Manufacturing techniques
Using a CNC mill to make components.
In this day and age you might at first think I'm going to 3d print components and I have considered this. Looking on Shapeways to find examples and trying to get the designer to make a 3mm version has proved fruitless. I also knew that I'd have to teach myself to use a CAD system to design any prints and then acquire a 3d printer and learn to use that. My first approach was to draw a flat 'kit' type design for the parts I would need on various sorts of drawing software that I was familiar with. This was printed out on labels and stuck to plasticard before being cut out and assembled...that works but it suffers from not only a bit too much hard work but more importantly, I can't cut it accurately to avoid having to 'fit' the pieces or fill holes. As a process It's just not accurate enough to make numerous examples, easily.
Looking for a way to cut out the parts in kit form led me to the card cutting hobby and a look at the Silhouette and Cricut machines. I know some people have got on well with these but I found they wouldn't cut the thickness of card I needed. I then considered milling the parts using a CNC (Computer Numerical Control) milling machine. That would cut the thickest of card although there is little online evidence of them being used for this purpose so I still didn't know if it would work...only one way to find out!
ICI hopper parts on Freecad.pdfI won't go down the long and winding road to explain my final decisions, suffice to say I ended up choosing a cheap Chinese CNC machine to test the practicality and using 'FreeCAD' to design the parts. The image to the left shows the parts as drawn to scale and laid out for cutting. There was a bit of trial and error resulting in several versions but that is how I learned to use the process. There is also a bit of 'Chicken and egg' in needing to know what the machine can do before you can produce a design which works but you can't cut anything until the design is sorted.
I started off simply by producing an overlay for the Tri-ang bogie which altered it's shape to one resembling a plate frame type. Two of these stuck either side of the central springs gave a good likeness of the original from the normal viewing distance. The first 8 for the prototype model were cut from 0.5mm plasticard. As I was building a batch of 5 hoppers, I needed 40 of these so automating their production was high on the agenda.
After drawing the part in FreeCAD it was cloned to make a job of four units. FreeCAD is able to generate the G code which is used to tell the mill how to cut the parts. The G code reaches the mill via some firmware called UGS, which controls the machine.
ICI hopper bogie sides on Freecad.pdf
One challenge to overcome was how to fasten a sheet of thin and flexible material to the machine bed which was able to withstand the cutting forces whilst being easy to get the parts off afterwards. Thanks to YouTube, I put masking tape on the machine bed and under the plasticard and then stuck the two, back to back with superglue.
It took some time to find the right cutter, cutting speed, feed rate and depth of cut to avoid a melted blob of plastic ruining the job but eventually I managed to produce the side frames and once painted, I suspect they'll do the job better than this cruel closeup of the original hand cut one shows.