Technical drawing
From Wikipedia, the free encyclopedia
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Technical drawing , also known as drafting, is the "art and practice of creating accurate representations of objects for technical, architectural and engineering needs." This definition includes the various fields and technologies underpinning electronics, which has in turn revolutionized the art with new tools in the form of Computer Aided Design. A practitioner of the craft is known as a draftsman, (or draughtsman in the UK), and recently, "drafter". In some fields, particularly electronics, draftsmen are also known by the ambiguous "designer", who's job would be distinct and separate from the engineers specifying and working out the design details. In short, draftsman are communicators that are part of an engineering team charged with producing specialty documentation packaged as a design, which following the standards of the field, can be understood by others with the same training. Most technical fields, even those not generally considered as highly techical such as plumbing have their own conventions and practices—the symbols and shorthand notations that convey a lot of information in minimal space. All of these are specialized communications. The foundation of the modern computerized world is directly founded on the accurate scale drawings on synthetic vellums that enabled the integrated circuits and printed circuit boards that underly modern electronics, as discussed in the section below Before CAD. The development and progress of the industrial revolution on the whole depends on the art, usually taught and classed as one of the Industrial arts. Engineering and architectural students are introduced to the art, generally in freshman level courses.
Today the mechanics of the drafting task have been greatly accelerated through the use of CADD systems, but regardless of whether a draft is drawn by hand or with computer assistance, the field-use-drawing must be reproducible with a version control system to maintain authorized and approved changes to the master document (or computer files, the modern analog).
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[edit] Manual drafting
The basic drafting procedure is to place a piece of paper (or other material) on a smooth surface with right-angle corners and straight sides—typically a drafting table. A sliding straightedge known as a T-square is then placed on one of the sides, allowing it to be slid across the side of the table, and over the surface of the paper.
"Parallel lines" can be drawn simply by moving the T-square and running a pencil or technical pen along the T-square's edge, but more typically the T-square is used as a tool to hold other devices such as set squares or triangles. In this case the draftsman places one or more triangles of known angles on the T-square—which is itself at right angles to the edge of the table—and can then draw lines at any chosen angle to others on the page. Modern drafting tables (which have by now largely been replaced by CAD stations) come equipped with a parallel rule that is supported on both sides of the table to slide over a large piece of paper. Because it is secured on both sides, lines drawn along the edge are guaranteed to be parallel.
In addition, the draftsperson uses several tools to draw curves and circles. Primary among these are the compasses, used for drawing simple arcs and circles, and the French curve, typically a piece of plastic with complex curves on it. A spline is a rubber coated articulated metal that can be manually bent to most curves.
Drafting templates assist the draftsperson consistently recreate recurring objects in a drawing without having to reproduce the object from scratch every time. This is especially useful when using common symbols; i.e. in the context of stagecraft, a lighting designer will typically draw from the USITT standard library of lighting fixture symbols to indicate the position of a common fixture across multiple positions. Templates are sold commercially by a number of vendors, usually customized to a specific task, but it is also not uncommon for a draftsperson to create their own templates.
| Drawing type "name" |
Dimensions (width X height) |
Drawing type "name" |
Dimensions (width X height) |
|---|---|---|---|
| A-size | 08.5 by 11.0 inches 022 cm by 028 cm |
B-size | 11.0 by 17.0 inches 028 cm by 043 cm |
| C-size | 17.0 by 22.0 inches 043 cm by 056 cm |
D-size | 22.0 by 34.0 inches 056 cm by 086 cm |
| E-size | 34.0 by 44.0 inches 086 cm by 112 cm |
F-size | 44.0 by 68.0 inches 112 cm by 173 cm |
| G-size | 88.0 by 68.0 inches 224 cm by 173 cm |
H-size | 68.0 by 136 inches 173 cm by 345 cm |
| As can be seen in the series, the width of the previous drawing size becomes the height of the next size in the sequence. |
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(Doubled dimension shown in italics in each pairing) The given series |
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This basic drafting system requires an accurate table and constant attention to the positioning of the tools. A common error is to allow the triangles to push the top of the T-square down slightly, thereby throwing off all angles. Even tasks as simple as drawing two angled lines meeting at a point require a number of moves of the T-square and triangles, and in general drafting can be a time consuming process.
