What is a Mill File?

Parker Brown

A mill file is a tool used in both wood and metalworking applications that removes small pieces of material from a workpiece—they are the most common type of file. At one point, mill files served a very similar purpose to a milling machine, mainly being used to finish the surfaces of parts. Since the development of the milling machine, mill files are no longer widely used for surfacing purposes. Instead, they are typically used for small finishing jobs, most commonly deburring a workpiece. Files in general coming in a wide variety of sizes, shapes and materials for different jobs.

Triangular files, also called three-square files, are specialized tools that have three sides used for trimming and sharpening edges.
Triangular files, also called three-square files, are specialized tools that have three sides used for trimming and sharpening edges.

These variances in file type are necessary to increase their use, as application can vary greatly. For example, small, narrow mill files with finer tooth arrangements are used for very precise finishing jobs. On the other hand, larger mill files with wider tooth arrangements are used for larger finishing jobs, such as surfacing a metal plate or the end of a block of wood.

Files vary in cross section, which allows them to perform more specialized tasks where a specific shape is required. Cross sections are most commonly flat, but can be round, triangular, or square. Sometimes, a file will be ground to a certain shape in accordance with a specific job. Mill files, however, do not have specialized or unique cross sections. Instead, their cross sections are rectangular in shape.

A mill file can also vary in tooth configuration to suit different cutting applications. The tooth configuration is known as the cut of the file, which refers to how fine the teeth are. Cut sizes are identified by name, which from smoothest to roughest, include dead smooth, smooth, and second cut. Bastard, middle, and rough come after. Dead smooth files are often used for very small jobs which require precision, while rough files are used for larger surfacing processes.

Much like the cut, teeth configuration can also greatly between files. The most commonly-found cut configuration is known as a single-cut, where the teeth are a single series of parallel lines. A double-cut, on the other hand, consists of two different sets of lines that are perpendicular to each other. Such an arrangement forms what are known as diamond teeth, which can create very well-balanced cuts. While a mill file typically has teeth on either side, this is not always the case; sometimes, one side will be left blank, which can help prevent unnecessary cutting in compact spaces.

Finishing work is not the only job that a mill file can perform. Mile files are also widely used for sharpening applications, mainly for blades found on circular and miter saws. They can also be used to shorten the end of a piece of metal or wood after being cut. Besides this, they are occasionally used for finishing the edges of glass and may perform other jobs where finishing work is required.

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