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I am surprised that no-one has come up with a method of 3D scanning and printing of knobs. Saying that you probably will still have the problem of mounting shafts etc as has been mentioned previously.
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Someone has. Forum member Jon, 'Ti Pwun', produces a small rane of 3-D printed replica items for Hacker & Roberts radios, such as these Roberts knobs (Sold out):
https://jonsprintshop.com/index.php?...hig3lgnut09d12
The original Roberts knob was a very poor design from an engineering point of view, as illustrated below with an original R200 broken knob, (still with it's knob 'bright!) alongside a replica I cast, but with a brass insert and grub screw. Tightening the grub screw into the captive nut on the original knob was destined to crack the fragile Bakelite off and that's what often happens, not to mention the knob 'brights' getting lost.
With any type of knob, if cosmetic appearance is of paramount importance rather than just functionality, generally (as with the Roberts knobs), if one is missing or damaged, you can't make an exact replacement, so would need a full set, the cost of which could well exceed what the set is worth. (I don't think I've ever paid more than a fiver for a Roberts set, nor would I).
It can end up being a lose-lose situation - not economically viable as a commercial proposition, yet too expensive to produce to be attractively priced for anything but the most expensive sets. (The same applies to other production methods of course, such as casting.
From what I've seen of 3-D printed items (not radio related), apart from the complexity for 'the lower orders' such as myself when it comes to the software aspects of scanning items for the 3-D items, there are two limitations: Firstly, you don't get a smooth finish as you would with cast items - there are tiny 'striations' from when the nozzle laid down the molten plastic filament. That might not matter with something like Roberts knobs, which have ribbed sides, and little else showing, but it would be very noticeable with others styles. Maybe the 'striations' can be smoothed out somehow - I've no idea.
Secondly, you can't easily replicate original colours, (as you can with cast materials). For example, at this link, there are 38 colours of PLA filament (Polylactic acid), but apart from black, white, grey, and maybe red, few other colours that could be used to replicate knobs for radios or test gear.
One thing that can certainly be said for PLA is that it's very tough stuff and has applications in the hobby that go beyond just replica knobs.