Dimensional metrology - Page 1
Forum section: electronics / general

author: rhb
author_rank: Super Contributor
timestamp: July 21, 2019, 01:42:27 am
content: Is dimensional metrology an acceptable topic here?In particular, squareness, flatness. angle and length. With current electronic components, things which were previously extremely difficult to measure have become much easier. And as JBeale and branadic demonstrated, the price performance for a good design is absolutely mind boggling.My personal interests are arc second angular measurements, interferometric length measurement and optical wavelength flatness of non-optical surfaces.I recently hit on a viable scheme for making 10-20 arc second accurate cylindrical squares from piston wrist pins for under $5 for a 3" tall cylinder. I need to rework the procedure because temperature effects from handling the square become a 20-30 arc second source of errors.By placing the sort of inclinometer JBeale demonstrated on a 3 point support, flatness to 50 millionths or better seems very possible if one has adequate environmental controls. The environmental control is *hard*. I've got a 1 F/ft gradient in a 10' x 7' room with a window AC running full time. I was quite shocked. That will cause serious curvature of a stone surface plate.

author: Dubbie
author_rank: Unknown
timestamp: July 21, 2019, 02:24:02 am
content: Fine by me! I love this stuff.

author: BravoV
author_rank: Super Contributor
timestamp: July 21, 2019, 04:07:17 am
content: Thanks for bringing this out, subbed.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: July 21, 2019, 05:56:52 am
content: I have some fascination with the subject of measuring flatnessI always wondered if anyone have been able to DIY visual autocollimators cheaply?my current understanding of this is that, the optics just needs to be fixed/stiff long enough as it (or the target point) is shifted around the DUTthis video not exactly about flatness, but the method is similar. looking at (or for?) the reflected target (to know the "cross hair" is now 100% reflecting back straight on itself)there is another tube video by oxtools about flatness calibration and the calibrator is also using autocollimatorI think robrenz has some toys/video on this too

author: lukier
author_rank: Unknown
timestamp: July 21, 2019, 07:10:47 am
content: Nice thread!Recently, I've started collecting Agilent laser interferometry bits & bobs, 5517x lasers and optical bits to play with. Got most parts for 3 linear axes. The angular interferometer or 10717A are very expensive and rare so now I focus on linear displacement.Don't have particularly stable lab, no machine shop with a surface plate or a stabilised optical bench so I expect a lot of noise in the measurements, but that's part of the fun. Should get some PCBs for making DIY STM32 based receiver next week.I recommend Sam's Laser FAQ on the subject, he also has eBay store with refurb lasers:http://www.repairfaq.org/sam/laserhst.htm#hst5517c

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 21, 2019, 12:47:05 pm
content: Autocollimators are reasonably easy to build and are also available surplus on the 'bay. I use them for camera repair, which is a bit different than flatness metrology. An autocollimator focused and zeroed out with a flat mirror makes an infinity source. If you put a camera in front of it, you can view the target image on the film and set infinity focus with great accuracy. If you build one, be sure to use afairly high power eyepiece, as that's where the precision/resolution comes from. Inteferometers are also reasonably easy to build, but can be hard to apply without a lot of bells and whistles, which is why finding the used HP stuff is so useful. You also need flat mirrors with some transmission, which can be expensive. I used to work for Burleigh Instruments, which made research grade interferometers with up to 70 mm mirrors. We did mirror pairs up to lambda/200 flatness and sometimes triple-passed them with corner cubes. The company is long gone now, but the stuff shows up on eBay now and then. There's a "not quite an interferometer" technique used for alignment that's little known. Get two mirrors and drill a small hole through one. Look through the hole at the other mirror. When they get close to parallel, you'll see the multiple reflections collapse. The sensitivity is quite good at that point. It may help to put a bright ring around the hole so you can see it better.

author: rhb
author_rank: Super Contributor
timestamp: July 21, 2019, 02:35:13 pm
content: Conrad's comment about making an autocoillimater makes me realize I've not done my homework despite having the books on the shelf. My ebay searches didn't turn one up at an attractive price, but I didn't make much effort to find one.For checking a surface plate I have in mind the following:Level the plate as precisely as possible with a 4 arc second per division vial on a 3 point mountingMove the vial across the plate in a grid or "union jack" pattern and note the readingsintegrate the slopes and contour the data4 arc seconds is 20 millionths per inch so an equilateral triangle base with carbide balls 2.5" apart gives a resolution of a half tenth per division. Dividing that visually in 4ths is reasonable. With a capacitance bridge arrangement such as JBeale documented resolution of 5 millionths should be practical though slow. The settling time for a high precision vial is rather long. My 10 arc second machinist's level takes 10-15 seconds to stabilize. I've got vials and a partially finished mount. Just too many competing projects.I've got an idea for a Talyvel style instrument with 0.1 arc second resolution and a settling time of a few seconds, but have not had time to work on it yet.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 21, 2019, 05:20:49 pm
content: Watch out for local surface variations (roughness) fooling you if you use point contacts or close to such (balls). I'm thinking some flat pads of small area might work better. FWIW, the big expensive profilometer at work scans the part by moving it over a 10" optical flat. The carriage has small Teflon pads running on the flat.

