When surface engraving metals, you may consider using a rotating diamond cutter. A diamond cutter provides a wider scratch in comparison to drag diamonds, and you can rotate the diamond to yield a deeper and wider cut line.
If depth is desired to paint-fill the engraving (at least .005"), you should use a carbide cutter. Carbide cutters are available with a range of tip sizes and cutting angles.
For best engraving results, select the cutter designed for your material (soft metals or hard metals). Stainless steel calls for more specialized quarter-round cutters and lubrication. Most brass available on the market today contains little lead; therefore, you should use lubricant to obtain the best cut and cutter life. Feed rates and cutter rotation speeds should be adjusted for the cleanest cuts. Engraving harder metals requires you to increase the spindle downforce spring in order to force the cutter into the material. If spring tension is too light, the cutter may not achieve, or maintain, a consistent cutting depth.
—Anthony Harris, Gravotech