To Define the Starting Point for Drilling
For each hole selected to be drilled, the system has to determine the starting point for drilling (output in the GOTO statement). This point is determined differently for different drilling types.
For Countersink drilling, you must specify the start surface. You can specify a surface or create a datum plane whose intersection with the hole axis will be used as the starting point. For blind holes, the starting point is specified automatically.
For geometry hole sets, you must define a start surface to set the hole depth. On the Depth tabbed page of the Holes dialog box, click 
to select the start surface.
to select the start surface.To define the depth for Blind drilling, you must specify the start surface after you specify the starting point. On the Depth tabbed page of the Holes dialog box, click 
to select a end surface or click 
and enter the drill depth. If you select a non-planar surface, the depth is defined by the point of intersection of this surface with the hole axis.
to select a end surface or click and enter the drill depth. If you select a non-planar surface, the depth is defined by the point of intersection of this surface with the hole axis.For other depth types, the starting point will be automatically determined as the point of intersection of the axis with the topmost surface among the workpiece(s) and reference part(s) that the hole intersects.
By default, all the reference parts and workpieces are used for determining the starting point; you can unselect some of the parts, if desired, or explicitly select parts, by using the Models collector of the Options tab in the Holes dialog box. The All Models option selects all the reference parts and workpieces. When you select a hole to drill, the system will look for the appropriate surface among the selected parts, and use its intersection with the hole axis as the starting point.
1. Selected axis
2. Default starting point (both the workpiece and the reference part are considered; top of workpiece is used)
3. Unselect the workpiece
4. New starting point (top of reference part is used)