Monday morning 8:30am and I find myself back at my old school Altrincham Grammar School for Boys in south Manchester, UK. With all the materials I need and all the schools machines available to me to use, I was set to get going. Big thanks to Alan Williams, Rob Baker, and Gary Binns for giving me the time, support, and access to machinery over the week; it's much appreciated!
Job number one was to turn down the reverse ring adapters. As you can see below right, the adapters arrived with their 67mm diameter threads protruding from their front face. I ordered this size specifically because I knew a ring with a 67mm thread would have an outside ring diameter of 70mm, ideal for my purposes. I mounted the rings in a brand new small high speed lathe and faced off the thread and front surface until smooth. Before and after shots, and how the process exposed the bare metal can be seen below.
To make the PMMA elements, I had intended to mount the acrylic rod into the lathe and turn down the face by hand according to a laser cut template of the surface curvature. However, luckily for me the school had a CNC lathe in the workshop not being used. From the sounds of it the machine hadn't been used for a proper job in a long time, so it took a bit of time working out how the system ran, but to me it was an absolute gift. It enabled me to export my elements surface curvature from my CAD directly into the lathes control software. From there I could cut the surface from a billet secured into the lathes chuck. The lathe is small however, and the max diameter chuck limits were around 40mm.
Instead of using the PMMA rod I had, I ended up using blocks of acrylic cut down to a cylindrical shape on a band saw and then turned down on a manual control lathe until I was left with 3 identical billets of 51mm diameter (.5mm extra all round for tolerance) with a step down to 30mm so that it could fit into the small lathe chuck. Below are images of the billets being turned down and them all stacked together:
Once the billets were ready it was simply a case of getting to grips with the CNC lathe software. Getting SolidWorks to talk to the gcode converter was pretty easy in the end, just a DXF export did the trick. The first design was so simple though we ended up rebuilding it with the new specification (f/1, 50mm entrance diameter) on separate software. We secured the billet, measured up the length of the protrusion, entered the numbers, and set the machine to cut the first front surface. The setup and the cutting process are shown below:
The result was good. At first there was a flat spot where the tool hadn't taken the surface off at the very centre of the lens, but after offsetting the cutting profile by a millimeter, the surface came out perfect. Using the CNC is good news for me because it's 100% accurate, very precise, as close to perfect as can be made with the resources I have. The results out of the machine were miles ahead of anything I could have achieved by cutting the profile on a manual lathe. The results are shown below, the profile clearly visible.
Finally, because i'm a bit keen, I decided to mount the cut billet into the high speed lathe and give it a quick polish. P1000, 6000, 12000 with water and soap, then onto the buffing wheel with plastic polish resulted in a near perfect finish. There were bits around the edges that needed a little cleaning up, but that could come later when polishing up properly. Below you can see the polished face next to a processed and an unprocessed billet.
End of day one! That was the easy bit done, now I had to magic up a way to cut the other side of the lens.... how could we mount the cut side in the lathe?? That was Tuesdays challenge.
