Workshop metrology and hand tools — hacksaw blade length definition: In hand fitting practice, the specified length of a hacksaw blade is measured between which features on the blade?

Introduction / Context:
Knowing how a hacksaw blade's length is defined is essential for correct replacement and for ensuring the frame can tension the blade properly. Toolroom and workshop standards specify the reference points used to measure blade length so that frames, pins, and hole spacing are compatible across manufacturers.

Given Data / Assumptions:

• Standard hand hacksaw blades used in fitting and fabrication.
• Blade has two mounting holes that fit on frame pins.
• We are identifying the correct metrological definition of blade length.

Concept / Approach:
Interchangeability requires a consistent datum. The only datum that is invariant to hole diameter and edge chamfers is the centre of each hole. Therefore, standards define blade length as the distance between the centres of the two mounting holes. References to outer or inner edges can vary with hole size and countersink, leading to mismatch and poor tensioning.

Step-by-Step Solution:
Identify fixed datums on the blade: the two circular holes for frame pins.Choose the feature whose position is independent of hole diameter: the centre of each hole.Therefore, blade length = distance between the centres of the two pin holes.Other edge-based measurements change with hole diameter or wear and are not standard.

Verification / Alternative check:
Check a blade and frame: if the hole-centre spacing matches the frame's adjustable distance, the blade tensions correctly; measuring to hole edges yields inconsistent fit.

Why Other Options Are Wrong:
Outside or inside edges depend on hole diameter and chamfer, so lengths won't be consistent.

Tip-to-tip toothed length ignores the mounting geometry entirely.

Frame window length varies with frame design and tension setting; it is not a blade standard.

Common Pitfalls:
Ordering blades by overall metal strip length instead of hole-centre spacing; mixing junior (150 mm) and full-size (300 mm) blades due to misreading specifications; assuming all frames can compensate for several millimetres of error in length—many cannot.

Final Answer:
the centres of the two pin holes