Your making only about 20 parts a year tells me that this one set of measurements represents the entire process run. I am guessing that means one series of set ups per part.
SPC and traditionally understood process capability (computed as cpk), however, assumes there will be many parts running on the same setup. So, such elaborate statistics don’t appear to suit your needs.
However, there is a misunderstanding that one needs x-bar and r charts and other elaborate data washing for control and improvement projects like 6 Sigma. Pareto charts, distribution charts, cost analysis and ROI predictions for choosing targeted controls and improvements can serve your needs without a whole lot of statistical gymnastics. The Cove has many tools that can help you. Click on the Post Attachments button in the site header and you can access a long list of tools to help understand and manage process capability.