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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?

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    "I need to drill a hole in the firewall to run a new harness" Is that even legal? The whole point of a firewall is to provide an unbroken wall to ward against fire. – nick012000 Apr 14 at 5:06
  • @nick012000 Do you know of a law? From Wikipedia - “The name originates from steam-powered vehicles, where the firewall separated the driver from the fire heating the boiler.” – HandyHowie Apr 14 at 11:58
  • @HandyHowie There might be roadworthiness regulations that it might break, if it would cause a hazard in the event of a collision. If you're trying to connect the engine well with the passenger compartment, you might be violating carbon monoxide regulations, etc. Quoting wikipedia (en.wikipedia.org/wiki/Firewall_(construction)): "Automotive firewalls have to be fitted so that they form a complete seal. Usually this is done by bonding the sheet metal to the bodywork using fibreglass resin." – nick012000 Apr 14 at 12:16
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    @nick012000 That section in Wikipedia is talking about competition rules. You may be correct, but normal road cars have lots of holes in the bulkhead that are not fireproof. I’d be surprised if adding another hole with a grommet for an extra harness was illegal in a road car. – HandyHowie Apr 14 at 12:38
  • Ideally you find an existing hole and feed your new wires through there. – Criggie Apr 15 at 12:48
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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 Step drill bit

Chassis punches

Chassis punches

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    The hole required for those chassis punches is the clearance hole for the bolt operating them - that way the hole is centred on the hole centre as punched. If you make a larger hole then the final hole can be off-centre and I had and still have a set of those that were used to make holes in car wings for aerials. – Solar Mike Apr 13 at 19:03
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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.

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