How does a repeat'o'meter work?
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  1. #1
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    Default How does a repeat'o'meter work?

    I have seen pics and reference to repeat o meters. I know they are used to check flat surfaces but how do they work, specifically? I remember reading that someone had made one, forgot who. They look simple but... I googled but found no good information on the tool. Thanks for amusing me. Steve.

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    A Repear-O-Meter is a Rahn-Federal trade name for a gadget that's benn around for a century. Look here for a demo about 30 minutes in.

    https://www.youtube.com/watch?v=NWObTpn6dTk

    Try to ignore the guy with the mustache: way too much talk for too little info.

    It will only sense departures in an otherwise perfect sphere. Since flat is a surface of a sphere of infinite radius the Repeat-O-Meter is handy for sensing rolled off edges, localised wear etc that the differential level, laser, autollimator, gadget du jur missed. You zero it someplace on the flat and scan around the surface looking for localized humps and divots. It will find them.

    It's as sensitive as the indicato, gage head and amp, whatever you put in it. Don Roberts uses a 0.0001 indicator and when that doesn't tell the tale, he uses an LVDT and gage head set for 5 millionths per graduation.

    You need both methods to calibrate a surface plate. One alone won't do it and met the spec. If you get a chance, look in Fed Spec GGG-456-P. World of info there.

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    I found this on Google.....Granite Flat Calibration and Lapping

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    Ill take some pics of mine and post em so you can see some detail .. they are really bone simple. STD practice is to go over all the lines on a moody check of the plate , pick the one with the least amount of deviation and set your repeat o meters zero on that then proceed to check all the little pie shaped areas and edges that are missed with the moody checking grid. The mil spec even has a chart for pre cal of a surface plate , if it checks with in "X %" of its specified cert with a repeat o meter then a full inspection is not required.
    Cheers Don

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    lowCountryCamo --

    The US Federal Specification for granite surface plates, GGG-F-463c, contains a Figure 7 that is near-enough to being a dimensioned drawing of a repeat-reading gage to build one (as well as a lot of other good information). Your favorite internet search engine will turn up a PDF copy you can download.

    John

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    From what I can see it's nothing more than a "gauge block" that's hinged so it doesn't drop on your toe when you get to an edge, attached to a stand to hold the gauge. The distance between the sets of feet are a known amount. I believe in the case of a Repeat-o-meter it's 5", but here's another version that's metric and 100 mm. Repeat Meter Obishi Keiki Seisakusho Japan

    It definitely doesn't strike me as something especially difficult to construct.

    Don I'm definitely keen to see pictures and details of when a full check isn't required.

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    A Repeat-O-Meter can be approximated with a surface gage whose mast is extended forward of the base with a dial indicator mounted some distance out to contact the plate. This expedient works pretty good if the base is flat.

    The Repeat-O-meter excells in this mode because it has several features that make it suited for micro-inch/40 nanometer resolution
    Last edited by Forrest Addy; 07-25-2014 at 08:36 AM.

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    Here is one picture from another thread.



    dee
    ;-D

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    I'm reviving this thread to ask a question. Figure 7 of GGG-F-463c shows that the Repeat-O-Meter has three fixed feet and one floating contact. I have reproduced that here:



    The fixed feet are only shown in a side view. The separation between the left pair and the right fixed feet is 5", and the separation between the right fixed foot and the floating contact is 5". But what is the separation between the two fixed feet on the left? I had assumed this was also 5", but the photo of the 1956 Repeat-O-Meter looks much skinnier in that direction. The Obishi meter linked above also looks much skinnier.

    Cheers,
    Bruce

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    http://www.tru-stone.com/pages/smp.asp#prod

    Quality Digest Magazine read the right side info


    Metrology--FI--Gauging Quality, Episode 1 - Surface Plates--EN--213-1-31 - YouTube around 20 min talks about the repeat-o-meter

    Saw this one too. Amazing how You Tube has helped us share info.

    YouTube

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    Quote Originally Posted by Richard King View Post
    http://www.tru-stone.com/pages/smp.asp#prod

    Quality Digest Magazine read the right side info


    Metrology--FI--Gauging Quality, Episode 1 - Surface Plates--EN--213-1-31 - YouTube around 20 min talks about the repeat-o-meter

    Saw this one too. Amazing how You Tube has helped us share info.

