[HN Gopher] The Origins of Precision [video]
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The Origins of Precision [video]
Author : gitgud
Score : 25 points
Date : 2020-10-13 06:11 UTC (16 hours ago)
(HTM) web link (www.youtube.com)
(TXT) w3m dump (www.youtube.com)
| mauvehaus wrote:
| If you're into this sort of thing and in the Northeast, the
| American Precision Museum in Windsor, VT is worth a visit. Among
| other things, they have Bridgeport milking machine number 1.
|
| A lot of the original work in precision manufacturing was in the
| arms industry. Interchangeable parts in rifles were a huge deal
| when they came about: no longer did you need a skilled gunsmith
| to individually fettle replacement parts when a gun needed
| repair, you could swap one worn part for another in less time and
| closer to the action.
| canadian_tired wrote:
| If you prefer a book, take a gander at "The Perfectionists" by
| Simon Winchester. If you enjoy the pedantry of accuracy vs
| precision (or wish you did), this book is for you.
| jeffreyrogers wrote:
| There is a really great book called "Foundations of Mechanical
| Accuracy" that shows in detail (and with a lot of very good
| photographs/technical illustrations) how extremely precise
| measuring tools can be built up from a simple flat plate. One of
| the most interesting things I've read recently.
| mvidal01 wrote:
| It's on Archive.org -
| https://archive.org/details/FoundationsOfMechanicalAccuracy
| CamperBob2 wrote:
| Archive.org is so weird. Henry Ford died in 1947, so _Moving
| Forward_ is out of copyright (and consequently available on
| Project Gutenberg, as another post points out). Yet on
| archive.org I can only "borrow" an encrypted copy.
|
| Meanwhile, _Foundations of Mechanical Accuracy_ was published
| in 1970 by Wayne R. Moore, who was presumably still alive at
| the time, so its copyright status is still very much in
| force... but archive.org says "Here, have a nicely-OCR'ed
| .PDF."
|
| I hope they're able to survive their present legal
| difficulties, but I'll be darned if I can see how.
| petertodd wrote:
| While that's an excellent book, one of the things that I find
| fascinating about it is what it doesn't cover all that well:
| how do you make an accurate screw thread? The usual way you
| make a screw thread is by cutting it on a lathe. But the heart
| of a lathe is an accurate screw thread...
|
| How to making flat plates from scratch with the 3 plate method
| is relatively easy to understand. But screw threads have a much
| more complex geometry, with no obvious way of making one from
| scratch. They also have many more parameters to consider,
| including relative pitch accuracy, and absolute accuracy.
|
| As far as I can tell achieving accurate screw threads it
| something that took a huge amount of work by many different
| people effectively _working together_ to get successively
| better and better threads, using each others ' screw threads
| made using multiple techniques to average out errors. But I've
| never actually seen anyone describe from start to finish out to
| go from nothing to an accurate screw thread. It's not even
| clear to me that it's possible to do alone in a reasonable
| amount of time.
|
| Relevant:
| https://freechaptersinbooks.wordpress.com/2012/09/18/screw-t...
| jeffreyrogers wrote:
| Haha, I wondered the exact same thing when reading it. Same
| thing about the spindle. It talked about how you could
| measure how accurate a spindle was but not how one was
| constructed.
| aaronblohowiak wrote:
| I imagine you can do this through gear reduction -- reducing
| the ratio of your leadscrew to the tool movement essentially
| reduces the inaccuracy of your leadscrew, iiuc. the percent
| error is the same, but you care more about absolute error
| than percent.
| petertodd wrote:
| That's a good idea. But I don't think it actually works, at
| least with the obvious way to do it.
|
| Suppose you have a lathe whose screw thread has errors in
| pitch such that the position of the middle thread is
| incorrect. With the right gears, you can use that lathe to
| cut a second thread with a different pitch. But regardless
| of what pitch you choose, the middle of that second thread
| will have the exact same _absolute_ error in that position
| as the original thread. So you haven 't actually improved
| anything.
|
| The best I can think of to improve the accuracy of a thread
| is to measure the thread against multiple length standards,
| and grind/lap away material by hand to bring the actual
| position along the thread length as close as possible to
| your length standards. However, as the number of length
| standards you can practically measure against is limited,
| you'll need to already be at a point where relative pitch
| error is small. Averaging out errors with a long follower
| nut is probably one way to do that.
| jbay808 wrote:
| I believe they said that their screw threads were lapped by
| hand while being checked by interferometer readings.
|
| An absurdly difficult process, but the result is a full-area
| bearing screw that never wears out.
| petertodd wrote:
| Yes, that's how the Moore Tool Company did it. However the
| first screw-cutting lathe actually dates back all the way
| to 1800. While as far as I can tell the first use of
| interferometry for distance measurement was the Michelson
| interferometer, in 1887; it took until 1960 for the meter
| to finally be defined in terms of wavelengths of light.
