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General Metal Finishing

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 I've used several companies in the past for Anodising but this last one seem to have a far better setup than the others. They told me they Hard Anodised the parts and it may have been because of the surface finish of the parts, I can't remember for sure but I think that was the case. As a layman though they may have just said that when it could have been Chromic Anodised, I wouldn't have known the difference! I did make it plain though that I wanted a grey finish. It seems to be a much tougher finish than the other stuff I've had done (black) with no powdery finish at all, I think I just gave it a wipe over with something to make it look nice. This was ten years ago so no idea if they're still around as I now live over 100 miles away!

Anodizing of aluminum comes in essentially two forms: sulfuric or chromic.  Although chromic anodizing is generally referred to as hard anodizing that is because of the chromic oxide surface.  The difference in hardness or abrasion resistance is fairly small -- Rc-58 vs Rc-60 quite typically.  As with any finish, the result is never better than the surface beneath it!

I designed/developed a piece of equipment for the US military some years ago.  The requirement called for chromic anodizing.  The problem was that the chromic anodizing chipped off in the field raising the potential of reflections that could give away their position.  Sulfuric anodizing, on the other hand, passed all the drop & hit tests with flying colors.

I often pre-treat any part I send out for anodizing by soaking it in a fairly high potassium hydroxide (potassium lye) solution for 10-12 hours before giving it to the anodizer.

An engineering company opposite where I used to work told me they used to ask for "deep etch" before anodising to get the finish they wanted.

I have a few bits on monthly cycle sulphuric anodised all colours. Grey being one of them and dependant upon time and aluminium grade.

Chromic is thinnest just for cosmetic purposes often painted over for that reason.
Sulphurics are thicker, longer lasting and more durable. If unspecified dare say this type 2 would have been done.
Hard anodising is sulphuric type 3 being thicker than type 2, colours wont look right except black. ie yellow (gold) will look pale mustard.

Type 2 straight in adds 0.04mm to each surface if comes out ok first time. Only need to machine diameters within 0.1mm.
Used to have parts etched 21 years ago after mop polish jobbies. Dependant upon dip time can bring mirror finish to satin or even just under matt. This removes metal so tolerances will be affected usually of order 0.1mm each surface.
Like wise of late I bead blast and have type 2 done. If unspecify 'Brighten' they come out rough matt, brightened dependant upon time can come out semi bright to satin. This also removes metal not good for threads and precision fit parts that will be scrapped. Not uncommon to see 0.3mm metal removed on each face so on a 1mm pitch thread parts will go an extra 2/3 of a rev.

Problem being you may get parts back from these monkeys and look fine until come to fit. When the job don't come out right it will be stripped removing metal again so the earlier 0.3mm reduction can now be 0.6mm not good for a 4k item.
Have had to scrap many a job over the years each could be months of work or they often loose a part more than once when replaced then get three back!
Law to themselves and why I had a few years doing my own anodising 2002 to 2006.


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