Making Parts....

Bill Clem

Member
Joined
Mar 14, 2004
Messages
126
Slowly I've accumulated a bunch of tools, a vertical mill, a lathe, bead blaster, horizontal and vertical bandsaws, drum sander, belt sander, downdraft painting table, 4 Scotchbrite grinding/buffing wheels, TIG welder and a bunch of other stuff that have bankrupted the piggy bank. But I haven't yet figured a good way to prep small parts for anodizing. The Scotchbrite buffing wheel is okay, but still leaves a slight visual tooling mark. I wandered onto reference to a vibratory tumbler, and I think I might be going nuts.... walnuts, that is. Apparently a highly recommended polishing method is to use rouge-impregnated walnut shells to buff up aluminum. Does anyone out there in Rotorland have any experience with prepping parts for anodizing? Or I may just have to go nuts and find out.
 
Bill,
I have not, I remember a DVD named "Deep inside your Rotax". The engine rebuilder stated he used walnut shells to clean the outside of Rotax crankcase.
I see that Eastwood products is using baking soda for a lot of fine finishing instead of glass beads. Sooo.......... whachmaking?
 
It probably requires a couple of acid baths.
To prepare parts for alodine we dip them in "smut go" and then phosphoric acid, then rinse with water and right into the alodine tank.
 
I spent years preping metals for plating. The best surface prep would be a cotton buffing wheel with white buffing compound. It is commercially sold as "stainless steel" compound and is a medium-light grit. It comes in bricks, ready to use. Most plating supply companies will have it.

You must degrease after buffing. We used to use a carbon tetrachloride fog bath, but that's not something you want to "try at home." Lacquer thinner is a reasonable alternative.

CAUTION: It is VERY easy to deform or even melt aluminum parts with overzealous buffing and too much pressure on the wheel. In the case of hardened aluminum, extreme care must be taken to avoid building excessive heat. It is remarkably easy to get the piece hot enough to de-temper, and a bad thing.

I have seen aluminum parts on gyros and other aircraft that have clearly been buffed to the point the temper was lost. Very pretty...very pretty indeed...but no longer 2024 or 6061.

Tumblers, by the way, are great for rocks. Not much good for metal parts. If you tumble a part long enough to get a good finish, it is also usually remarkably deformed.

*JC*
 
On my parts I use "Manual Labor"
I use dish soap and 220 grit wet and dry sandpaper to get all the oxidation and flaws out of the surface then I switch to scotch brite to put on that satin like finish.
You must always sand in the same direction and with the grain of the aluminum in nice even strokes all the exact same direction if you want it to come out looking professional.
after you have your finish wipe the part down with lacquer thinner and your good to go.
It is a lot of work but the results are great.
 
Polishing the effortless way : )

Polishing the effortless way : )

HI guys, I have used tumblers for years to clean, and polish brass and steel ammo casings. You just through them in the media ( there are lots of styles ) sometimes even add a small amount of liquid abrasive compound and leave it for several hours... nice shiny stuff comes out at the end.
In industry they also use vibratory tumblers and have done for many years to polish and even de-burr edges in cast components in Magnesium, aluminum and many other alloys.

Have at it... it's a low effort way of polishing. :eek:
 
"Effortless",... I like that!

"Effortless",... I like that!

I just ordered a Thumler vibratory tumbler. Don't know why they call it a "tumbler" since nothing tumbles, just vibrates. Thanks for the comments. I've been a little worried about heat generation using the Scotchbrite wheels but even though the parts are hot enough to create steam, they don't stay hot enough, long enough to affect the temper. My fingers, however, are another story. Had to start using welding gloves to hold the pieces to the wheels, both for heat and protection from buffing the skin right off my pinkies. The local anodizing shop has said that any striations from sanding or buffing will show through the anodizing, in other words, a mirror finish is needed.
 
Bill,
One thing you will have to watch for, is unlike polishing brass, aluminum will give off a black powdery residue. This will build up and fill in all the voids in your finishing media. This begins to look like a graphite powder and acts like it too! The powder acts as a lubricant and any polishing/ deburring action quickly ceases.
The big production houses use a continuous water spray on the vibratory bowl which keeps the parts clean and also the media clean so that it maintains a good sharp cutting/ polishing edge.
If you don't keep this washed out and clean, you are going to end up with a big black powdery mess which is gonna suck to clean up!

