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What is a three square file?

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Image of a DIYer using a three square file to smooth the inside of a shape cut into a thin piece of wood Three square files are machinist’s files. Their triangular shape makes them perfect for shaping and deburring internal angles and corners.
Image of a DIYer starting the squaring process by cutting a triangular notch with a three square file They are used to start the process of turning round drill holes into square slots.

For a step by step guide, see: How to square a round hole with files

Confusion with taper saw files

A taper saw file which is not to be confused with a three square file Taper saw files have the same cross section as three square files, but have more of a taper. It’s important not to confuse the two! Taper saws are single cut and more suited to sharpening than three square files.

For more information, see: What are taper saw and double ended saw files?

What are the characteristics of three square files?

Diagram of a triangular cross section

Cross section

Three square files, also called ‘triangular files’, ‘three corner files’ or ‘three quarter files’, are triangular in cross section.

A triangular groove cut by a three square file As their cross section is an equilateral triangle, they can be relied on to create notches at an accurate 60° angle.
Illustration of an equilateral triangle with 60° angles in all corners The cross-sectional shape of this file earned it its name: “three square” means a shape with three sides of equal length, which perfectly describes an equilateral triangle.
Image showing a three square file's outline and cut

Profile and cut

They are tapered in width and thickness, which allows them to fit into tight spaces. They are double cut on all three faces, and single cut on all three edges.

An indication of the range of lengths in which three square files are usually available


Three square files are usually available in lengths from 100mm (4 inches) to 350mm (14 inches).

American and Swiss flags, illustrating that this type of file is made in either American or Swiss pattern

Swiss or American?

Three square files can be either Swiss or American pattern.

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