Cleanly drill firewall

Question

I recently drilled a 1/2in hole in the sheet metal on the inside of a door panel, and the result was less of a hole and more of an ugly, asymmetrical gash as the metal deformed under the stress of the drill. I had to use cutters to cleanup metal which bent out of plane.
Now I need to drill a hole in the firewall to run a new harness, and I'd like to do it right this time. How can I ensure that I drill a clean, circular, 1/2in - 1in hole in the firewall? Should I use a specialized bit? Do I run the drill at max speed?

MTA · Accepted Answer

To make a clean hole in sheet metal, use a step drill bit. Put a dab of oil on the drill bit and keep the speed down. To make a hole larger than your largest step drill bit, first make a hole with your step drill bit, then use a chassis punch, which requires access to both sides of the sheet metal.
Step drill bit

Chassis punches

HandyHowie · Answer

In metalwork class at school we were shown how to drill round holes in thin sheet steel with normal metal twist drills.  After drilling a small pilot hole, the teacher cut a small piece of Emery cloth and placed it abrasive side down between the drill bit and the sheet steel.  The drill bit then produced a perfect round hole.
This article suggests the same thing, but with just a small piece of cloth, not emery cloth.

Cleanly drill firewall

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