The reason for this phase of prototyping is to address the surface quality issues which caused my last 2 lenses to under perform compared to their design specifications. After looking at the processes I used to make the optics, I concluded that hand polishing the elements was the cause of the problem, as it resulted in uneven deformation of the elements surface curvatures. The solution to this problem was obvious, the optics had to be machine polished.
To do this I would use the CNC lathe, and include a polishing process directly after cutting the face of the element. This would involve running a custom polishing tool along the same profile used for the cutting process, resulting in even polishing across the face.
The CNC machine isn't really designed for polishing, so there isn't a standard tool provided with it that does what I want. Therefore, I have to design and make my own to insert into the auto tool turret. The requirements are:
- The tool needs to be sprung so that it can adjust as the surface is worn down. This removes the need for complicated and unpredictable x axis programming to ensure that the pad is always in contact at the right pressure with the face of the optic.
- The head/pad needs to pivot so that it is always in contact at the correct angle with the profile of the surface curvature.
- Needs to have easily removable/replaceable pads which can take discs of abrasive paper from 240 upto potentially 12000.
- Thin tip to ensure the pad can pivot back on it's self without exerting excess pressure on the face when polishing concave faces.
In the end I cut a sprung shock-absorber/damper on the manual lathe with a thin blunt tip, which I then placed the pads onto using the worlds most trusty material... bluetack. While a little primitive, it actually worked a treat. Because the machine isn't really designed for complexly new custom tools, I had to tell the control software that the polishing bit was in fact a boring tool. This is the easiest to configure to fool the machine into running on the path I want without the software throwing up error. This is why on the screen, the software simulations of the polishing process shows a pointed tip tool rather than a pad.
The tool worked well, with bluetack providing a surprisingly good support for the pad, as it kept it held onto the tool as it naturally pivoted back and forth with the surface curvature. The spring system worked well also, allowing the pad the freedom to find it's own pressure. Although, I am a little concerned that the spring was compressing when the tool passed near the edges, but this was being caused by the blue tack restricting the pads movement as the angle increased. After a few passes across the optic, the bluetack moulded to the form of the tool causing less compression. I programmed the machine to run the polishing pad up and down the face of the optic 5 times before pulling out and pausing the spindle so that lubricant can be added, or the pad could be changes/cleaned.
Below is a video of the tool in action (skip to the end to see the resultant optic after having been machine polished and then passed under the buffing wheel to remove micro scratches):
