The compas is a measuring instrument, used as a symbol, since time immemorial. There are paintings that locate it more than 5,000 years ago, although this date surely does not indicate the creation of this valuable tool that remains valid to this day.
Used by construction professionals, architects, textile artisans and almost in any application in which it is necessary to find the center or transport the measurements from a pattern which they maintain without graduation. They are particularly useful for measuring distances between surfaces, on surfaces or also for comparing measurements based on a pattern, such as graduated rulers. They are eventually used when inspecting parts on lathes, compasses should never be used while the part is turning since the readings will be inaccurate and illusory, but even more importantly, there is a danger that the tool will be torn from the hands of the operator.
When using the compass to take a proportion or measurement, it is important that the contact with the piece is very soft but firm. One way to check if we are doing it correctly is by using the sense of touch to verify that the pressure can be felt at the tips of the fingers.
To transform the proportion into a measure with numerical indication, the correct thing is to use a graduated ruler and make the ends coincide with the graduated scale. In this way we will obtain the correct measure of the piece. Measurements made by this means can be performed with very little risk of error, if the operator has the necessary skill and care.
Although there are different classifications for measures, we will propose one below that can serve as a guide.
Compasses with Flat Arms
Used for both interior and exterior measurements, they include a spring in the arch and an adjustable nut by means of a screw to regulate the pressure on the arms and thus achieve the desired position.
They are used to obtain measurements between lines or points; to transport measurements taken from a steel ruler, and to draw circles or arcs. The tips are winged and the parallel arms allow measurements to be made by visual comparison rather than by touch. Point compasses are limited in their reach due to the opening of their arms, and become less efficient in strokes and similar applications when the points are decidedly inclined in relation to the surface being drawn.
Combining a straight tip with a curve, they are often used to draw parallel lines from an edge or to locate the center of a cylindrical piece.
There are also other ways to classify the measures, as we will see in the figure below, into inner and outer measures. That is, according to the type of measurement to be carried out.
When using any measuring instrument it is important to remember that it is impossible to achieve a perfect measurement, however, tolerances and limits must be respected; understanding tolerance as the error or permissible variation in a given measure and limit as the largest and smallest measure that can exist in a piece.