Adding Skins to Carriages
Entry posted by MikeOxon
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One idea leads to another; in this case, I have tried extending the idea that I showed in an earlier post of adding cladding to a brass-tube boiler by 3D-printing an outer sleeve.
3D-printed boiler cladding
During my early ‘learning curve’ with 3D-printing (i.e. about a year ago), I made some broad-gauge carriages, as described in a short series of blog posts. Printing the complete carriage as a single task had several advantages, such as including internal partitions and seats, but also created difficulties in making window openings and other features on the vertical sides. For the window openings, I had to include support structures which proved quite difficult to remove without leaving rough edges around the windows.
3D-printed Broad Gauge Carriage
There is the additional point that each print took about 8 hours to complete, which is a serious deterrent to much experimentation - my preferred method of working.
As a result, the completed carriages lacked detail, so one solution that I explored was to use my Silhouette cutter to add outside frames to plain carriage sides, as shown in my Pre-Grouping blog
My new ‘cladding’ idea, however, is to add a thin skin to the previously printed carriage body. This ‘skin’ can carry the additional detailing to represent external mouldings and other fittings. My initial experiment was to make a ‘skin’ only 0.5 mm (20 thou) thick, which could be laid flat on the printer bed. With this method, cut-outs for windows could be made with no requirement for any support structures, while slots in the surface could be made to mark the edges of doors. External mouldings could also be extruded, by simply increasing the thickness of the skin in the required locations – again with no need for any support structures. The other major advantage is that a detailed skin of this type prints in a few minutes, so it is easy to correct mistakes and make improvements, without committing to extended time-scales.
As usual, there were some lessons to be learned. I made the initial drawings of the carriage side in ‘Autosketch’ and then transferred these to ‘Fusion 360’ in DXF format. I placed the drawing on the horizontal (XY) plane and then extruded the main panels to a depth of 0.5 mm, excluding the window areas and door edges and mouldings. After this first extrusion, I selected each of the door edges and extruded these by only 0.25 mm, so that they were recessed below the main surface. In a similar way, I extruded the mouldings by 0.75 mm, to raise them above the panels.
3D-model for side overlay
For the louvres above the doors, I raised each rectangle and then applied the ‘chamfer’ tool to achieve the required angle of each louvre individually.
chamfered louvres above window
My first print, which took about 9 minutes, showed that some of my details, especially the raised mouldings, were too fine to appear in the final print. In fact, I should have used the pre-view facility in the ‘Cura’ slicing software, which would have warned me that this was going to happen.
It is all too easy to draw details that are too small to be realised by my printer, in which the print head has a diameter of 0.4 mm. It is interesting to note that narrow slots (0,25 mm), as at the edges of doors, do appear but similarly-sized raised features do not appear. Modifying the drawing meant that I had to get to grips with the drawing tools in ‘Fusion 360’, which are broadly similar to those in ‘Autosketch’ but with some key differences.
One important aspect when creating drawings in a 3D program is that you have to decide in which plane the drawing is required to appear. This can sometimes be tricky, especially after the initial drawing has been extruded to create new surfaces above the original plane. The first action, when entering ‘drawing mode’ is to select the drawing plane. By selecting the top surface of the carriage side, I could draw new profiles for the arc-shaped mouldings at each end of the carriage. I could then extrude these new sections by 0.25 mm above the top surface, to match the original mouldings (which had been extruded by 0.75 mm from the original drawing plane). It is essential to keep a clear head when making these modifications!
For the straight side-mouldings, I used a different method by selecting the vertical edges of the moulding, where they rise above the carriage side. The ‘Move’ tool can then be used to move these edges in order to increase the width of the mouldings to a printable size.
Changing the width of a moulding
After making these changes, I transferred the 3D model to ‘Cura’ and this time, I remembered to use the ‘Preview’ mode, to check that the details would actually print! I also set the line-width to 0.3 mm, even though my print head is 0.4 mm diameter. This can improve the smoothness of very thin panels like these. I suspect this is because the nozzle is circular, so that a rectangular grid of lines benefits from slightly closer spacing between successive passes of the print-head.
All looked good in the pre-view, so I proceeded to another print, which again took only a few minutes to complete.
When printing these very thin ‘skins’, the ‘Cura’ software automatically produces frames around the window openings and the edges of the sides, which, in the case of the windows, provides a fair representation of the bolections. The main panels are very thin indeed and peel off the printer bed like plastic tape but I found they were sufficiently robust to resist tearing.
Peeling side-skin off printer bed
I smeared some PVA adhesive on the original carriage side and carefully laid my new cladding in alignment with the existing window openings, which already had the droplights represented. Since I had previously sprayed the carriage body with red primer, the droplights automatically appeared in a suitable red colour. As early GWR carriages were painted brown overall, the final painting job was very straight-forward. Although diagonal lines from the printing process were visible on the surfaces of the printed panels, these were sufficiently fine to cease being noticeable under a coat of paint.
Skin applied to Carriage Body
I shall continue to experiment with different parameters for the printing process and, if making new carriages, I would make the original sides a little thinner, to allow a thicker surface skin to be applied without increasing the overall thickness by too much. I think my present skins are on the limit of what my current printer can reasonably accomplish.
Mike
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