Imagine a space marine chapter which has lost contact with most of the Imperium (in particular, with Mars and other forge worlds) for centuries or even millenia, or a chapter which has fallen out with the Adeptus Mechanicus and can no longer send techmarines to it for training.
Assuming such a chapter had a well-staffed, knowledgeable Armoury to start with, is it possible for its techmarines to train new techmarines? Simply take the battle brothers who would normally be sent to Adeptus Mechanicus for training, and train them fully within the Armoury itself.
Assume the techmarines and the entire chapter are pragmatic enough to not care about the formal/ritualistic aspect of things, as long as the end result functions fully equivalently to a "standard" techmarine.
I've seen the related question on obscure chapters obtaining techmarines, but that (and its answer) are geared towards "how to get to the Adeptus Mechanicus when you normally can't", while I'm more interested in "what do you do when you really can't."