2.3. Dimensioning technical drawings
Dimensioning
The most important function that dimensioning of a technical drawing has, is enabling manufacturing by using given measurements and to fulfill its function when added to an assembly. Dimensioning exactly defines the final state of a model.
Dimensioning of technical drawings can be done in three variations, namely according to function, according to manufacturing, or accoring to testing. All three methods can be applied simultaneuosly, but over-dimensioning by using additional dimensions should be avoided.
Important:
Only add as much measurements as necessary to a technical drawing!
Over-dimensioning has to be avoided - it often causes conflicts with tolerancing.
Function based dimensioning: Toleranced dimensioning is used. If dimensions of the model do not meet these measurements, its function is no longer guaranteed.
Manufacturing based dimensioning: Dimensioning is dependent on the type of manufacturing. It is used when the model should be manufactured without recalculating exact measurements.
Testing based dimensioning: Used when manufactured parts or assemblies should be tested after correct manufacturing or mounting.
Depending on the type of manufacturing, conventional or with CNC-Machines, you should determine wether to use reference dimensioning or coordinate dimensioning. For coordinate dimensioning, a point of origin is firstly specified, to which all other measurements refer. During the CAD course, we only use standard reference dimensioning, which will be explained using drawing examples.
Basic elements for dimensioning technical drawinings
The following figures show basic elements of dimensioning, according to DIN ISO 129-1.
Basic dimensioning
Every distance is described in technical drawings using this dimensioning. A complete Measurement consists of a measure line, measure aid lines, measure arrows and a quantity.
The measure line is aligned parallel to the dimensioned distance or displayed as an arc when dimensioning angles. Measure aid lines should be aligned perpendicular to the measure line. Measure arrows create boundries for measure lines, which touch measure aid lines.
A quantity is displayed above the measure line. Its size is standardized according to DIN ISO 6428 and its font according to ISO 9175-1.
Diameter
The circle with a slash indicates a diameter, the following quantity indicates the size of the diameter.
A spherical element is indicated by an additional S. If a radius is used, the S is placed in front of an R.
Radial dimensioning
The letter R in combination with a quantity indicates dimensioning of a radius. The measure line originates in the arc center and the arrow points to the respective arc.
Arcs
The length of an arc is indicated by a small arc in front or above the quantity. The arc length is limited by two measure aid lines.
Chamfer
A chamfer is dimensioned by specifying an angle and a distance, differentiating between symmetric and asymmetric chamfers. A symmetric chamfer means that the angle is 45° and therefore vertical and horizontal distances are equal. An asymmetric chamfer has an angle greater or lower 45° which results in different horizontal and vertical distances.
General information on technical drawings
Some general information is included in a complete technical drawining, which is placed next to the title block.
- General tolerances according to DIN ISO 2768-m
Every dimension is toleranced by general tolerances with a lower and upper deviation. For doing so, a valid general tolerance should be next to the title block, e.g. ISO 2768-m or general tolerance ISO 2768-m. The letter m represents usual chart values (f=fine (ger. "fein"); g = rough (ger. "grob")).
- Surface symbols
The most commonly used surface symbol should be placed next to the title block as general information, in front of all other used surface symbols in braces.
