Flatness "Backstory"
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 wandering over and placing my hand on the smooth flat surface.  It's a place of control, discipline, and the best place to have a chat about the issues you're having with a part.  It's where problems are solved & friends are made.  As a youth on vacation my parents always dragged me on tours of local manufacturing facilities and my visit to a granite query in Barre VT still stays prominent in my memories (Mom found a nice pic below).
As of this writing, July-2020, 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 five granite surface plates sizes of 18"x12", 24"x18"(2x), and 36"x24" along with one 36"x24" Taft-Peirce Cast Iron plate which I have dated back to WWII.  Each plate is old and of unknown quality but they have nevertheless served me well when needing ~0.003" accuracy of inspection.  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 this project with the goal of resurfacing to AA grade quality my 'junk' granite surface plates. ​
The Surface Plate ... some background
  • The surface plate is a solid flat plate used as the main horizontal reference plane for precision inspection, layout, & tool setup. 
  • Used as the baseline for all measurements, the surface is extremely flat with a 24” x 18” plate having 80 mike (2 µm) overall flatness & 35 mike (0.9 µm) local flatness.  I'll be using the shorthand "Mike" in this article (1 mike = 1 millionth of an inch = 0.000001")
  • The Granite surface plate was introduced in 1940 due to scarcity of metal during WWII.
  • Although granite has many benefits, cast iron is still preferred for “master surfaces” in high-accuracy industries as they can be more easily worked to higher grade of flatness, provide better bearing surface & optical properties.
  • Surface plate flatness is created by a principle of “Automatic generation of gages”.  This is unlike most mechanical precision instruments which derive their precision from more precise standards. 
  • In this process, three approximately flat surfaces are progressively refined to precise flatness by rubbing them together in pairs and manually removing material. 
  • This process was developed during the Industrial Revolution and still plays a critical role in all modern manufacturing
  • Granite Surface plates are worn out of spec relatively fast due to abrasion from normal use.  Always clean a surface plate prior to use and keep it covered
  • Machine shops typically have specialists come in yearly to calibrate plates and will often need to resurface the plates with specialized tools & techniques.
Project Goals
  1. Learn the Whitworth Three-Plate Method to produce 3 flat planes from 6" cast iron discs.
  2. Produce a rectangular cast iron lap, again with the 3-Plate Method, which will be used to recondition my granite surface plates.
  3. Recondition my granite surface plates to AA grade.
  4. Learn the Optical Flat inspection techniques.
  5. Learn how to use Autocollimator in the Moody Method to measure flatness.
  6. Design & Build my own Repeat-O-Meter to measure local flatness of a granite surface plate.  Incorporate an LVDT indicator.
  7. Design & Build a Spherometer and Two-Footed-Twist-Level.
  8. Scrape my 24"x18" Taft-Peirce cast Iron surface plate using the bluing technique on my newly surfaced AA granite table .
The below picture is a nice "summary photo" showing an item from each of the 8 tasks above which was taken at project completion; all of which will be explained in this series.
Project Justification
After some investigation into the scope of this project it became clear that it was not a trivial undertaking.  Below you'll see the 4 options available for me to recondition granite surface plates - of course I chose the last option in green box.
Useful Links
Video: The Foundation of Metrology, by Mitutoyo
Granite Surface Plate Calibration, By Jim Bible of DGI
How to Calibrate surface plates in the plant, By J.C. Moody
​Book: Foundations of Mechanical Accuracy, By Wayne R. Moore
Book: Fundamentals of Dimensional Metrology, By Ted Busch
Publication: The Art of Hand Scraping, by Robert R. Wade
Publication: How to measure flatness, American Machinist, Aug 1966 #591
Video: Surface Plate Calibration and Conditioning, By OxTools - Tom Lipton
Video Series by Robrenz: DIY Surface Plate Lapping #1 / #1b / #2
​Tolerances according to ASME B89.3.7-2013, Mitutoyo paper
A Paper on Plane Metallic Surfaces or True Planes, Joseph Whitworth
Reference Planes, Alessandro Anzalone - Hillsborough CC
​The Whitworth Three Plates Method, Eric Weinhoffer
Video: Flat From Scratch, Accidental Science
Video Series by OxTools: Making flat Lapping Plates using the 3 plate method #1 / #2 / #3 / #4
Video: Repeat O Meter, by OxTools
Video: Repeat-O-Meter, by Robrenz
Video: Autocollimator by Robrenz
Video: 3 Plate Method (2 Footed Twist Gage), by Robrenz
Optical Flat Interpretation, Advanced Optics
Fringe Interpretation, Starrett
Taft Peirce Handbook, 1945
Busch Precision Duplex Bench Blocks
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