Square machinist

The machinist square, a precise right-angle tool, is essential in metalworking and woodworking for verifying perpendicularity.

About this subject

The machinist square, also known as a steel square or try square, is a precision measuring tool used primarily in metalworking and fine woodworking to check right angles. Unlike common squares, it is made from hardened and ground tool steel to ensure accuracy on both inside and outside faces. Its main function is to verify a 90-degree angle between two surfaces, such as machine guides, machined parts, or assemblies. The blade is usually thin with a beveled edge to fit into tight spaces, while the stock (or head) provides a stable reference.

Historically, the machinist square became essential during the Industrial Revolution as interchangeable parts and mechanical precision gained importance. Manufacturers like Starrett, Mitutoyo, and Brown & Sharpe set quality standards for these tools. Advanced models include squares with adjustable blades for variable angles (like the universal square) or with graduated scales. In workshops, the square is used alongside height gauges, dial indicators, and gauge blocks.

Notably, the accuracy of a machinist square is specified by the maximum perpendicularity error, which for Class A (high precision) can be less than 0.02 mm per 100 mm length. Calibration is performed against a flat reference surface, such as a granite surface plate. Some squares have a beveled back to allow scribing perpendicular lines or checking chamfers. Proper care includes avoiding drops and keeping faces clean, as any particle can compromise measurement accuracy.

Frequently Asked Questions

What is the difference between a machinist square and a try square?

In modern usage, 'machinist square' and 'try square' are often used interchangeably for precision squares. However, a try square traditionally refers to a woodworking square with a brass strip, while a machinist square is all steel and made to tighter tolerances.

How to calibrate a machinist square?

Place the square on a flat surface (like a granite surface plate) and check the blade against a known right-angle standard or use a dial indicator to measure deviation. Adjustment requires grinding the stock or blade face.

What is the purpose of the bevel on a machinist square blade?

The bevel along the blade edge allows the square to fit into internal corners, such as slots or guideways, without touching burrs. It also aids in scribing accurate perpendicular lines.