Wow, fascinating thread. What you are wanting to do is sometimes referred to as spiral, or helical, milling. As others have pointed out, 4 axis CNC provides a way of performing this – “thread milling” – which I have never done. With such a long component, it may present a challenge with tool overhang.
The picture you showed and much of the discussion is around conventional, manual spiral milling – which I have done, once. That is quite a fine thread to attempt by mechanical thread milling. For example the coarsest “thread” I would be able to mill is 8 tpi – using the standard gear set for my machine. The coarser of the threads you are looking at is between 12 & 13 tpi. I appreciate that what you want is a metric thread, but that can be converted using a 127 tooth gear (and add to the gear set to go beyond what it was intended for). There are probably numerous ways that the different manufacturer’s have configured their offerings – on my machine the short lead selections (less than 3” lead in one turn), is that the dividing head is turned by hand, which feeds back through the end gears to move the table. For longer leads (greater than 3”) the table is fed (hand or power) and the dividing head get turned by the gearing from the lead screw.
I get it that you have the opportunity to purchase all sorts of tooling for that mill you have, and you should seriously buy as much as you can afford. However, I would caution you against getting set up for spiral milling. There are many other milling machine operations that would typically be more common / useful than setting up for spiral milling. Typically spiral milling jobs would be “once in a blue moon”. Since you indicated you are tight on cash, I suggest you share what tooling you have the opportunity to purchase and a list of jobs you hope to undertake and the people in this community would be glad to recommend what would generally build that mill into a widely versatile machine.
As you said in your first post (and numerous others have agreed) – what you want to do, would be way easier on a lathe. You said you had seen lathe(s) in your area starting at about $2000 (and you would need to add some for tooling, depending on what it came with) would be less expensive than most of what you have been considering to make that part with out a lathe. The people who have replied that what you want to do is “possible” are correct. I agree it is
possible, but wow. Certainly it would depend on what opportunities you are able to track down – buy cheap, fix, repurpose etc. However, if you are going from scratch that is a serious undertaking. One consideration I have not seen anyone share is that you could use your mill to make a “lathe like” machine. More like a threading bench than a lathe. Perhaps a thread chasing set up instead of end gears, lead screw etc.
I have never seen any active machining set ups (i.e. home hobby shops) with just a milling machine. I have seen numerous with just a lathe and various small support machines – that works well (that is all I had for many years). Perhaps others here could comment on the versatility of mill only work shops?
Attached are pictures of my one spiral milling job, recutting the ramps on a Reeves drive (mechanical variable speed pulley).
Let us know how you make out?