A question that we sometimes get is “whats a basic dimension?” – These dimensions can be fairly confusing as they look quite similar to other dimension types that you’re familiar with.
There are three different types of dimensions found on prints:
- Reference Dimensions
- Tolerance Dimensions
- Basic Dimensions
We’ll have brief description of each then get into Basic Dimension.
A Reference Dimension is a dimension given to any features that’s just for reference – or for visualization purposes. These are called out on a drawing using parenthesis (5.125 for example). It can also be a dimension that is described on another drawing or elsewhere.
Either way, the dimension does not get inspected. Think of a Reference Dimension like a foot long sandwich – sure, it’s called a “foot long” but, that’s just a close-enough referenced size, so you know how much food you are getting.
A Tolerance Dimension is a dimension used if a feature’s size is to needs to be called out or specify the location or something without applying Geometric Dimensioning and Tolerancing principles to it. They are typically given a plus/minus “±” tolerance but can have asymmetric limits as well.
What is Dimensions and Dimensioning Systems?
A Dimension is a defined numerical quantity in the appropriate units of length for specifying the size, position, or other geometric qualification of a part.
Or, in other words, the method of indicating on a drawing the sizes of the object and other details necessary for its construction and operation by means of lines, numerals, symbols, notes, etc., is called dimensioning.
In order to construct an object, it is necessary to be acquainted with its shape and size. Hence an engineering drawing is prepared giving a representation of the shape, size and other details most significant for the construction of the object.
Dimensions are given for the purpose of indicating the sizes of the various features of the object and their relationship in position. It is principally necessary in ascertaining the size of the object, the names of the parts and the diameter of the holes, etc.
What you’ll learn in this post:
- Elements of Dimensioning
- Units of Dimensions
- Dimensioning common features
- Types of Dimensioning Systems
- Type of Dimensioning
#1. Elements of Dimensioning.
It consists of projection line, leader line, termination of the dimension line, the origin indication, symbols and the dimension itself.
- Dimension Figure: These are drawn may be vertical or inclined to indicate the height of the dimension figure.
- Dimension Line: Dimension line is a continuous thin line. it is indicated by arrowheads, it is drawn parallel to the surface whose length must be indicated.
- Projection or Extension Line: A projection line or extension line, it is a thin line. It is drawn perpendicular to the surface which is to be dimensioned. The projection line slightly extends than the dimension line.
- Constructions line: A construction line is a thin line drawn to indicate the dimension line. The construction line is extended slightly beyond the point intersection.
- Arrowheads: Arrowheads are used to indicate the dimension line. Usually, the arrowhead must include angles of a minimum of 15°. The Arrowhead may be open or close or closed and filled.
- Leader or Pointer Lines: These are thin continuous lines drawn from a dimension figure to the feature to which it refers. Leader line is drawn may be 30° or 60° to the bottom of dimensions.
#2. Units of Dimensions.
The dimension unit is millimeter. The unit of the dimension is not written in writing the dimension fig. and a footnote for saying “ALL DIMENSIONS ARE IN MM” is written at a prominent place on the drawing sheet.
#3. Dimensioning common features.
Dimensioning diameters
Circular features or cylindrical parts like shafts, pipes, rods or any other round shape are always dimensioned by indicating their diameter. Because it is easily measured.
The diameter is indicated by the ∅ sign. The diameters on the circular objects may be indicated in any one of the following ways as shown in fig.
Dimensioning Radius
The curved, fillets and round figures are shown in drawings by arcs or circles. The dimensioning is done by giving radii.
The measured radius is indicated by R. The dimension line is drawn radial with the arrowhead touching the arc.
When the centre is located by the projection lines, the centre shall be marked by a clear dot and the arrowhead may be reversed.
When the centre of the arc falls outside the drawing limit the dimension line of the radius should be broken or interrupted according to whether or not it located the centres.
Dimensioning Arcs
When dimensioning the length of an arc, the dimension figure is identified by the symbol and is written above the dimension.
Dimensioning Chords
The chord is the linear distance between any two points of an arc. It is dimensioned as shown in the figure.
Dimensioning Angles
The angles are dimensioned in the same manner as that of linear dimensions.
#4. Types of Dimensioning systems.
There are two types of dimensioning systems – Aligned system and Unidirectional system.
Aligned System
In this type of dimensioning system, Dimensions are placed above the dimension lines which are drawn without any break and written parallel to them.
That they may be read from the bottom or any other side of the drawing sheet. Dimensions are placed in the middle and on top of the dimension lines.
Unidirectional System
In this type of dimensioning system, dimensions are placed in such a way that they can be read from the bottom edge of the drawing sheet. Dimensions are inserts by breaking the dimension lines at the middle.
#5. Types of Dimensioning.
The different methods, arrangement and the indication of dimension are
- Chain Dimensioning
- Parallel Dimensioning
- Combined Dimensioning
- Progressive Dimensioning
- Dimensioning by Co-ordinates
- Equidistant Dimensioning
- Repeated Dimensions
Chain Dimensions
Chain dimensioning is a dimensioning scheme that dimensions from point to point. Or it is a series of adjacent dimensions arranged in one horizontal row.
When chain dimensioning is used the location of one feature is based – or referenced – on the location of the previous dimensioned feature.
Chain dimensioning is used when it is important that two features are located a certain distance from one another, for example, two fixed holes with two fixed pins might require a chain dimensioning.
Parallel Dimensioning
When a number of dimensions are measured in the same direction from a common surface or line, the method of indicating all the dimensions from the same feature is called parallel dimensioning. The dimension lines are parallel to each other and equally spaced.
Combined Dimensioning
In this method, those of the chain and parallel dimensions, are used in the same drawing.
Progressive Dimensioning
This method is adopted when dimension has to be established from a particular datum. The overall dimensions are placed outside the smaller dimensions this dimension is showed by the common reference line.
Dimensioning by Coordinates
Dimensioning by a coordinate table may be used in place of other styles and can be used to make the drawing easier to read, this method is done when a number of holes of different sizes have to be dimensioned.
Equidistant Dimensioning
The dimensioning simplified by given the product of the number of spacing and the dimension value. A point is said to be equidistant from a set of objects if the distance becomes that points and each object in the set are equal.
Repeated Dimensions
When, certain features or elements of the same size, are repeated a number of times on drawing, in order to avoid the repetition of the same dimension everywhere, the product of a number of repeated features and the dimensions value may be indicated only at one such feature.