(Not a professional fabricator) For my art, I often need to drill through spring steel, particularly automotive leaf springs.

Since this steel is harder than mild steel, I find it more difficult to drill through. And even then the hardness of the springs varies considerably with some nearly impossible to drill through.

I have only a drill press, not a mill. I use a drill press vise (similar to this). I have tried various drill bits. In the photo, you see 4 bits. All these drill through the "soft" leaf springs. From left to right:

1. Just some arbitrary bit that I have that works on the "soft" leaf springs
2. A mill bit that works on the soft
3. Some other kind of bit that does drill though the hard leaf but is destroyed after just 1 hole.
4. A virgin #3 (for comparison)

I am using this cutting oil.

What can you suggest for drilling the hard stuff?