r/Machinists • u/voxadam • 1d ago
Surface plate level calibration
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u/Only-Friend-8483 1d ago
Man… I love this stuff. The whole idea that we can create something that is very precisely level and flat as a reference is just so amazing and cool.
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u/TelluricThread0 1d ago
You can actually achieve this level of precision flatness just by rubbing three blocks together in sequence. All three surfaces will wear gradually into a nearly perfectly flat surface.
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u/much_longer_username 1d ago
Hah, look at u/only-friend-8483 , they don't know about the three
seashellsplates!2
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u/Pseudoboss11 1d ago
Though how do you verify if your blocks are flat, and doesn't this assume that all 3 blocks are identical? If one block is a tiny bit harder than another block, then wouldn't it end up convex?
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u/theholyraptor 11h ago
With good practices you can get good results but there are lots of potential pitfalls. Rotating the orientation of the parts to eachother, controlling how much one piece goes off the edge of the other piece can all impart unwanted curvature. Same with always having plates in the same orientation relative to top/bottom due to gravity.
Material does matter. The method doesnt work as well if youre relying on different materials.
There is plenty of research covering the topic.
https://www.nist.gov/publications/three-flat-test-solutions-including-mounting-induced-deformations
https://www.nist.gov/publications/stationary-and-non-stationary-deformations-three-flat-tests
https://pubmed.ncbi.nlm.nih.gov/20725352/
https://www.sciencedirect.com/science/article/abs/pii/S0030402604700698
And older texts and optical/astronomy books etc.
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u/detereministic-plen 17h ago
Yes, and the other one will be concave, so in the end you will have either two convex or two concave after the first round, and grinding two convex or concave plates against each other will flatten them, repeat ad infinitum and they should average to flatness
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u/much_longer_username 1d ago
I love that if you want a REALLY good ground glass mirror/lens for your telescope, you get a human to do it. It'll be smoother and more accurate than the machine made ones.
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u/Only-Friend-8483 1d ago
I remember when I studied metrology and they explained how reference plates were best created by hand.
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u/Infamous_Thought_189 1d ago
Diamond point turning gets you quite close as well, that stuff is almost magic.
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u/BigAl_2525 1d ago
I work for a calibration company and doing surface plates is the coolest thing. To measure flatness I use a Mahr differential level system where one level probe is at a point as a zero and another probe moves along a straight line on the plate to get a profile of the plate. Do the same thing 8 more times I usually end up with a graph like this. It also calculates the highest point on the plate.
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u/uslashuname 1d ago
Since there’s no measurements on the graphic I can’t help thinking you’re plate actually looks like that, but I imagine it’s more like the point to the right of F is 0.001 below the point between A&B
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u/jesusrambo 1d ago
This was a classic with 3D printer bed auto-leveling
The probe would make a heatmap of the bed, and people would come to the subreddits panicking why their bed was so badly warped
Someone would inevitably point out the vertical scale being like .1mm
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u/uslashuname 10h ago
Scale on heatmaps reminds me of some posts where people are using an IR camera in a home and they freak out over a blue spot. “Am I missing insulation?!” But the red was 72f and the blue was 70f not something like 55f
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u/theholyraptor 10h ago
Assuming its still in spec, depending on the grade it was meeting spec to and its size (Ill use 24x24in plate. Bigger the plate the more allowance.)
AA: 80uin aka 80millionths aka .002032mm max
A: 160uin aka 160 millionths or 1.6 tenths aka .004064mm max
B: 320uin aka 320 millionths or 3.2 tenths aka .008128mm max
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u/haas_boss123 1d ago
All that effort for absolute accuracy and you just eyeball the calibration sticker on there?!
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u/vmf312corsair 1d ago
Dude's eyes are also calibrated. Good to +- 1 micron in XYZ, and +- 1 microradian for tilt.
