I needed to go back to tap the thermistor thread in one of the Coaxial8ors and, while I was doing that, I thought "I just need a pilot thread to get started and I can be on my way", referencing to tapping the plates. I soon had the idea to print such a jig, but first I tested the idea out with an M6 bolt and learned that I'd need to consider how to keep it in place.

Me1: If only I had a plate with a bunch of starter threads. Then I could put them both in a vice.

Me2: We have plates with starter threads at home.

The plates with starter threads at home:

Yes, I retrieved and cleaned up the clamp plate from Coaxial8or r0. I pried off the plastic and cartridges from the clamp plate, then put it in the above tub along with a shallow amount of kettle water so that the PETG softened and I could clean off the rest.

Then I asked for some new plates to be cut, where the holes in the CAD were offset by 0.05 before being exported to .DXF for the waterjet:

I started off with the M3, and seeing that worked, tried a pilot M6 which seemed to work too. Thus, I switched back to the M3 tap, broke off a plate and threaded all the M3 holes on the new coupler plate.

I started the threads using the old coupler plate, then went back to finish them:

This plan all changed from the M4 tap, where the starting tap doesn't actually finish the threads like the M3 one. Only the finishing tap cuts the threads fully. I couldn't even properly thread in the finishing tap into the starter threads, so I instead decided to just use the finishing tap and fully cut the threads in one pass.

By the time I got to the all-important M6 threads, I had a strategy:

1. Get the tap into the cloner plate.
2. Align tap with the hole so there's no XY jiggle movement but the plates don't have a gap in them.
3. Place in vice.
4. Turn about 4 - 5 times.
5. Turn back by a quarter.
6. Turn until there's like 4ish threads left visible on the tap (see image below)
7. Turn back till about 2mm of the tap tip is visible.
8. Then you slowly turn until the tapping plate comes off. 
   1. It's then faster to start the next one because the tap height is approximately enough to have the two plates flush and there be no jiggle with the untapped hole.
9. Then it feels like a candy crush level selector as I go though the other 7 holes.

It took me 35 mins to thread the M6es of 2 plates. I think I switched to a new cloner starter tap every 12 or so threads to make sure that the tap was still going in straight.

This is the result:

Surely there's probably an actual specific plate that allows one to hand tap perfectly straight, but I don't know what it's called (or if it even exists).

In other news, I also printed the new holder+cover, and saw that the PTFE gasket actually got squashed thin from the top side:

[May 22] It all goes together, so this strategy seems to work at the moment: