ME 3263 Introduction to Sensors and Data Analysis (Fall 2018)

Lab #1 - Measurements of machining precision and accuracy

How can you measure something?

All measurements have traceable standards. There are seven base units in SI - meter (length), second (time), Mole (amount of substance), Ampere (electric current), Kelvin (temperature), Candela (Luminous intensity), and kilogram (mass) 1. Any measurement you make should have some method to check against a reference. In this lab, we will use calipers that measure dimensions i.e. meter*10$^{-3}$ (length). Calipers can always be verified to work with gage blocks.

Sources of measurement variations

No measurement is exact. No surface is compeletely flat. Every measurement you make has two types of uncertainties, systematic and random. Systematic uncertainties come from faults in your assumptions or equipment. Here are some examples for caliper measurements:

Your calipers were dropped on the floor and bent, now all of your measurements will be too long or too short.
The bar stock was not machined with square edges so each measurement along the sides increases or decreases
You align the calipers with the bar stock, instead of measuring point-to-point. Now your measurements are only the maximum length from edge-to-edge

Random uncertainties are associated with unpredictable (or unforeseen at the time) experimental conditions. These can also be due to simplifications of your model. Here are some examples for caliper measurements:

You assume the surface is flat, but it is in fact rough
The temperature in the room changes the dimensions through thermal expansion
Your calipers are not aligned parallel to the edges of the bar stock

In theory, all uncertainies could be accounted for by factoring in all physics in a problem e.g. Temperature-dependence, Coriolis effect, electrical disturbances in your readings. In practice, the diminishing return on investment prevents this practice. For something like measuring bar stock, it is more than sufficient to report the average and standard deviation with good calipers.

rjk14002/ME3263_Lab-01

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