There are a number of tools available for shop personnel to evaluate the right angle relationship between two surfaces.
The basic machinist square has a number of variations, the most common being the hardened steel square used to check right angles and set up milling and drilling machines. The hardened steel square consists of a thin blade and a thick beam that are set at precise right angles to each other. The square has no scales and is not useful for linear measurements. To evaluate the right angle, the user holds the thick beam on the reference surface and the blade against the side of the part. The user then looks for light between the blade and the part or slips feeler stock between the two. These types of squares are usually used on work where tolerances of 0.001 inch are called out.
These so-called “squares” are not really squares, and they are actually used to check the right angle relationship between the two surfaces, known as perpendicularity. Most prints have a call-out requiring a right angle relationship, but one right angle does not make a square. There may also be a call-out for a part to be square, but this refers to the geometric shape of the part. It may be that the reason for calling this tool a “square” is that it’s too difficult to say, “Hand me that perpendicular.” Because this language is used in the shop, we can talk about inspecting for the right angle as a squareness check.
A cylindrical square can be used in a similar manner. By placing the cylinder next to the part and using the same visual or feeler stock check, the operator can get a good sense of right angle.
There are a number of other hand tools used to inspect for the square form in a part, including combination squares, linear and digital protractors, and even electronic levels. But if parts are more than 8 inches in length, hand tools cannot cover the range needed, and surface plate tools are required. In addition, hand tool methods all rely on the observation and skill of the operator to interpret the angle. None provide any empirical data that can be analyzed or used to begin controlling the process.
For large part/surface plate work, one of the best means to inspect for squareness is to use the precise vertical ways that are built into a height gage or master squareness gage. Both have a precision slide to which a dial or test indicator can be mounted. This allows the indicator to be moved in an accurate, vertical line of travel when both the gage and part are on the surface plate. The advantage of this type of squareness gage over handheld, visual squares is that the dial test indicator allows the operator to read the exact amount of error instead of eyeballing it.
Because both the master squareness and height gages use the same reference surface as the part (such as the surface plate), and because the gages themselves provide a precise reference for the vertical axis, both are capable of measuring the perpendicularity of the side compared to the base. On the downside, master squareness gages are not capable of measuring the horizontal axis of the part, and in neither case are the vertical and horizontal readings tied together, so the user has to plot the individual values to come up with a measurement.
Those really interested in data collection will want to use a motorized electronic height gage. These gages not only allow automated positioning but also have the capability of using a high-resolution linear encoder for positioning the indicator along the vertical axis and a similar linear scale for the horizontal axis. Most electronic height gages have dedicated, preprogrammed functions for checking perpendicularity. The operator just enters the length of the path to be inspected and the number of readings to be taken along that path. The slide can be positioned manually along the path (as with the master squareness gage) to let the gage controller collect the data, or the gage can execute an automated data measurement routine.
Once the measurement cycle is completed, the processor can provide the actual angle measured, the full table of the test part or even a graph of the part profile. This can be invaluable for large parts where lapping can be performed to fix demonstrated high or low spots. The plate is the reference for both the part and the height gage; therefore, a clean plate and a high degree of flatness are essential when making precision checks.
In sum, with the handheld, machinist square measurements, operator influence and visual techniques limit the process to 0.001-inch tolerance levels. Height gages with high performance digital encoders, long-range measurements, automated gaging routines and computing capabilities can bring surface plate measurements to levels of 0.00005 inch or better.