author: rhb
author_rank: Super Contributor
timestamp: July 24, 2019, 12:28:12 am
content: I bought 4 mm carbide balls which I plan to press into pockets and then finish on a surface grinder to make small flats. Lacking that, I think some fine silicon carbide paper wetted both sides and placed on a surface plate would make satisfactory flats with a few quick figure eights. I paid attention when Conrad mentioned that issue before.I've been thinking about a 2D Talyvel style level using a precision ball on a single music wire flexure with limit stops to protect the wire and 3 point support. Because levels are self proving, it doesn't even require an especially high precision ball. Just fling a small MCU and an LCD at it.That's amenable to using a pair of capacitance bridges, but also better grade optical proximity sensors than the first ones I tried might work. I'm trying to steel myself to reading datasheets all day to select a better sensor.I find my 10 arc second per division level painfully slow. I don't want to think what the settling time for a 0.1 arc second bubble level would be.It's amazing how much time it takes to do things when you get demanding. I hate to think what a lambda/200 flat cost.I'm hoping to be able to work tomorrow on a setup for testing a cylindrical square made from a wrist pin without spoiling it by the heat from my hands. I found an Indian company that makes wrist pins up to 120 mm diameter and 250 mm length. But one email addressed bounced with "mailbox full" and I got no response from the yahoo.com address.I've mentioned all this to Conrad privately as part of another conversation, but for the benefit of others I'll give a brief explanation.Piston wrist pins (gudgeon pins to Brits) are ground to *very* tight tolerances. You can get used ones for free from anyone who rebuilds engines. New ones 3" long are under $3 US. So if you grind one end so that you have 3 raised spots ~120 degrees apart and place it in a fixture so it can be rotated on a surface plate with a tenth indicator at the top edge, you can lap the feet with silicon grit paper, or scrape with a carbide scraper until you don't have measurable runout at the top. A tenth TIR in 3" is under 7 arc seconds error. But at ~1" diameter, a 16 F difference in temperature between the ends is 7 arc seconds of error.That is an extremely accurate reference square for $3 and 20-30 minutes of your time. It's actually too accurate to use to test a square visually to the same accuracy. My experiments indicate that a gap smaller than 0.0005" is too small to see.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 24, 2019, 03:01:09 am
content: Nonsense, you just need a brighter light!

author: CatalinaWOW
author_rank: Super Contributor
timestamp: July 24, 2019, 03:50:18 am
content: Or, since light transmitted by the gap should be roughly proportional to the gap, just an appropriate light sensor (appropriate covers all sorts of details including alignment sensitivity and stability among others) with more gain. A nice metrology rathole to dive into, figuring out the error sources and coming up with calibration methods.

author: rhb
author_rank: Super Contributor
timestamp: July 26, 2019, 01:01:19 am
content: I don't have a part number to cite, but if you look at the linearity of some of the sensors over a 1 mm or smaller range it's quite amazing. With a 12 bit ADC, 10 microinch resolution is quite plausible. At least if the datasheets are telling the truth. That's better than 1 arc second if you use the full 1 mm range and a 3" wrist pin.Will it work? I have no idea. My crude experiments with a $0.45 US sensor and a 0.0001" reading micrometer suggest it will. What the tests really told me was I needed to be a lot more sophisticated in my test setup and needed a better sensor.

author: Vtile
author_rank: Super Contributor
timestamp: July 27, 2019, 06:43:51 pm
content: Quote from: CatalinaWOW on July 24, 2019, 03:50:18 amRatholes are surprisingly interesting places to visit.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: July 27, 2019, 07:19:25 pm
content: Quote from: Vtile on July 27, 2019, 06:43:51 pmTotally agree. Not always rewarding, but usually fun and occasionally a diamond is found.

