I had a step drill whose smallest step was 1/4" - this seemed as if it would be OK for enlarging the #30 hole to 1/4" when shaping the HS forward spar bend relief, but going directly to 1/4" resulted in the step drill slipping and I felt that if I continued with it I could irrevocably damage the part.
A quick web search later and I had ordered a set of three metal cutting imperial step drills from Amazon - bargain!
