The Union Butterfield 4591 high-speed steel left-hand spiral flute taper pin reamer is an uncoated (bright) round shank tool with a square end, used for reaming holes that are intended to receive taper pins. This tool is often used in a drill press, screw machine, or lathe.
A taper pin is a standardized tapered fastener that has a slightly larger diameter at one end than the other so it can be inserted into a reamed, correctly tapered hole, creating a tight connection due to friction. Some taper pins have threaded ends to receive nuts. Spiral flute reamers are useful for interrupted cuts because they prevent whole flute contact with the leading edge of an interrupted cut, as in a keyway. This prevents damage to the interruption and the reamer flute. Left-hand spiral tools are good for use in through holes (extending through the workpiece) because they push chips ahead of the tip of the tool. High-speed steel (HSS) is a general-purpose steel for cutting tools that is compatible with many metalworking materials. It is often lower in price than other cutting tool substrates. Square-end shanks help prevent rotation in the tool holder.
The number of flutes on a reamer depends on the cutting diameter of the tool. Reamers in this series with taper pin sizes 0 to 8 have six flutes and those with taper pin sizes 9 and 10 have eight flutes.
Reamers are used to finish existing holes. When holes are created using common applications such as drilling or punching, they often have minute imperfections. Their roundness may be flawed, or their diameters may be slightly smaller than required. Reamers are made to exacting tolerances, so they can correct such errors and create precisely round holes with exact dimensions. Reaming is often used before tapping screw threads.
Precision manufactures machine tools under the following brands: Precision Twist, Precision Dormer, and Union Butterfield. They have locations around the world, and provide tools adhering to ANSI, NAS, and DIN standards.