The actual mechanism of desire according to Gilles Deleuze and Félix Guattari. Deleuze and Guattari give as their example the machine formed by the child and the maternal breast. The idea seems to derive in part from a reaction to Melanie Klein's overly Oedipal theory of object relations (particularly her case study of Richard), but more directly from Bruno Bettelheim's case study of a schizophrenic boy, little Joey, in The Empty Fortress (1967). Looking at these case studies of how children play, Deleuze and Guattari argue that the objects that the children play with are not symbolic (e.g. they aren't representatives of the phallus), but machinic, meaning they give desire the means to not merely express itself, but form something constructive. It is in light of this they would later describe their work as constructivist. The concept first appears in L'Anti-Oedipe (1972) translated as Anti-Oedipus (1977), but interestingly is dropped thereafter. In the sequel to Anti-Oedipus, Mille Plateaux (1980), translated as A Thousand Plateaus (1987), they instead speak of abstract machines and assemblages, but they retain the core idea that desire's basic function is to assemble and render machinic.
I. Buchanan Deleuze and Guattari's Anti-Oedipus (2008).