The bite is one of the essential qualities of a rasp. It is its ability to bite the wood, that is to say, to remove material without having to exert excessive pressure. The tooth quality is the result of many factors:
- the surface of the blank prior to stitching must be perfectly smooth, otherwise any surface defects prior to stitching may end up being on the tip of the tooth after stitching.
- the sharpness of edge of each tooth.
- the correct orientation of the teeth,
- the correct shape of the teeth
The last 3 points are directly depending on the skills of the « stitcher » that hand-cuts (or hand-stitches) the teeth.
For pity’s sake, no acid! Of course, if very well controlled, submersing a rasp in an acid bath can help to clean the grooves without attacking (too much) the edges of the teeth. But it is showing very little respect for your tool. The best solution is frequent brushing with a brass brush.
Grain of barley
This verb is used to describe a rasp that works fitfully or jerkily. This is due either to the poor quality of the teeth, or because the rasp has clogged. That is to say that the teeth are filled with chips, which takes away their bite. Rasp juddering, in addition to the fact that it is inefficient and very uncomfortable to use, gives a poor quality surface finish.
The length of a rasp is measured from the shoulders to the tip. For example, an 8″ rasp would be approx. 11″ long if you measure it including the 3″ handle.
The arrangement of the teeth of a hand-stitched rasp is called « random ». This is a misnomer, as in production the teeth are stitched in rows. However because the teeth are individually stitched, the end result is that the teeth are never perfectly aligned. You can try but it’s simply impossible to align them perfectly. But that’s a good thing because this slight variation greatly enhances the smoothness of the resulting surface.
Whereas rasp-making machines always strictly align the teeth : if you look at a machine-made rasp from different angles, there are usually at least three different angles in which the teeth are strictly aligned, much like a newly planted forest (at least in France, I don’t know about other countries :-)). Because the teeth in a machine made rasp are perfectly aligned, the rasp can produce that has streaks or lines on the finished surface.
Rasp (just in case…)
A rasp has teeth and is made to shape wood or soft materials (aluminium, lead, plastics, resins,…), whereas files are designed for metalworking.
Right-handed / Left-handed
Our rasps, because they are made to order, have teeth slightly oriented on one side or the other, depending on the hand of their future user : to the right for a right-handed user, to the left for left-handed people. This allows both an increase in the bite of the rasp and improved chip clearance to prevent the teeth from clogging. Machine-made rasps are not handed.
Apart from the « square » (as the name suggests, an area that begins above the tang that has the height done equal to the width of the rasp), a good rasp is stitched all over, including the edges, sides (except for specific models) and until the tip. In addition to the tip problems mentioned below (see Tapered), rasps mass-made by machines have always a few millimeters without teeth on the edges, which prevents them to work in the corners.
Store your rasp in the rigid box that is provided with the tool. This will protect the tool when it is stored in your tool box or your drawer, and prevent the teeth accidentally colliding with other hardened tools which could damage the teeth. From time to time, before storing your tool, oil or grease the surface ; or wrap it in an oiled rag. This will help to protect it against rust.
A rasp is called « tapered » when its section is being progressively reduced till the very tip. All rasps made in our workshop are tapered (Cabinet Makers rasps, Modellers rasps, Half-round, Rat Tail, Square, etc.). Hand-stitching a rasp allows for a tapering till the tip. This tip, however, essential for precision work, is missing on machine-made rasps (ie 99.9% of rasps in circulation). Indeed, they are truncated because the machines cannot raise the teeth in the narrow and often strongly rounded surfaces of the tips.
The teeth are numbered according to their size, ie the number of teeth per square inch. No. 1 corresponds to the largest (coarsest) teeth, No. 15 the finest. The finest teeth of machine-made rasp are the sizes of No. 5 in size. Only the hand of a man can producer smaller (and much sharper) teeth : click on the picture to enlarge it.