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General Metal Finishing

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Jonny:
Certainly need a self etching primer and have used canned acrylic spray over the top, not very durable unlike the car wheel paint.

Op posts 7 yrs ago concerned about the crap these pros are saying http://www.finishing.com/186/55.shtml#new
That's why anodisers scrap parts, take it to a proper anodisers who know what theyre doing ie have taken spin polished, bead blasted of different grades plus a strip and re anodise, all came back perfect match.
Usually take in 60/120gr bead blasted parts to tolerance and brightened to give a satin/sheen, one anodiser can remove 0.3mm on a 0.75mm pitch thread = scrap. Take to another and no loss of tolerances can go either way adding or reducing tolerances.
In fact took one part in Friday and it wont come back matt or grey and still have the beaded look.

Arbalist:
I had some stuff hard Anodised and it came out great except a couple of reamed holes had reduced in size!

DMIOM:

--- Quote from: Arbalist on June 07, 2015, 05:41:54 PM ---I had some stuff hard Anodised and it came out great except a couple of reamed holes had reduced in size!

--- End quote ---

Dimensional changes can vary depending on which anodising process was used!

Whereas plating is purely additive, anodising is a conversion of the surface layer.   The layer is formed by consumption of the surface aluminium to fuel the growth of the aluminium oxide matrix, and typically the thickness of the resultant layer is about double the amount of substrate consumed.  Growing a 1 thou layer will have typically consumed 0.5 thou of the underlying part so every surface will now be 0.5 thou bigger - so if you anodise a part all over to a 1 thou layer, outside dimensions will be 1 thou bigger and the actual bore diameter of a hole will be 1 thou smaller.

Anodising to take colour usually uses sulphuric acid electrolyte (known as Type II anodising). For lighter/brighter colours which benefit from light being reflected from the underlying aluminium, 0.5 thou ano layer is often about right. To get black parts, you need to ano to a full 1 thou layer to get enough of those big black molecules into the dye matrix to block out all of the light.

"Hard" anodising can take several forms. Type IIB is basically just the same as a thin, undyed Type II but at a reduced temperature; Type III is again the same as Type II but much thicker - maybe up to 5 thou - and needs near-freezing electrolyte. Type I hard-coat anodising on the other hand uses chromic acid and builds a far denser, thinner layer which can be as little as 1/4 or 1/2 a thou thick.

So, if your "hard coat" was produced by Type I anodising, the reduction in bore might have been as little as 0.25 thou - but if it was Type III anodised the bore could have reduced by 5 thou.

If you have dimensionally-critical parts, it isn't just the actual anodising that needs to be considered. Anodising needs perfect electrical contact between the electrolyte and the whole surface so, after degreasing, parts almost always have some form of etch dip to remove the random oxide layer that has formed by exposure to atmospheric oxygen. This is a purely consumption process, so makes parts smaller; and if over-done can (a) reduce dimensions, and (b) reduce surface quality.

So, usually anodising gives you a slightly bigger part - but not always!

Dave

Arbalist:
I didn't actually specify at the time what type of Anodising I wanted, I just said I wanted a dark grey finish to the largely sand blasted items. Most of it came out very well but a couple of sections of angle were a bit patchy, obviously not such a good grade of alloy for anodising.

DMIOM:

--- Quote from: Arbalist on June 08, 2015, 04:06:36 AM ---I didn't actually specify at the time what type of Anodising I wanted, I just said I wanted a dark grey finish to the largely sand blasted items. Most of it came out very well but a couple of sections of angle were a bit patchy, obviously not such a good grade of alloy for anodising.

--- End quote ---

Couple of thoughts:

Dark grey may not have been genuine "hard" anodising.

Not sure from what you said how they were made, but castings are notorious - anodising can expose non-homogeneous mixes, though usually that shows up on a dyed part as white spots where the non-aluminium specks didn't anodise & there was nothing to take the dye (welds with different filler rod to the base alloy can also show differences when anodised).

Also you mentioned sandblasting - that's pretty much an absolute no-no pre-anodising. Sand blasting leaves microscopic particles embedded in the surface - and even worse, if the media has been used for other parts, you can get other metals embedded, worst of which is any ferrous matter.  If you need to blast a part to clean it or to give a matt surface, glass beads are good - but the blasting rig must never have had sand or grit through it. Glass beads are expensive in small quantities but you can buy 1/2 cwt bags from the likes of MSC for not much more than a small plastic tub from some DIY suppliers.  If a part has been sandblasted, it can take a lot of etching to get down to un-tainted metal.

You mentioned angles - not sure if its relevant here but sharp corners are the hardest to get a good robust anodic coating. The oxide layer is a fairly in-flexible high aspect ratio hexagonal matrix - try to imagine a ceramic honeycomb around an infinitely sharp edge. Anything we make which we're going to anodise, for parts that will be handled or otherwise rubbed, we always try to get at least radius of 0.5mm on any edge. Crests of threads, where possible, are cut using full-form inserts; and any knurling is 'softened' to remove sharp peaks.

Of course, a patchy appearance might also just be down to poor anodising - especially poor circulation / poor temperature control.

Dave

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