Between March and August this year, Paul Morris used The Edge’s Laser Cutter to cut-out approximately 500 components for a 1:250 scale model of Brisbane’s, Story Bridge. In this post, Paul shares his inspiration for the project, decisions around the choice of material, plenty of archive and process of photos, and all the digital files you need to make your own Story Bridge replica!
Written by Paul Morris
A bit of background…
The Story Bridge was designed by Sir John Bradfield between 1933 and 1935, having previously completed the design of the Sydney Harbour bridge in the 1920s.
In researching the design of the Story Bridge, I discovered that the original tender design drawings were housed at the State Archives of Queensland. The State Archives are open to the public on selected Saturdays, so I visited the Public viewing room with the intention to photograph a number of the drawings. As it turned out, a copy of the design could be requested in digital format at no cost (BYO USB disk). A total of 92 drawings made up the tender set, dated 1935. A copy of the cover sheet and the drawing block (which shows the design approval by Dr Bradfield) are reproduced below.
As an engineer, I find blueprints fascinating. This set, signed by Sir John Bradfield have considerable nostalgic value for Queensland engineering. The first few sheets of the tender design pack are reproduced below.
How it all started…
When shopping with my wife at a homewares store, she spied this $300 metal bridge shelf and said she was keen to buy it for one of my sons. Looking at the flimsy sheet metal construction, I naturally said – “…oh, I can make a bridge shelf much better than that, and probably half the cost….”, which evolved into a 6 month project (that has yet to yield a metal bridge shelf!).
After some investigation, I established that I could fabricate the metal in nominally 4mm, 2mm and 1mm gauge mild steel via a metal fabricator employing a laser cutter. Design was relatively no problem, as I’ve been using AutoCAD for a couple of decades now.
So I spent a fair few evenings designing the metal components and got a couple of quotes for metal laser cutting. No real surprise, the cost to cut ranged from $150 to $600, for a scale ranging from 1:500 (900mm long) to 1:250 (1,800mm long). I eventually opted for a 1:250 scale.
Let’s talk material…
Having completed the design, I contemplated how I could check that all of the components would actually fit together before committing to the metal laser. One option was to model each of the components (as designed) in 3D and using a modelling package such as SolidWorks to check for constructability (ie. that the cut parts fit together). This wasn’t a particularly difficult exercise as I had drawn all of the components in 2D and only needed to extrude each 2D surface to the thickness of the mild steel plate. Needless to say, this is a robust, highly accurate and thoroughly professional approach, but where’s the fun in building a virtual model?
In order to prototype the steel bridge model, I wanted to find a cheap material that was easy to use. Plywood was a natural choice, as were a range of polymer sheet materials. Surfing the internet identified a few laser cutting firms, albeit, when initial pricing came back, I discovered that the cost to cut plywood and acrylic was only marginally cheaper than steel!
After much more surfing of the internet, I stumbled across the State Library of Queensland site with an obscure reference to a laser cutter. Once in The Edge Fab Lab webpage, all was revealed – “…Conceived as a model for the library of the future, The Edge, launched by State Library of Queensland (SLQ) in 2010, is at the forefront of re-imagining libraries for the 21st century. With a mandate to empower Queenslanders to explore creativity across art, science, technology, and enterprise, The Edge is a visionary space for ‘creating creatives’; a melting pot of ideas and innovation, capacity-building, experimentation and innovation….”. Or in other words, The Edge has a laser cutter that the public can use for free!
With induction completed, I experimented with plywood and acrylic and very quickly landed on acrylic as a much cleaner and accurate material to model with. It also had the advantage that I could also model in the same gauge as proposed for the mild steel – that is, in 4mm, 2mm and 1mm thickness.
Time to cut…
After some effort in shopping around, the thinner 1mm acrylic was sourced, however, the very thin nature of this material, coupled with the fine detail proposed for the pieces to be cut meant the 1mm acrylic just couldn’t be cut to the 0.5mm width sought without significant distortion (warpage) of the material.
- The best result in the thin material was achieved through the following cut measures:
- Multiple passes at high speed in lieu of less passes
- Scoring the acrylic to about 0.8mm thickness, then snapping the remaining 0.2mm uncut section – this actually gave quite a good finish with zero distortion of the material (ie. the distortion only happens when thermal stress was relieved when the laser eventually cut through
- Zig-zagging the cut, which is achievable with AutoCAD, whereby a few centimetres were cut in one location, then the cutting head was moved to another location to allow the original cut to dissipate heat, then the remaining cuts zig-zagged all over the sheet
- Although a departure from the original proposal, I opted for 1.5mm thick acrylic, which actually proved to cheaper than 1mm acrylic.
Tech specs and files…
Design of the model was completed in AutoCAD 2014, then dwg files imported into The Edge’s Corel Draw 7, used as the primary software interface with The Edge’s Trotec Speedy 300 laser cutter. Only problem encountered importing AutoCAD files into Corel Draw was with respect to hatching. The imported hatch areas would be corrupted and could not be used for engraving. Engraved hatching was imported as single hairlines, offset at 0.05mm to create a thick engraved line (eg. a hairline offset 20 times at 0.05mm generates a line of 1mm thickness). I’m sure there is an easier way, so will investigate on the next model.
Files are attached in the zip file, and tabled below. Total cut length was 216m involving 3.6sqm of acrylic, albeit actual acrylic used is probably another 50% on this area with the trials that were conducted.
Cad files are offered for use under a Creative Commons Attribution-NonCommercial-ShareAlike license
In addition to the acrylic as a material used, Weld-On 3 Acrylic Solvent was used for all bonding. This solvent proved to be extremely easy to use and effective in bonding the acrylic. A syringe was used to inject the solvent into the joints. Initially a hypodermic needle was used, but eventually I ordered larger / longer printer-ink needles that were better suited to applying the solvent. In fact, by simply touching the needle end to the joint to be bonded, the capillary tension created by the joint between the acrylic pieces was enough to draw the solvent out of the need without the need to actually depress the needle plunger.
The beauty of Weld-On 3 is that it is dichloromethane solvent which has a very low viscosity and is highly flammable. Or in other words, it evaporates very quickly which means there is no clean-up required. Working time is about 1 minute, with bonding time about 2 minutes. Unless a glue, solvent bonding involves momentarily chemically dissolving the acrylic, after which it re-solidifies. This is more akin to a weld as opposed to a chemical adhered bond. The bonded interface is normally very clear, so the bond is in fact invisible.
Paul’s Story Bridge replica will be on display in The Edge for the next few months! Make sure you pop down and check it out!