There is a skin on rolled a/iron. Try firm pressure and stop/start the drill to keep the speed down to just tuning till you break the into the softer stuff inside.
I tried a machine but it was rubbish the stones were too coarse. Best of luck.
On the the other hand one of the shopping channels occasionally has sets of drills that go through Breeze block, glass, brick iron and steel sandwiches like butter. Must get some.