Metrology: The Tools, Terms, and Essential Concepts
Dec 10, 2012CMS North America’s Quality Assurance and Calibration expert, Marty Sutten, helps prepare you for a discussion on Metrology by discussing the tools, terms, and essential concepts in this month’s CMS North America blog …
We started off our newsletter technical section with a bang last month. What I would like to do this month is set up a preview of what to expect for next year. There are a lot of topics to discuss and I have built up some good stories over the last 30 years to share with you.
We are going to delve into the idiosyncrasies of machine tool calibration and the equipment needed or not needed to give yourself a good and accurate machine. We will immerse ourselves in the many types of motion control including motors, drives, bearings, and slides. Feedback devices from encoders, resolvers, and glass scales, to magnetic scales and inductive scales, and even interferometric scales. We will see how analog is different from digital and why there is open loop and closed loop systems. What is a master slave system and if a syncro motor system is needed.
Once we have been exposed to the various systems we will then discuss how these systems are integrated. How they are matched to the application and how we make these darn things so accurate. Compensation tables will be dissected and the how, where ,and why we do what we do will become a little clearer to everyone. Standards such as ASME and JIS and ISO and their histories will be brought to your attention.
After our foundation is set we will get into the metrology of calibration. What we need to measure, how we need to measure it, and the options of how it can be measured. You don’t always need the newest million dollar piece of equipment to get very credible results. If you understand exactly what it is that needs measured you will find there are many ways to skin that cat!
We will divide our calibration into several sections including dimensional, geometric, and volumetric. Here is a brief summary of these areas of interest:
- Dimensional is the measurement and calibration of linear distance. For example, if I want to drill 2 holes, 4 inches apart and they are 4.1inches apart I need to make some adjustments.
- Geometric is straight, flat, square and parallel. If we ask the cnc to cut a square panel then we would like it to be square not a parallelogram or polygon or any of those other terms we learned in high school.
- Another example is if we had 2 panels stacked on top of each other and we cut down the right hand side then took the top panel and flipped it over and pushed the 2 cut sides together they should touch completely with no gap anywhere along the edge, right? What if the axis was not straight and it cut an ever so slight arc. Depending on if the arc we cut was concave or convex, we would either have a sea-saw or a football shaped gap. It didn’t take a million dollar laser to see that.
- Volumetric, it seems as if this has been the buzz word in the cnc world for the last several years. I don’t claim to have all the answers on this subject but it’s because no one does! This is still a very new field and is being explored very intensely. It is the last frontier in our world of cnc and the pioneers working toward it are doing so with the enthusiasm of a Christopher Columbus, Magellan, and Amundsen, but along the way there are bound to be some Shackletons, Cabots, and Sir John Franklins (I’m not footnoting, look them up). These are the successes and failures of such a work. The intent and purpose of volumetric compensation is to be able to inspect and correct the machine’s dimensional and geometric inaccuracies throughout the entire volume of the machine. Simple isn’t it? The biggest difficulty in this endeavor isn’t to make the calculations and algorithms; it is to do the metrology. The larger the machine the more difficult the task.
We will explore the many methods tried, the ones that failed, and the ones that show the most promise
Along the way we will explain tricks, techniques, and tools needed to obtain the euphoria of a correctly aligned machine tool that will produce exactly what is asked of it every time. We have a lot to do so enjoy the end of this year and get ready to step it up a notch next year.
