Surface Finish question 3

[doohicky]

Would anybody be able to tell me what the prescribed parameter of measurement of surface finish is to be used?

I know that it used to be RMS, but was changed to Ra in 1995, is this still the method in use today for evaluating your standard metal machining finishes?

Thanks for the help

Mike

 

[ctopher]

try these:

http://www.engineersedge.com/surface_finish.htm

http://jjjtrain.kanabco.com/vms/engineering_surface_finish.html

Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP1.1 / PDMWorks 05
ctopher's home site

 

[unclesyd]

You can still use any of the callouts that you desire making sure your supplier understands and has the capability of measuring same.

 

[jsummerfield]

The roughness average Ra is expressed in micro meters or micro inches roughness. Other finish terms include the maximum depth and radial projection of imperfections. ASME B46.1 discusses the roughness average and RMS roughness Rq.

John

 

[doohicky]

Ok, what I'm looking for is the industry standard on the "correct" method of calling out surface finish.

Is there a difinitive answer to this in B46.1 - 2002?

In the 1995 version, most if not all the examples are for the use of the parameter "Ra" .

I understand that in 1995 ISO had deemed that the correct medium for surface finish evaluation is to be Ra, where as the old method was RMS.

Mike

 

[jsummerfield]

Although not mentioned, ASME B16.5 - 2003 specifies various surface roughness
- not to exceed 3.2 micrometer (125 micro in.) roughness
- ... serrated spiral finish having a resultant surface
finish from 3.2 to 6.3 micro m (125 to 250 micro in.) average roughness.
The term Ra was not used.

Interpretation: 4-4
Question (3): According to ASME B16.5-1996, para. 6.4.4.3, may a flange with less than 45 grooves per inch or more than 55 grooves per inch meet the flange facing finish requirements as long as the resultant surface finish has a 125 to 250 micro in. average roughness?
Reply (3): Yes.

BTW, The character mu was used throughout ASME B15.5 instead of my spelling micro. I prefer not to find the ASCIIish code to insert the character.

John

 

[codeeng]

The correct term is Ra values but it is recognized that either can be accepted as there is only 15% difference approximately.

 

[davidribeiro]

While we on the topic of roughness on flange faces, what type of finish would be best suited to a gasket under vacuum conditions?

Dave

 

[codeeng]

Dave
First off you need to select a gasket to suit your service conditions, then provide the recommended surface finish for that gasket. Vacuum should not be a concern (after all its only 15 psi). I do recall however that Flexitallic recommend an inner ring on spiral wound gaskets in vacuum service.

 

[ctopher]

Just my 2 cents,
I hear "Surface finish" all the time, but in reality it is not a finish, it is "Surface Roughness". I see dwgs all the time with a note under "finish" calling out 32,64,etc. It is not correct. It should not be called a finish.
Just wanted to pass on. Thanks.

Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP3.1 / PDMWorks 05
ctopher's home site (updated 06-21-05)

FAQ559-1100
FAQ559-716

 

[unclesyd]

The last time I checked Surface Roughness was just one aspect of the Surface Finish equation. What happened to Surface Smoothness, Surface Waviness, Aspherics, and several others.