Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing since 1989
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Maintaining a Smooth Surface Finish after Anodizing
Q. I have an Aluminum (7075-T73 extruded bar) machined part. After anodizing per MIL-A-8625 / MIL-PRF-8625 [on DLA] type II, the finish on the part looks roughly etched along with the grain of the material. On the sides of the part the etching looks linear and on the ends look finely pitted. Has anyone had a similar condition.
David Luna- Duarte, California, USA
2003
A. Extrusions are better than cast but they can be considerably worse than wrought. It would sound like you had copper precipitation in layers. The anodizing ate the copper out. It is possible that the operator recycled the part more than once or left it in a process tank or foul rinse tank for an extended period of time. A metallurgical examination would give you a strong clue. Summary: it is either bad metal or bad anodizing.
James Watts- Navarre, Florida
2003
Q. We are going to be producing a cylindrical part from 6061-T6 and the requirement for the finished part is 55 Ra to 85 Ra. We can create whatever finish is required during machining and/or brushing, but we are uncertain as to how much this will shift during anodizing (Type II, Class I; also Class II Black). Customer has inserted the term Bright into the description, yet they also want the textured surface referenced above. One intuitively thinks that the finish will become incrementally smoother through anodize. What is the reality?
Murray RitlandMachine Shop QA - Burnsville, Minnesota, USA
February 11, 2009
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A. Few, if any, specifications provide a definition of bright. What is bright to me might not be to you. - Navarre, Florida February 12, 2009 A. Anodizing will very slightly smooth the surface, depending on the film thickness. Leo Herringonconsultant - Grand Rapids, Michigan, USA February 12, 2009 |
A. To clarify a bit what Leo stated:
Unless one is starting with an extremely rough surface, the anodizing process will roughen it a bit.
It's been my experience that a sandblasted surface will smoothen a bit after anodizing (due to the pretreatment, and etching reaction that goes on during the anodizing process). But a smooth surface RA will definitely increase with anodizing, especially with a typical hard coat process of .001" or more. Hence the normal requirement to mask O-ring surfaces prior to anodizing.
Marc Green
anodizer - Boise, Idaho
February 16, 2009
Multiple threads merged: please forgive chronology errors :-)
Improving surface finish on 6061 Aluminum Anodized bore
Q. I have a aluminum part, (6061) that has been anodized and the bore finish has become rougher, (18ra). The bore size after anodizing is 1.168 x .900 deep with a 18ra finish. The part final size can be 1.1690-1.1695 with a finish of 8ra. Is there a way to improve the finish to a 8ra without damaging or removing the anodized finish?
Eric SmithEngineer - Omaha, Nebraska
May 3, 2010
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May 5, 2010 A. 8 RA is a pretty tall order for a standard .002" hardcoat. Typically the finished product will be double the starting roughness. Usually with a minimum of 16 RA. Marc Green anodizer - Boise, Idaho A. Eric - Colorado Springs, Colorado May 5, 2010 A. It can be honed with the proper "stones" and lubricant. I will guess that you can meet the finish requirement at the high end of the bore dia. James Watts- Navarre, Florida May 5, 2010 |
What parameters can affect the Rmr (Tp) ratio on an anodized surface?
Q. My component has an anodized internal bore which has a Tp ratio of 99.7%.
The problem is that the component drawing states a Tp ratio in the range of 50-90%.
What factors/parameters affect this ratio?
Any advice would be much appreciated.
Engineer - St.Helens, Merseyside, England.
February 12, 2012
February 13, 2012
Hi, Alan. Forgive my ignorance, but I had no experience with the terms Rmr and Tp, and had no idea what you were even talking about. But my googling eventually yielded:
www.bcmac.com/pdf_files/surface%20finish%20101.pdf
... which I find to be the clearest presentation on surface roughness I've yet seen. Other readers who are also unfamiliar with those terms, or who find themselves confused by the many different surface roughness standards may find that page enlightening.
But being able to generally understand the question now doesn't mean I can answer it, except to perhaps suggest that I am not confident that you are measuring what the designer wanted you to measure in the way that s/he wanted it measured :-(
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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Ed. note: Unfortunately that very helpful page seems to have been dropped :-(
January 28, 2014
Q. Dear friends,
Shall we need to consider the roughness of the Aluminum surface, does it consider for surface finishing, if yes then how much it should be?
- Abu Dhabi, UAE
A. Hi Aijazullah. We appended your inquiry to a thread on the same topic which may help provide some answers. You may also be interested in letter 22123, "Hardcoat anodizing surface roughness, Ra / RMS issues".
The short answer is that anodizing tends to roughen the surface, and the significance of this issue will depend upon the application. For example, as Marc Green implies, anodizing of O-ring grooves can roughen them to unsuitability.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
January 30, 2014
Q. We have recently encountered a problem in our anodizing process. The aluminum profiles show a sporadic rough surface. Sometimes the problem is evident in the entire batch, but other times it only occurs in some pieces. Please, I would like to know if you can recommend a solution for this problem. The profiles are extruded, aged and afterwards anodized.
CRISTIAN HIDALGO- CALI COLOMBIA
October 15, 2020
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