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Chisel Sharpening: Maintaining the Right Angle
When your chisel, or nomi, first arrives from the tool maker the angle of the blade will tend to be slightly higher, or less acute, than you may want it to be for optimal use. In other words, it will need to be sharpened. Because sharpening tools, techniques and preferences vary from one woodworker to the next, this is just a basic guideline to use as a starting point.
To find the right angle for your chisel, you will first need to determine three things: the type of chisel, its intended use, and the type, or hardness, of the wood you will be working. Generally speaking, a lower angle is preferable for finer operations and softer woods, while the tougher demands of rough work and/or harder woods require a higher (more blunt) blade angle to better hold its edge.
There are two basic chisel types we will be discussing here:
Push/Paring Chisels (Kinari Nomi, Usu Nomi, Shinogi Nomi, Sashi Nomi and Tsuki Nomi): Either long or short handled, these tools are intended to be used alone, without the aid of a mallet or hammer. Finer shaving, paring and cleaning operations are all done with push chisels. Push chisels require a relatively low (or acute) angle for their cutting edge. We suggest an angle anywhere between 26 and 28 degrees depending on your preference and the other factors (i.e. usage and wood) mentioned above.
Striking Chisels (Oire Nomi, Chu Tataki Nomi and Chu Usu Nomi) and Timber Chisels (Atsu Nomi, Tataki Nomi): As the name implies, these are meant to be used with a hammer to make deep cuts and in the removal of more material, as is required when mortising or cross grain shaving. Because they are required to withstand a bit more abuse between sharpenings, we recommend that a striking chisel have a slightly more blunt (less acute) cutting angle – somewhere between 28 and 30 degrees, depending on your preference and choice of material.
When sharpening your chisels, it is important to always use water stones (natural or synthetic) or a water lubricated grinding wheel to prevent overheating of the tool metal. Do not use a non-water cooled grinder, felt or leather buffing wheel to sharpen or hone your chisels as this may cause overheating (and thus change the temper) of the tool. Neither Suzuki-ya/suzikitool.com or the blacksmiths will be held responsible for damage caused by such misuse.
Typically, you will notice that the flat, bottom face (urasuki) of a chisel has been “hollow ground” - that is, it shows a slight depression along its interior length (on wider blades, more than one “hollow” may be evident). This is to reduce surface area that needs to be worked in the sharpening process. It is very important, however, that during grinding or sharpening, no hollow should be created on the top face (kireba), or cutting surface of the blade as this may cause the cutting edge (kissaki) of the blade to become brittle.