Many users of Blender see the program as a tremendous tool for creating stunning artworque or animations. Many modelling techniques are covered in tutorials for building models using methods such as box modelling, building shapes and forms polygon by polygon or tracing over a background image, used as a reference for the models shape and proportions. These are great methods to build an artistic representation of a product, but what happens when you want the model to be used for engineering and have the precision of a CAD model?
Fortunately, Blenders arsenal of internal modelling tools has been quietly growing and with a little forethought and planning Blender can produce models to a far higher dimensional accuracy than parts could ever be manufactured to. To start drawing with precision we need to consider the real world dimensions one Blender unit will represent. In architectural worque it is common for one Blender unit to equal one metre, but this leaves the smallest dimension that can be viewed using the "Edge Length" feature as 1mm. In Engineering, parts are often produced with dimension tolerances of 0.02mm and sometimes as small as a few microns. To achieve this level of accuracy in Blender we would need to assign a dimension of one Blender unit to equal one millimetre, allowing you to view an edge length of one micron and model to sub micron accuracy (1micron = 0.001mm).
To help you visualise just how small one micron is, image 1 shows a size comparison related to a human hair.
To enable you to see large components at this scale you will need to increase the view cameras clipping distance View>>View Properties and increase "Clip End" to a size larger than the model.
If you intend to model with CAD precision you need to adopt a workflow similar to those used in industry standard CAD packages. CONSTRAIN EVERYTHING.
Extruding with Accuracy
When Extruding, Rotating or scaling a set of vértices, Blender allows you to constrain their positions using numerical input. It's better to type in the length of an extruded edge rather than dragging the edge until the edge length looks right.
To constrain an extruded vertex to a set dimension, simply select the vertex you wish to extrude [RMB]. Press [E] for Extrude. [X or Y or Z] to constrain to a set axis. [12.513] or any other dimension for the length, (minus figures extrude in the opposite direction) then press Enter. The new vertex will be positioned the exact distance you entered along the axis.
If you need to extrude an edge at an angle to an axis simply extrude the vertex along the axis at the required length then rotate the edge to the desired angle. This is a little more complex because you need to use Blenders Snap tools. Say you wanted to extrude an edge 35mm long and 45 degrees above the X-axis. Select the vertex to extrude [RMB] snap the cursor to the vertex [Shift-S] Cursor>Selection, the cursor will be used as the centre point of the rotation. Extrude the vertex [E] constrain the extrusion to the X-axis [X] and a length of  press Enter, an edge has been created 35mm long on the X-axis. With the Extruded vertex still selected we need to rotate it -45degrees from the cursor. Change the Pivot Point to 3D Cursor, so any rotation will be centred on the cursor position and press [R] for rotate then [-45] to rotate the vertex 45 degrees in an anticlockwise direction, press Enter to accept the rotation.
Constraining moves to an axis and dimensional input works for extrusion [E] rotation [R] move (grab) [G] and scale [S]. However with scale you will need to use a scale factor rather than a dimensional input. This is easily calculated by dividing the required finished size by the existing size of the object.
Using these simple procedures you can create the basic geometry of components to extrude or spin into accurate 3D models. But first the sections will need a little refinement. Very few components have sharp edges and it is normal for corners to be rounded off with fillets or chamfers. In
CAD packages the Fillet and Chamfer tools automate the process of rounding off corners to a set radius or chamfer. In Blender you can achieve the same result but need to construct the geometry manually.
Adding Fillets and Chamfers.
It's possible to accurately position fillets using Blenders mesh tools, but you will need to know the angle between the edges to fillet, the fillet radius you require and one of the edges must be aligned to a known axis.
To create a fillet, extrude a vertex from the intersection of two edges 1.5 times larger than the fillet radius. Rotate this vertex 1/2 angle A.
From the intersect extrude a vertex perpendicular to the edge on the known axis and the length of the fillet radius.
Extrude another vértices from this parallel to the edge on the known axis. Where these two lines cross is the centre point for the fillet.
Using the knife tool [K] snap a cut line [Ctrl-LMB] between the two reference vértices parallel to the known axis line and cut a new vertex on the centre point of the fillet.
From the centre point extrude a vertex perpendicular to the edge on the known axis at the length of the fillet radius.
Set the cursor on the centre point and spin the vertex on the end of the radius edge through an angle of
180 degrees minus Angle A.
Delete the reference vértices. Remake the edges between the fillet and surrounding vértices.
Its possible using basic geometry techniques and constrained dimensional input to accurately create almost any component. For the model to be useful in manufacturing it must be manifold (no internal faces) and contain no holes. Many rapid prototyping machines will accept a 3D model that has been exported as a .stl file and cutter paths for CNC machines can alos be produced directly from the model.
To output a model as an engineering drawing layout that will print at an accurate scale I find it easiest is to set the camera in orthographic mode, with a camera scale set at a múltiple of 25.4. Then set the size X and Size Y in the Format panel of the Render Buttons to an equivalent múltiple but this time of 300 (printers tend to print at a resolution of 300 dpi). So if you wanted to have a 1:1 scaled image fit onto an A4 page of 11" by 8" you would need to set the camera scale to 25.4 x 11 = 279.4, the Format panel Size X: to 8 x 300 = 2400 and Size Y: to 11 x 300 = 3300. There is one further requirement as the image is set to print at the screen resolution not 300dpi. This requires the use of an image-editing program such as the Gimp or Photo Shop, to change the output size of the image to 300dpi.
The above techniques are a brief extract of the information contained in the eleven-part tutorial Modelling a 608 Bearing. Which details the process and techniques needed to precisely model the bearing and produce an accurate scaled drawing. The 608 Bearing tutorial is the first part in my engineer's guide to Blender, more tutorials and techniques will be added as I worque through the project of redesigning my CNC router.
The tutorial is available on the 3D graphics section of www.rab3d.com
Or directly via http://homepage.ntlworld.com/r.burke2/tutorial.html
Technical Manager of a leading Building Products manufacturer.
I use Blender alongside 2D and 3D CAD packages to both quickly prototype new products and assemblies and produce graphics for both marketing and technical literature. I have found blender an extremely productive tool and I am currently experimenting with techniques that will allow blender to be accepted as an alternative engineering design tool for hobby and model engineers. People who cant afford the excessive costs of 3D parametric modelling programs.