The Union Butterfield 4579 high-speed steel left hand spiral flute bridge reamer is a Morse taper shank tool with a black oxide finish, commonly used for reaming interrupted cuts in structural steel. Bridge reamers are tapered to allow the reamer to enter overlapping and misaligned holes where extremely precise tolerances are not necessary. Spiral construction prevents whole-flute contact with the leading edge in an interrupted cut, as in a keyway. This reduces the possibility of damage to the interruption and to 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 common, general-purpose steel for cutting tools and is compatible with a variety of materials. With no coating or surface treatment, this uncoated tool can be used on a broad range of materials and provide an economical alternative to coated tools. Taper shank tools can be self-holding or self-releasing based on the degree of taper: Small tapers are self-holding within a tool holder, while large tapers are self-releasing and will easily release from the tool holder. The shank end of a Morse taper shank tool is self-holding because friction between the tool and the tool holder prevents slippage. It is tapered to ensure accurate alignment of the tool in the socket of the tool holder. Black oxide improves lubricity, resists corrosion, and will not chip or flake; it also helps to prevent buildup and welding of material on the tool’s cutting edge when it is used on iron and steel.
In the Union Butterfield 4579 series, all reamers have three flutes. Tolerances for all reamers in this series are +0.0100” to -0.0000”.
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.