I didn't know about the Whitworth 3 plates method and enjoyed reading about it a lot. It answers a question that's been on my mind since I was a child. Thank you for posting.
Simon Winchester wrote an enjoyable book about the history of precision that starts with Wilkinson’s boring of steam cylinders to improve upon the efficiency of the first steam engines and rifles, through engineering history to ASML’s nano-level IC fab machines.
In America, it's called The Perfectionists: How Precision Engineers Created the Modern World. Elsewhere, the title of the same book is Exactly.
Some reviewers said that. I didn't feel that way, or notice a lack of continuity, possibly because I enjoyed each chapter on its own merits. For example, I didn't know about ASML at all, even though I've been in the computer and software business fora. long time, so I was just fascinated to learn details about something I'd taken for granted, but which is clearly a peak achievement in the history of human precision manufacturing.
Right!? He discusses flatness, but not planarity, or levelness. Not even a glimmer of an attempt to describe the bootstrapping to precision. What’s weird is he starts talking about electronics when he gets to the 20th c., yet the 20th c. is perhaps the century where good old fashioned precision has improved the most!
Thanks, haven't seen his yet; my suggestion is Clough42 - especially for the HN crowd since he's a software engineer, electronics hobbyist, and has a 3D printer too, so it's a really nice mix - not just in the sense of (metal) machining one video and something else the next, but unexpected combinations of them (e.g. 'I need a fixture to hold this - over to the 3D printer') that make it really interesting and more varied than most others I watch on YouTube who mostly have their one thing. (Marius Hornberger is another quite like that - but swapping electronics for mechanical stuff, and metalworking for woodworking.) A/V production quality is great too.
Yes, James is excellent, I picked up most of my Fusion360-fu from watching him design (in 360) and then 3D print or machine components. Very thorough engineer!
Also, check out Robin Renzetti, (here he is lapping a surface plate: https://www.youtube.com/watch?v=6nY7uW1uG9s). Lacks the entertainment and dad jokes of ToT, but is an absolute genius. Absolutely amazing attention to detail. Top Lipton has a lot of good videos too, and has written a couple of books.
> Typically made of granite, surface plates act as a datum, or the basis upon which precise measurements and movements can be made.
That's cool! My father was an electronic engineer and one of his projects was quite a compact device (as big as 2 decks of cards) that was able, IIRC, to detect a slope of a few microns over a meter, and to take multiple measurements to make a 3d model of the surface. This was paid for by a company [1] that made big slabs of granite, which I reckon must have been these datums referenced in the article.
This was almost 20 years ago, and I was in my teenage years at the time. I had built the C# interface to download the data from the device, but I never understood or even though about the purpose of the whole endeavour until this article.
I'm always fascinated by these old techniques. Two surfaces can slide along one another if they are both spherical, but not three. You can make three wooden straight-edges this way too. Instead of blue ink, you can use chalk.
One thing I'm curious about, is whether this is actually how granite surface plates are still made. Or have they switched to some kind of interferometry.
Interferometry still only acts as a comparison (to a reference mirror). So there's still the 3 plate method at the heart of it, although I expect most commercially produced surface plates are just machine-ground and then measured against a master, rather than hand-made three at a time.
Edit to elaborate -- to make those ultraflat mirrors, you can also use an interferometric variant of the 3-plate method when, instead of granite or cast iron surface plates, you're making optical flats. Instead of looking at contact patterns with dye, you can observe the interference bands that show up between two optically flat, smooth surfaces to compare them, which is generally more sensitive. But it's still a comparison process. You still proceed by lapping the high spots and checking again...
I'd love to know the answer to your question too. I initially learned of this method while reading through Wayne Moore's "Foundations of Mechanical Accuracy" which was written in 1970 afaik.
It is a wonderful feeling in machining when you figure out how to transfer features mechanically without going through a numerical intermediate. You are essentially working with a compass and straightedge in your mind, comparing surfaces, lines and single points of contact directly. There is no accumulation of error from approximate measurements. Regardless of accuracy, your precision is infinite.
Look up a 'sine bar' and gauge blocks for inspiration. :)
It is also interesting to note that functional surfaces are very simple, being composed of flats, rounds and inclined planes. This is why you can make nearly anything with a lathe (and a mill to spare your sanity).
Wyler makes inclinometers that are accurate enough to measure the change in angle of a surface plate directly. It's amazing technology. Here is a YouTube video from 2011 that talks about how they were developed. I remember that I was watching videos about surface plate calibration, and this came up.
(home-made)! Although for the sake of relevance to this topic, it's worth noting that although Dan hand-scraped many of the surfaces, and employed the spirit of these techniques to measure and align the lathe, he didn't actually need to use the 3-plate method to build it because he was working against an existing master surface plate.
A similar process can also get you a master square, although there are other ways to do it (such as via circle division).
You start with a surface plate and three "pretty good" squares. Then put the squares against each other, and look at the contact pattern, and so on...
Also, IIRC, Whitworth improved the method by introducing scraping to the process, which makes it much faster, but did not invent it; the same fundamental technique was already being done in Maudslay's workshop using lapping when he started working there.
I don' get how this works. How exactly do you put the squares together?
Stacking them (even with rotations by mutlplies of pi/2) will not detect the case when every edge has the same kind of deviation from the straight line, right?
