Thetis had the gear made for Achilles because his first set was worn by Patroclus when he went to battle and taken by Hector when he killed Patroclus.
θˈɛɾiz hæd ðə ɡˈɪɹ mˌeɪd fɔːɹ ɐkˈɪliːz bɪkˈʌz hɪz fˈɜːst sˈɛt wʌz wˈoːɹn baɪ pˈætɹɑːkləs wɛn hiː wɛnt tə bˈæɾəl ænd tˈeɪkən baɪ hˈɛktɚ wɛn hiː kˈɪld pˈætɹɑːkləs.