Study on Repeatability in Measurements – Human Variability
Written by Joanna Carbajal
We continue to research to deliver functional and high-quality parts in the maritime spare parts industry. We conducted two studies to determine human variability in measurements using digital calipers. We were interested in determining if there were measurement differences between person to person and day to day. Our goal was to use our results to provide recommendations on how to minimize measurement differences.
There were five people involved. Two were in Mexico and three were in California. Ten parts, found in both offices, were chosen as standard parts to measure. Standard parts were used to eliminate the variability in exact dimensions, which allowed us to focus on those participating in the study.
Here were the results:
Day one: the difference in measurements between employees was between 0 mm and 5.71 mm. The average measurement difference was 0.64 mm.
Day two: the difference in measurements between employees was between 0.02 mm and 5.49 mm. The average measurement difference was 0.74 mm.
Comparing days: the difference in measurements from one day to another ranged from 0 mm to 0.42 mm with an average difference of 0.06 mm. No one had the same measurements from day one to day two.
We found that even with the labeled pictures, not everyone measured the same dimensions. There was some confusion that we do not expect from an operator at a micro-factory as they will be getting measurements from an engineering drawing which should be more clear.
Each person took the same measurements on two different days that lead to different results. The reason for this could be: squeezing the calipers together tighter on one day, not checking whether the calipers are zeroed before each measurement, parts that were difficult to keep parallel to the jaws and repeating a single measurement. We know the parts were machined, stamped, or injection molded, and we assume the variability in the parts from California to Mexico is negligible. We are assuming that the calipers we have are new enough that they have not worn down.
The more difficult dimensions to measure had the greatest variability. To improve, we discussed a standard use of the calipers. The fan bracket thickness was the only measurement every person had the same from day one to day two.
We did a second study to eliminate the most difficult parts to measure and parts with unclear dimensions. The study was smaller. We used only four parts instead of ten. We had the same participants from Mexico and California.
Here were the results:
Day one: the difference in measurements between employees was between 0.00 mm and 0.50 mm. The average measurement difference was 0.15 mm.
Day two: the difference in measurements between employees was between 0.02 mm to 0.86 mm. The average measurement difference was 0.14 mm.
Comparing days: the difference in measurements from one day to another ranged from 0.00 mm to 0.20 mm with an average of 0.05 mm difference. Everyone had the same measurement for the bolt diameter from day 1 to day 2.
When using the depth gauge, the calipers should be held perpendicular to the object. The gauge should be aligned with a straight surface if possible for a more accurate measurement. Measurements like heights that have a curved edge, like the fan bracket height, are difficult to measure because the straight edge of the base is not parallel to the top of the bent part. This means using the depth gauge would help assume the part is kept flat on the surface, and the gauge is flat against the wall of the object and the surface. The step gauge may be more accurate or easier to use for some dimensions.
Softer materials require more precision when measuring them as the jaws of the calipers are stronger and can manipulate your results if you are not careful. Overall, the majority of our measurement variations were within 0.10 mm of other people and across days. To improve, we determined which part of the calipers should be used for various types of dimensions and updated our measurement rules.
With our vision to have micro-factories all over the world, we want to ensure that there is repeatability in everything we do. Using a rapid prototyping approach, we will continue to research so that we can serve the maritime spare parts industry seamlessly. We intend to provide the same level of high-quality parts anywhere in the world, and studies such as these are quick ways for us to identify and solve for challenges around repeatability and reliability.