# What is the unit "my" in the context of lapping abrasives and roughness? **URL:** \ **Category:** Factual Questions\ **Created:** [September 14, 2015, 8:12pm UTC](https://boards.straightdope.com/t/what-is-the-unit-my-in-the-context-of-lapping-abrasives-and-roughness/731248 "2015-09-14T20:12:27Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [September 14, 2015, 8:12pm UTC](https://boards.straightdope.com/t/what-is-the-unit-my-in-the-context-of-lapping-abrasives-and-roughness/731248/1 "2015-09-14T20:12:27Z")
I keep seeing what appears to be a unit called “my” in discussions of lapping, abrasive powders, and surface finishes. It seems to apply to particle diameters and also to surface roughness amplitude. It might be of Swiss origin. Anybody know what it is? Thanks! ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [September 14, 2015, 8:14pm UTC](https://boards.straightdope.com/t/what-is-the-unit-my-in-the-context-of-lapping-abrasives-and-roughness/731248/2 "2015-09-14T20:14:46Z")
Sure it isn’t just microns? ---
**Author:** ![donkeyoatey](https://avatars.discourse-cdn.com/v4/letter/d/e47c2d/32.png) [@donkeyoatey](https://boards.straightdope.com/u/donkeyoatey)\ **Post date:** [September 14, 2015, 8:32pm UTC](https://boards.straightdope.com/t/what-is-the-unit-my-in-the-context-of-lapping-abrasives-and-roughness/731248/3 "2015-09-14T20:32:49Z")
Are you looking at this in a mirror “µm”? ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [September 14, 2015, 10:11pm UTC](https://boards.straightdope.com/t/what-is-the-unit-my-in-the-context-of-lapping-abrasives-and-roughness/731248/4 "2015-09-14T22:11:37Z")
Thought about micrometers but I don’t think that’s it. In one document I see “particle size 150 my, lapping time 0.1 s” and “roughness Rz = 0.8 my” and then “tolerances within 0.002 mm” and “allowance of 4-8 microns”. So, they are expressing micrometers in other ways, not in “my”. There’s a table that gives values of Ra in “my (m)” and “my (inch)”, which makes it look like perhaps “my” is not a unit. There are plenty of places where they write “µm” (I hope that displays to you as a mu, as it does to me). They also write helpful things like the line below, which I’m not gonna add quotes to: 1 µ = 0.000’001" = 0.025’ 4 mm And there are also obvious photographs of machine printouts or displays with text in them like: VER 2.50 YM HOR 500.0 YM So, long story short, they are assuming I get a great deal more meaning out of this than I do. There’s so much of it, the document looks very useful, but this weirdness is woven through it. I seem to have found the Voynich Manuscript of Abrasive Finishing. ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [September 14, 2015, 10:20pm UTC](https://boards.straightdope.com/t/what-is-the-unit-my-in-the-context-of-lapping-abrasives-and-roughness/731248/5 "2015-09-14T22:20:05Z")
Well, a yottameter is Ym. But fuck if that isn’t in the realm of the super tiny. ---
**Author:** ![FordPrefect](https://avatars.discourse-cdn.com/v4/letter/f/e99b99/32.png) [@FordPrefect](https://boards.straightdope.com/u/FordPrefect)\ **Post date:** [September 14, 2015, 11:34pm UTC](https://boards.straightdope.com/t/what-is-the-unit-my-in-the-context-of-lapping-abrasives-and-roughness/731248/6 "2015-09-14T23:34:19Z")
If metal workers spell half as poorly as blacksmiths, I think I have your answer 🙂 ---
**Author:** ![Nefario](https://avatars.discourse-cdn.com/v4/letter/n/59ef9b/32.png) [@Nefario](https://boards.straightdope.com/u/Nefario)\ **Post date:** [September 15, 2015, 12:00am UTC](https://boards.straightdope.com/t/what-is-the-unit-my-in-the-context-of-lapping-abrasives-and-roughness/731248/7 "2015-09-15T00:00:23Z")
Could this be a document of foreign origin that used “mi” for microinches, then was translated by a nontechnical person who thought it should be corrected to “my”? Does “my” = microinches make sense? ---
**Author:** ![thelurkinghorror](https://avatars.discourse-cdn.com/v4/letter/t/7c8e57/32.png) [@thelurkinghorror](https://boards.straightdope.com/u/thelurkinghorror)\ **Post date:** [September 15, 2015, 12:20am UTC](https://boards.straightdope.com/t/what-is-the-unit-my-in-the-context-of-lapping-abrasives-and-roughness/731248/8 "2015-09-15T00:20:49Z")
WAG/using the German Wikipedia: “My” appears to be the German for the Greek μ, and is thus a prefix meaning “micro” or 10^-6. [German page](https://de.wikipedia.org/wiki/My). [Google translated](https://translate.google.com/translate?hl=en&sl=de&tl=en&u=https%3A%2F%2Fde.wikipedia.org%2Fwiki%2FMy). Without a unit, context may be important. Either that, or they are using the capital Μ instead of lowercase μ, and “y” is some unit in German? Micrometer? ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [September 15, 2015, 1:46am UTC](https://boards.straightdope.com/t/what-is-the-unit-my-in-the-context-of-lapping-abrasives-and-roughness/731248/9 "2015-09-15T01:46:56Z")
Wow, maybe we’re on to something. Thelurkinghorror, you have hit on a little gold mine there. I think this is of Swiss origin and they do speak German (among other things). Nefario, I think the answer to your question about whether “my” = microinch is that it would make sense. If I clean up the earlier line a bit, it is correct: 1 microinch = 0.000001" = 0.0254 mm The magnitude of the microinch is physically plausible. Right now the most plausible explanation, to me, would be that it means microinch, and that there are various unexplained exceptions such as the column headings with other contradictory units. Still seems a bit nutty, though. ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [September 15, 2015, 2:58am UTC](https://boards.straightdope.com/t/what-is-the-unit-my-in-the-context-of-lapping-abrasives-and-roughness/731248/10 "2015-09-15T02:58:38Z")
I did a google search, and it did come up with the _pronunciation_ of a micro- unit (μ) spelled as **my** -crow. Don’t know if that helps. [linky.](https://books.google.com/books?id=8gfiBAAAQBAJ&pg=PA1516&dq=milli-yocto+%22my%22&hl=en&sa=X&ved=0CCsQ6AEwAWoVChMI4Z6Y24T4xwIVRguSCh1_PwY1#v=onepage&q=milli-yocto%20%22my%22&f=false)