When a player taps the Grindclock ability to pull X cards from a player's library and place them in the graveyard, does the counter reset?
In MTG, counters are physical markers that are counted, added and removed; they are not abstractions that store a number that can be manipulated. As such, it makes no sense to talk about resetting a counter; you want to know if the counters are removed.
Nothing instructs you to remove the the counters, so you do not remove them. Just like attacking doesn't remove +1/+1 counters, using Grindclock's ability does not remove charge counters.
The counters on Grindclock do not reset. In fact, "Reset" is not a special keyword that means anything to a Magic player. The short answer is that if nothing says to remove counters, they don't get removed. The long answer is that they remain until one of two things happen:
- An effect removes the counters
- The object upon which the counters were placed cease to exist
An example of the first bullet is Power Conduit, which has an activated ability that removes counters. Coincidentally, it creates counters that are interchangeable with those generated by Grindclock.
An example of the second bullet would be if I cast Naturalize on your Grindclock. Your Grindclock would be destroyed. Any counters on it cease to exist. This is not the same as "removing" counters.