French term (edited): posage d'usinage

On the basis of what you have identified as the meaning, I'd expect this to be called a 'jig' — something that in some way holds something in the right place etc. for machining.
Another term also used is 'machining fixture' — especially appropriate, perhaps, where the notion of 'fixing' comes in. This might perhaps be what you need for 'montage' to distinguish it from 'posage'.

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Note added at 1 hr (2018-04-04 10:23:27 GMT)
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To answer Marie-Christine's point about 'gabarit' — in engineering, and in both FR and EN, these sorts of terms are used in many different ways: a 'jig' may be for fixing / holding something, or for measuring something; in the former sense, it can also be referred to as a 'fixture'; in the latter, as a 'gauge'.
Likewise, a 'gabarit' can be a 'template' or a 'model', and even a 'gauge' too — cf. railways.
As so often, there is no such thing as a simplistic, 1:1 correspondence between these terms when you start getting right down into the nitty-gritty of the subject — and believe me, I've been getting my hands dirty in engineering long enough to be all too painfully aware of this!