# Small surface plate tester



## 38super (Apr 30, 2018)

Was watching Rahn Repeat-O-Meter vids from Oxtools and NYC CNC .  Great for 24x36 plates or larger but too big for a 12x18.  Did a little resizing and used set screw/ball bearing pivot instead of a flexure.  Fleabay 0.0001 dial indicator ($26).  Button on side locks the pivot arm when not in used, unlocked it limits 0.030" travel.  Not a daily use tool, but handy if buying a used surface plate.  

http://s1333.photobucket.com/user/F-4CWeasel/library/Rahn Style Surface Plate Tester?sort=2&page=1


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## Ray C (Apr 30, 2018)

Looks really good.   Ever since Tom Lipton and Rob Renz have been dedicating time to surface plate analysis tools, I've been tempted to know how flat my plate is which is 18x24".  

If you took photos while you made those, consider posting them here.   Folks would love to see it.

Welcome to Hobby Machinist...

Ray


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## Eddyde (Apr 30, 2018)

Welcome HM! Nice project and and please post some pictures, we'd like to know more about your project? I have a 18x24 plate I bought new 25 years ago, curious to know how accurate it still is.


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## 38super (Apr 30, 2018)

Pls use the PB link, saves bandwidth.  Made one for a friend, borrowed mine to verify (0.0001-0.00015 overall).  'Fraid to say mine looks more like a motocross track.

This might be a better PB link: http://s1333.photobucket.com/user/F-4CWeasel/Rahn Style Surface Plate Tester/story


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## 38super (Aug 12, 2018)

Sent one to Stan at Bar Z, here's his vid:




He has posted a link to the PDF files.


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## 38super (Aug 24, 2018)

Robrenz made mention of something I hadn't thought of.   He mounts the carbide inserts upside down so the cutting edge acts like a squeegee.  This prevents dirt from getting under the carbide feet.


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## Bob Korves (Aug 24, 2018)

Note that the Repeat-O-Meter is only half of the equation to check a surface plate.  It only shows if you have hills or valleys.  It does not give their relative heights.  Typically, a autocollimator is also used to test the actual geometry of the plate from one position to another.  Neither of these tools will do the job of calibrating a surface plate by itself.  Beyond the tools, a clear understanding and interpretation of what the tools are telling you is also necessary, along with the skills, tools, and knowledge of how to use the information to actually make the plate flat.


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## Technical Ted (Aug 24, 2018)

I love your project; you did a great job! I downloaded your plans and hope to make one myself some day. But what I have done in the past is simply mount my tenths indicator on my Starrett surface gauge and done the same check. In fact, I can get a lot longer reach if I want to with that. I don't really see any difference between the two methods other than a repeat-o-meter might be a little easier to setup. Am I missing something???

Ted


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## 38super (Aug 24, 2018)

Should be the same provided there's no gravity induced sag. I sweep an area that gives a consistent reading to have establish a known good reference.  From there you can sweep more area to get a surface profile.  My surface plate was bought used with out the means to verify flatness, caveat emptor.  
Optical lab rats may find this handy to characterize lens alignment.


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## 38super (Jan 4, 2019)

So has anyone built one yet?  Please share your thoughts, good or bad.


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## Bob Korves (Jan 9, 2019)

Take the surface plate to a pro shop and let them do the calibration and certification.  My 18x24x4" 2 ledge plate cost me just over $100 for the job, took them about 20-25 minutes to get it correct, from a .003" hole in the plate center error.  We had Standridge Granite from So. Cal. come here on one of their road trips, and we had 5 plates that belonged to 4 of us for them to do.  Without the quantity we did, the minimum order charge and travel fee would have killed the deal.  If you live close to a shop that does the work in house, you can take it there and get it calibrated while you wait.  Call first to get price, availability, and other information.  Those guys can do the work in their sleep and get it right after calibrating  hundreds or thousands of them.  Friendly workers, fun group collaboration.


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## 38super (Mar 1, 2019)

FYI - This RoM design was driven by "on hand" material, none of my ideas are cast in concrete.  US Marine motto: Improvise, Adapt, Overcome.


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## 38super (May 24, 2019)

Per Bob's comments, a long gradual trough will lead you astray or produce confounding readings.  Found this on a recent used surface plate purchase.  Surfing for reconditioning quotes.


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## 38super (Jul 8, 2020)

Brownells has 1/2-40 taps, typically used for tactical tupperware.


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## Optic Eyes (Apr 28, 2021)

Bob Korves said:


> Note that the Repeat-O-Meter is only half of the equation to check a surface plate.  It only shows if you have hills or valleys.  It does not give their relative heights.  Typically, a autocollimator is also used to test the actual geometry of the plate from one position to another.  Neither of these tools will do the job of calibrating a surface plate by itself.  Beyond the tools, a clear understanding and interpretation of what the tools are telling you is also necessary, along with the skills, tools, and knowledge of how to use the information to actually make the plate flat.


Good points, many autocolimators give angular readings, alignment scopes are metric or Imperial and others have no internal micrometers so you have to use  targets that have micrometers attached, this is the type I have.


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## Optic Eyes (May 12, 2021)

Bob Korves said:


> Note that the Repeat-O-Meter is only half of the equation to check a surface plate.  It only shows if you have hills or valleys.  It does not give their relative heights.  Typically, a autocollimator is also used to test the actual geometry of the plate from one position to another.  Neither of these tools will do the job of calibrating a surface plate by itself.  Beyond the tools, a clear understanding and interpretation of what the tools are telling you is also necessary, along with the skills, tools, and knowledge of how to use the information to actually make the plate flat.


Bingo, no true reference plane, the Feds use a Mahr indicator and a heavy base, then you check by indicating a pattern, if the readings are acceptable you are OK for checking Govt. parts, again this magnifies error but does not give a measurement of the outage.


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## macardoso (May 12, 2021)

Bob Korves said:


> Take the surface plate to a pro shop and let them do the calibration and certification. My 18x24x4" 2 ledge plate cost me just over $100 for the job, took them about 20-25 minutes to get it correct, from a .003" hole in the plate center error. We had Standridge Granite from So. Cal. come here on one of their road trips, and we had 5 plates that belonged to 4 of us for them to do. Without the quantity we did, the minimum order charge and travel fee would have killed the deal. If you live close to a shop that does the work in house, you can take it there and get it calibrated while you wait. Call first to get price, availability, and other information. Those guys can do the work in their sleep and get it right after calibrating hundreds or thousands of them. Friendly workers, fun group collaboration.


I'd really be interested in any tips for finding someone to lap and qualify a granite plate. If I could get my 18x24 done for ~$100 I would do it in a heartbeat.


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## Optic Eyes (May 12, 2021)

Couldn’t agree more, Know a guy who bought a TruStone lab grade. I bought an Enco shop grade for a tenth of the price, neither one could get the Mahr indicator to move


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