# Surface Finishing Measurement



## lgaunt (Aug 6, 2012)

I am working in a machine shop in a remote area in Thailand. I have several measuring tools including a micrometer, vernier calipers, and a dial gauge. We also have several magnify glasses that are useful for looking at smaller features. I am currently designing a shaft that will fit through two bearings in a vibrating assembly. The shaft needs a surface finish of  32 μ-inch, and what I need is a way to measure this finishing with the tools I have available (i.e. no computers) . Unfortunately, I am not even sure what tools I need use to do this. Any help would be appreciated.


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## jgedde (Aug 6, 2012)

They do make surface finish measuring tools, but they are very pricey.  A more budget minded method is to use a surface finish comparator like this: http://www1.mscdirect.com/cgi/NNSRI...re=ItemDetail-_-ResultListing-_-SearchResults

All you do is find the closest finish to what you have by eye.

John


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## Bill Gruby (Aug 6, 2012)

John is correct. The Surface Standards are actually the cheapest way out. A Surface Test Indicator will be much more than that. The tools you discribed will not be much if any help to you. Surface Standards are a bare minimum.

 "Billy G"  :thinking:


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## Tony Wells (Aug 6, 2012)

A profilometer (I have one made by Mitutoyo) is what is normally used to measure deviations in surface finish. It operated somewhat like a phonograph needle and traces the surface and the electronic signals are graphed and measured in different terms, depending on the specifications. 

As John and Bill said, they are rather expensive, but a sample plate with various standard specified finish examples (16, 32, 63, 125, 250, 500 etc.) can be had pretty reasonably. Some also give examples of various machining methods. Turning doesn't "look" or feel the same as milling or grinding, so you can get a fingernail and eye comparison with what you need.

Edit to add:

An old timer's trick that might or might not help. I know you are looking for a 32 finish, but rarely is a better finish objectionable. I have a story about that, sometime though. A 16 finish will barely, if at all, pick up a #2b pencil lead if you try to draw a line perpendicular to the lay of the cut, whatever cutting method is employed. So if you can't get a line on it with a pencil, it's probably better than you need at a 32. I suppose experiments could be done with harder and softer leads, like with hardness files, to get a close idea, but at least this is better than just a guess.


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## 4GSR (Aug 6, 2012)

Tony,

That's interested using a 2B pencil to check surface with.  I'll have to dig up my old drafting tools and see if I have some 2B lead and give it a try. 

Or, I'll use my calibrated finger to check surface finish with.:biggrin:


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## Tony Wells (Aug 6, 2012)

One shop I worked in we did a ton of honing on some pretty good shop built equipment. Our parts were a series of sleeves made of Incoloy that went into SII's downhole mud pulser. They required a 16 finish, and for a time, all we had was a comparator plate. One of the QC guys at Smith told me about the pencil trick, and once I got the profilometer, found it to be pretty accurate as far as a 16 or better finish. Of course, if you really bear down, you can transfer a little graphite, but using normal writing pressure, you can't really draw a line. Once it gets into the 30+ range, you can clearly see a line, and 63 or more and you can write your name if you want....with a 2B lead. When you get down to a 2-4, you can't transfer any graphite to speak of.

When I worked QC myself, I used a pencil if there was a doubt in 'O' ring grooves that were spec'd to 32. Usually, if the program was written to profile each side and then sweep the bottom (my preference) we got a far better than 32 finish even if the insert wasn't in really good shape. I think there may have been some burnishing effect.


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## fgduncan (Sep 15, 2013)

lgaunt said:


> I am working in a machine shop in a remote area in Thailand. I have several measuring tools including a micrometer, vernier calipers, and a dial gauge. We also have several magnify glasses that are useful for looking at smaller features. I am currently designing a shaft that will fit through two bearings in a vibrating assembly. The shaft needs a surface finish of  32 μ-inch, and what I need is a way to measure this finishing with the tools I have available (i.e. no computers) . Unfortunately, I am not even sure what tools I need use to do this. Any help would be appreciated.



I have very limited experience with this type of requirement, but the usual way, I understand, to express surface requirements on the contract I saw was 32u-inch OR BETTER. Emphasis added. A 32 u-inch finish can barely be felt with the fingernail test, while a 16u-inch finish is simply smooth. If you can go for a better finish, then do so, although it will take longer. Then run a fingernail across the machine tool marks, not with them. If you cannot feel any roughness, then you are very likely at or better than the 32u-inch finish.

Unfortunately, the equipment you describe is worthless for this type of assessment. You really need a Surface Roughness Scale at the absolute minimum.  They are fairly inexpensive and last forever. Hope this helps.

FGD


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