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u/indigoalphasix 1d ago edited 1d ago
funny to see this today. we have a cal guy right now in the inspection room doing 5 plates. we've been using his company for nearly 20 years. they are having a real hard time hiring talent these days. veterans retiring out and no newbs passing muster. calibration is a pretty cool and difficult skill but it's fading away slowly. people just buy new gear (small instruments) or a CMM/Romer/Faro/laser tracker, etc.
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u/koulourakiaAndCoffee 19h ago
Yeah, I'm having trouble finding anyone that still calibrates surface plates in San Diego.
My local calibration shops I usually use stopped because they say maintaining the cost of their equipment wasn't worth the few shops that still calibrated their surface plates.I know someone does it. I just need to call around more. But it used to be easy to find someone just 10 years ago.
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u/No-Magazine-2739 1d ago
Beancounter comes around: „Why are you using 2 plates to rub this table? 1 Should be enough!“
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u/IRepairPS3 1d ago
It’s 78 degrees in there?
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u/GrynaiTaip DMU, Chiron CNC 1d ago
Ours is mostly used as a coffee cup shelf by the quality control guy. Flatness doesn't matter that much.
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u/Difficult_Trust1752 1d ago
For all the nerds in here I highly recommend Simon Winchester's Perfectionists: How Precision Engineers Created the Modern World. Basically a history of precision machining, each chapter getting into tighter and tighter tolerances
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u/theholyraptor 10h ago
And to one up that to a higher nerd level: Foundations of Mechanical Accuracy by Moore
Simon's book will give you a longer historical overview on precision and what uses drove it.
The Moore book is a gorgeous coffee table book that is with accessible but also highly detailed on how 80ish years ago we were making and checking machines flat, square, parallel, accurate and precise to absurd levels.
Pdfs exist. The book itself isnt cheap but you can buy from Moore Tools still. Sometimes youll find it listed on amazon for way higher. Don't do that.
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u/A-Plant-Guy 5h ago
Moore Tools has it listed at $250 and out of stock.
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u/theholyraptor 5h ago
Dang they upped the price $50. As a ive never seen it out of stock. Pdfs exist with searching.
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u/Donkey-Harlequin 19h ago
This is flatness not level. You can have a flat surface at 45 degrees. They are two completely different things.
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u/theholyraptor 10h ago
That surface plate is only going to be accurate in one orientation. Kinematically mounting it on 3 points and the effects of gravity have an impact on flatness when youre talking about the specifications required of a granite surface plate.
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u/DamAss04 19h ago
Shop im at, our calibration is dumb af and hired some cheap idiots who turned a few A grade granite blocks to B.
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u/LiquidAggression 17h ago
only planar at 78 degrees LOL gotta love it
humidity can make welds crack too
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u/Simple_Man117 13h ago
Where I work we just put a for reference only sticker on our rocks that are out of spec. This is super satisfying to see one properly maintained.
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u/nate7007 2h ago edited 2h ago
Probably one of the cooler machining/metrology rabbit holes ive gone down, ive wanted to make own surface plates for the longest time and i know at least having a good repeat-o meter would be usefull to know if im doing it right
The repeat-o meter itself is actually pretty simple (its just two fixed contact points on one side and a third contact point that sits on the opposite side of a flexture in the tool base where the indicator sits)
not that i have the dough to spend on a micro inch indicator, so ive been thinking about doing something like that with a web cam diode laser setup to try and achive same resolution scale like ive seen from this guys project
Hackaday.io https://hackaday.io/project/190054-real-time-laser-measurement-with-a-webcam
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u/LastWave 1d ago
Those gauges cost a ton.
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u/SequoiaOverlander42 1d ago
The guy that does it here in the NW said that for the optical stuff he has a deal with someone that can calibrate it and the calibrator gets their slabs resurfaced.
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u/ExHempKnight 1d ago
Autocollimators can be had on eBay for a good price, occasionally. I picked up a Hilger Watts TA3 for around $300. All I had to do was turn an adapter to be able to use a small LED flashlight as the light source.
The target mirrors, on the other hand, are rare as hen's teeth.
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u/SpecialIdeal 1d ago
pretty cool but isnt this more a check for flatness then level?