As I have unhappily found over the years, where you use a fixture to measure, it is typically for reason beyond holding it which you can use a vice for (in the case cited). I'm not saying one cannot do certain studies on the individual instruments which make up the gage - that's a given in high rel shops. The system as a whole is important, though.
In the simple situation described, you would probably get away with it because of the simpicity - but as a QS assessor I would want studies on parts in the holding fixture. As soon as you increase complexity slightly over the simple torgue example you're asking for trouble if you dont. Let's say in the torque test described you are testing to a certain 4" bolt. The fixture may have a built in height that determines how much of the shaft is exposed during the test. What I'm trying to point out is more often than not a fixture has an effect on a measurement and sometimes it's dramatic.
A good example is a spring tension test holding fixture. In cases where you have small springs (such as those which return brake shoes to their inward 'standby' position after breaking the car on the 'old' foundation {drum} brakes) if the 'hook' that goes around the spring end is rounded you will get a different reading than if it is squared off. If the spring is tested a bit off center, the reading will differ. If the 'hook' that goes through the spring end is wider (if square) or the diameter is larger (if it is round) than a hook on another 'fixture' it will read differently. In one case I was involved in, both the manufacturer and we had identical spring test machines. The only way we could get the same readings is when we shipped the holding fixture with the springs to be tested which made things interesting because the springs were made in Germany and shipped here.
I point this out only to ensure you understand that a fixture ALONE can affect the reading to such a very large degree that while you want to do certain studies on the individual instrument (or instruments) attached to, or used in conjunction with, the fixture, discounting the fixture and not doing your study with the fixture could come back to bite you.
One size does not fit all. Make sure you understand how the system as a whole functions in your specific case. Spring engineers and automatic transmission part engineers will tell you that there is seldom an acceptable alturnate to using the fixture except in very basic cases.
All this said, fixture design its self is often very important. In the case of vibration testing - back in the 1980's - I was involved in test fixture design for holding CCAs (circuit card assemblies) during vibration and thermal shock testing. The test fixtures had to be designed with their resonant frequency taken into cosideration along with the resonant frequency of the CCAs. We used magnesium for vibration fixtures because other metals do not have the damping effect (most metals tend to 'ring' like a bell) magnesium has. This is basically it (from a 1988 Mac picture):
Since we were montoring vibration characteristics and testing for effects to the CCAs themselves, the fixture was one of the most important parts of the 'system'. We even did a finite element analysis on the fixture to ensure we well understood its characteristics.
Summary: Never under estimate the importance of the effects of a test or holding fixture as a part of a system as a whole. And if you do not understand the significance of saying you are measuring something in a 'restrained' configuration (as is often done with very flexible parts), you may need to learn more about the role of fixturing in measurement and test of parts.
Just my 2 cents :thedeal:
