For some time, I have been feeling dissatisfied with the shaping of the frames on my model of the broad gauge engine ‘Rob Roy’ but couldn’t think of any ways to improve them, with the limited tools that I have.  The construction of my model is described earlier in my blog.

 

 

 

Recently I started to think about whether my 3D-printer might be able to help. I do like engines to be made of metal, so a complete plastic print wasn’t my favourite option, although I did find it a useful exercise for testing my modelling skills.

 

I had the idea that it might be possible to make a tool or template, to help in forming brass sheet to the correct curved shapes. I couldn’t work from published drawings because I have modified the frames to suit some slightly over-size Tri-ang wheels, which I chose because they have the correct number of spokes for the GWR ‘Waverley’ class. These wheels are a very prominent feature of the prototype.

 

My starting point was a JPEG image of the frames that I made for my model, which has some compromises in dimensions, to accommodate the wheels. It proved quite difficult to turn this into something that I could ‘extrude’ into a ‘solid’ model by using my 'Fusion 360' software.

 

Rob Roy Frames (modified) – JPEG image

 

Different software packages have their strengths and weaknesses. For this application, ‘Silhouette Studio’ has an excellent ‘trace’ function but has very limited export capability.

 

My first step was, therefore, to open the JPEG image in ‘Studio’. One pitfall is that the image had be scaled to 72px/inch, which caught me out because I usually use 300 for printing. I was initially puzzled because the image appeared 4.2 times too big!

 

Once I had a correctly-sized background image, I use the ‘trace tool’, with the various filters turned off. The result was an outline drawing, which I saved in ‘Studio3’ format.

 

 

 

To get this drawing into ‘Fusion 360’, it needs to be converted to SVG, which proved tricky.There is, however, a website  that will do an on-line conversion of ‘Studio’ files to SVG format, so, by using this, I now had the drawing in a format that I could insert into ‘Fusion 360’. 

 

In principle, the extrude tools in ‘Fusion 360’ can be used to transform an imported drawing into a solid object. In practice, however, my drawing turned out to have tiny gaps in the lines, which did not create the closed areas that are needed for extrusion to work.  There is an ‘inspect’ tool, which identified a very large number of places where such gaps occurred but I don’t know of any easy way to close gaps in ‘Fusion 360’, other than on a point-by-point basis. Since the gaps are very tiny, it is difficult to find where to apply the editing tools such as ‘extend’ and ‘join’ and, in some places, they did not seem to work on the imported drawing. Failures seemed to occur where lines met some types of curves and would not connect .

 

I needed another piece of software, to try and resolve the problem. So, I opened my SVG drawing in ‘Inkscape’ and explored the various ‘repair’ tools in that software. The ‘edit paths by nodes’ tool revealed that there was a very large number of nodes in the traced drawing. The ‘simplify’ command on the ‘path’ menu did a good job in reducing these to a more manageable number. By zooming in on the drawing to look at the detail of the nodes, it was easy to see where some nodes did not link up and it was easy to move node points so that they ‘fused’.

 

I re-saved the drawing and inserted the new version into ‘Fusion 360’. Overall, the situation was now much better in that the main area could now be selected as a closed object. The ‘inspect’ tool revealed just a few problem areas and it was now feasible to give these points individual attention. In some cases, it was quicker simply to delete a short section and replace it with new lines. This method was sufficient to ‘close’ all the separate areas.

 

It was only when I came to transfer the design to my slicing software, ‘Cura’, for printing that I realised that the scale had somehow changed during the transfer from ‘Inkscape’ to ‘Fusion 360’.  In my previous work, I had always used DXF files from ‘Autosketch’ and these transferred correctly to scale. As a check, I tried saving the file in DXF format from ‘Inkscape’, which solved the scale problem, but the other problems of ‘loose ends’ appeared again and, in the end, I found it easier to re-scale the printer file within my ‘Cura’ software, before finally converting the model to ‘gcode’ for my E180 printer. The printed tool is shown below.

 

 

 

For my purpose, the most important part is the curved top surface, which provides a firm base on which to construct my curved splashers.

 

My first step was to glue a sheet of 10 thou (0.25 mm) brass sheet to one face of the tool. I used ‘UHU’ adhesive so that, after processing, the brass could easily be removed by immersion in hot water.  I then used my Dremel ‘Moto-Saw’ to make a rough cut around the main features. This wasn’t as easy as I had hoped, since the saw operates with a vibrating motion and tended to pull at the thin brass sheet. It was, however, adequate for making a rough outline, which I could then refine by means of jewellers’ snips..

 

 

 

I found it easy to use the snips, now that the brass sheet was firmly attached to the tool, which I could hold comfortably during cutting.

 

 

 

For the final trimming, to match the edges of the tool, I used a selection of needle files. Although the tool is, obviously, very soft, it was sufficiently firm to provide feedback when the brass edges had matched the tool surfaces.

 

 

 

Once the frames had been shaped to my satisfaction, I started to add the curved top surface to form the splashers. For this, I used lengths of 5 thou (0.125 mm) brass shim. I used separate lengths for each section of the splashers, as I had done in my original model, but I feel it would be possible, with care, to fold the whole top as a single sheet. I provided a series of tabs along the back of the splashers that I folded down for attachment to the frame. Because the  tool is plastic that melts easily, I could not solder these tabs in situ but, once everything was correctly shaped, I could remove the components from the tool, by immersion in hot water, and solder the parts together subsequently.

 

 

 

I treated this as a ‘practice run’ and propose to try it ‘for real’ on some future engine builds that are in the pipeline. In fact, having looked at my ‘Rob Roy’ again, it doesn’t look nearly as bad as I thought and I shall finish it in its present form, while using the new techniques to build different designs.

 

Having got this far, I decided to see how much extra work was needed to create a complete 3D-printed frame. The answer was not a great deal and, as a training exercise, I made a complete set of frames and splashers with ‘Fusion 360’, as shown below.  It was necessary to extrude selected parts of the drawing by different amounts to create the 3D structure.

 

 

 

I took the opportunity to add sand-boxes and rudimentary springs to my original drawing. One advantage of using computer-aided design is that producing a pair of right and left handed frames is simply a matter of pressing a ‘mirror’ button! So, here’s a pair of frames, straight from the 3D-printer, with Tri-ang driving wheels in place on one side.

 

 

 

Although the splasher tops are rather ‘thick’, to allow successful printing, they are also surprisingly robust and this would be a feasible method to use … providing you are content with plastic engines.  I intend to continue with brass construction but with the assistance of 3D-printed tools, to help in forming complex shapes.

 

Mike