Heat does not seem to be an issue, in fact I have used heat to soften and repair some chips. The important thing is to support the sign so it does not bend during drilling. Any deflection of the metal will chip off chunks. The last drilling I did was one of my homemade address numbers that I forgot to punch before enameling. Using a plain HSS bit and drilling on a steel plate the hole appeared clean with light chips around the edges.

Antique? It's new to me.