GD&T - Part 1; Linear Dimensions

In a Perfect World...

As you sit down and begin to sketch out your design, whether that’s in a 3D CAD software or hand drawn on the nearest piece of scrap paper, all component parts are considered perfect, in size and function.

However, as soon as the design leaves your screen and is passed on to the required manufacturing processes, those perfect intensions that you spent hours constructing can very quickly become mis-read or mis-interpreted leading to form errors and parameter changes that can have a great effect on the functionality of the final part/assembly.

Thankfully, for both us as the designer and our manufacturing suppliers, a standardised set of requirements has been produced to link design, manufacturing and quality known as Geometric Product Specification (GPS).

Examples of these standards are ASME Y14.5 or BS8888. These standards help us link;

• Nominal Features - Our ideal, exact geometry according to our CAD model or 2D drawing
• Real Features - Our non-ideal geometry produced during manufacture
• Extracted Features - Non-ideal geometry extracted from the manufactured part
• Associated Features - Ideal, non-exact geometry best fit to the extracted geometry

Nothing is ever Perfect!

As the designer, you must always assume nothing can be made to an exact size!

Below are example tolerance ranges expected with some common manufacturing methods

To ensure that our design is manufactured as close as possible to our intended requirements we can communicate instructions, traditionally detailed on a 2D drawing of the component, highlighting which areas of our design are of the highest importance.

The simplest way of achieving this is through dimensional tolerances. 

A dimensional tolerance is defined as the allowable variation from the nominal or theoretically exact dimension. In most cases dimensional tolerances are applied as Bilateral dimensions e.g. 35 +/- 0.2mm or 35.2/34.8mm where the theoretical exact dimension is at the mid range of the values given.

However in some cases the dimension may be given as a Unilateral dimension e.g. 35 +0/-0.2mm. Unilateral dimensions are most commonly used when defining the fit between two components e.g. a shaft into a hole (H7/f7). Although a unilateral dimensions may be specified on the 2D drawing, at The Engineering Notebook we would typically ask our designers to always model their 3D CAD models mid range of all dimensional tolerance bands.

Using dimensional tolerances (red) we can develop our previously dimensioned drawing to show that the position and size of the diameter 10mm holes in our bracket are important to us and the intended use of our component. For any bolted flange, we would always like to have better control of our bolted flange thickness also. This will help us control any build up of tolerances in the assembly stage.

Reference dimensions (green) can be used where we have dimensional stack ups within the component, these allow us to avoid over constrainting the component. As shown above, using a reference dimension of (55mm) informs our manufacturing supplier that the individual dimensions (20, 35 and 20mm) of the upper profile are more important that the overall length of the part.

Alternatively, if overall length was more important, and we wanted to add an additional level of control to that length, we could dimension the part as shown below:

Reference dimensions can also be used for having a quick way of showing the overall size of a component, a useful way of sizing stock material from which the part is going to be made from.

General or Untoleranced Dimensions

Where possible all features of a component should be dimensioned on the 2D drawing using either linear tolerances or geometric tolerances. However, sometimes this is not always practical and can sometimes lead to over crowding of the drawing and lead to further confusion. Depending on the size of the part and the intended manufacturing process some dimensions may be left as general or untoleranced dimensions. 

This class of dimensions can be controlled by a statement or table typically within the drawing boarder. ISO 2768-1 outlines the following tolerance bands for uncontrolled standard features: