Project 24 - Battledown Flyover #2 (design approach)

With only photographs of the bridge to hand the challenge is to accurately determine dimensions of it. First stop was the satellite image on Google maps and using the scale measure provided I estimated the length of a side to be 150 feet, that's 600 mm in 4 mm scale and since the sides are skewed the model is going to be huge. Next, I needed to work out its height and radius of the curved top.

My photo of the north side was opened in Photoshop and scaled by reference to the brick pillars and a standard brick size and as a check measured the length for comparison with the previously determined 600 mm. It was 601.5 mm - good enough. 

The arc radius was estimated by overlaying a circle and enlarging it until it mirrored the arc. This gave an arc radius of 1600 mm.

I now needed to consider how I was going to fabricate it since it is too big for my 3D printer! I found two natural break points which resulted in splitting it into three equal parts. The two ends are nearly identical in construction so, I only need to design one end and print twice per side. The center section is of a different form.

I use the FreeCad Application 'Part' tool to design models. This utilises three dimensional geometric elements (blocks, spheres, cylinders etc.) to construct a model. I soon found a limitation in trying to create a 1600 mm radius part and had to learn the 'Sketch' feature of the Application to create it in two dimensions like a normal  drawing. Then it had to be extruded using the 'Part Design' tool into a three dimensional object that I could integrate with other parts using my normal approach. Oh dear, have I lost you with this esoteric discourse. Just ignore that and lets move on.

Here is the completed end design. The strut that seems unconnected is a separate piece that is fitted when the end is glued to the centre span. What is missing is the inside face of this side. I decided to make this a separate piece (easily created by mirroring the design of this side), with a view to painting the inside faces for full paint coverage of hidden areas. However, I reasoned it would not matter. I am glad I made a separate piece though, as you will learn.

I noticed in photographs that there is a difference between the two ends in that one end has one upright with double thickness sides made from rivetted plates. To achieve this in the model it was simply a case of creating two designs of the upright swapping them around and exporting two different .stl files. A .stl file is needed by the 3D printer Lychee Slicer Application for preparing the design for print and outputting machine readable code.

And this is what the Lychee did to it. All those orange bits are supports necessary for the printing process. I used its automated facility to both orientate the part and determine where it will place supports. However, I am learning to supplement the result manually, placing supports in places where I think more are needed since relying solely on the automated facility can result in deformations in areas of the model.

The Application tells me that this part will take 6hr 43 mins and 9 seconds to print! Time to print is dictated by the overall height of the part above the print bed. Subsequently, with growing experience, I now set the orientation manually. For essentially flat parts like this I set X & Y to 10 degrees each and this gives a lower print time without degradation. I noticed that the side connected to supports does not sustain the same quality of fine relief detail as the non support side. To overcome this we should always apply supports on the side with least detail - and this is a good reason why I made the inside face a separate piece since it has the same high level of detail as the outside face.

There is a lot of waste in this process. There is probably more plastic in the throwaway supports than in the model! The quantity may have reduced if I had inclined it more but, that would take much longer to print.

Rivet counters can have a field day spotting the difference in rivet numbers between the prototype and model (right). For others, be amazed at the level of detail 3D resin printing offers.

To Part 3.

To Part 1.

Read about the SLA (stereolithography) resin 3D printer I use here.