If you drill large holes in thin sheet metal, the result is unsatisfactory: The brinks are ragged and wavy.
The process is essentially better done with a step drill and the result looks more pleasing.
For the following steps I usually use no drill press. It is enough by drilling in alumnium to wedge the drills into a file handle. In addition curved sheet metals can not be clamped in a drill stand. Only by drilling in steel I use a drilling machine.
If you want to work on very thin sheet metals (< 0.5 mm), then put before boring a thicker sheet metal under it and wedge both together with one or two small fastening clamps. Otherwise it goes without.
Step drills are only usable starting from hole diameters of 4 mm. Therefore a small drill is necessary for better guidance with a normal metal drill. Mark the center of the drilling with a center punch. Also a pointed thin nail goes. Afterwards bore trough the sheet metal in the marked place with an 1 mm drill. Now extend this drilling in two further steps with suit drills first on 2 mm and then on 3 mm.
Now the drilling is large enough, in order to use the step drill. Turn the drill carefully under constant pressure always in the full circle. Especially when boring by hand avoid it to make only half rounds and to turn instead the drill forward and backwards! Otherwise no round holes develop.
If you nevertheless uses a drill press, then use with steel sheets at the most 500 revolutions per minute, with VA steel even only 250. When boring in aluminum proceed carefully, so that you don't implement several stages at one time with the drill and at the end becomes the hole too large.
Usually all shops, which act with tools, offers also such step drills. There are it in two constructions: with 1 mm steps and with 2 mm steps. The first ones can be used only for holes up to 12 mm, whereas the other ones are usable mostly up to 20 mm.