Translation:My grandpa has a machine which makes tasty coffee.
Somehow you missed the 'b' in "robić", I don't know what your keyboard did :D
Yes, the verb "robić" takes Accusative, and "robiącą" here is an adjectival participle, which you can just imagine to be an adjective created from a verb. "robiącą" describes "maszynę", and the phrase "maszynę robiącą" is in Accusative because of the verb "mieć" (to have). Then "smaczną kawę" is also Accusative, but this time because of "robić".
thanks, Jellei! So those participles like robiąc work like adjectives here and get the endings according to the gender and case of the nouns they refer to. And they themselves can also produce case shifts, according to the (sorry, German term) Verbvalenz (should be the valency of the verb in English) of the verb, they derive from. It´s always helpful for visual learners like me to point these relationships out with arrows and highlighted text, which I hereby try to imagine in my head, as we don´t have those options yet in the discussion section.
btw: I really like the tipps each thematic subsection (or how they are called) explain those grammatical phenomena in other languages on Duolingo (for instance French). Is this planned to be implemented for Polish in future times? Would be really nice!
As long as we still wait for that, I can highly recommend this 500 pg. strong pdf offered by the University of Pittsburgh, which I stumbled upon in another discussion thread:
The participle system is explained on page 300 et seqq. - although the author(s) don´t offer a more visual based approach for persons like me (but then the pdf would have gotten even more volumenous...)
The link shows an error for me.
Actually "robiąc" is a different thing, "robiąc" is an adverbial participle, translating to "while doing"/"while making". Adverbial participles do not undergo declension.
"Robiąc kanapki, słuchałem muzyki" = "While making sandwiches, I was listening to music".
The basic form for the adjectival participle, as we usually consider masculine the default form, would be "robiący" - this one behaves like an adjective.