A diamond bit is going to cut very shallow and may or may not give the results you are looking for?... a small dia. end mill or a half round conical engraving cutter 60degree .005 tip (for small text) may give better results with deeper engraving with a light sanding or polish to de-bur before anodizing, that's one of the main uses for these cutters. Proper feed and speed with a clean up pass leaves very little bur.http://www.2linc.com/engraving/conical_1-4.htm
.......They sell ones for soft and hard materials (S is what I generally use)
This company sells spring loaded engraving tool for uneven parts but they are pricy and designed for fairly low rpms on the smaller versions........ I just use the cutters for engraving flat material (acrylic, aluminum, wood) at high feeds and 10,000-15,000 rpm.
If you have a diamond drag bit, try it... if you don't like the depth I would try the conical bits or small ball designed for aluminum (coolant will help prevent clogging the mill, I use the conical bits dry)
Many other company's sell as well.