After reading some recent posts about horse drawn wagons and the like, I started to wonder if it would be possible to 3D-print my own horses.

 

A look at the 'Cults' website yielded a 3D-printable horse  by David Mussaffi, described as ‘FDM printer ready’, so I thought that this would be a good place to start.  I looked at the file after loading it into my ‘Cura’ slicing software and found that the model was cleverly split into three parts, such that there were flat surfaces to lie on the printer bed, with no overhangs that would require support structures.  The original design was rather large for 4 mm scale, so I adjusted the ‘scale’ setting in ‘Cura’ until I felt it was a reasonable size for use on my railway. The following illustration shows the three parts, as they appeared on my printer bed,

 

Three components of downloaded horse by David Mussaffi – as printed

 

Once separated and carefully trimmed along the edges, these three parts fitted together very well, to make a complete model – in fact, I was pleasantly surprised by the result and decided that a few of these 3D-prints would be useful additions to my railway, for pairing with various vehicles. Differences in texture are visible on this unpainted model but I felt that, with a little more fettling and painting, a very satisfactory model could be produced.

 

3D-printed horse after assembly of 3 components

 

I also realised that by adjusting the overall scale and/or individual parameters such as girth, it would be possible to produce a range of different types of horse from the same basic design.

 

3D printed horse variations

 

Horse Shunting

 

A few years ago, when I set up a scene on my layout involving horse shunting, I found myself musing on whether there might be any way of simulating a walking motion.  I wonder if any readers remember the little ‘ramp walker’ toy that once came in cereal packets? A string could be attached to a model horse and hung over the edge of a table, attached to a small weight. This would cause the horse to ‘walk’ to the edge and then stop.

‘Cereal Packet’ Horse Walker

 

When I first thought about it, a few years ago, I could see no way in which I replicate a model of this type at an appropriate size. Now, however, having gained some experience in using a 3D-printer and having created a 3D-printed horse, the thought entered my mind again and I thought it was worth ‘giving it a go’.

 

Well, I’ve had a go but still haven’t succeeded in making it walk! I suspect that there is a problem of scale. If I halve the linear dimensions, then the surface areas decrease by a factor of four and the weight by a factor of eight. I suspect such changes upset the relationships between movement, weight, and friction that make the model work.

 

I have documented the steps I’ve taken so far in the hope that someone will be able to suggest which parameters to change, to make it work. After around 3 months of fiddling with it, I’ve run out of ideas!

 

Downloaded 'Walker'

 

The trouble from the outset was that I had only a vague idea of how the walking action worked, so I looked on the web and found another model on the 'Cults' website, described as a '4 legs walker'. This model consists of a simple ‘body’ and a set of pivoted legs, which enabled me to examine the principles behind the ‘walking’ motion. I knew, however, that I would have to produce a much more realistic looking ‘body’!

 

Downloadable 4legs-Walker, showing shaped feet

 

Now the fun began, as I contemplated how to adapt the ‘realistic’ 3D-printed horse to the ‘walking’ principle! I was encouraged by the fact that the horse model I had downloaded was already divided into separate parts that I might be able to adapt to a suitable new configuration.  So, I started on a new ‘learning curve’! What follows has taken a few months of ‘trial and error’ (mostly the latter) but the main steps are summarised below:

 

Meshmixer

 

The first hurdle was to get the downloaded horse model into my 3D-modelling software ’Fusion 360’.  It is possible to import a mesh (STL) file but, when I tried to convert this model into an editable ‘body’, I immediately got a message that there were too many ‘faces’, so that conversion was not possible. It was clear that some basic ‘editing’ was needed before I could start to use my ‘Fusion 360’ modelling tools to adapt the components of my downloaded horse. This sent me on a search for software that could help me to solve this difficulty and I found the very useful (and free) ‘Meshmixer’ program by ‘Autodesk’.  Some of the useful functions of this software are described below:

 

Separating the three components using 'Meshmixer

 

After opening the original model that I had downloaded from the ‘Cults’ site, I found that I could use the ‘Select’ tool in ‘Meshmixer’ to separate the three parts of the original model, so that I could work on each of them independently.

