The affix ukañ comes after the word karmaṇa in the same sense of 'able to effect that'.,
This debars ठञ्. As कर्मणो प्रभवति = कार्मुकं धनुः ॥ This word कार्मुकम् means always a 'bow', and is never employed to designate anything else.,
कर्मण उकञ्
Adhyāya 5 · Pāda 1 · Rule 103