Generally, the '-er' suffix is attached to a verb to indicate occupation, so that 'finger,' 'grocer' and 'hammer' as nouns (granted, there is 'to hammer' as a verb) would not qualify. Most often if not always, from such as (to) hammer - as in 'hammerer'; (to) teach; (to) write; (to) speak; (to) drive; (to) manufacture; etc. (Of course: '-or' serves as the alternative suffix to '-er' when the phonetics of the case call for a sound more convenient than the '-er' sound, as in (to) direct; (to) conduct; (to) operate; (to) supervise; (to) manipulate; etc. Exceptions of verbs where neither '-er' nor '-or' apply include (to) study, as in 'student,' so much preferable to 'studier'; etc. BTW, where occupations involve nouns in the sciences, etc., '-cist' '-ian' and even '-er,' or '-or' apply : Editorialist; Scientist; Economist; Statistician; Optician; (politician, even); Astronomer; Doctor; etc.