A solution to these problems was the introduction of the mechanical "drafting machine", an application of the pantograph (sometimes referred to incorrectly as a "pentagraph" in these situations) which allowed the draftsman to have an accurate right angle at any point on the page quite quickly. These machines often included the ability to change the angle, thereby removing the need for the triangles as well.
In addition to the mastery of the mechanics of drawing lines, arcs and circles (and text) onto a piece of paper—with respect to the detailing of physical objects—the drafting effort requires a thorough understanding of geometry, trigonometry and spatial comprehension, and in all cases demands precision and accuracy, and attention to detail of high order.
Although drafting is sometimes accomplished by a project engineer, architect—or even by shop personnel such as a machinist—skilled drafters (and/or designers) usually accomplish the task and are always in demand to some level.
[edit] Before CAD
Before CAD, master technical drawings were produced on a either paper or vellum within a technical field. There was great need to be able to "pick off a dimension" from a drawing accurately, requiring in turn a need for extremely accurate master drawings. While the blueprint process enabling 1:1 scaled copies dates from the 1840s, large accurate-to-scale drawings were technologically handicapped because quality natural vellums were limited in size and expensive, paper shrinks and expands drastically with humidity, and initial accuracy quickly degraded in the field situation where such documents made or broke a product.
The problem was resolved in part by the invention of a modern imitation "vellum" made from plasticized cotton. Like natural vellum, the synthetic is more dimensionally stable than a linen or paper sheet, which is frequently critical in the development of large scaled drawings and plans such as Blueprints. It was also extremely important in that reproduction technology for dissemination of the plans as like a high quality natural vellum, it could be produced in a thin enough sheet to be virtually transparent to strong light enabling a source drawing to be used directly in the blueprint reproductions of field-used drawings.
During the last century, antedating integrated CAD and modern laser printing which only came about after development of VLSI based microprocessors, synthetic vellums were at the heart of any large engineering or architectural project. "Blueprints" are a copy of such master drawings, and are used as the field and day to day references originally drafted on the vellum masters. Large paper drawings require an additional step (tracing paper amenable to letting light pass through it, and hence is more error prone)
Drafting vellums eventually came to be standardized into a series of drawing sizes known as "A-size", "B-size", ..., "G-size" drawings which doubled in sheet size area with every step. Indeed, VLSI microcircuits themselves were layed out on such vellums layer by layer, "masked" to the dimensions of the given layer (a tracing step of sorts), and those masks photographed, all to scale in very large specialty light boxes. The negatives (known as photomicrographs, photo+micro+graph) thus obtained, were then step-reproduced in carefully aligned arrays and etched onto a glass plate Master of that layer.
In short huge dimensional drawings representing the guts of an integrated circuit were scaled down optically and reproduced to produce each layer of the computer chips which eventually came to be part of the systems which replaced the vellums that made the CAD technology possible. Large scale hand drafted drawings in today's world are unusual and rare, but the old technology still exists and is the foundation upon which the modern computerized world is built. It is still common for engineers and architects to work out the details of a concept, so called "Sketches" on paper drawings before going to CAD. Even in the heyday of hand drafted blueprint technology technical workers found that working with a sketch was an aid to clear thinking.
[edit] CAD
Today, the mechanics of the drafting task have largely been automated and accelerated through the use of Computer Aided Design systems (CAD).
[edit] Projections
Common views of objects include:
- Floor plan (or bird's-eye view)
- Orthographic projection
- Axonometric projection
- Orthogonal projection
- Oblique projection
- Perspective projection, Perspective (graphical)
- Engineering drawing
- Auxiliary view
