Author Topic: General Metal Finishing  (Read 9906 times)

Circlip

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General Metal Finishing
« on: December 07, 2008, 06:47:07 AM »
 Don't know if anyone else has seen this, but it certainly answers a lot of questions.

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

   Regards   Ian.

Offline Darren

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Re: General Metal Finishing
« Reply #1 on: December 07, 2008, 07:01:29 AM »
I might be able to add a bit of exp here   :D

Depends on what you are trying to finish of course but here's my discoveries over the years.

I once found a very rusty steel milk churn that I wanted to put in the garden for no other reason than curiosity.

I did very little prep, just washed it really, and painted it right over the heavy rust with a product called POR-15.
That was around 15 years ago, it's still in the garden and the paint is still on it. This paint brushes on really well leaving a very smooth gloss finish, but it does not like UV rays which make it go milky coloured. But you can overpaint it to get around this.

Inside of course this isn't a problem. I have a steel cabinet done at the same time and it still looks fine.

Another paint I tried about a year ago was floor paint. Johnsons I think it was.
I painted my S&B lathe with it to tidy it up a bit. It sticks like S*&t to a blanket, covers well, even slightly oily bits too. Oops, but it hasn't come off.
Best of it is though, it's very easy to clean/wash down and very very tough. Drop tools on it no problem, won't chip or even mark much.

Only problem I found was it left some slight brush marks, but not too bad, you've seen the lathe.

Darren  :wave:
You will find it a distinct help… if you know and look as if you know what you are doing. (IRS training manual)

Offline Darren

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Re: General Metal Finishing
« Reply #2 on: December 07, 2008, 07:20:20 AM »
Oh and I can also personally recommend Marine Clean and Metal Ready, they all work very well.

Metal ready is used for new metal prep, not rusty stuff btw. Por-15 likes rusty metal and water makes it cure harder. Opposite to other paints so great in damp conditions.

http://www.por15.com/
You will find it a distinct help… if you know and look as if you know what you are doing. (IRS training manual)

Offline PTsideshow

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and I'm working on the first two!
glen

Offline mechman48

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Re: General Metal Finishing
« Reply #4 on: June 07, 2015, 06:16:26 AM »
Thanks for all the links & info... but I'm not intending to do any anodising / powder coating, don't have the facilities nor the finances, just want to paint the likes of inside of aluminium flywheels ( 6" dia) so would some form of etching paint & spray car enamel be suitable (Halfords / B&Q  :scratch: ).

TIA
George
George.


Always look on the bright side of life, & remember.. KISS..' Keep It Simple Stupid'

Offline Jonny

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Re: General Metal Finishing
« Reply #5 on: June 07, 2015, 02:23:51 PM »
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.

Offline Arbalist

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Re: General Metal Finishing
« Reply #6 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!

Offline DMIOM

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Re: General Metal Finishing
« Reply #7 on: June 08, 2015, 02:58:45 AM »
I had some stuff hard Anodised and it came out great except a couple of reamed holes had reduced in size!

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

Offline Arbalist

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Re: General Metal Finishing
« Reply #8 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.

Offline DMIOM

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Re: General Metal Finishing
« Reply #9 on: June 08, 2015, 04:45:32 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.

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

Offline Arbalist

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Re: General Metal Finishing
« Reply #10 on: June 08, 2015, 11:37:14 AM »
 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!

Offline Lew_Merrick_PE

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Re: General Metal Finishing
« Reply #11 on: June 08, 2015, 05:21:03 PM »
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.

Offline Arbalist

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Re: General Metal Finishing
« Reply #12 on: June 09, 2015, 04:57:42 AM »
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.

Offline Jonny

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Re: General Metal Finishing
« Reply #13 on: June 15, 2015, 07:27:09 AM »
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.