Beaver away
Definition 1 of 1
Pro Tip 1/3Continuous Effort
Implies ongoing, persistent work, like a beaver building a dam. 
She's been beavering away at her thesis for months.
Pro Tip 2/3Neutral Tone
Not formal or informal, can be used in various contexts. 
During the team meeting, he said he'd been beavering away on the project.
Pro Tip 3/3No Negative Connotation
Doesn't suggest the work is unpleasant, just hard or diligent. 
After beavering away all week, they were ready for the launch.