Layout tools — primary use of a scribing (surface) block: In bench layout on a surface plate, a scribing block is primarily used to

Introduction / Context:
Bench layout relies on a flat reference (surface plate) and height-based marking to transfer dimensions to workpieces. The scribing (surface) block, often with a scriber and fine adjustment, is central to this process. Knowing its primary purpose avoids misuse and improves layout accuracy.

Given Data / Assumptions:

• A standard surface plate and scribing block with adjustable scriber.
• Work requires parallel lines, datum transfers, or height-based marking.
• Ancillary tools like V-blocks can be used with it when needed.

Concept / Approach:
The scribing block is essentially a height gauge for marking. It sets a precise height relative to the surface plate and draws or transfers lines onto the work. While it can help check flatness by feel against the plate and can assist in finding centres of round bars when used with a V-block, its primary, exam-definitive function is accurate height-based scribing and layout.

Step-by-Step Solution:
Place work on the surface plate and set required scriber height on the scribing block.Move the block along the plate to mark a parallel line at the set height.Use auxiliary V-blocks to support round bars if centre lines are required.Therefore, the principal use is to mark accurate layout lines at a set height.

Verification / Alternative check:
Tool catalogues classify scribing blocks/surface gauges under marking-out tools, not holding or direct roughness measurement tools.

Why Other Options Are Wrong:
Holding round bars is the function of V-blocks, not the scribing block.

Surface roughness is measured with comparators or profilometers.

Locating centres of round bars requires a V-block and scribing block together; the block alone is insufficient.

Thread pitch measurement uses thread gauges, not scribing blocks.

Common Pitfalls:
Dragging chips under the base and scratching the surface plate; using a blunt scriber leading to thick, inaccurate lines; failing to clamp the scriber height properly before marking.

Final Answer:
mark accurate layout lines at a set height from the reference surface