What does 5-∅5.5 hole thru 90° c'sink ∅10.4 mean on the drawing?

This notation is a concise callout for a specific, multi-step machining operation on an engineering drawing. It describes a hole that begins as a pilot hole, is enlarged, and is finished with a countersink. The instruction "5-∅5.5 hole thru 90° c'sink ∅10.4" should be interpreted as a sequence: first, produce five holes, each with a diameter (∅) of 5.5 millimeters, drilled completely through the material ("thru"). Subsequently, each of these five holes is to have a 90-degree countersink applied to its entrance (and potentially its exit, though typically only one side unless specified) to a major diameter of 10.4 millimeters. The depth of the countersink is implicitly defined by the 90-degree angle and the 10.4mm diameter; where the cone of the countersink meets the 5.5mm hole, that intersection point sets the depth automatically.

The mechanism and geometry are precise. The 90-degree angle refers to the included angle of the countersinking tool, meaning the sides of the conical depression form a 90-degree angle to each other. Given a 5.5mm pilot hole and a target outer diameter of 10.4mm for the countersink, the depth of the conical cut can be calculated. Since the countersink angle is measured between opposite sides, the tool's cutting edge forms a 45-degree angle relative to the axis of the hole. The required depth is therefore the radial difference between the countersink's radius and the hole's radius ( (10.4/2) - (5.5/2) = 2.45mm ) divided by the tangent of 45 degrees. As tan(45°) equals 1, the depth is simply 2.45 millimeters. This creates a conical seat perfectly sized for a flat-head screw with a 90-degree head angle and a 10.4mm head diameter, ensuring the screw sits flush with or below the surface.

The implications for manufacturing are significant. This callout dictates a two-tool process: first a 5.5mm drill bit, then a 90-degree countersink tool adjusted or selected to produce a 10.4mm diameter. The order is critical, as countersinking first would leave no center for the drill. The notation's compactness places responsibility on the machinist to understand the sequence and the resulting geometry. It also specifies a critical fit: the 10.4mm diameter is the final controlling dimension for the screw head's seating surface, which is often more critical for assembly than the theoretical cone depth. A deviation in the pilot hole diameter would alter the depth at which the 10.4mm diameter is achieved, potentially affecting the screw's protrusion or recess.

In practical terms, this specification ensures proper function in assembly, typically for securing components with flat-head fasteners in a flush manner. The "thru" designation means the 5.5mm hole passes through the entire workpiece, which may indicate the fastener is intended to clamp another part or that a nut will be used on the opposite side. The presence of five such holes implies a bolt circle or a specific pattern for a cover, plate, or housing. The precision in the callout—specifying the final countersink diameter rather than just the angle—removes ambiguity, controlling the final appearance and function more tightly than a note specifying only the angle would. This level of detail is common in mechanical and product design drawings where controlled assembly and a finished aesthetic are required.