In working on a positioning issue with my laser I discovered that one or more of my rulers is off:
Which 15cm / 150mm ruler mark is accurate? Or are both off?
The top steel 6 inch ruler has Otto Frei’s name on it, the bottom paper/plastic ruler says Rio. Neither seller actually made these so I’m not faulting either one. They disagree by a little over 2.5mm.
I used digital and analog calipers set to exactly 150.0mm and both matched the steel ruler, so that’s the only one I will use now. It is good to know.
Neil A
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Neil… That is a dilemma. You put your faith in something or someone only to find that it or they are off, but who or what is off? I have simple wooden rulers nailed to the edge of my benches as points of reference. I also have a Starrett ruller glued to the front of another. They are always off relative to each other by a bit, the significance of which is to be determined by how important the measurement is that I am taking. In the end, most of what I do is by eye and that is how it will be measured when I am done, so I don’t worry too much about it. One day maybe I will make them all the same, but who is to say that they are right and does it matter? Forgive me, I have been watching Wolf Hall…Rob
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Anyway, you can use them to measure something that is a known dimension?
Brennan, I posted about the two rulers being off just as a heads-up to others. That’s something I discovered in the process of trying to center a design on my laser and having a lot of grief. Details about that are not of much interest to most on Orchid so I’m leaving that out.
Neil A
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Ah that makes sense. Yeah I love details like that 
Neil A,
That’s crazy. Thanks for letting us know!
Jeff
If precision is needed it is imperative to use the same measuring device in both ends.
Or that the devices are calibrated and/or verified.
In the old days when inches was all over the world they were often different.
The old Norwegian was 26.14 mm while the Swedish was 24.74 and the imperial was 25.4.
So in the old days this was a source of frustration.
Especially since the measuring devices lasted a long time and they were not marked well.
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Now, I’m obsessed with comparing 6" rulers. So far the Otto Frei ruler is matching up exactly with all of the other 6" steel rulers that I have floating around. I don’t think I have any of those yellow flexible Rio Grande rulers anymore.
Ha ha!
i am visualizing 1000’s of orchidians comparing rulers right now! 
julie
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I reached out to Mark Nelson, Education Director of Rio Grande to ask about the ruler discrepancy.
Here’s what Mark said. He also attached a picture.
“Hey Jeff
We checked stock and they are all good. We measured from the mark where the measurements start and not at the end of the ruler.
Not sure how neil_a3 is doing it.
I hope this helps.”
Yesterday I compared an Otto Frei 6" stainless ruler to a Contenti 6" stainless ruler. Those two rulers also matched up perfectly.
Maybe Neil A got a bad one?
Jeff
I mean, those two rulers are absolutely different but it’s only off by 1-2 mm.
It’s a conspiracy!! 
You’re right! I didn’t see that until I blew up the image. Those two Rio Grande rulers are not a perfect match, close, but not perfect. I’ll double check later today, but I’m pretty sure that the Contenti and Otto Frei rulers that I have are a perfect match.
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Hi Neil,
another thing that caught my eye is the thickness of the lines…ie: where you put your scribe within that thick line could impact things…
this is an example of why i got into CAD…
the thickness of the pencil lead impacts the total length
ie: physically line up (15) 1.5mm melee diamonds
then, draw out (15) 1.5mm melee diamonds
…the drawn line of melees is longer, due to the thickness of the pencil lead…even using very thin pencil lead…
not always an issue, depending on the design, but…
julie
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We [Rio Grande] measured from the mark where the measurements start and not at the end of the ruler.
Not sure how neil_a3 is doing it.
Mine do not have the unruled margin at the end as in Rio’s photo. Here’s mine:
I used the ruler as it was…ruled. Mine are between 15 and possibly 20 years old so Rio’s current flexible rulers are new & improved.
I have another narrower 6 inch steel ruler stamped “Freemans” and its markings match perfectly with the Otto Frei-labeled steel ruler.
For most usage the minor discrepancy is no big deal but my CNC mill is precise to hundredths of a millimeter so if I want to mill a seat for a cab for example, I need to get reasonably accurate measurements.
Neil A
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This has been a very informative thread. Thanks for bringing it up Neil A!!
Jeff
