How are files made?

29th February 2020


	How are files made?


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			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.

			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
			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’.

			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
			Annealing is a process through which steel is softened so that it is easier to work with.

			The file blank is heated until it turns a deep red colour, and then left to cool at room temperature.

			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
			At this point, teeth are cut into the file at regular intervals using a chisel.

			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?

			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.

			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).

			These files are usually cut with fine teeth, and used to produce a very smooth finish.

			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
			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.

			The file is heated once again.

			Once it has reached the desired temperature, it is plunged into a large bath of brine and cooled rapidly

			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.

			This process is repeated multiple times to make sure the steel is hard enough to be used as an abrasive.

			Softening the tang
			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.

			As the tang of the file is thinner than the rest of the body, it is a potential weak point.

			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.

			Files that have undergone this part of the process are sometimes referred to as being ‘variable heat-treated’.