author: rhb
author_rank: Super Contributor
timestamp: July 27, 2019, 08:19:26 pm
content: Quote from: CatalinaWOW on July 27, 2019, 07:19:25 pmI must demur, not always *successful* but almost always educational. And that is its own reward ;-)I'm fairly certain that a substantial portion of the errors I encountered were because the micrometer spindle face was not *perfectly* perpendicular to the axis of the screw. That would not be an issue for the pendulum sensing case.I think I need to go out to the shop and make the fixture for testing the squareness of the end of a wrist pin. The real aggravation will be hunting down a suitable rubber roller.

author: Mr. Scram
author_rank: Super Contributor
timestamp: July 27, 2019, 10:28:27 pm
content: Interesting thread!

author: rhb
author_rank: Super Contributor
timestamp: July 28, 2019, 01:20:14 am
content: I spent about 90 minutes today in my shop. I thought it would be OK as it didn't seem all that hot, just mid 80's. I was absolutely parboiled and soaked from head to toe when I came back inside the house.I did manage to make a failed prototype of a base for checking 3 point supported cylindrical squares. I really needed a piece of 2" x 3/8" angle, but could only find a piece of 1.5" x 1/4". So I used that. I'm hoping I can salvage it, but not sure yet. I may have to start over with a heavier piece of metal.Problem is, the next larger angle I can find is 6" x 1/2" which is rather large for a 4" x 6" bandsaw.. It's also larger than I care to do by hand. I would like to practice at the bench with files, but for a piece that large I might wimp out and use the mill.I'm afraid I'm on a bit of an Alexander Weygers kick at the moment. In "The Recycling, Use and Repair of Tools" which is reprinted in "The Complete Modern Blacksmith" with his other two books, he presents an exercise for the reader. He attended marine engineering school in Holland in 1916. The exercise is to take a flat steel plate and a round bar. Drill a hole in the plate and file it hexagonal. Then file the round bar to be a close sliding fit over a length of 3" in all 6 positions. That's a 0.001" tolerance using files and a spring caliper.I've not worked up the nerve to attempt that. I can do a rectangular piece to those tolerances, but that's *much* easier than hexagons. But I'd like to before I die, just to say I did.If I can do that, turn a series of 1/4" wide collars on a 1.5" bar 18" long to 0.0001" and pick up a thread on a machine without a thread dial. I'll be satisfied I can claim to be a "machinist". Sadly, today most people who claim to be machinists merely operate machines. Give them a lathe without dials or a DRO and lots of backlash and they are helpless.If the engine broke down in mid-ocean 100 years ago and the marine engineer could not fix the engine with whatever tools he had, everyone on board died. The machine tools at the school were worn out by design. To graduate you had to be able to do accurate work despite limited and worn out tools. The marine engineer was responsible for making sure the crew did not die if something broke.Today, many "machinists" can't solve a problem in high school math, algebra, geometry and trigonometry, without a computer program to do it for them.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: July 28, 2019, 03:54:11 am
content: I am stuck in some sort of middle land from where you describe. Most of my tools are old and worn (the cross feed on my lathe has 40 thousands of backlash) so I have lots of practice compensating for both wear and lack of fancy equipment. But I was born several decades too late to have the direct hands on teaching from those who did the work you describe. I have several of the books and they are helpful, but nothing like the monkey see, monkey do school of learning.P. S. I don't know the technique for doing a hex that your text recommends, but if your lathe has indexing holes as mine does it can make that task much easier. I have done it several ways including totally freehand, and don't have the patience to do it freehand to the tolerances you speak of. At least not without the motivation of being in the middle of the ocean without motive power.

author: SilverSolder
author_rank: Super Contributor
timestamp: July 28, 2019, 11:32:44 am
content: Back in the engineering college days, one of the instructors would always come down hard on students that made parts more precise than necessary (not that it happened too often, lol, but there was always someone that went for broke).With that educational background, I have found that you can do a surprising amount of quite decently precise work with a humble angle grinder!e.g.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 28, 2019, 12:37:24 pm
content: In theory you can make just about anything, to any degree of precision, with nothing more than a file, maybe a scraper and some simple measuring tools. But why? I'd build a planetary lapping machine before ever scraping three plates together to make flats. Heck, it would take less time. Things like surface grinders and honing machines exist. Until you get to the last millionths, they're fast and do a great job. Life is too short for reenacting early industrial history; there's new stuff to do!

author: tomato
author_rank: Regular Contributor
timestamp: July 28, 2019, 03:12:03 pm
content: Quote from: Conrad Hoffman on July 28, 2019, 12:37:24 pmI prefer a slight variation to your statement.QuoteCouldn't agree more.