    YouTube
    Second (bottom) youtube link not working. Maybe the video was removed or only part of the address copied?

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    Rich, thanks for posting the links, but it doesn't really answer my question. It appears that the Repeat-O-Meter has about a 5" x 1" (125mm x 25mm) base which is rectangular. But what's the story with the meter in Figure 7 of GGG-F-463c? How far apart should the two rear feet be? It seems like these two tools are similar but not identical.

    (Or perhaps the two rear feet are supposed to be almost touching. Since they are 3/8" diameter, this would make the total width about 1 inch. So then Figure 7 of GGG-F-463c *would* look like the skinny thing in the pictures. Is that right??)

    Cheers,
    Bruce

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    On the You tube from Ox I saw when the tester pulled off the meter the 2 points were near the hinged split so those 2 near the end must make it 3- points? I don't own one. Jeff Thiele I believe made one after we had a plate lapper lap and test a plate in a class we had several years ago. I'll email him and ask him to comment in this forum. Rich

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    I made a feeler gauge hinge and drilled and tapped holes in some bits that I had laying around
    so I could check my Grizzly surface plate
    After some messing around I got the repeatability down to about three divisions of the indicator
    Way better than needed to measure the. 0005 error I found in the plate20170110_100725.jpg20170110_100815.jpg

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    Quote Originally Posted by ballen View Post
    I'm reviving this thread to ask a question. Figure 7 of GGG-F-463c shows that the Repeat-O-Meter has three fixed feet and one floating contact. I have reproduced that here:



    The fixed feet are only shown in a side view. The separation between the left pair and the right fixed feet is 5", and the separation between the right fixed foot and the floating contact is 5". But what is the separation between the two fixed feet on the left? I had assumed this was also 5", but the photo of the 1956 Repeat-O-Meter looks much skinnier in that direction. The Obishi meter linked above also looks much skinnier.

    Cheers,
    Bruce
    I think you could make the width any size that's convenient. What Hbjj posted looks to be about 6" wide. The three pads on the left side are defining a plane the single pad on the right side is measuring the deviation from that plane. If I was making one I would make it about 2 1/2 inches wide. Wide enough to give you a stable base but not so big that your pushing a bunch of mass around.

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    Rich. I drew one up, got some cast iron for the body and stress relieved it- and that is as far as I have gotten. Not a high priority project.

    As SIP6A indicates above, the 3 feet on the one side are to define a plane. An amendment to GGG-P-463C June 15 1977 adds paragraph 4.5.5.1 indicates the position of the 2 feet may be at the end or in the middle.

    An image search found one picture showing the feet. It is an e-bay ad. RAHN REPEAT O METER SURFACE PLATE CALIBRATION TOOL

    The diameter of the feet are 3/8 inch. I measure the center to center distance of the pair to be about 1 1/4 inches.

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    Quote Originally Posted by ballen View Post
    Rich, thanks for posting the links, but it doesn't really answer my question. It appears that the Repeat-O-Meter has about a 5" x 1" (125mm x 25mm) base which is rectangular. But what's the story with the meter in Figure 7 of GGG-F-463c? How far apart should the two rear feet be? It seems like these two tools are similar but not identical.

    (Or perhaps the two rear feet are supposed to be almost touching. Since they are 3/8" diameter, this would make the total width about 1 inch. So then Figure 7 of GGG-F-463c *would* look like the skinny thing in the pictures. Is that right??)

    Cheers,
    Bruce
    I remember a thread about that a while back that i participated in. am sure it gave dimensions and more all taken from a genuine Rahn. if you get desperate have a look through my history

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    ok, so i didnt participate in it .

    Should be all the info you need here. Enjoy

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    Bruce --

    Take a look at Figure 10 "Pad locations on mirror or detector carriage" in the US Federal Standard. Although it's not explicitly identified as a bottom view of the Figure 7 "Repeat reading gage", I think it reasonable to impute that the term "detector carriage" covers the repeating reading gage.

    If so, the magic answer to your question is 2 1/4 inch.

    John

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    John, Demon, Rich, J.R., thank you all for the very helpful replies and information! I am going to go with the roughly 1 1/4" separation, probably with the two feet near the flexture for better stability in that region. This is a nice project, and will be a good way to monitor the health of my surface plate.

    For closure I am reproducing here a photo from the thread that Demon referenced.



    Cheers, Bruce


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