Machined surface finishes for Aeshetic Anodising 2

[EnGinEar]

Can any one suggest, or help point me in the direction of, information that will enable me to decide on a Machining surface finish for a selection of small aluminium components that require Anodising.

The anodising I intend to use is Sulphuric - Dyed in various colours, and serves 2 purposes:

1. Eliminate the possibility of electrolytic reaction in a mixed metal water-cooling system.

2. To improve the visual appearance of the components.

The components can't be subjected to extensive etching due to the risk of eroding Small tapped threads. Therefore I am looking to specify a machined finish that will enhance the final anodised finish.

I don't currently have access to gauging or visual references of surface finishes but from what information I have found, I’ve been tempted to use a finish of 64 MicroInch but am hesitant until I can establish this value is ideal, and how it will effect machining.

Thanks in advance

 

[drawoh]

EnGinEar,

I have a surface roughness gage sitting on my desk. This is nothing more than a set of example finishes which I can examine and compare with the finish on actual parts. This is ideal for deciding what aesthetic finish I want on something I am about to fabricate.

Check with your local tool supply shops.

JHG

 

[kenvlach]

“The components can't be subjected to extensive etching due to the risk of eroding Small tapped threads.”
--- It is customary to plug threaded holes, especially small diameter ones, on parts to be anodized. Hence, no risk of etching or anodize buildup.

Re “finish of 64 MicroInch.” -- I realize that that this doesn’t fully describe the surface microprofile, but to me, as an anodizer, it seems pretty awful. I would expect poor appearance and corrosion resistance from anodizing such a rough surface. The anodic oxide forms via nucleation, coalescence and growth phases. At the metal/oxide interface, a barrier oxide layer of less than 5 nm (0.2 microinch) thick forms, from which the bulk of the anodic oxide grows outward, perpendicular to the surface. The growth replicates and magnifies any metal surface asperities. A rule of thumb is that hard anodizing (0.002 inch thick) will double the initial surface roughness or R_a value. Conventional anodize will have a proportionately smaller effect. Because of the perpendicular nature of the anodic oxide growth, which occurs both inward and outward, defects occur due to colliding growth fronts at sharp inside corners and outside edges, which is the reason for minimum radius of curvature values given for hard anodizing in MIL-A-8625F. At the inside corners, the external surfaces of the 2 planar growth fronts mechanically collide. At outside edges, the inward growth fronts collide and ‘starve’ the edge of aluminum.

Now consider what this means for anodization of rough finished aluminum. The anodize will begin growing perpendicularly outward from all the jagged little planes comprising the metal surface. The collision of all the adjoining planes gives a grayish, weak anodize rather than the ~clear, hard normal anodize. It looks ugly, especially at greater anodize thicknesses.

I would suggest machining to as fine a finish as your budget allows (perhaps 8 or 16 microinches). Then, plug the holes and do a light etch prior to anodizing. In lieu of etching, a bright dip to chemically polish the surface would improve both appearance and ultimate corrosion protection. Also, you may wish to chemfilm the holes for corrosion resistance. If so, this is done first: chemfilm the entire part, insert plugs and then follow the anodize procedure.

Hope this helps. Please give details and alloy if you have further questions.
Ken

 

[sreid]

I'd suggest vibro tumbling in an abrasive media. This will remove external cutting tool marks and should not effect internal threads.

 

[kenvlach]

Vibratory tumbing (a type of mass finishing) is a good suggestion. Some additional resources on finishing and anodizing:

Machinery’s Handbook and ASM Handbook, vol. 13, Surface Engineering both have charts showing the range of surface finish R_a obtained from different metal forming and finishing operations. The latter book includes chapters on mass finishing operations and “Surface Engineering of Aluminum and Aluminum Alloys.” ASM Handbook, vol. 16, Machining is probably useful (although I don't own).

Nova Finishing Systems sells vibratory equipment and mass finishing media. See “MEDIA GUIDELINES FOR VIBRATORY DEBURRING/POLISHING EQUIPMENT” at

http://www.novafinishing.com/supplies.html

The Anodizing Reference Guide at

http://www.anodizing.org/reference_guide.html

gives the results (e.g., colors) expected from anodizing various Al alloys. It also gives some of the finish designations from “Designation System for Aluminum Finishes”, DAF-45. The Aluminum Association (

http://www.aluminum.org)

sells DAF-45 as hardcopy (paper), but it can purchased and downloaded from

http://global.ihs.com

“Surface Smoothness of Anodic Coatings” at

http://www.metalast.com/technology/pdf/surface_smooth.pdf

gives R_a before and after Type II (conventional) and Type III (hard) anodizing. Note that initial sample R_a’s were in the range 4.1 to 20.9.

For an advanced study on the formation and structure of the anodize film, see

http://www.nucotec.de/pdfs/AnodicOxideFilm.pdf

Check the Letters and answers, and consider posting your question (with a little more info), at this forum:
“1b. PRETREATMENTS
[PICKLING, PASSIVATION, PHOSPHATIZING, CLEANING, ELECTROPOLISHING, ACTIVATION]” at

http://www.finishing.com/Letters/index.html