CAD Drawing Standards

       Drawings standards help Industrial Designers and Engineers create a seamless transition form design to prototype to manufacturing.  These standards, forms, unit scale and size of drawings, views dimensioning and tolerancing are used through out the product development and CAD-CAE-CAM industry.  Typically drawing plans are created from hand drawn, pencil or technical pen on vellum or mylar or printed on paper and take the form of 3D CAD models or an electronic digital file .dwg .dxf or pdf.   These drawing import and export formatting standards help communicate a design and enough information to allow a manufacturer to build or fabricate shape, cast or mold the product. With 3D CAD files there is no misinterpretation of the drawings or design intent, you get what you deliver.

See  ID Design Control Drawings
See  ME  Engineering CAD Drawings
See  CAD-CAE-CAM

• Drawing – Formatting
             Scale & Units
                      United States and Imperial units
                      Metric units
             Scale
                      Architect's & Engineers scale 
                      Choice of scale

• Views and Projections

• Multiple views and projections

• Showing dimensions

• Geometric Dimensioning and Tolerancing

• Notes

• Sizes of drawings
             Orthographic projection
             Pictorials
             Perspective

• CAD File Formats

Drawing - Formatting

Plans are often prepared in a set. The set includes all the information required for the purpose of the set, and may exclude views or projections which are unnecessary. A set of plans can be on standard office-sized paper or on large sheets. It can be stapled, folded or rolled as required. Blueprints were formerly printed using a chemical-printing process that yielded graphics on blue-colored paper or, alternatively, of blue-lines on white paper, have been superseded by more modern reproduction 4 color processes that yield multi-color coded lines, shading and textures on white paper.

Scale & Units

United States and Imperial units  In the United States, and prior to metrification in Britain, Canada and Australia, architect's scales are/were marked as a ratio of x inches-to-the-foot (typically written as x"=1'-0"). For example one inch measured from a drawing with a scale of "one-inch-to-the-foot" is equivalent to one foot in the real world (a scale of 1:12) whereas one inch measured from a drawing with a scale of "two-inches-to-the-foot" is equivalent to six inches in the real world (a scale of 1:6).  Typical scales used in the United States is full scale, with inches divided into sixteenths of an inch

The following scales are generally grouped in pairs using the same dual-numbered index line:

• three-inches-to-the-foot (3"=1'-0") (ratio equivalent 1:4)/one-and-one-half-inch-to-the-foot (1-1/2"=1'-0") (1:8)

• one-inch-to-the-foot (1"=1'-0") (1:12)/one-half-inch-to-the-foot (1/2"=1'-0") (1:24)

• three-quarters-inch-to-the-foot (3/4"=1'-0") (1:16)/three-eighths-inch-to-the-foot (3/8"=1'-0") (1:32)

• one-quarter-inch-to-the-foot (1/4"=1'-0") (1:48)/one-eighth-inch-to-the-foot (1/8"=1'-0") (1:96)

• three-sixteenths-inch-to-the-foot (3/16"=1'-0") (1:64)/three-thirty-seconds-inch-to-the-foot (3/32"=1'0") (1:128)
   

Metric units

Architect's and engineering scales used in Britain and other metric areas are marked with ratios without reference to a base unit. Therefore a drawing will indicate both its scale and the unit of measurement being used.  In Britain the standard units used on architectural drawings are the SI units millimeters (mm) and meters (m), whereas in France centimeters (cm) and meters are most often used. In Britain, scales often found on architect's scales are:

• 1:1/1:10

• 1:2/1:20

• 1:5/1:50

• 1:100/1:200

• 1:500/1:1000

• 1:1250/1:2500

Scale

Plans are usually scale drawings, meaning that the plans are drawn at specific ratio relative to the actual size of the place or object. Various scales may be used for different drawings in a set. For example, a floor plan may be drawn at 1:50 (or 1/4"=1'-0") whereas a detailed view may be drawn at 1:25 (or 1/2"=1'-0"). Site plans are often drawn at 1:200 or 1:100.

