unless the brass sheet is perfectly flat you will get uneven engraving. A better result would be to use a drag engraver. Best to use a dedicated tool that will allow for uneven sheet, I think they use some form of spring loading.
On my gravograph the cutter has a holder that has a larger face area that bears onto the sheet to be engraved with just the tip of the cutter showing this allows the depth of the engraving to be adjusted, only downside is that it can sometimes mark the surface.

Peter