I'm not going back to the 1800s with Maudslay and Whitworth here but instead explain where my personal flatness story starts. For some reason I've always been attracted to the massive granite block central to all inspection rooms... I find myself wondering over and placing my hand on the smooth flat surface. It's a place of power, grounding, calmness, and always is the best place to have a chat about the issues you're having with a part. It's where solutions are found & friends are made. As a youth my parents always dragged me on tours of local manufacturing facilities while we were on vacation and my visit to a granite query (I think in Vermont) still stays prominent in my memories. The massive size and polishing machines were impressive.
To the present day, over the last 5 years I've collected a number of used surface plates for very reasonable cost. It seems that each auction I attend there's a lonely piece of granite stuck at the low starting bid of $10 and I simply can't help myself to add this to my purchase. I'm now the perplexed owner of four granite surface plates ranging from 18"x12" to 36"x24" and one 36"x24" Taft & Peirce Cast Iron surface plate. Each plate of course is of unknown quality but have nevertheless served me well when needing >0.002" accuracy of inspection or sand lapping. Anything requiring better accuracy simply leaves me feeling uneasy with the results. In my quest to remedy this situation along with my interest in lapping has brought me to outline a new project which at the end I hope will result in me resurfacing my 36"x24" Rahn surface plate to AA grade.
Now in order to achieve this goal I need to "work up" to a level in which I feel confident. Below is a sequence to-do list I've outlined for this project. I'll likely have a separate blog post for each step outlining the basic process, lessons learned, etc.
After some more investigation I'll be needing to make some tools.
1 - Two Footed Twist Gage
2 - Repeat-O-Meter
3 - Spherometer
4 - Charging Roller
Then as for the overall flatness inspection I'd like to perform this with both an Autocollimator and Differential Levels to gain experience with both methods and as they will typically be used equipment also as a sanity check that the equipment is in good working order.
Below are some very useful links which inspired my effort. Hope you find them useful also.