Exegesis:

The structure of v. 17 is, syntactically, as follows: hou … autou is a relative clause, ‘in whose hand (is) the shovel,’ with estin ‘is’ understood; this is followed by two final infinitives: diakatharai ‘in order to clean,’ and sunagagein ‘in order to bring together,’ both dependent upon the preceding clause. Then follows an independent clause to de achuron … asbestō ‘but the chaff he will burn with unquenchable fire.’ This structure, brings out that the primary purpose of the winnowing is to clean and to gather the grain (indicated by the two final infinitives), but that he will inevitably also burn the chaff (indicated by co-ordinate clause).

hou … autou ‘whose … his.’ The relative pronoun hou goes either with to ptuon ‘whose shovel’ (so, with shift from the relative to independent clause, Revised Standard Version, Translator’s New Testament, New English Bible) or with cheiri ‘in whose hand’ (so Nieuwe Vertaling, Willibrord, Bible de Jérusalem, Zürcher Bibel). Its antecedent is autos in the preceding clause and to this autou also refers. Clauses of this type are best understood as Hebraistic and the relative pronoun refers either in a general way to the clause as a whole (like the relative noun ʾasher) or is taken up again by the personal pronoun. It is therefore preferable to connect hou with cheiri, and not with to ptuon.

ptuon (only here and parallel Mt. 3.12 in the N.T.) ‘(winnowing) shovel,’ a fork-like instrument with which the threshed grain was thrown into the wind. Here as in Jer. 15.7 the winnowing symbolizes judgment.

diakatharai tēn halōna autou ‘in order to clean his threshing-floor,’ indicating the purpose of the winnowing announced in the preceding clause.

diakathairō ‘to clean out,’ ‘to clear.’

halōn ‘threshing-floor,’ or ‘threshed grain still lying on the threshing floor.’ The latter is preferable.

sunagagein ton siton ‘in order to gather the grain.’

sunagō ‘to gather,’ ‘to bring together,’ ‘to call together.’

sitos ‘grain,’ either a specific sort of grain such as ‘wheat,’ or grain in general.

eis tēn apothēkēn autou ‘into his barn,’ or, ‘granary.’

apothēkē ‘storage place,’ here for grain, ‘barn,’ ‘granary,’ probably a large pit with a building overhead (cf. IDB II, 469 s.v. ‘granary’).

to de achuron katakausei ‘but the chaff he will burn.’ The place of achuron in the clause indicates emphasis, cf. Bible de Jérusalem, ‘and as to the chaff, he will burn it.’

achuron ‘chaff.’

katakauō ‘consume with fire,’ ‘burn up.’

puri asbestō ‘with unquenchable fire’ or “fire that can never go out” (New English Bible), i.e. the fire of everlasting judgment (cf. Is. 66.24), or of hell (cf. Mk. 9.43).

Translation:

One can use this metaphor in many cultures, where necessary shifting to another cereal, such as rice, see below on “wheat”. Such a shift usually entails adjustment in the rendering of the accompanying terms; in some cases the tools and processes can be referred to by terms associated with rice, although for the cereal the term ‘wheat,’ known as a foreign product, is used. Where such rather close cultural equivalents are lacking, the translator may choose some other kind of fruit or agricultural product that is stored after its waste products have been removed, adjusting the terms for tools and processes accordingly. In some cases translators have introduced foreign terms, e.g. in Pohnpeian and Trukese, transliterating English ‘shovel’ and ‘wheat,’ and clarifying the winnowing process by illustrations.

His winnowing fork is in his hand, or, ‘his hands already hold the winnowing basket’ (Balinese). Often it is better to have a personal subject, e.g. ‘he is holding the winnowing fork in his hands,’ ‘he has already taken the winnowing basket’ (Manobo), where necessary shifting to a simile, e.g. ‘he is like a man who is holding….’ Winnowing fork, or, ‘(winnowing) basket/tray,’ or an instrumental derivation of ‘to winnow’ (Batak Toba). Where such a technical term is lacking one may use a descriptive phrase, e.g. ‘dirt shaking out thing’ (Ekari), or a generic term such as ‘tool’ (Kekchi), ‘thing’ (Kituba), the specific use of which is sufficiently made clear by the subsequent clauses.

Wheat, or the (indigenous or borrowed) name of another small grain used for food in the culture or known to the receptors as being used for food in other cultures, e.g. ‘rice,’ ‘millet,’ ‘barley,’ ‘corn/maize’; or an expression built upon such a name, such as ‘seed like rice’ (Korku), ‘foreign rice’ (Uab Meto).

Clear his threshing floor, or, preferring the other meaning of Gr. halōn, ‘winnow (or, sift) his threshed grain,’ cf. ‘winnow what-has-been-threshed (lit. trodden-out)’ (Toraja-Sa’dan), ‘remove hulls of rice that he has pounded’ (Kituba), or the reference to threshing or its equivalent, ‘clean-well skin-of wheat’ (Trukese), ‘fan the rice’ (Sranan Tongo). Some versions apparently try to combine the two meanings that Gr. halōn can have, cf. ‘winnow the heap on the threshing floor’ (Kannada), ‘clean his wheats in their place’ (Pohnpeian).

Gather … into his granary, or, ‘into his house-of storage’ (Pohnpeian, Trukese), ‘into the storeroom,’ or simply, ‘into its place’; cf. also on “barns” in 12.18. In some places threshing or its equivalent is not done before but after the storing in the granary, e.g. in Bali, where bunches of rice are kept in the rice-barn unpounded, and every few days the quantity needed is brought down, pounded and winnowed. Hence the phrase in question had to be rendered in Balinese by, ‘the grains … he gathers and stores-away’ (i.e. in a big vessel in the house where the pounded rice to be used in the next days is kept).

The term the chaff summarizes all the waste products, i.e. empty ears, stalks and straw, and can, if necessary, be described with one or more of these terms, cf. ‘its empty ones and straw’ (Toraja-Sa’dan), ‘the straw’ (Manobo), ‘the hulls’ (Kituba, similarly Ekari, Pohnpeian).

Unquenchable, or, ‘not to be put out,’ or, in active construction, ‘that is not dying’ (Kituba), ‘that never goes out’ (Nyakyusa), ‘that you can’t quench (lit. kill)’ (Sranan Tongo).

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.