Mill bastard files are a type of hand file widely used in metalworking and other related industries. They are part of the American pattern files, which follow a specific design and cutting characteristics. 
The term "bastard" refers to the coarseness of the file's teeth, indicating that it is neither too rough (coarse) nor too fine (smooth). These files are typically around 8 to 14 inches in length, and their cross-sectional shape resembles a rectangle with two rounded edges and two flat edges. The file's surface is covered with parallel rows of teeth that extend across its length.
Mill bastard files have a medium coarseness, making them suitable for a variety of tasks, including removing material quickly, shaping metal, and achieving smooth finishes. The teeth of the mill bastard file are cut at an angle to the file's axis, forming a diagonal pattern known as the "single cut." This type of cut allows for aggressive material removal and is ideal for rough shaping and fast stock removal. Due to their versatility and efficiency, mill bastard files are commonly used by metalworkers, machinists, and carpenters. 

They are particularly useful for: 

1. Shaping metal: Mill bastard files are excellent for rough shaping tasks, such as rounding edges or forming contours in metal pieces. 

2. Removing burrs and imperfections: After cutting or machining metal, burrs and rough edges may be left behind. Mill bastard files can quickly smooth out these imperfections, creating a clean and polished surface. 

3. Preparing surfaces for further finishing: The medium coarseness of these files makes them an ideal choice for preparing metal surfaces for additional finishing processes, such as sanding or polishing. 

4. Woodworking applications: While primarily designed for metalworking, mill bastard files can also be used in woodworking for shaping and smoothing wood surfaces. 
5. Sharpening tools: Mill bastard files are sometimes used to sharpen certain cutting tools like chisels and hand saws.