Now it is true that any machining marks or scratches are going to show through the finish. I have found that a mirror finish is not necessary however.

The picture shows some aluminum parts that I produce in house.
The part in front is the raw machined part.
The middle part has been finished with a "one step vibratory finish" it has a frosted finish, almost like a galvinized or cold zinc plating finish. This is what I do to all my components that get anodized. I believe it is a ceramic media used.
The part in back has been further polished with a walnut shell media. It does produce a slightly glossier finished product, but not necessary...After all, the anodizing process is etching the surface removing this "mirror" finish.

Also, I would shy away from anodizing any "flight critical" components, as the anodizing process has negative effects on the material properties.

Hope this helps.
Denis
 

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Question? Is around 130 deg F enough to adversly affect aluminum plate. I buffed my cheek plates and mast/frame plates to a mirror finish holding the parts in my bare hands at all times. It got hot but if it felt too hot to handle I stopped and let the part cool down. So did I weaken or seriously weaken the parts?
 
Good advice

Good advice

Thanks Denis. I have wondered about the issue of anodizing "flight critical" parts recently. Most tempers for 6061 and 7075 require heating at temperatures above 375 degrees for a considerable period of time. When I looked into powder coating, it appeared that the oven time and temperature was probably less than that needed to affect the temper, but I'm not willing to take that chance. Also powder coating has the potential for covering tiny cracks. I did powder coat a 4130 engine mount recently, white, so that any cracks would be more visible. As far as anodizing parts, it is likely true that anodizing might have a small effect on strength as it does compromise a tiny bit of the surface, a few thousandths perhaps. My rule of thumb in the past for life safety products has been to use material capable of holding at least 5 times the working load, in some applications I have seen designs calling for 10 times the working load. Aviation design seems to cut these numbers a bit, and fatigue life, given the vibratory loads seems to be a more important issue. But the simple fact is that if the compromise induced by anodizing is sufficient to cause failure, then I think that the part was greatly under sized for the load. I recently replaced some mast brackets of 2024 with 7075-T7 aluminum of greater thickness. 7075 has greater tensile strength but is known to be more brittle, but the T7 temper has less propensity for crack spread. A trade-off, like most things.

It is a rule of thumb that most aluminum tempers are unaffected by temperatures less than 200 degrees Fahrenheit. Above that some effect can be seen, dependent on the temperature and time. 212 degrees, boiling water, would have minimal effect unless maintained for a long period of time. 600 degrees would have a much quicker effect. The heat treatment for 6061-T6 is 990 degrees, followed by 320 degrees for 10 hours. It is unlikely that most things that we might do would reach these temperatures, with the exception of exhaust exposure. Otherwise we couldn't make engine cylinders from aluminum.
 
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But the simple fact is that if the compromise induced by anodizing is sufficient to cause failure, then I think that the part was greatly under sized for the load.

I thought the same thing too until Minnesota Mike turned me on to this...
www.experimentalhelo.com/Anodizing&Fatigue.pdf

Very interesting. It still seems to me that the weakening would be negligible, but, I still won't take the chance just for a little "cool" factor.

Denis
 
Hey Denis,
I don't know what the parts are in the photo but they look so nice I'm tempted to buy one and figure out what to do with it latter. :^) nice work.

Scott
 
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Thanks!

Thanks!

Lol!
Thanks Scott. Yeah, it did turn out to be a pretty cool looking part....The only bad side is that it took forever to tweak the CNC program enough to get it down to decent cycle time.....And then as soon as I did, we discontinued that model! :mad:

Here is a link to a link if you are interested...
Thanks again,
d

http://www.rotaryforum.com/forum/showthread.php?p=310703
 
A bit more information...

A bit more information...

I have read the article about the Safari control rod failure. A further discussion can be found here:www.finishing.com/444/19.shtml. They discuss the inherent differences between sulfuric acid and chromic acid washes. Apparently the aerospace industry, ie. Boeing, has addressed this issue. I don't know the answers, only the questions. I would be real reluctant to anodize some flight critical parts, like rotorheads, although I have seen some parts anodized by overseas manufacturers (who will remain nameless to avoid a whole bunch of squabbling.
 
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