The difficulty here is the group, or cluster, of consonants in the middle of “grindstone”. Just take out the /d/ sound and it gets a lot easier, with no risk of an extra syllable creeping in. Notice the weak vowel in “to” as well.
This strange idiom in full is “keep your nose to the grindstone”, and means “continue working”, or “work harder”. It’s often, even usually, used with irony.