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How are files made?

How are files made?

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Illustration of the way a chisel is used to cut teeth into a file The basic principle of file-making is to cut teeth into a metal strip to provide a rough tool that can wear away material from a softer surface.
Image of a machine used to chisel teeth into a file While files were produced by hand for hundreds of years, they can now also be mass produced by machine. Either process follows the method below.

Creating the blank

Image of a blacksmith forging a piece of metal to the right size and shape to later be turned into a file The first step in the file-making process is to create a strip of metal that is roughly the right shape and size of the finished file. This is called the ‘blank’.
A file blank being forged into shape by the use of a power hammer To achieve this result, steel can be forged, melted and poured into a die to set, or pressed between two heavy rollers and then cut to shape.

Annealing the file

Image of a piece of steel being heated so that it can anneal, or cool slowly at room temperature. This makes the steel softer so that it is easier to work with Annealing is a process through which steel is softened so that it is easier to work with.
File blanks that have been heated to red hot and then left to cool at room temperature so that they will be in a soft state and ready to have their teeth cut The file blank is heated until it turns a deep red colour, and then left to cool at room temperature.
Image of a file blank being ground back into shape after the annealing process is complete As heating the metal blank can cause it to deform, it is ground or filed back into shape once it has cooled.

Cutting teeth into the file

Image of a cold chisel, which is used to cut the teeth into the face of a file At this point, teeth are cut into the file at regular intervals using a chisel.
Close up of a file's teeth showing the 40-55° angle The angle of the teeth is usually around 40-55 degrees to the file’s surface, depending on the type of pattern being cut onto the file. This angle is referred to as the ‘rake’ of the file.

For more information, see: What is a file’s cut?

File teeth that have broken because they were cut at too deep an angle to the body of the file
If the angle of the teeth is too narrow, they are more likely to get lodged in the surface of the workpiece. If the angle is too wide, they are more likely to break and snap off the body of the file.
Illustration of what negative rake looks like on file teeth Some files can be made with a negative rake, meaning that the teeth actually point away from the workpiece rather than towards it.

In this case, the teeth do not cut material away, but instead scrape across the surface, scratching off any irregular lumps (high spots) and forcing the offcut material into any tiny dents (low spots).

A DIYer checking the finish on a piece of metal that they have filed with a smooth file These files are usually cut with fine teeth, and used to produce a very smooth finish.
A blacksmith cutting teeth into a rasp face with a triangular punch. This process is known as stitching.

Cutting rasp teeth

Rasp teeth are made using a triangular punch, which cuts each tooth individually.

For more information on rasps, see: What is a rasp?

Tempering the file

Image of the first part of the tempering process for steel, which is to heat the metal until it is red hot Once the teeth have been cut, the file needs to be tempered, or hardened, so that it can cut through other materials without taking damage.
A file that has been heated in the first part of the tempering process The file is heated once again.
A file blank being plunged into a bath of brine to cool it rapidly Once it has reached the desired temperature, it is plunged into a large bath of brine and cooled rapidly
Image of a tightly packed molecular structure illustrating the effects of water hardening on steel Rapid cooling like this causes the grain in the steel’s molecular structure to become finer, which makes it harder and gives it a greater tensile strength.
Repeat the process This process is repeated multiple times to make sure the steel is hard enough to be used as an abrasive.

Softening the tang

A file that has broken due to a sudden impact One side effect of the tempering process is that it can cause the steel to become brittle, making it more likely to shear or snap if dropped.
Close up of the tang of a file As the tang of the file is thinner than the rest of the body, it is a potential weak point.
Image to illustrate that the tang must be heated again and allowed to anneal so that it is soft enough to withstand the pressures of being inserted into a handle As such, once the rest of the heat treatment has been completed, the tang is heated again and allowed to cool at room temperature. This softens the tang once again, making it less brittle and more resistant to damage.
Thermometers at hot and cold temperatures illustrating variable heat treatment Files that have undergone this part of the process are sometimes referred to as being ‘variable heat-treated’.

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