"Backstory" - The beginning of Flatness

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.
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 sizes of 18"x12", 24"x18", and 36"x24" along with one 36"x24" Taft & Peirce Cast Iron 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.

1. Learn the Whitworth three plate method to produce 3 flat planes from raw stock.  I based my process on a great YouTube video by OxTools.  This will allow me to get a feel for the lapping process, use of different media, and find out what I'm getting into.  I'll be using 6" cast iron discs for this process.
2. Produce a cast iron flat plane which will then be used as a lap for the granite surface plates.  I've obtained two 14"x12" cast iron laps from Craigs List ($42 each) which I'll use for this step.  These laps are from Busch Precision "1600 Series - Duplex Bench Plates" with both sides produced to 0.0003" flatness and one side grooved.  The grooved lapping side of these plates are in relatively good shape (flat to 0.002") but unfortunately the opposite flat side lived it's life either face down on a bench or as a hammer surface.  My plan here is to resurface the beat up flat surfaces with a fly cutter on my benchtop mill (no surface grinder here) and then perform the 3 plate method.  Couple challenges here... First, I don't actually have 3 independent surfaces which is what is required for the Whitworth method so will be using 4 surfaces; A, B, C', C" and modifying the process - hoping that C' & C" can be pre-lapped well enough to represent the C surface to A & B.  Second, is simply the quality of the surface I'll be starting with off the mill - this may lead to extended lapping time.  And finally, the fact that I'll be trying to lap in rectangular plates... as the Whitworth method requires 90 deg rotation within the cycle this posses an issue with overhang.  An alternative method is to learn some scrapping but this requires a known good granite surface plate and it becomes a chicken before egg situation.  Also, I really want to produce a simply flat surface as I think it will hold the charging diamond better/more for the granite lapping process.
3. Now that I have a 14"x10" flat cast iron surface I can get to my final step and recondition my granite surface plates.  Within this step is some considerable learning with respect to inspecting the surface quality.  ​​​I'll have to keep my eye out for some inspection equipment but so far the pricing even on ebay or auction is out of my wheelhouse.  

So that's the plan.  As of this writing I've just completed step one with very good results so will write that blog post now.

Below are some useful & informative links on the subject

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
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
Duplex Bench Blocks  Challenge Precision  Busch Precision