Metallurgist question about CNC aluminum (NE1 CNC?)

[kirchner@upstat]

While doing some machining on cast aluminum wheels I have come across a problem. The parts are coming out looking like they are water spotted. I am told that it is called a galvanic occurrence.
I’m wondering if:
1. I am doing something to cause this. To slow a speed, too fast, the item is getting to hot etc.
2. The source material is old and of poor casting.
3. Wrong bit in the CNC machine

Anyone have experience with this? Any suggestions on who to contact that might shed light on eliminating or reducing this effect.

I'm just working on it with someone who should know more but its a Hass machine and we have used carbide, ceramic and diamond tips. The diamond seems to destroy itself rather quick.

FWIW the base metal is old (over 40 years). I suspect it might be the metal but I want to be able to rule out the fact that I might be causing it.

If I posted in the wrong forum please advise.

Thanks In advance --

[Geof]

What do you mean by; "The parts are coming out looking like they are water spotted"?

Do you mean the freshly machined surface has a mottled finish? A picture or two could be helpful.

When cast aluminum is machined with sharp tooling at the correct speed so the surface finish is very good it is often possible to see the crystal structure of the metal. This is very obvious on some cast alloys after they have been heat treated to raise the temper from the as-cast annealed state. The heat treating involves holding the alloy at a high temperature so that the crystal structure reorganizes in a process called precipitation hardening. This is also known as age hardening and it is a process that occurs faster at higher temperatures but also takes place over a long time at room temperature. This process improves the yield strength of the alloy but at the same time reduces the ductility; with some alloys when it is taken to far they can become very brittle.

You say these parts are 40 years old cast aluminum wheels...I don't want to worry you unnecessarily but I will say I might be very reluctant to ride in a vehicle fitted with these wheels.

[kirchner@upstat]

Sorry, Here are some photos.
Notice the lip of the rim. It is freshly machined and it has the apperrence of being galvanized. Im afraid to keep cutting deeper.

I have sprayed a clear coat over it just for protection but it has not changed the finish. On one photo you can see the fine machine lines and where they repositioned the bit to compensate for the curve.

Any suggestions? Im afraid I might be doing something wrong.

[Geof]

Originally Posted by kirchner@upstat ....Any suggestions? Im afraid I might be doing something wrong....

No you are not doing anything wrong; that is a very nice example of what I was describing, you are seeing the grain structure. Your comment; "it has the apperrence of being galvanized" is right on the mark; the mottled appearnce on hot dipped galvanized is the grain structure (crystal structure) of the zinc. If you lived further north you would have seen a similar mottled appearance on your windows due to frost forming on a cold night, just larger crystals. Incidentally the very small whitish spots are probably hydrogen porosity which is very common in cast aluminum.

You will not change the appearance by taking any more material off, this is the crystal structure of the metal all the way through. Also I will say now having seen a picture I would not be worried about using these wheels, that grain structure is probably bigger than it was 40 years ago but it is still quite small. But I do hope that you have not taken off more than a few thou just to clean up the surface.

[kirchner@upstat]

Thanks! Hmm it doesnt look good for me.

I Just took a tiny bit off.

I had hoped to achieve a nice finish much like the newer alloy type wheels but it looks like this is impossible with machining if I'm reading this correct?

I guess my only course of action is to progressively polish the surface with ever decreasing grit until the desired finish is achieved.

I guess I already know the answer but are there any machining techniques that can work this out?

Thanks -

[Geof]

Originally Posted by kirchner@upstat Thanks! Hmm it doesnt look good for me. I Just took a tiny bit off.-

Good.

Originally Posted by kirchner@upstat I had hoped to achieve a nice finish much like the newer alloy type wheels but it looks like this is impossible with machining if I'm reading this correct?-

Yes. EDIT: New wheels will have a finer crystal structure so not as noticeable, but it is there.

Originally Posted by kirchner@upstat I guess my only course of action is to progressively polish the surface with ever decreasing grit until the desired finish is achieved.-

Yes but see comment below.

Originally Posted by kirchner@upstat I guess I already know the answer but are there any machining techniques that can work this out?

As I mentioned this is the crystal structure of the metal all the way through. If you used a tool with a larger nose radius or one of the wiper inserts from Sandvik you will get a much smoother surface for final polishing but you will still see the grain structure.

My suggestions is do not machine any more off, just polish with finer and finer grit and then finishing up with proper polishing compound to put on a mirror finish. Once you have the polished mirror finish the grain structure is not as visible. You can see it if you look closely and have the light coming at an angle but you have to know it is there to recognise it.

[kirchner@upstat]

Thanks Geof
Looks like Im going to wear out my finger tips.... or hire someone else to do it.
Oh well I heard of a process called Electropolishing on line. I wonder if this might be able to do anything?

[Geof]

Originally Posted by kirchner@upstat ....Oh well I heard of a process called Electropolishing on line. I wonder if this might be able to do anything?

Maybe not; electropolishing aluminum is somewhat similar to anodizing and cast aluminum very often does not anodize well. Even if it would work you would still have to polish the tool marks away first.