Architect's & Engineers Scale 

An Architectural scale 1/4" = 1'-0" is US based fractions of an inch based on increments to foot  An architect's scale is a specialized ruler. It is used in making or measuring from reduced scale drawings, such as blueprints and floor plans . It is marked with a range of calibrated scales (ratios).  A device for drawing straight lines is a ruler.  In common usage both are referred to as a ruler not to be confused with a weighting scale.

An Engineering scale defines scale based on the U.S. inch or EU meter divided into tens or thousands, increments to the inch/foot or meter (cm/mm)  One half scale 0.50 = 1.0  or one tenth scale  0.10 = 1.0  or one tenth =  one foot/meter or one-tenth size"  The scale was driven from mass production when machined parts and components and manufacturing equipment required a greater precision to accommodate tolerance fits, for example, in laying out printed circuit boards with the spacing of leads from integrated circuit chips as one-tenth of an inch.  The engineering scale is the standard used in industrial design, mechanical and manufacturing engineering to define, design and measure parts to see if they meet specifications.

In Canada and the United States, this scale is divided into decimalized fractions of an inch, but has a cross-section like an equilateral triangle, which enables the scale to have six edges indexed for measurement. One edge is divided into tenths of an inch, and the subsequent ones are directly marked for twentieths, thirtieths, fortieths, fiftieths, and finally sixtieths of an inch.  On a metric engineer's scale, common scales are 1:100, 1:200, 1:250, 1:300, 1:400, and 1:500.

The terms and the means of writing them down have changed, and for model kits they are now standardized for the European Union.   In English-speaking countries, such terms as "1/72" were used, but the format with a colon as "1:72" is often preferred. The slash format is usually avoided with decimal fractions: "1/76.2" is usually not used; it's "1:76.2" instead. That hybrid 00 gauge can also be expressed by explicitly using a mixed system of units as "4 mm:1 ft" or "1 mm:3 in", but the dimensionless form makes comparison with other scales easier.

Choice of Scale

The nominal height of a man is simple in the inch-based system: six feet. Many traditional scales are derived so that a figure of such a height against the model can be readily imagined as a simple relation to an inch. Although the metric system has specified a limited series of scales for blueprints and maps, when it comes to models, there may be a problem with these scales for a readily imagined person of 180 centimeters. Model railways have the additional difficulty of having to present the rail gauge as a simple number, the height of a person being secondary. Trade authorities in metric countries are attempting to specify scales that are simple multiples of 2 and 5, but neither tracks nor people seem to fit. In such cases, rationalization may actually be invoked for competitive advantage, to prevent interoperability with products from another manufacturing country.

Military and industrial development can traditionally been traced to metric system where the number of millimeters relate to the relative height of the human figure based on 180 cm standard man. Therefore 25 mm scale refers to 1:72 scale, whilst the 15 mm scale (nowadays the most popular scale in ancient, medieval and Renaissance wargaming) refers to 1:120 scale (Many manufacturers refer to 15 mm as 1:100 scale). Likewise, 50 mm scale is the same as 1:35 military model scale, and 5 mm equals 1:350 naval scale.

Views and Projections

Because plans represent three-dimensional objects on a two-dimensional plane, the use of views or projections is crucial to the legibility of plans. Each projection is achieved by assuming a vantage point from which to see the place or object, and a type of projection. These projection types are:

• Orthographic projection, including:
  • Plan view or floor plan or top view
  • Elevation, usually a 'head-on' view front of an exterior of the product
  • Section, a cutaway view of the interior of the product

·        Perspective views

A variety of line styles are used to graphically represent physical objects. Types of lines include the following:

• visible – are continuous lines used to depict edges directly visible from a particular angle.
• hidden – are short-dashed lines that may be used to represent edges that are not directly visible.
• center – are alternately long- and short-dashed lines that may be used to represent the axes of circular features.
• cutting plane – are thin, medium-dashed lines, or thick alternately long- and double short-dashed that may be used to define sections for section views.
• section – are thin lines in a parallel pattern used to indicate surfaces in section views resulting from "cutting." Section lines are commonly referred to as "cross-hatching."

