Metrology fixtures — function of a V-block: In inspection and marking work, a V-block is primarily used to

Introduction / Context:
V-blocks are precision fixtures with a 90-degree V-groove used extensively for holding cylindrical work. They are indispensable during inspection, layout, and light machining because they provide stable and repeatable location for round sections.

Given Data / Assumptions:

• Standard V-block with clamp.
• Operations include marking centre lines, concentricity checks, and light machining or drilling.
• Use on a surface plate or drill press table.

Concept / Approach:
The V-groove supports a cylindrical workpiece along two lines of contact, centring it with respect to the block. When used with a scribing block, it enables accurate centre-line marking. In inspection, it provides a repeatable nest for dial indicator checks. It is not used to measure roughness or angles directly; other gauges are required for those tasks.

Step-by-Step Solution:
Place the round bar in the V-groove; clamp if needed.Use a scribing block to mark diametral lines or to find the centre.Alternatively, set up a dial indicator to check runout while rotating the bar.Hence, V-blocks primarily locate and support round rods for marking or inspection.

Verification / Alternative check:
Inspection manuals list V-blocks as supports for cylindrical work, not for surface roughness or general flatness testing.

Why Other Options Are Wrong:
Flatness checks use surface plates, straight edges, and feeler gauges, not V-blocks.

Surface roughness requires comparators or stylus instruments.

Angle measurement needs protractors, sine bars, or angle gauges.

Sheet holding for shearing is a function of the shear's hold-down, not V-blocks.

Common Pitfalls:
Forgetting to deburr the bar before seating; using damaged V-blocks that introduce error; not clamping during drilling leading to spinning of the work.

Final Answer:
locate centres of round rods or support them for machining/inspection