 

Simplifying the model mesh

 

The next step was to use the ‘Edit’ menu within the ‘Select’ tool in 'Meshmixer', to reduce the complexity of the mesh, choosing the ‘max deviation’ reduce target, so that deviations of less than 0.2 mm were ignored. This seemed a reasonable figure, to match the capability of my 3D printer. This process allowed me to reduce the number of facets on each component, so that they became suitable for importing into ‘Fusion 360’.

 

Converting the Mesh

 

Once the STL mesh file has been imported into ‘Fusion 360’, there is an option to convert a ‘mesh’ model into a ‘body’, which can then be operated upon by means of the usual ‘Fusion 360’ tools. During this process, I discovered that my ‘Windows 10’ operating system includes a 3D Viewer App, which not only displays STL files but also shows statistics, such as the number of faces and vertices in the model. The original downloaded horse had 50,474 triangles in its mesh whereas, after simplification as described above, this was reduced to 2,634 triangles.

 

After carrying out these modifications, I had a model that could be imported successfully into ‘Fusion 360’, where I could start to make the changes needed for a ‘walking’ model.  Before I could do much more, however, I realised that I needed to make the upper body of the horse ‘hollow’, so that it could contain the tops of movable legs and their pivots. My aim was to provide the functionality contained in the the basic ‘4-legs Walker’ that I showed above.

 

Making a hollow model with 'Meshmixer'

 

I found that ‘Meshmixer’ contains a simple tool within the ‘Edit’ menu to make a mesh model ‘hollow’, with options to define the thickness of the remaining side walls and the complexity of the inner mesh. This procedure is illustrated below:

 

Using Meshmixer to create a hollow model

 

In 'Meshmixer', the inner mesh is described in terms of ‘mesh density’ so, by trial and error, I selected a value that seemed to match the number of triangles in the outer mesh.  Now, I could import the hollow mesh model into ‘Fusion 360’, where I could use the ‘Hole’ tool to make apertures in the sides of the horse body, to accept axles for the pivoted legs.

 

Adding legs

 

For the initial trials I created simple rectangular legs with open rings at the top to act as pivots – these were similar to the ones used by the ‘4legs-Walker.’ I reamed out the holes until the legs swung freely on 2 mm diameter steel axles. When printed, the upper body now looked like this:

 

My 3D printed Hollow Body with Legs

 

I did quite a lot of fettling of the various components, to make sure that the legs swung freely within the upper body shell.

 

Lower Body

 

I imported the original solid version of the lower body into ‘Fusion 360’ and used the ‘Split’ tool to cut off the original legs from this part of the body, just below the belly of the model. I then drew rectangles on the flat top surface of the lower body, to define where the movable legs could pass through, while limiting their angle of travel. I extruded these rectangles into apertures, through the depth of the lower body, using trial and error to adjust the size of these apertures so that they allowed an appropriate amount of free leg movement, fore and aft.

 

My 3D Printed Lower Body with Slots

 

Printing the Feet

 

The shape of the feet is critical to the working of the model, so I started by modifying the ones used by the ‘4legs-Walker’, downloaded from the ‘Cults’ site. Because the lower surface has to be angled, I separated the feet from the legs and provided a flat upper surface to each foot, which could lie on the bed of my 3D printer, when I printed the feet ‘upside down’. I also provided a rectangular socket in the top of each foot to hold a leg securely.

 

Assembly

 

Once I had designed all the individual components in ‘Fusion 360’, I printed each of the components: upper body, lower body, head, 4 legs and 4 feet. As mentioned above, a considerable amount of trial and error was needed to achieve the clearances needed for free movement of the legs within a controlled arc of swing.  Fortunately, each of these small parts only took a few minutes to print, so it was easy to make successive ‘tries’.  I show below this collection of parts, as they emerged from the printer.

 

My 3D printed ‘Walking Horse’ Components

 

Lessons Learned

 

I’ve learned quite a lot about how to modify a 3D-printable model downloaded from the web, by means of tools such as ‘Meshmixer’, which I hope will be of interest to other 3D modellers.

 

Help Requested

 

Unfortunately, though,  I’ve still not succeeded in achieving an effective walking motion so, if any of my readers can make useful suggestions, I’ll be very pleased to receive them.

 

In the meantime, I’ll keep trying to find the ‘magic solution’

 

Mike