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A hollow chisel is a woodworking tool for cutting single square holes in timber or, if used in an overlapping pattern, rectangular mortises. The tool consists of a hollow, square chisel with a rotating drill bit at its center. The tool is used in a dedicated mortiser machine or in a standard drill press with a special adapter. Downward force on the tool causes the chisel to cut out a square shape while the rotating drill bit clears all material from within the four sides of the cut through a port in the chisel body. Hollow chisels are available in a wide selection of single sizes or in sets.
Drilling round holes into timber is a fairly easy operation. Square holes, on the other hand, require special tools and techniques to achieve. Square openings or slots in timber are commonly known as a mortise and are used extensively to fit door hinges and locks and in the construction of mortise and tenon joints. These mortises can be cut out with a conventional hand chisel or a specialist tool known as a hollow chisel. The hollow chisel is basically a combination power tool bit used in a mortiser machine or, when paired with an adapter, in a regular drill press.
The hollow chisel consists of a steel tube of square cross section that has a scalloped cutting edge ground into its working end representing the chisel component. A flat edged rotary drill bit is positioned in the center of the hollow chisel with its cutting edge positioned slightly back from that of the chisel. During cutting operations, the square chisel body is clamped in a static position while the drill bit rotates within it. Downward force on the tool causes the sharp chisel to cut a square hole while the drill bit clears all the internal material out of the cut. This waste material is carried up the drill shaft by its flutes and is ejected from the tool via a port cut into one side of the chisel body.
This tool may be utilized to cut a single square hole or used in an overlapping pattern to cut longer or wider slots. The hollow chisel is made of hardened tool steel which holds a sharp edge well. It is available in a large selection of sizes either as a single item or as sets consisting of several consecutive or common sizes. The cutting edge on a hollow chisel is extremely sharp, and care should be taken when handling or storing it to avoid accidental contact with the edge.