higher. The rasped surface may look rough, but the sharp teeth leave tiny hills and valleys. Finer rasps leave more shallow grooves. After the initial cut, you aren’t working on a solid surface, you’re only shaving off the high spots. A card scraper makes quick work of shaving off the tiny hills left by the rasp. (Yes, there actually is a use for those curved scrapers that came in the set you bought.)
From the June 2013 issue #204
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