| jbay808 wrote:
| Oh, I see what you mean now; I misunderstood your
| question!
|
| Here's a screw cutting machine from Da Vinci's notebook:
|
| https://www.ssplprints.com/image/100676/leonardo-da-
| vinci-sc...
|
| But as you observe, it already has a screw in it. Two
| screws actually, one on either side, which advance a
| carriage that cuts a new screw in the center from a
| blank. It can either duplicate the pitch, or using a gear
| ratio, it can cut a different pitch than the two master
| screws.
|
| The central screw averages the error of the two master
| screws, so with certain assumptions, the new can end up
| more accurate than either of the masters; the master
| screws can then be swapped out for new screws made this
| way, and so on. But there are limits to that approach,
| because common errors won't get averaged out. However you
| could, for example, assume that two screws made this way
| with the same masters are accurate duplicates of _each
| other_ , then when you replace the masters with those two
| new screws, you can mount one of them flipped end-over-
| end, or with a rotational offset of eg. 90 or 180 degrees
| (offsetting the leadnut to compensate), and gradually
| average out more errors that way.
|
| Another way as you noted in a different comment is to use
| a long, soft leather lead-nut to average multiple
| threads, and use that as a master to create a new thread
| (assuming a low cutting force -- for example with force
| amplification, or for checking a thread, and so on).
|
| There are some first principles approaches you could use,
| like making a master cylinder and then wrapping a wire of
| constant-diameter around it to make a literal 'thread',
| and bonding it in place. It wouldn't be the most robust
| threadform but that's another way.
| mauvehaus wrote:
| With a screw origination machine, of course ;-)
|
| Henry Maudslay invented the first one that saw widespread
| use.
| petertodd wrote:
| ...and notice how his screw-cutting-lathe has an accurate
| lead screw in it? :)
|
| https://www.ssplprints.com/image/100286/henry-maudslays-
| orig...
|
| https://www.gracesguide.co.uk/Henry_Maudslay:_Machine_Tools
|
| I've never actually seen a detailed explanation of how he
| cut that first accurate lead screw! Like I say, as far as I
| can tell it took an iterative variety of processes that
| made a variety of screw threads, with no one method alone
| being enough by itself.
| mauvehaus wrote:
| The top two pictures of the gracesguide.co.uk page show a
| screw originating machine, not a screw cutting lathe. And
| that's the machine you need.
|
| AIUI, it's used to cut a lead screw in relatively soft
| material by holding a cutting tool to it at an angle.
| Once you have that, you can generate a screw in harder
| material from it (carefully, one has to assume).
| dylan604 wrote:
| I learned this lesson with the difference in an all-thread
| rod compared to a lead screw. even though both were at a
| typical 1/4"-20 pitch, the all-thread had too much slop in it
| to make for an accurate, repeatable motion. I went down the
| same rabbit hole of reading about how accurately turn a
| screw.
| thechao wrote:
| Currently priced at 2349.56$ with 3.99$ in shipping, on Amazon.
| SAI_Peregrinus wrote:
| Or $150 from Moore. http://mooretool.com/publications.html
| mikewarot wrote:
| One of the things he mentions is a chapter in Henry Ford's Book
| "Moving Forward" about the _very practical_ need to be able to
| measure to a millionth of an inch... I highly recommend reading
| that chapter
|
| http://gutenberg.net.au/ebooks17/1700321h.html#ch14
| seiferteric wrote:
| He mentions that all bricks in a building are relative to the
| first brick... I am not an expert at all, but aren't bricks laid
| using a leveling line? And the mortar will provide some "wiggle"
| room to fix small mistakes?
| dylan604 wrote:
| Also known as the cornerstone. Even though the mortar might
| wiggle, if the first stone is not correct,the building will not
| be in the correct spot.
| petertodd wrote:
| You're absolutely right:
| https://www.youtube.com/watch?v=lORIZ1shRIM
|
| Given that mortar is semi-solid, there's no way you could ever
| reliably get the same thickness of mortar without an external
| reference.
|
| Older tall chimneys are often visibly crooked in parts. I'm no
| chimney builder, but it's easy to imagine it being difficult to
| get the alignment perfect when you're trying to make an angled
| structure, high up in the air, using primitive tools. (remember
| that tall chimneys are narrower at the top than the bottom to
| save money, so you can't directly compare it to a vertical
| plumb-bob line: you have to offset each layer slightly from
| vertical)
|
| Equally, once the error builds up to the point where you can
| see it from the ground, getting the rest of the chimney back to
| true would be easy: just lay the next layers of bricks slightly
| offset until it looks right again. Results in an slightly ugly
| looking chimney. But a lot cheaper and faster than demolishing
| the crooked bit and starting over.
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