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Small surface plate tester

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38super

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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
 

Ray C

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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
 

Eddyde

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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.
 

38super

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Sent one to Stan at Bar Z, here's his vid:
He has posted a link to the PDF files.
 

38super

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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.
 

Bob Korves

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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.
 

Technical Ted

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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
 

38super

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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.
 

38super

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

Bob Korves

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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.
 

38super

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

38super

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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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