Flat From Scratch, revised - The 3 plates method

Here I share my experience on making flat the ways of my home made lathe with the 3 plates method, and a discussion about some caveats and the history of the method.

So how to make a true flat surface without a flat surface? Before any true flat surface was ever made there were only pseudo-flat surfaces, far to be really flat, a fact that had many implications in hampering the technological development. Who, for the first time, formalized a method to obtain a (quite) true flat surface (from scratch) was Joseph Whitworth who revealed his 3 plates method in a paper addressed to the British Association in Glasgow on 1840. There are no much information online about this method, and the few lines mentioning it, that one could find in a book of mechanical technology, are not enough to let understand how actually this method can be carried out. Eventually I found some information online, enough to try myself the method on the occasion to make the ways of my home made lathe.
This video is a revision of a previous one with the same title where some issues have been addressed. I provided both historical e technological context, and a detailed illustration on how I carried out this method, with some variations introduced to speed up the work.
Some caveats related to the described method are also discussed.

I made this video with the hope others will find it interesting and useful.
00:00:00 Introduction
00:01:00 About flat surfaces
00:03:05 History of the method
00:05:54 Why we need flat surfaces
00:06:13 Definitions
00:07:01 Description of the method
00:07:18 Caveat on the description
00:11:27 Discussion on the limits of the method
00:14:49 My experience and trick to overcome the limits
Acknowledgments

There is no much information online about this method, but digging the net eventually I came up to the original paper (a scan still available through Archive.org), its transcript on Wikipedia, and an useful article written by Eric Weinhoffer that directed me to an other document, a PDF titled “Reference Planes” written by Doctor Alessandro Anzalone, for the Hillsborough Community College - Brandon Campus, both helped me a lot and confirmed what I intuitively figured out when I read the paper for the first time.
Many thanks to both the cited authors, without their work I would not have been able to get the results I got.
Lately I had some more insights and many thanks goes to Johan (DIYSwede) that directed me to the good video of Robin Renzetti on the 3 plate method. Many thanks even to them.
Also I have to thanks “12L14” from Homemadetools forum who directed me to the comment of Forrest Addy at the Practical Machinist forum.
Links & References
[1] The Whitworth method presented in 1840 in “Miscellaneous Papers on Mechanical Subjects”, with the paper titled “A Paper on Plane Metallic Surfaces or True Planes”:
https://en.wikisource.org/wiki/Miscellaneous_Papers_on_Mechanical_Subjects/A_Paper_on_Plane_Metallic_Surfaces_or_True_Planes
Blog article of Eric Weinhoffer
http://ericweinhoffer.com/blog/2017/7/30/the-whitworth-three-plates-method
Reference Planes by Alessandro Anzalone, Hillsborough Community College, Brandon Campus

https://etshare.pbworks.com/f/Chapter%2014%20Reference%20Planes.pdf
Wikipedia page on Sir Joseph Whitworth:
https://en.wikipedia.org/wiki/Joseph_Whitworth
[2] “Foundations of Mechanical Accuracy” Wayne R. Moore – 1970
http://mooretool.com/publications.html
[3] Robin Renzetti ROBRENZ ATD #1 Intro, 3 Plate Method
@ROBRENZ
https://www.youtube.com/watch?v=Va4TGDnQqDs&t=202
[4] Forrest Addy, his comment on Practical Machinist forum (post #11):
[quoted post:]

«Square or round or rectangular plate makes no difference in the final flatness provided the work is rotated a RANDOM amount. The object of rotation of the plates in successive cuts is to prevent "radial lobing" of the surface. If the plates are rotated exactly 90 degrees, 72 degrees, 60 degrees, or any other angle corresponding to a regular polygon, there is the hazard of radial "lobing" to appear in the surface. » [...]
«I disagree with the Moore assertion of only square or round plates being capable of generating truly flat surfaces. Square or round plate shapes may be helpful but not essential. »
https://www.practicalmachinist.com/vb/machine-reconditioning-scraping-and-inspection/whitworth-three-plate-method-330634/#post2912880
Other sources: http://www.circuitousroot.com/artifice/machine-shop/surface-finishing/hand-scraping/

Видео Flat From Scratch, revised - The 3 plates method канала Accidental Science