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Thread: 125-250 RMS Surface Finish
08-09-2007, 08:39 PM #1
I have a customer that wants a 125-250 RMS serrated-concentric finish put on a 316L SS plate. The only problem is, I have never done this before and am having trouble finding a concrete answer or standard. I thumbed through the Machinist Handbook, but couldn’t find anything (of course I could have been looking in the wrong place). I eventually found ASME B16 – Standards of Pipes and Fittings, but of course it is just an outline, and I don’t want to spend 100’s of dollars on something that might not answer my question.
I did a quick search here under the general and general new for 125-250 RMS, but nothing came back.
So, can anyone tell me what 125-250 RMS serrated-concentric is (I also understand that there is a 125-250 RMS serrated-spiral as well, what’s the difference)? Pictures would be a huge help. Thanks in advance for the help.
08-09-2007, 09:32 PM #2
By your question I could not tell if you were having a problem understanding the RMS or the serrated concentric, or both. I found a website that may help some with regards to machining, web page
The RMS (Root Mean Square) is measured per DIN4762/Ansi B46-1. On most surface finish gage settings it is also known as Rq, which according to my info at hand is about 25% greater than Ra. -Mike
08-09-2007, 10:44 PM #3
Thanks for the response.
After I posted the question here, I went to the public library and looked up ASME B16.5-1996 (I had seen a few references to this standard in my web research). Section (or whatever you call it) 188.8.131.52 Other Flange Facings states, "Either a serrated concentric or serrated spiral finish having a resultant surface finish from 125 micro in. to 250 micro in. average roughness shall be furnished. The cutting tool employed should have an approximate 0.06 in. or larger radius, and there should be from 45 grooves/in. through 55 grooves/in."
08-09-2007, 11:52 PM #4
Experience has shown that the finish you describe holds gaskets well sgainst moderate pressure in bolted flanges connections.
Concentric serrations are a huge PITA to make requiring a zillion plunges with a tool to a constant depepth. I don't know who came up with it but the confusion of this particular call out is good for at least 4 hours of lost produiction until the issue is settled. "Or spiral serrations" - read tool feed marks - are added words that have been eliminated this particular hassle forever - or will once they've become universally promulgated.
Many years of making these very same flange finished have boiled it down almost to a cook book entry for me. 125 to 250 microinch finish may be approximated with a 0.015 t0 0.020" feed per rev with a 0.062 tool radius. The finish spec represents a tool mark trough to cusp height. If you have a finicky inspector looking over your shoulder, be sure to prove the finish before you take the final cut. Materials, surface speed, and circumstances may require changes in parameters.
08-10-2007, 01:12 PM #5
Most flange finish specs refer back to ASME section IX (boiler & pressure vessle code). If the customer wants concentric, there's usually a reason for it. IIRC, ASME requires concentric on flanges of Class 900 or higher. We don't do phonographic anymore. Of coarse, I can't tell you or show you how we put the concentric on because it's considered a trade secret.
08-10-2007, 01:57 PM #6
Plunge grooving the concentric type with a thread mill insert might take some of the tedium out of the job. You'd have to come up with a lathe style holder, but that should not be too difficult to make up. A 20 tpi insert could be used, splitting the grooves with a .025" move would give you 40 grooves/inch....you get the idea. [img]smile.gif[/img]
08-10-2007, 06:42 PM #7
HU. I phonographed many a 600 and 1200 PSI flex gasket face using a tap silver brazed to a holder or a geometric thread chaser, etc. Pilot snugly in the bore and spin it with a #3 air motor.
I think phonograph finishing is BS. I tested smooth and phonographed high pressure flexitallic gasketed flanges to leakage with a Haskel pump and found no real difference so long as the bolts were torqued the same. I think that traditional "wisdom" of ages past triumphed over technical evaluation and comparison testing.
03-02-2012, 12:03 PM #8
250 rms finish
I also am looking at doing a job for a customer who wants a 250 rms finish on some flanges. I would love to see a close up picture of a part with this done to see what it looks like.