7 ms·
The Whitworth Three Plates Method (2017)
- sgc 6y agoAn interesting concept, but I wonder how many repetitions for which types of applications are required?
- opwieurposiu 6y agoI have done it, it took a couple weeks. Recommend getting a power scraper if you want to make a large plate.
- yetihehe 6y agoHow many stackings were needed?
- opwieurposiu 6y agoIf 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. It mostly depends on how far you have to go.
- analog31 6y agoI'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.
- ethanwillis 6y agoI'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.
- jbay808 6y agoInterferometry 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...
- mikewarot 6y agoWyler 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. https://www.youtube.com/watch?v=0doCHgJa3Ps https://www.youtube.com/watch?v=0doCHgJa3Ps
- imtringued 6y agoYou only need this method to create the first one. Based on that reference surface you can scrape as many flat surfaces as you want. If you build a flat machine like a surface grinder then that surface grinder can transfer its own flatness onto a surface plate.
- ganzuul 6y agoIt 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).
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- shureluck 6y agoCouldnt 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?
- ethanwillis 6y agoDepends on the level of accuracy you want. There will be imperfections in the surface from the castings derived from the ice based flat surface. This method can take you to much more flat surfaces from first principles. Also there will be imperfections in the ice based surface itself too. 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.
- donavanm 6y ago>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.
- adrianN 6y agoI don't know much about metallurgy, but perhaps sufficiently slow cooling would have the desired effect.
- ganzuul 6y agoThe 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.
- jbay808 6y agoA 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.
- praptak 6y agoI 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?
- yetihehe 6y agohttps://etshare.pbworks.com/f/Chapter%2014%20Reference%20Planes.pdf https://etshare.pbworks.com/f/Chapter%2014%20Reference%20Pla... page 5.
- linsomniac 6y agoSmarter Every Day has a video about the "Dawn of Precision" and "How to measure a millionth of an inch", including mentions of the Whitworth method. https://youtu.be/T-xMCFOwllE https://youtu.be/T-xMCFOwllE
- lsllc 6y agoIf you are interested in machining and the minutia behind machining (squaring and tramming), check out This Old Tony. Definitely worth a watch! https://www.youtube.com/watch?v=tW8HNAlUXxU https://www.youtube.com/watch?v=tW8HNAlUXxU
- mberning 6y agoAnd Abom79. He has a great video of using dye and power scrapers to flatten a Cincinnati Shaper. Great stuff.
- lsllc 6y agoWhile we're at it, Alec Steele is pretty interesting too! (he's a blacksmith, so forging).
- infogulch 6y agooxtoolco has a series on the 3 plate method from start to finish https://youtu.be/rHmsQEAx16o https://youtu.be/rHmsQEAx16o love all his videos on tools and toolmaking
- danw1979 6y agoI knew before I clicked through to the comments that ToT would be right here at the top.
- OJFord 6y agoThanks, 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.
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- leeoniya 6y agoHigh precision air bearing CNC lathe and grinder: https://m.youtube.com/watch?v=sFrVdoOhu1Q https://m.youtube.com/watch?v=sFrVdoOhu1Q
- jbay808 6y ago(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.
- mikewarot 6y agoI 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.
- imtringued 6y agoMaking 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.
- yetihehe 6y agoI'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).
- mikewarot 6y agoBear in mind that temperature changes may unevenly affect the plate if the material isn't consistent. It's unlikely but possible.
- codeulike 6y agoSo what does 'lapping' actually mean? There's a lot of lapping in that process.
- rriepe 6y agoRubbing two surfaces together
- SamBam 6y agoI'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.")
- mauvehaus 6y agoI'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. To be clear, I haven't done either :-)
- mhb 6y agoI 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.
- bacon_waffle 6y agoBoth surfaces do wear, however any high spots will wear faster.
- 1_player 6y ago> 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. 1: https://www.microplan-group.com/en/ https://www.microplan-group.com/en/ EDIT: it might have been a C interface coupled with Gtk, not C#. In retrospect, neither was a very smart or stable choice 20 years ago.
- wombatmobile 6y agoI 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. https://www.amazon.com/Perfectionists-Precision-Engineers-Created-Modern/dp/0062652567/ref=sr_1_1 https://www.amazon.com/Perfectionists-Precision-Engineers-Cr...
- wbl 6y agoI wasn't a fan. It was a bunch of disjointed essays rather than a real history. The later parts really deviated from the promise.
- thechao 6y agoRight!? 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! A complete dodge & waste of money.
- wombatmobile 6y agoSome 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.
- dekhn 6y agoThis 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.
- dekhn 6y agoIf 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 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).
- egberts1 6y agoMakes me wonder if that’s how the Ancient Egyptians managed to get their limestone blocks in perfect surfaces and at right angles.
- yetihehe 6y agoIf 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 https://www.youtube.com/watch?v=BNSb5gPdqsA you can find more by searching "egyptian lathe".