author: rhb
author_rank: Super Contributor
timestamp: July 28, 2019, 04:54:37 pm
content: Many years ago I came across the website of someone who builds custom CNC machines for a living. He uses square tubing for the ways and poured concrete for the supports. He finishes the ways to a thou over the full length with an angle grinder. The heavy stuff is amazingly straight to begin with. The 2 x 3/8" stuff Dad left me is straight to a thou in 3 ft which is the current limit of what I can measure.The more you know and the greater your skill, the less you need to accomplish a task. We live in an era when the need for efficiency and low cost has made most people entirely dependent on exotic tools and test gear for even simple tasks.Unless you're just doing it for practice, things should only be as precise as needed. I made metal bases to hold some dev boards by hand to 1-2 thou using a spring caliper just for practice. And the fact that my eye saw it was not square and it bugged me.Rookie engineers over specifying parts is *very* costly in industry.The hex in Wygers book is *all* done with files and caliper. Only other tool allowed is a drill to make the initial hole.Why learn to play a musical instrument? Why learn mathematics? Why make things?Most people just go to the store and buy things they don't understand. They think owning tools and test equipment is crazy. But they think McGyver is cool even though the stuff in the TV show is impossible. I watched it once for about 5 minutes while flipping through channels. He had just been locked in an attic until it was time to kill him. So he made a harpoon cannon out of a brass refracting telescope. Used it with lighter fluid for propellant to fire a 10-15 lb harpoon and some 1/2" rope 40-50 ft into a tree and escaped. I never looked at the show again.The day the trucks stop delivering food to the grocery store all those pre-industrial skills will be the difference between life and death. I hope I'm dead by then, but it's just a virus mutation away and could happen at any time. And then there's major geological events like the Valles, Long Valley, Yellowstone and Mazama calderas, asteroids and solar flares.Today's effort at the indicator stand base came out nicely except for the piece of angle being too small. A piece of 3" x 3/8" would be more appropriate, but I don't have any on hand. It's got 3 carbide ball feet with ~1 mm flats. I still need to scrape the transfer surface so I can use it for testing regular squares against the cylindrical square. Some testing showed that its pretty much impossible to test a square visually to better than 1 minute of arc without optical equipment for the task.It's entirely kinematic except for the transfer surface. I've still got to make the indicator holder and thermal isolation pads so that my hands don't touch the wrist pin or the indicator stand when rotating the wrist pin.As for originating a surface plate, I'd just use a precision level on a 3 point mount and an angle grinder to make a single plate.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 28, 2019, 06:51:11 pm
content: Lacking a proper scraper, I once flattened the sole of a Stanley plane with a coarse rat tail file. Sounds awful, but worked great. The metal won't tell anybody how you got it to the precision needed. The problems in my real job aren't so much that the needed precision is so difficult, but that the precision is needed in parts that are too flexible to be stable, or too small to use the methods known to work. With our own projects we can usually make things whatever size and mass we think is appropriate.

author: View[+]Finder
author_rank: Unknown
timestamp: April 13, 2025, 08:46:00 pm
content: Warning: this topic has not been posted in for at least 120 days.Unless you're sure you want to reply, please consider starting a new topic.This comes from the fountain of "new must be better" and like much "new and improved" hardware and software, it serves to improve the lifestyle of the developer. At the expense of continuity of user-experience for everybody else.This post is about the metrology of the physical world--SPOILER: it is ahead of electrical metrology by about 3000 years.In particular, my recent interest in the use of laser measurement to calibrate things like milling machines and CNC cutters. On eBay, of course, I found an interesting HP vintage "measurement display" the HP-5508. Finding it could measure and record four different temperature sensors, I went on the hunt for one. On eBay under $150. The hunt ended with my saving $$$ by getting a 5508 with a HP-5518 laser and cables to boot.One pesky problem with precision equipment from reliable manufacturers is that the equipment has it's own idea of "reliable" and refuses to work at a lower level than designed. Understandable. Now the laser needs a tuneup and with the help of the photos attached and the manual, all will be working soon.

author: View[+]Finder
author_rank: Unknown
timestamp: April 13, 2025, 08:48:02 pm
content: And a couple more photos for the whole PCB on the HP-5518 Metrology Laser

author: ivo
author_rank: Regular Contributor
timestamp: April 16, 2025, 11:03:02 am
content: This is a very interesting series recently started on youtube, showing a construction of a "hobby" laser interferometre. Should be good to follow along.

