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Profilometers Question
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Posted by: prop154

12/01/2003, 07:58:12

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Can anyone explain the difference between Ra values and Rz values on a profilometer? I am measuring a shaft that calls out for a 10-25 Ra finish in an area that has a lip seal- The problem is we having a problem with leaking units and I think we may be using the wrong scale to measure the shafts with?:






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Re: Profilometers
Re: Profilometers -- prop154 Post Reply Top of thread Forum
Posted by: RandyKimball
Barney
12/01/2003, 22:38:42

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ANSI B46.1-1978 covers geometric irregularities of surfaces of solid materials, physical specimens for gaging roughness, and the characterictics of the instruments for measuring roughness. The Ra scale is actually the amount of surface roughness arithmetic average (Roughness Average - the "R" and the "a") as read in micrometers of instrument stylus motion. In most cases the peak to peak surface roughness will be four times the indicated average roughness. A micometer is one millionth of a meter. (.000001). Sometimes the rougness is specified as microinches, one micoinch is .0254 micrometer. A mirror finish is about 4-Ra. A well lapped and polished steel surface will be between 16 and 8 Ra. Reading or markings in microinches will generally be written with a decimal in them as .20 which is about 8-Ra
Now all this said, you could have a bad blip on the surface and still get a good Ra -average-. Your stylus could be damaged.
You could be checking the wrong direction. Your gain or other testing adjustments, if there are any, could be set incorrectly.

Are you turning the diameter in question? A good answer to a finish that seals well is to burnish the surface. Think of the surface of the metal as the finish on a record album, actually this is a very close example. Burnishing presses down the upper peaks of the surface roughness to fill in the valleys. This causes the material to look shiny to us because light reflects off the flattened peaks. It also makes the depth of the roughness less deep than the molecular structure of most liquids, not allowing them to enter the "canyons" and sneak through. By the way an old turntable is nothing more than a glorified surface roughness tester on steroids, except you need to be testing against the grooves, the direction that would scratch the record.

In short, you could have some sharp points on the peaks of your surface roughness and still get a fair average reading. I suspect you need to change the final surface finishing process.
.. a telling test is to see if the surface files your fingernail ... if it does...or if you see fingernail on the surface... you have a problem.
You can burnish a surface by placing a cam bearing against it on a lathe. You may need to start with a slightly larger diameter to allow for the surface "shrinkage". The shrinkage won't be much but it will shrink as the peaks are pushed into the valleys. Another way is to spin the diameter against a rubberized burnishing wheel spun in a table tool grinder. These come in grit values and are easy to get. They do a great job of removing the peaks of the roughness to a smooth texture that tends to seal well, and is easy on seal materials. A semi-skilled operator can hand spin the work against a tool grinder spun burnishing wheel with good results as it hardly changes the diameter, but makes instant improvements in the finish that can easily be seen with the eye. The Ra reading on your instrument may not change much but the peaks will be gone and the results are absolutely there.

-randy-




The worst idea of you lifetime may be the catalyst to the grandest idea of the century, never let suggestings go unsaid or fall on "death ears".

Modified by RandyKimball at Mon, Dec 01, 2003, 22:47:58

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