You analyze the raw data from a single run. Do not subgroup the data, but analyze individual measurements taken at specific intervals.
Your understanding of the interpretation is correct.
If this analysis tells you that you must wait 55 sec. between samples to see true product variation, you have two options: 1) use an I-MR chart, or 2) collect 1 measurement every 55 seconds and group x number of them into a subgroup. Normally, the period of autocorrelation is much greater than 50 seconds, so it usually makes more sense to use an I-MR chart.
Regarding Repeatability, the variation that you see during the 50 second interval is pure repeatability variation. As you describe your measurement device, there is no reproducibility variation.
Once you have this, you can perform your capability analysis. Just remember that each "part" is 50 seconds long. Allow at least 50-55 seconds between measurements for a subgroup, and longer times between subgroups.
Your understanding of the interpretation is correct.
If this analysis tells you that you must wait 55 sec. between samples to see true product variation, you have two options: 1) use an I-MR chart, or 2) collect 1 measurement every 55 seconds and group x number of them into a subgroup. Normally, the period of autocorrelation is much greater than 50 seconds, so it usually makes more sense to use an I-MR chart.
Regarding Repeatability, the variation that you see during the 50 second interval is pure repeatability variation. As you describe your measurement device, there is no reproducibility variation.
Once you have this, you can perform your capability analysis. Just remember that each "part" is 50 seconds long. Allow at least 50-55 seconds between measurements for a subgroup, and longer times between subgroups.