my glasses (pupdate)

The grade 5 titanium arrived and was clearly a much better fit for the frames than grade 1. It is far less ductile and has some of the 'springiness' of stainless. There isn't too much to say regarding process as it remains practically identical to my last post, which is the beauty of CNC machining. Modelling and setup take a long time, but you only really have to do it once barring minor changes to the part and stock. Speaking of minor changes, tabs were added to increase the surface area of the glue and the depth of cut was increased to ensure full penetration of the stock (kinky). 

This doesn't capture the silkiness of
the sandblasted finish 

fig 1.

This second run came very close to being the last but unfortunately there were some issues: 

1. On test fitting the arms whilst the part was in the machine it seemed tight. Instead of trusting the measurements we opened it up 0.25mm (the diameter of an accupuncture needle) and consequently the fit was sloppy which meant the arms had far too much play up and down. Edit: I've just realised that this probably meant 0.5mm total which would explain it 
2. The hinge snapped in exactly the same place as my original glasses (fig 1.). This is evidently a weak point which I will have to strengthen. I'm fairly confident that this piece can't be solid as I think it is used to open the frame up for lenses. 
3. On closer inspection of my canvas (shown in my last post) I had neglected to model a small indention of about 1mm at the end of the hinge which allows the arm to slot in and rotate around. This was cut with a dremel on this part but this is easily added to the model and fusion will update the toolpaths to suit automatically.

Solutions and other points:

1. For the fit of the arms, stick to the measurements! There is always self destructive tendency to assume you know better than the tools which is apparently emboldened by constantly being proven otherwise. 
2. For the snapped hinge there are a few options but I think the most logical is probably just to offset the toolpath by 0.5mm making the frames that much thicker. Titanium work hardens very easily. At a molecular level a work-hardenable (new word?) material exhibits a 'defect free pattern' which is to say it's molecules are aligned in a consistent pattern that is deformed when the material is work hardened through say, bending, squeezing, sheering etc. These deformations create obstacles and 'pinning points' in the materials structure which compound, impeding the motion of other molecules and therefore increase the materials strength through resistance to further change. Unfortunately by deforming the material and introducing these deformations, you also make the material more brittle as the molecules are less able to slide past one another as they do in a ductile (pliable) material. Instead the molecules are forced up agaisnt one another until breaking point and the material snaps as it did in the hinge of these glasses. If it seems like I just read the wikipedia article before writing that, it's because I did. Fortunately there is a process (and another wikipedia article) called Annealing that involves heating a material to above its recrystalisation temperature which allows the molecules to 'relax'  back into alignment reintroducing ductility. Unfortunately, for titanium this occurs at 600-750C which are the kind of temps I do not have access to. 
3. The finish this leaves had me swearing oaths. I'm not one for finishing (honesty of process and all that) but this may have changed my mind, it doesn't hurt that it's also a very practical way of removing burrs left by the machining process.