One of the easiest ways to corrupt a spring result is to confuse the testing machine's coordinate with the spring's movement from contact. A clear datum convention prevents that error.
Absolute machine position
The machine controller needs an absolute coordinate to control motion. STM-X labels this value Machine Z. It remains the unchanged machine coordinate in compression, extension and manual operation.
Machine Z contains fixture height, installed tooling and the current crosshead location. It does not, by itself, say how far the spring has been compressed or extended.
The preload datum
A preload establishes controlled contact and removes ambiguity from approach. At the first eligible confirmed preload point, the software stores the corresponding Machine Z value as the test datum. That accepted datum is immutable for the run.
Test displacement
The magnitude is nonnegative. Compression or Extension remains the physical-direction label. This avoids depending on whether a particular machine axis counts upward or downward.
Why subtraction alone is not enough
Changing the sign, adding an arbitrary offset or relabelling Machine Z as displacement makes a screen look plausible while weakening the evidence. The raw coordinate and the derived displacement should both remain available with clear provenance.
What a report should preserve
- The operating mode and physical direction
- The accepted preload datum
- The absolute coordinate and derived displacement convention
- The force and position units
- The points used for calculations and acceptance
This guide provides general engineering information. Apply the governing drawing, licensed standard, risk assessment and approved procedure for the actual test.