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.
Manufacturing variations¶
Different production methods will often exhibit varying degrees of precision and accuracy. It’s important to understand these differences, and how to measure them. In this lab, you will investigate several different sets of round stock. Some have been cut with a bandsaw, some with a CNC mill. You will identify and examine the differences between the two methods.
Deliverables¶
For this assignment, you and your lab partner will submit a joint report as a .pdf to your HuskyCT section within one week of your lab date. It will only be necessary for one of you to submit the report. Please clearly mark, either in the body as footnotes or in the appendix as a separate section, what each of you contributed to the overall report.
You are limited to 3 pages (not including the title page, references, or appendix) and 2 figures. Additional data, figures, and information can be put in an appendix. The appendix will not be graded, but you may refer to it to explain data, methods, or other relevant information.