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Tuesday, 5 May 2015

'N' Thornycroft J Class Lorry - 3D Print. Part 5a

In this series of postings I shall be making this:

as a kit of parts to a scale of 1:148. Now that is small and will stretch the limits of our printer capability, as you will see.

First thing we need is a dimensional drawing. A chassis dimensioned drawing is freely available from Hampshire Museum Service here. With regard to the bodywork I shall use the specified wheelbase dimension to scale the above photo.

Now I need a Computer Aided Design Application, one that can create 3D models and has an export facility for Stereolithography files. This is a data file that defines 3D products built up one layer of material at a time, being what 3D printers do. I reviewed popular open source CAD Applications and choose FreeCad, which seemed to have good tutorials. Help is also available from YouTube videos. Even so, CAD can be quite complicated and the method I used from the plethora of options available in the Application may not be the best or even most effective means. I used a facility called 'Part'. The drawn model is created by adding and subtracting geometric shapes rather than starting with a 2D line drawing. It sounds a simple method and it is, although I have not yet needed to create convoluted shapes.

These photos demonstrate the principal as applied to the front wheel of the lorry:


1. A blank cylinder 5.94mm diameter and 1mm thick.
2. Another cylinder is superimposed (height arbitrary but, more than the first cylinder) that is embedded 0.5mm into the first cylinder.
3. The second cylinder is 'cut' from the first, which leaves the impression it made in the first cylinder.
4. A cone is added, representing the hub of the wheel.
5. 9 more cylinders superimposed and embedded the full depth of the part.
6. the 9 cylinders are cut from the wheel leaving an axle location hole and 8 vents.

I decided to create a group of objects in the same drawing for quickness although it is quite acceptable to have separate drawings for each object.


On the left are the four wheels, a steering wheel and petrol tank. The steering wheel is solid as it is not practical to print the spokes for something this small. Each rear wheel is a set of two in tandem with the rear one being a solid disc. The right hand image shows how the wedge shaped petrol tank was made from two rectangular boxes. The angled box is cut from the other to form the wedge.

Having exported a steriolithography file it needs to be converted to a G code file that is specific to the printer in use. It sets parameters that heat the plastic, moves the printing nozzle in x-y-z planes (within the bounds of the available printing bed) and extrudes a string of plastic the length of which is set for each segment of the printed object (the object is created by laying down strings of plastic in rows and layers).

The conversion utility is Slic3r. The utility needs to be configured and a configuration file compatible with our printer was supplied by the printer manufacturer. All we need to do is adjust parameters when necessary for optimum printing of specific objects.

The next posting will present all the remaining drawn parts for the kit.

To Part 5b

To Part 1 (About the printer)


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