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Anybody used Dremel effectively to sharpen knives and things like chisels and scrapers? What kind of bits are good for that application?

Also, what do you call an electromotor that is clamped to a table (like a vise) and has a grinding/sharpening wheel? I was thinking of getting one of those if Dremel (with an adequate bit) won't do.

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The motorized grinder you are refering to is a "bench grinder". They will come in a variety if sizes. Most homeowners use a 7 or 8 inch model. You can purchase accessories like a wire brush or a buffing pad. Most models will have shafts extending from both sides of the motor. This allows for having a fine and coarse stone installed at the same time. I prefer the bench grinder over a Dremel as the bench model has a "toolrest" built in which allows for more precise grinding. For sharpening expensive wood chisels and gouges I prefer a hand stone and a jig that sets the tool at the precise angle for sharpening.

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First item in sharpening things. When you're sharpening chisels and knives, you don't remove that much metal that you require powered implements. Manual movement on broad, flat stones is sufficient.

Second item in sharpening things. Using a powered grinding implement will destroy any edged tool by heat buildup. A very light touch is needed, once you've exceeded the temper temperature, toss the tool.

A dual sided carborundum water stone will fill the bill for most people. The coarse side takes out the nicks (sign you're maltreating your edged tools) and the fine side puts the honed edge on the implement. Get some polishing compound and a piece of leather to make a strop if you want a really fine edge, it takes the burr off that forms during sharpening.

This is one of those situations where the beauty of having a tool spinning a grinding disk at 10,000 rpm will quickly cause the need for reacquiring everything you attempt to sharpen with it. Any heat buildup takes the temper out of the edge, ruining the tool. Typically, you don't have enough surface area to keep the edge straight either which destroys the straight edge needed on chisels and plane irons. They appreciate the broad, flat surface of any number of sharpening stones.

Same thing with axes, I've seen them destroyed with a common bench grinder, any time the metal turns blue, you've removed the hardness required to keep the edge, at which point, there's no point trying to sharpen it. Best to use a mill file for first sharpening and one of those Norton Carborundum handled file stones for the sharp edge. These tools work best for axes and soil implements.

Start with the manual methods, learn control and what a properly sharp edge for that tool looks like. Once you've graduated up to where you're sharpening lawn mower blades and other blades (such as brush hog blades) that get driven into media under mechanical power, then start considering using a bench grinder. You need to remove larger amounts of metal to restore the edge, you also need to learn now to take a reasonable amount of material off so you don't overheat the edge and ruin its temper.

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An additional note to add to the good answers already here:

I've worked as a carpenter (forming, framing and finishing) some, and love sharp edges. I've never used a Dremel to sharpen; it is too small in comparison to most blades that I use. However, for a new tool or one with a badly dulled edge, I often use a "bench grinder".

For my convenience, I like to divide the sharpening process in my mind into three parts: grinding, sharpening and stropping.

Grinding

Grinding is where the tool is given its general shape and the cross section of the edge is decided. A cutting edge should have as small of an angle as it can while still keeping its strength. This depends on the material to be cut; a kitchen knife used for slicing tomatoes will have a smaller angle between its two faces than an axe used for splitting firewood. This grinding and shaping of the edge can be done, as Fiasco's answer suggests, by hand with a file. This avoids the danger of over-heating. However, in most shops and factories, it is done using a powered grinder with a spinning stone like the bench grinder. If you use a bench grinder freehand, it takes patience and practice, practice, practice to learn the right angle and light pressure to hold your tool against the stone. In lighter tools like knives, chisels and plane blades, the overheating danger is much greater, and you only need to overheat the blade once to ruin it. Also, you can quickly remove too much material from the edge, and reduce the lifetime of the tool. Or you can accidentally grind too much off of the corner of a plane blade, and need to tediously grind it straight again. I repeat again from Fiasco: when grinding your tool or shaping the edge with a powered grinder, use a very light touch. However, in the case of a badly damaged edge, using only a flat stone could take a very long time to restore the edge geometry, and careful use of the bench grinder can help reduce that time. The bench grinder does not finish the edge for use; the resulting edge is uneven and has aggressive burrs that you can feel (if you touch the edge, touch softly and move your finger across the edge, not along it!) and often even see.

Sharpening

Sharpening is where the edge itself is prepared for use. The edge of a cutting tool is where its thickness is reduced gradually to almost nothing (almost because it still needs strength at the very edge, and where there is no material left it cannot give strength!). A general rule that I use is that if a tool makes sparks, it is too hot for sharpening. I prefer a flat stone like this:

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Stones like these can be found in hardware stores all over the Americas, and probably wherever else people use knives. They are usually made of resin-bonded aluminum oxide or silicone carbide grits. You can use them with water or oil as lubricant, or dry. An important advantage of these stones over the Dremel grinder (beside not endangering the temper) is that they are wide and long enough to help you keep the angle of your tool with the stone. I often use finer stones after the one pictured, but this is good enough for many uses.

Stropping

Sometimes a third step is included in sharpening: stropping or "polishing" the edge. This is traditionally done using a leather strap and very fine abrasive powder, removing the last invisible burrs left by whatever stone was used in sharpening and leaving a mirror finish on the "cheeks" of the edge.

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Trying to "free hand" a Dremel to sharpen knives, blades and tools is not a very effective way and works lousy. I tried this once trying to put a better edge on a round point dirt shovel and even that was a lousy method. For the shovel I found that a large hand file worked so so much better.

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    I have a 10 inch double-cut bastard file I use for shovel sharpening. It speedily removes metal for quick sharpening and leaves a slightly serrate edge that cuts through tree roots quite nicely. If I want a smoother finish, a Silicon Carbide Norton Utility File (sharpening stone with a handle) works wonders for the finishing pass or, once you've got the initial sharpening done, touchup. Commented Jan 6, 2013 at 6:02
  • And for those wondering, yes, "bastard file" is a legitimate technical term for a type of file.
    – keshlam
    Commented Mar 11, 2023 at 17:44

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