Accredited Surface Roughness Measurement - Requirement for 1Ra surfaces

M

Mandell

#1
We have a requirement for 1Ra surfaces, and although what we've been doing in the past is OK per our customer, I know we are whistling in the dark about our stated values. We want to do better, but the problem starts with the uncertainty we can get from accredited sources.

2Ra is about the limit for uncertainty, unless you have NIST do it ($1650) or Oak Ridge (dunno but it ain't cheap). We are thinking of doing it ourselves, we have a new lab coming up with an isolated slab, and I'm looking at isolation tables - I think we can do it with a high end lab grade profilometer or with a laser based device.

I'm guessing we can do a roughness sample for about $100, with an uncertainty (if it's a good patch) as low as .5Ra and values down to 1Ra or so.

Do you know anyone that might be interested in a service like this? (other than us, and we are a little nutty)
 
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D

DIBOB

#3
Zygo can do it for about $1k. I have a small part with a 1.97 Ra spec. I had base-line samples measured by Zygo and used them to set bias on a Laser-Scan LED system (@ $3.5k). The laser repeated to about .1 micro" and never needs calibration once the bias is set for the surface type (finish & material reflectivity) and is uneffected by vibration & temp. Scan time is about 1 second and out-put is digital to Excel files.
 
M

Mandell

#4
Well, we went ahead and bought an isolation table and built a new facility since my original post, and my initial results are we can get to .5Ra with little trouble and some inexpensive Mitutoyo equipment.

We looked at laser and at Taylor Hobson's white light system and decided not, due to limitations for both for part geometry, and our geometry is usually an involute gear. I did talk to Don Cohen at Michigan Metrology who chairs the B46 committee, and I think the state of the art needs a few more years development before we can use it. Although he uses a laser system in his business.

One thing that is true is by definition in the standard, scratch and sniff systems take precedence over light based.
 
J

JAltmann

#5
First question that comes to mind is are you trying to measure surfaces with 1 Ra? or just trying to make your scope go that low? (I assume your talking microinches.)

If your trying to measure a 1 Ra surfaces then you will need a laser system to do this with any degree of confidence.

If your simply trying to make your lab's scope go that low then you are really pushing the envelop and honestly not going to do this very well with stylus intrumentation.

Zygo's are good, as are the Wyko and Veeco systems (Don uses Wyko), but any laser system is going to be expensive and a little more cumbersome when compared to a stylus system, but they can give you a greater wealth of information over a stylus system. And truly for anything that requires a 1Ra, you need more information then can be obtained from the Ra measurement.
 
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M

Mandell

#6
No, I have no idea what uncertainty I'll have, but I can see .5 Ra now with an inexpensive system - I think with a decent system like a Kosaka or T-H I might push half that. We might have a need to better understand our surfaces, as we are developing products for some demanding applications and plan to do more. We have a scanning electron microscope to do some of that, but a laser gives better 3d info and the nature of an areal check I expect you are exactly right, much more repeatable results.

However, when we offer accredited services (which I know you are familiar with) you will get calls to referee a dispute between vendor and customer and in that case (by definition in the spec) mechanical systems have authority.

My real motivation in the short term is actually checking product, and staying ahead of my customer, checking better with lower uncertainty.

And finally, how the hell you been, James!? :tg: My name is Sam.:lmao:
 
J

JAltmann

#7
I've been good!

Back to the topic and some brief explainations for others. Your probably best sticking with a decent stylus instrument for the type of work your going after. Yes the 3D systems will give you a lower uncertianty, but with a price that a typical aboratory can not afford. I say this because those systems start at about 150k (USD) and are best for surface texture of 80 microinches or less. And generally smaller parts.

Where as the decent stylus profilometers start around 10k (USD) for a system that can output more than Ra and Rz, with an average price about 30k (USD) for a lab grade model.

But for your situation and other an uncertianty of about 0.5 microinch is about as good as you can expect without paying a small fortune in reference materials and their calibrations and realistically this puts you at doing work down to 2 microinch range. This is the realm of superfinishing and is not terribly common work either. This finish is about the level of glass to give a reference.

So usually those who can do this level of finish have the right equipment or if not i would recommend sending them to the labs who specialize in surface texture measurement, as those looking for this level probably are looking at a truly functional surface of which Ra alone is of little value.
 
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