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Polishing Aluminum

Posted: Wed Aug 31, 2011 3:05 pm
by Phil3
How does one obtain a smooth finish on an aluminum part produced on a milling machine? As an example, a 2" cube of aluminum with a 1/2" notch cut along one side, where all surfaces need to be aesthetically pleasing with minimal machine marks. A four flute end mill does well, but if I wanted a polished surface, what methods and how to get into inside corners or angles?

- Phil

Re: Polishing Aluminum

Posted: Wed Aug 31, 2011 3:26 pm
by JimGlass
A fly cutter works good for this. Use a toolbit with generous tool nose radius. Your milling machine head will need to be square as possible to the table.


Re: Polishing Aluminum

Posted: Wed Aug 31, 2011 3:36 pm
by RSG
Not sure just how fine a finish you are looking for but I haven't found a method yet that doesn't require allot of fine sanding with various grits down to 1000 grit then its off the vibrartory chamber in corn cob media then fine walnut. As Phil3 mentions having a mill square and trammed to the table is a good start.

Re: Polishing Aluminum

Posted: Wed Aug 31, 2011 3:36 pm
by Conrad_R_Hoffman
Similar to a flycutter, those expensive insert cutters about 3.5" diameter with 8 or so inserts give almost a mirror finish. My guess is buffing would be short work after that, though I find the finish very nice as is.

Re: Polishing Aluminum

Posted: Fri Sep 02, 2011 10:16 am
by david5605
Something that size you can throw in a rock tumbler with the correct grit and let it tumble for a day or two. The tumbler will polish it but will not remove any milling marks. All it will do is polish them up.

I've used the tumbler on some aluminum turners cubes I made with good results. It does get in to all the nooks and crannies.

Re: Polishing Aluminum

Posted: Fri Sep 02, 2011 10:16 pm
by Orrin
I realize my technique is not always possible or practical; but, when it will work in your situation I recommend it.

I use a HSS fly-cutter on aluminum whenever possible. The edge is as keen as possible. I always brush on kerosene, never allowing the cutting edge become dry.

I never touch the aluminum, afterward, with bare hands. I always wear soft cotton gloves or use facial tissue to handle the part. The reward is a brilliant surface that looks somewhat like a CD and causes refractions that are all colors of the rainbow.

When this isn't possible, I buff out machine markings with Cratex tips, followed by polishing with any kind of brightwork polish. I remove the residue with kerosene, followed by a wipe-down with denatured alcohol, all the while making sure my hands never touch the aluminum.

I used these methods in building my Miser engine, seen here:

At one meet an old-timer took a look, snorted, then stomped off, accusing me of taking things a bit too far by having the flywheel chrome-plated.


Re: Polishing Aluminum

Posted: Sat Sep 10, 2011 6:58 pm
by seal killer
Orrin and Phil and All--

Getting a great finish on aluminum was a difficult task for me. I finally managed to get an acceptable result after using 1000, 1200, then 1500 grit wet/dry. Next, I moved to the buffing wheel and used Tripoli 327 and finished with Jeweler's Rouge. I learned to dedicate a wheel to each different buffing compound . . . never mix them.

Perhaps, if I had known what I was doing, I may have been able to eliminate the 1200 and 1500 grit sandpaper. Still, the aluminum turned out pretty good.


Re: Polishing Aluminum

Posted: Sat Sep 10, 2011 11:31 pm
by RSG
Looks pretty good Bill, some wax will help keep that shine too. Before I ever started anodizing my own stuff I did a buff polish like yours. Keeping it shiny was easy with wax.

Re: Polishing Aluminum

Posted: Sun Sep 11, 2011 8:31 am
by steamin10
Aluminum oxidizes very quickly, and usually, any chemicals cause moisture to destroy the surface luster. I have been curious about home anodizing, as opposed to clear coat painting.

Is there any doable information out there?

Re: Polishing Aluminum

Posted: Sun Sep 11, 2011 10:50 am
by roundnose
For perhaps a little humor, though some may not find it real funny.
Through the 90s, I made considerable profit doing a final polish in the combustion chambers in motorcycle cylinder heads.
This was after flow balancing the ports. working the valves and seats," the real work".
This polishing is not always the best thing to do, on higher compression engines the polished surface can cause a heat barrier, reducing the cylinder heads ability to absorb heat, and can cause a piston to burn.
I found I could not get that point across to a majority of customers.
So, often I had to polish them up, so the customer could see them and feel good, then on engine assembly, I would rough up the polished surface to prevent over heating and engine failure.
Bikers are like bower birds, they like shiny things.

To get this polish. The final two steps, by hand, use 600 wet/dry paper with WD 40. Last- Mothers polish and a flannel rag.
The steel valve heads were done on a lathe. ... bay006.jpg

I dont know if this is a tip or a confession!

Re: Polishing Aluminum

Posted: Sun Sep 11, 2011 10:58 am
by seal killer

I don't know if it's a tip or a confession, either. I do know it is beautiful work! Can you do my bling-bling Sportster? Chrome makes it run faster, so polishing the combustion chambers will, also! :)


Re: Polishing Aluminum

Posted: Sun Sep 11, 2011 12:40 pm
by roundnose
Straying off the subject.
The best gains on Harleys, is on the heads, not a bad design, but terribly executed in manaufacturing.
Nothing lines up, the manifold to head interface, valve seat edges poking into the ports, shrouding of the seats from the head casting in the combustion chamber, CC volume difference in both heads, the worst in the industry.
Take care of that mess, will go along way in over all running of the engine.
The object being to reduce port turbulence, causing a bad fuel mix resulting in detonation. This by increasing fuel mix velocity when the valve first cracks open, and on through the valve cycle.

One of my favorite photos from those profitable days. ... ots017.jpg

Oh ya. it only took money, a plenty of it, to fix that disaster.
I know, I have a bad attitude!