Chippers are used to chip sticks and branches or mulch leaves. There are usually two types: chippers and chipper-shredders. There are different blade set-ups, but both are characterized by a rotating disk or drum covered with pivoting knives in a flail or shredder pattern. These knives can usually be removed when they are ready to be sharpened. Some chippers even have double-sided knives that can be switched around and used twice before sharpening.
When to Sharpen
The ideal shredder or chipper comes with knives that are easy to remove; they have quality steel edges that can be sharpened multiple times before being replaced. You should always know how many time the knives can be re-used before complete replacement, and what the best type of replacement knife may be in case one of the knives should break or notch during the sharpening process. Some manufacturers help out by including a sharpening device along with the chipper.
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The primary way you can tell if the blades need sharpening is by the quality of the chips that the device produces. Generally the duller the blades, the larger the chips will be and the more they will start to resemble chunks instead chips. Many chippers are actually designed so that they will slow down or stop when the blades need to be sharpened. As a rule of thumb, blades often need to be sharpened after six to eight hours of use.
How to Sharpen
If you decide to sharpen the blades yourself (instead of taking them to a professional) then make sure you have the right equipment. Since chipper blades are made out of high-quality steel, a normal grinder or grinding belt can be dangerous to use. If the knife becomes too hot under these devices, then the heating process used to make the steel and original edge can be affected, causing a serious decline in knife quality. Some sharpeners use a professional grinder and simply take frequent pauses to ensure that the blade stays cool. Otherwise, you can use a wet wheel. If you want to sharpen by hand, you may want to use a diamond whetstone to speed up the process.
The chipper knives should only be sharpened down to about one inch of their original size. This is a common rule among machinery-based knives, and ensures that the knives remain as efficient as possible. When originally designed, the edge of the knife and the tempered steel used to make them only went down about an inch, so trying to sharpen past an inch down will do no good and will probably cause the knife to break. If you have any doubts, take the knives to a professional.