For fine engraving work often a floating head is used. The reason you need a floating head/depth controller is simple. When making for example a line thickness of 0.2mm with a V engraving bit, the width of the line will change if the table or material is not exactly flat. Since no table of a CNC machine is flat over the entire surface within a hundreds of a mm., a solution needs to be found to keep the tool at the exact same depth over the entire area. This floating head offers this solution. Using a fine thread the depth can be adjusted. Then since the head is mounted using springs, it's possible to engrave even if the surface is not exactly flat, a few mm deviation is possible. To prevent scratching of the surface, using a spring and thread, it's also possible to adjust the surface pressure. Also the engraving head can be locked in case you need to do normal milling or drilling, so it is not required to remove the floating head when not engraving.