What is an engineer's straight edge?
Engineer's straight edges are precision measuring devices used to check that surfaces are straight.
Engineer's straight edges can take several forms; some look like rulers, others like spirit levels, rectangular pieces of stone or I-beam girders. But all will have at least one precision edge that is straight, while some may have two or three.
They are used for checking the flatness of mating machine surfaces, as a guide for making straight lines, and when checking the straightness of other tools. The precision with which straight edges are made means that they can be used to check or calibrate other tools that require a straight edge.
A ruler is essentially just a straight edge with measurement increments marked along its length. However, rulers are rarely made to the same degree of accuracy as engineer's straight edges.
Whilst some engineer's straight edges may be thin like rulers, most have a much wider working surface, allowing them to rest upright on a workpiece without falling flat. Engineer's straight edges also often have two working surfaces parallel to one another, allowing a spirit level to be placed on top.
Straight edges for plastering
Likewise, a straight edge used for plastering will not be made to anywhere near the same degree of accuracy as an engineer's straight edge.
Because of the extra work involved in manufacturing engineer's straight edges to such high levels of accuracy, they cost considerably more than straight edges used for plastering and screeding.
An engineer's straight edge should never be used for plastering or screeding, as doing so is likely to damage the working face, making it useless for precise engineering jobs.
Why use an engineer's straight edge?
The main advantage of using an engineer's straight edge over, say, a ruler or spirit level, is the accuracy that can be achieved. Even grade B (workshop) straight edges will be made to an accuracy far higher than a ruler.
A brief history of the engineer's straight edge...
Engineer's straight edges originated from rulers and other straight edges, which have been in use for centuries, with ivory rulers being used by the Indus valley civilisation in the region that is now north west Afghanistan, Pakistan and India.
Before 1500BC, wooden and stone rulers were used in ancient Egypt. One of the best examples is Egyptian ceremonial rulers, where the graduations were carved into stone.
Straight edges improved in accuracy over time with the discovery of new materials and better methods of manufacturing. Precision engineer's straight edges, however, were only possible with the invention of precision measuring devices such as micrometres and dial gauges.
The industrial revolution (1760-1840) led to a desire for improved accuracy of many tools and this meant engineer's straight edges became important for setting up machine tool ways in order to ensure the machine tools were able to produce the most accurate parts.