Putting them edge to edge will assure every edge is straight and the same length (within observation limits) but may get you a rhombus which is not a square.
Do you somehow iteratively combine both of the above?
If you're into precision, personally I think the story of the Michelson interferometer used to "disprove" cosmic aether is probably my favorite. There is also with the creation of the first high quality diffraction gratings (https://en.wikipedia.org/wiki/Henry_Joseph_Grayson and Rowland at JHU).
The Michelson interferometer was incredibly sensitive for its day but required heroic efforts to use: it was deep in a subbasement, floating on a pool of mercury, and was routinely unusable during the day due to horse and carriage deliveries a few buildings over. My friend who was a physics major at Princeton said they built one in a day for their physics lab (these days, vibration isolation tables and alignable optics are far, far easier).
I intend to scrape my way to 3 surface plates, instead of lapping.
I have 3" and 6" cast iron bar stock cut offs that I picked up for this purpose, along with Prussian blue, and something that should work as a scraper... what I don't have is a brayer, but I'm told you can improvise with a rolled up and tapped shop cloth.
Making a surface plate is kinda like hand sanding a knife blade. You start with a coarse grit and progressively move onto finer grits. Lapping is just the finest "grit" possible. Even Whitsworth probably scraped his plates flat using a "poor" reference plate and only then did he start lapping.
If you can find a flat piece of glass then use that as your starting point and then apply the 3 plates method.
I'm planning to make granite surface plates myself too. Where I live, we have granite tombstone makers in every small city and several in every big city. You can rather cheaply order any size slab you want and it's typically pretty flat too (polished).
If your plates were close to flat to start with it would only take 5 or so stackings. I had one plate with a very low corner and so it took around 50 stackings.
I'm still a little confused. In my mind, if I hold one surface still and rub the other surface on it, it's exactly the same as if I reversed their roles -- the surfaces shouldn't know which one is moving and which is stationary. For example, if I was sanding a block of wood, it wouldn't matter if I moved the sand paper on the wood or the wood on the sand paper.
Yet several statements in the article make it clear that my understanding is not correct (e.g. "Next, the Green plate acts as the control and the Red plate is lapped against it," or "Next, the Green and Blue plates are lapped against each other in an alternating manner.")
I'd you look into the community that hand grinds and figures telescope mirrors, you can learn a lot about how which piece is on top and what motion you use influences the shape you get. Of course, they're going for a curved surface, not a flat one, but I suspect some of the same applies.
I have ground a mirror and that doesn't provide the answer to his question. Yes - the top glass becomes concave and the bottom convex, but this is symmetrical and doesn't explain what a control surface means in this context.
Sorry, I must admit to not reading the OP before replying. Not sure what the OP is on about; of course both surfaces wear, the important thing is the sequence of which surfaces are compared/lapped against each other, and how they are oriented. The orientation is quite important - if each pair of plates are always at the same orientation relative to each other (or I suppose 180 degrees from that) then the plates may meet evenly, all having the same twist.
Foundations of Mechanical Accuracy is a great book if you're in to this sort of thing.
It doesn't matter. This will produce three flat surfaces. It looks like he's just using the word control to denote whichever plate is on the bottom. The top plate will become concave and the bottom one convex due to the overhang of the top plate as it is being ground, but that is not relevant here.
This is known as the automatic generation of gauges. It's mentioned in Feymann's There's Plenty of Room at the Bottom.
There are still people who hand scrape plates, but these days, there are a few flat grinding masters used to make nearly all plates, and those masters can traceability back to hand-scraped plates.
Couldnt one just use ice in a pond as a basis for a beginning perfectly flat surface? Then perhaps use clay or something similar poured onto it to create an initial flat surface and build up harder materials from there?
Float glass gets you a fairly flat (+/- 0.001"-0.0001") surface, but it's kinda fragile and bendy. You need something sturdy and flat underneath it, which sort of begs the question.
A good surface plate will get you 0.000001"-0.000005"
>Because you could also just pour molten metal into a mold and let it cool to get an initial "flat surface" but there are limits to that.
I dont know what metal would work for that. There's still significant surface tension in an open pour. I was casting some copper a few weeks back. A ~2CM piece had about ~3mm of surface curvature to it. Plus plenty of distortion from unequal cooling where it varied thickness as well.
The closest would probably be glass on a bed of mercury.
Differences in rate of cooling leads to uneven stresses and minute bends. Similarly while handling and working the surface plates you need to let them reach thermal equilibrium.
Large forgings which need to be machined to close tolerances are left to sit even for months in room temperature to reach thermal equilibrium.
If you look very closely at some artifacts, you can even see what looks like marks of power tools. Small introduction of those artifacts and techniques: https://www.youtube.com/watch?v=BNSb5gPdqsA you can find more by searching "egyptian lathe".
Simon Winchester wrote an enjoyable book about the history of precision that starts with Wilkinson’s boring of steam cylinders to improve upon the efficiency of the first steam engines and rifles, through engineering history to ASML’s nano-level IC fab machines.
In America, it's called The Perfectionists: How Precision Engineers Created the Modern World. Elsewhere, the title of the same book is Exactly.
https://www.amazon.com/Perfectionists-Precision-Engineers-Cr...