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.
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
8/14/2021 04:21:15 am
Man , now you have m exited. i have bean playing with scraping and finding flatness. if you dont have some were to start as far as reference surface its quite hard.i devised i thing to do the same as auto collomator. works on the principal of a vblock with a laser pointer. v block must have known footprint.also a reeding surface . then at the ends of the measured surface to prisms, so as to reflect the light in a full loop. the point of this is as your setup move forward the light path stay the same. so the one leg of the triangle you measuring with is constant. makes the math some what easier. as the v block change angle according to the surface the read surface the spot will move up and down. with this setup one can map any surface ore even machine ways with cheap setup and easy to do. takes some math.....cant have it all. i used a 30mm vblock at first, but ended with a 50. gets silly amount of sums . the light works as an 'optical lever' so if you know the length of the path around you have the length of the one side constant. as the v block change angle the displayed light spot moves up and down. if the v block is 50mm and the light path (optical lever) is 5m then the error will be amplified 100x. which makes it easy to measure very small deviations with out hell off expensive equipment, and well curate enough for the home shop.
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