author: EC8010
author_rank: Frequent Contributor
timestamp: April 16, 2025, 01:50:45 pm
content: Quote from: rhb on July 28, 2019, 01:20:14 amI think that's a bit unkind. I've spent some time modifying my Hobbymat MD65 lathe to get rid of most of the backlash and improve the feel, making it much easier to use. But the Colchester Bantam has some backlash I can't get rid of (well, not without serious modification). Also, I discover that when you move the Bantam cross slide, you slightly stretch the lead screw, and when you start the lathe, the vibration allows the stiction to be overcome and the slide moves 5um. I now have a dial gauge to monitor what's going onbeforeI take the cut. Yes, a poor workman blames his tools, but life's an awful lot easier with better tools.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 17, 2025, 12:45:25 pm
content: Building a Twyman-Green interferometer is quite easy and I've used them for small distance measurements with nothing more than a scope to count the fringes. For longer distances you need a counting setup, but with a HeNe laser you have a really accurate system with no NIST required!

author: mawyatt
author_rank: Super Contributor
timestamp: April 17, 2025, 03:50:39 pm
content: This is something that may be of interest to some folks. Quite some time ago we were developing various camera imaging systems for taking high resolution chip images. These custom systems (developed at home) were based upon reuse of Linear Rails (used for wafer positioning) mostly from THK with 1mm pitch. These rails were driven from 400 step motors with custom controllers utilizing Trinamic chips employing "Micro Stepping" which could produce 256 microsteps per Step for an overall effective Linear Rail resolution of ~10nm!! Of course this was theoretical resolution and we had no way to verify this level, sorta like a 32 bit ADCLong ago Rik Littlefield posted about using his focus stacking software (Zerene) to resolve camera/subject positions using a clever technique of image correlation by means of pixel shifting. We decide to try this in a unique way where a "subject" was placed upon a Piezo Electric Fixture we had constructed from Physik Instrumente surplus and designed and constructed a custom controller. The "subject" was small white photo-paper printed with a grey pattern from a laser printer. The laser printer produces high contrast random scattered carbon black bonded to the paper when viewed under a microscope. The paper was bonded to a flat glass slide and tilted wrt to the camera lens optical axis which is directly attached to the Linear Rail.The general idea was to move the Linear Rail with micro steps then have the Piezo Fixture move the subject a number of positions and capture images with high magnification for a very shallow Depth of Focus, then repeat at another micro step position.All these images were loading into the focus stacking software which would attempt to align in a sliding window with each image (thin sliver of in-focus area) by shifting the pixel positions including fractional amounts for a 2D best fit to create a final rendering of a fully in-focus result (took awhile for this to converge on each image). With the amount of recorded 2D pixel shift required at each captured image one can resolve the physical position change knowing the lens magnification and camera pixel dimensions. Took awhile to get this to work, but it did and we were able to achieve 10~20nm resolution and plot how well the stepper motor micro stepping and Linear Rail behaved.The key for this working is the highly random high contrast subject black "dots" printed by the laser and the narrow thin slice in focus area between images. By tilting the subject this gave the software a somewhat controllable transition in/out focus band to work with across each image and with fractional pixel positioning a high degree of resolution.Later we were able to remove the Piezo Fixture and just use the Camera/Lens Stepper movement with less lens magnification and achieve similar results as we did with the Piezo Fixture.Anyway, thought this might be of interest to folks in this thread.Here's some links about the Piezo Development, and below is a plot of the resolution achieved.https://www.photomacrography.net/forum/viewtopic.php?f=25&t=40510https://www.photomacrography.net/forum/viewtopic.php?f=25&t=40679https://www.photomacrography.net/forum/viewtopic.php?f=25&t=40681https://www.photomacrography.net/forum/viewtopic.php?f=25&t=40682Best

author: EC8010
author_rank: Frequent Contributor
timestamp: April 17, 2025, 05:11:55 pm
content: Wow! Very elegant. And I was pleased with my use of a mechanical dial gauge. . .

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: April 18, 2025, 07:16:51 am
content: Precision machinists say thateverything is rubber.Quote from: EC8010 on April 16, 2025, 01:50:45 pm

author: mendip_discovery
author_rank: Super Contributor
timestamp: April 18, 2025, 07:55:34 am
content: To a metrologist everything is just a thermometer.Quote from: Leo Bodnar on April 18, 2025, 07:16:51 am

author: EC8010
author_rank: Frequent Contributor
timestamp: April 18, 2025, 08:39:41 am
content: Quote from: Leo Bodnar on April 18, 2025, 07:16:51 amAnd lathes are made of cheese. I've certainly learned not to lean on the tail stock whilst attempting accurate work.Recent lathe work has been an attempt to counter the effects of draughts and temperature variation. . .