Factorio Forums

I've noticed ( or failed to notice during the 300+ hrs playing) in .15, the mining drill has gained a purple bar, what is this?

It is the productivity from the mining productivity research. It works in the same way as the purple modules:

When the purple bar get full you get an extra ore for free.
More research=faster bar.

And you can also further improve productivity with Productivity Modules, but those slow down mining speed. Those can be used to make mines last longer, but have a slower output (useful for uranium mines).