Update on Structures

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Our first update on technical things. This time from the structures team looking at the process of drilling sensitive optical components – at PROVE Pathfinder, that means lenses! Both our thermal infrared (TIR) and our optical (VIS) camera lenses are Commercial Off The Shelf (COTS) and are not designed to be used in a vacuum environment! This results in several sealed air volumes that have to be vented, and the way to do this is by drilling a small hole into the lens.

We started by taking x-ray images of each lens in our Physics department, and from that, inferring how many volumes needed venting. We show below our visual lens – guess how many volumes we need to vent?

Figure 1: X-ray image of the visual camera lens.

We thought 3, and so spent several hours drilling holes in the lens through layers of aluminium and air voids (and Teflon in the TIR lens).

Figure 2: Drilling process of lens for venting.

After this, it was noticed that there was a substantial amount of swarf (debris from drilling) inside the lens. Clearly this wasn’t the best way to do this, and as such, we needed to find a better way to vent the lens.

Figure 3: Looking through the optical lens, the swarf can be seen.

Upon consultation with staff and some members of our technical team, we decided to attempt to dissemble the lens to allow us to clean the swarf away, and to drill the second and third lenses with creation of minimal debris.

By creation of custom jigs to bind into both lenses, a lathe, a strap wrench, and a concerning amount of torque, the lenses were opened. The strap wrench was used so the torque could be spread over the whole circumference to avoid crushing the lenses like a traditional clamp would.

Figure 4: Strap wrench used to disassemble the lens
Figure 5: Lens jigs to fix the end of the lens to allow disassembly
FIgure 6: Lens jig assembled with the back of the visual lens

This allowed us to clean out the debris that we could, and disable the lenses as far as possible, but still the trapped volumes couldn’t be vented in this way – the first and third sections are still contained volumes. This does, however, allow us to drill through just one layer of material.

Figure 7: Disassembled optical camera lens

Following this, we devised a plan to allow debris to be minimised. In order:

  1. The lens is opened as much as possible using the procedure briefly described above.
  2. The lens is drilled straight down using a 2 mm diameter drill bit with airflow perpendicular to the surface to aid in swarf removal. This hole does NOT pierce the surface, leaving it in the region of the last 0.2 mm of depth.
  3. The last 0.2 mm of hole depth was drilled using a 0.6 mm diameter custom carbide drill bit. This ensured the volume of debris was at most 28 x 10-3 mm3, but when performing this, we observed the debris coming out of the hole and not going into the lens.
Figure 8: Practise hole on the bottom to investigate the different layers, the 2/0.6mm procedure was used on the top

It is particularly important to note that the 0.6mm drill bit should be fresh and as sharp as possible to give the best chance for a clean hole and zero swarf.