Example of an engineering drawing with different line types are color coded for clarity.
Black = object line and hatching
Red = hidden line
Blue = center line
Magenta = phantom line or cutting plane

Sectional views are indicated by the direction of arrows, as in the example above.

The objects can be represented with different orthographic projection views (front, rear, top, bottom, left and right side). There are two ways to place the different views on the drawing:

• The ISO standard considers a projection on the opposite direction, like an X-ray radiography; the top view is under the front view, the right view is at the left of the front view... This is called FR or First Angle Projection.
• WD uses the American standard (called Third Angle Projection) which places the left view on the left and the top view on the top. The standard in use is represented by a truncated cone.

Multiple views and projections

In most cases, a single view is not sufficient to show all necessary features, and several views are used. Types of views include the following:

• orthographic projection - show the object as it looks from the front, right, left, top, bottom, or back, and are typically positioned relative to each other according to the rules of either first-angle or third-angle projection. The former is primarily used in Europe and Asia, the latter is primarily used in the United States and Canada. Not all views are necessarily used, and determination of what surface constitutes the "front," etc., varies from object to object. "Orthographic" comes from the Greek for "straight writing (or drawing)."
• section - depict what the object would look like if it were cut perfectly along cutting plane lines defined in a particular view, and rotated 90° to directly view the resulting surface(s), which are indicated with section lines. They show features not externally visible, or not clearly visible.
• detail - show portions of other views, "magnified" for clarity.
• auxiliary projection - similar to orthographic projections, however the directions of viewing are other than those for orthographic projections.
• isometric - show the object from angles in which the scales along each axis of the object are equal. It corresponds to rotation of the object by ± 45° about the vertical axis, followed by rotation of approximately ± 35.264° [= arcsin(tan(30°))] about the horizontal axis starting from an orthographic projection view. "Isometric" comes from the Greek for "same measure."

Isometric projection of the above example object.

Dimensions

The required sizes of features are conveyed through use of dimensions. These measurements or distances may be indicated with either of two standardized forms of dimension: linear and ordinate.

• With linear dimensions, two parallel lines, called "extension lines," spaced at the distance between two features, are shown at each of the features. A line perpendicular to the extension lines, called a "dimension line," with arrows at its endpoints, is shown between, and terminating at, the extension lines. The distance is indicated numerically at the midpoint of the dimension line, either adjacent to it, or in a gap provided for it.
• With ordinate dimensions, one horizontal and one vertical extension line establish an origin for the entire view. The origin is identified with zeroes placed at the ends of these extension lines. Distances along the x- and y-axes to other features are specified using other extension lines, with the distances indicated numerically at their ends.
• Circular feature sizes are indicated using either diameter or radial dimensions. Radial dimensions use an "R" followed by the value for the radius; Diameter dimensions use a “D” or outside or overall dimension “OD”. Diameter can also be represented with a circle with forward-leaning diagonal line through it, called the diameter symbol, followed by the value for the diameter.  A horizontal, vertical or radically-aligned line with an arrowhead pointing to the feature is called a leader, is used in conjunction with linear, radial or diameter or circumference dimensions.  All types of dimensions are typically composed of two parts: the nominal value, which is the "ideal" size of the feature, and the tolerance, which specifies the amount that the value may vary above and below the nominal. Geometric Dimensioning and tolerancing is a method of specifying the functional geometry of an object.

Geometric Dimensioning and Tolerancing

Geometric dimensioning and tolerancing is a symbolic language used on engineering drawings and computer generated three-dimensional solid models for explicitly describing nominal geometry requirements and allowable variation.  It is often referred to by the abbreviation, GD&T.  Geometric dimensioning and tolerancing is the practice of applying mathematics and descriptive geometry with engineering drawings to define the nominal geometry of parts and assemblies, to define the allowable variation in form and size of individual features and to define the allowable variation between features to assist in assembly, fit and feature alignment.  Geometric dimensioning and tolerancing and specifications define the nominal, as-modeled or as-intended geometry.  There are several standards available. One such standard is ASME he International Standardization Organization.

