Initial Prototyping - Day 2

Tuesday was a short day, I was only in for the morning. However I was faced with the biggest challenge of the project. The design of the lens is convex convex, with a 2mm ledge joining them. The question is, one you've turned the front face, how the hell do you turn the back face??? After talking over a ton of ideas including sticking the lens to an inverse version using industrial double sided tape, or making soft jaws that I could cut into at the same time as cutting the lens, we settled on Gary's idea of tooling a type of collet to hold the outside rim of the lens. The theory is, if it holds 1mm of the 2mm ledge, all the way around the diameter of the lens, then if the bush is exactly the correct size, it will generate enough friction to hold the lens in place while machining.

Gary made this piece and had it turned down so it could fit into both the manual lathes and the smaller chuck of the CNC, hence the step down at the back. It also has 3 slots cut down the barrel of the collet, allowing it to crush when tightly secured in the lathe, crushing down around the lens and holding it in place so there was no chance of the lathe tool knocking the lens off axis (which would ruin the system as the front and back faces need to be perfectly inline).

Here you can see the perfect fit of the processed billet being inserted into the bush/collet. Because of the perfect size the lens would slide in and out without trouble, but when crushed in the chuck there was no chance of it moving. Making the collet took all day but was worth it as it can be used over and over again for any lens with a 50mm diameter.