Fig. 1461.
[Fig. 1461] represents the T-square, whose blade is some distance from the end of the back and is sometimes placed in the middle. When the square edges are at a true right angle the square is said to be true or square, the latter being a technical term meaning at practically a true right angle.
The machinists’ square is in fact a gauge whereby to test if one face stands at a right angle to another. It is applied by holding one edge firmly and fairly bedded against the work, while the other edge is brought to touch at some part against the face to be tested.
If in applying a square it be pressed firmly into the corner of the work, any error in the latter is apt to escape observation, because the square will tilt and the error be divided between the two surfaces tested. To avoid this the back should be pressed firmly against one surface of the work and the square edge then brought down or up to just touch the work, which it will do at one end only if the work surface is out of square or not at a right angle to the face to which the square back is applied.
Fig. 1462.
An application of the T-square is shown in [Fig. 1462], in which w is a piece of work requiring to have the face a of the jaw c at a right angle to the face b c. Sometimes the L-square is employed in conjunction with a straight-edge in place of the T-square. This is usually done in cases where the faces against which the square rests are so far apart as to require a larger T-square than is at hand. It is obvious that if the face a of the work is the one to be tested, the edge b is the part pressed to the work; or per contra, if b c is the face to be tested, the edge of the blade is pressed to the work.