The purpose of GD&T is describing the geometric requirements for part and assembly geometry. Proper application of GD&T will ensure that the allowable part and assembly geometry defined on the drawing leads to parts that have the desired form and fit (within limits) and function as intended.

These dimensioning and tolerancing rules for application to models and drawing documentation should be followed:

• All dimensions must have a tolerance. Every feature on every manufactured part is subject to variation, therefore, the limits of allowable variation must be specified. Plus and minus tolerances may be applied directly to dimensions or applied from a general tolerance block or general note. For basic dimensions, geometric tolerances are indirectly applied in a related Feature Control Frame. The only exceptions are for dimensions marked as minimum, maximum, stock or reference.
• Dimensioning and tolerancing shall completely define the nominal geometry and allowable variation. Measurement and scaling of the drawing is not allowed except in certain cases.
• Engineering drawings define the requirements of finished (complete) parts. Every dimension and tolerance required to define the finished part shall be shown on the drawing. If additional dimensions would be helpful, but are not required, they may be marked as reference.
• Dimensions should be applied to features and arranged in such a way as to represent the function of the features.
• Descriptions of manufacturing methods should be avoided. The geometry should be described without explicitly defining the method of manufacture.
• If certain sizes are required during manufacturing but are not required in the final geometry (due to shrinkage or other causes) they should be marked as non-mandatory.
• All dimensioning and tolerancing should be arranged for maximum readability and should be applied to visible lines in true profiles.
• When geometry is normally controlled by gage sizes or by code (e.g. stock materials), the dimension(s) shall be included with the gage or code number in parentheses following or below the dimension.
• Angles of 90° are assumed when lines (including center lines) are shown at right angles, but no angular dimension is explicitly shown. (This also applies to other orthogonal angles of 0°, 180°, 270°, etc.)
• Dimensions and tolerances are valid at 20° Celsius unless stated otherwise.
• Unless explicitly stated, all dimensions and tolerances are valid when the item is in a free state.
• Dimensions and tolerances apply to the full length, width, and depth of a feature.
• Dimensions and tolerances only apply at the level of the drawing where they are specified. It is not mandatory that they apply at other drawing levels, unless the specifications are repeated on the higher level drawing(s).

Notes

Notes--textual information--are also typically included in drawings, specifying details not otherwise conveyed. Notes are almost always in completely uppercase characters, for uniformity and maximal legibility after duplication of the drawing, which may involve substantial reduction in size. Leaders may be used in conjunction with notes in order to point to a particular feature or object that the note concerns.

Sizes of drawings

Sizes of WD drawings typically comply with either of two different standards, ISO (World Standard) or U.S. customary, according to the following tables:

The metric drawing sizes correspond to international paper sizes.  Engineering drawings could be readily doubled (or halved) in size and put on the next larger (or, respectively, smaller) size of paper with no waste of space. The U.S. customary "A-size" corresponds to "letter" size, and "B-size" corresponds to "ledger" or "tabloid" size.

ANSI Y14.2, Y14.3, and Y14.5 are standards that are commonly used in the U.S.

Orthographic projection

Orthographic projection is a means of drawing a three-dimensional (3D) object in two dimensions (2D).  It is a form of parallel projection, where the view direction is orthogonal to the projection plane. It is further divided into multi-view orthographic projections and axonometric projections. With multi-view orthographic projections, up to six pictures of an object are produced, with the projection plane parallel to one of the coordinate axes of the object   WD uses the Cartesian axes (x, y, and z) for defining the ground and sky or length, width and height.

Example:  Machined Aluminum, based on 3D CAD Model
Power Tool 7" Angle Grinder Head