Question 66

The Correct Answer is C **Explanation for Option C (Counterdrill):** The illustration labeled "B" typically depicts a stepped hole where the initial larger diameter hole is drilled to a certain depth, and then a smaller diameter hole continues from the bottom of the larger one. When this type of stepped hole is created using standard drilling techniques (often used before tapping or for clearance), the feature is sometimes referred to as a **counterdrill**. This term specifically denotes a step-down feature created by subsequent drilling operations, resulting in a cylindrical recess before the smaller through-hole begins. **Why the Other Options are Incorrect:** * **A) Counterbore:** A counterbore is a cylindrical flat-bottomed recess machined into the opening of a hole, designed to house the head of a fastener (like a socket head cap screw) so that the head lies flush with or below the surface. While the feature shown has a larger diameter section, a true counterbore typically has a precise, flat bottom (a 90-degree intersection), which is not usually what is implied by a standard "counterdrill" label unless specific geometry is provided. In simplified drafting or machining terminology, if the intent is simply to label a stepped hole created by sequential drilling, "Counterdrill" is often the intended label for the specific feature "B." * **B) Countersink:** A countersink is a conical (angled) recess machined into the opening of a hole, designed to accept the conical head of a screw (like a flat head screw) or to provide a lead-in for tapping. The feature "B" is shown with perpendicular sides (cylindrical), not angled sides. * **D) Spotface:** A spotface is a shallow, flat-bottomed circular surface machined around a hole opening, typically only deep enough to provide a clean, smooth, and square bearing surface for the underside of a bolt head or nut on an otherwise rough or uneven surface (like a casting). Feature "B" shows a significantly deeper recess than a typical spotface.