How to Use a Square Head Protractor

The square head protractor is a practical tools for builders, architects, machinists and draftsman.

A square head protractor most commonly refers to a specific type of measuring device employed by a draftsman or architect to measure an existing angle on paper. It is a handy tool for transferring or reading angles on blueprints, mechanical drawings or building plans. The flat, adjustable tool can also be used in the field to record numerical values for an existing angle. In certain situations (obtuse angles) it can be used like a bevel gauge.

Things You'll Need

• Square head protractor
• Pencil
• Blueprints or building plans

Instructions

1. Loosen the adjusting nut that holds the swing arm tight to the plate. The long arm should now slide back and forth.

2. Pull out a blueprint, mechanical drawing or building plan and locate any angle. To get an accurate value for the angle, place the bottom edge of the rectangular plate on one side of the angle and the long swing arm on the opposite side. Make sure both edges of the protractor line up precisely with each line. Then, make the numerical reading. It will be between 0 and 180 degrees.

3. Tighten the nut where the long arm joins the rectangular plate.

4. Remove the square from the spot on the drawing and transfer the angle to a board, piece of metal or another part of the drawing.

Tips & Warnings

• A D-head protractor is similar to a square head protractor, except it looks like one of the protractors used in high school geometry class. Both square head and D-head protractors consist of a long arm attached to a flat plate inscribed with accurate markings that run from 0 to 180 degrees. High quality square head protractors are sometimes made from stainless steel. A square head protractor can be used with accuracy to measure the angles of outside corners.

• Combination squares have a protractor device that can slide onto the flat bar of the tool. This tool is handy on the job site, but may not be of much use to the architect or draftsman who has to draw out the plans. Be sure to read the correct angle. Because the dial reads on a scale of 0 to 180 degrees, it is possible to record the value for an obtuse angle (greater than 90 degrees), when you are actually measuring an acute angle (less than 90 degrees).

References

• General Tools
• VictorMachinery Exchange