Aluminum Finishing

By now I have made enough reel parts of aluminum to have established a routine procedure for finishing. My goal is to arrive at a finish that is uniform but not highly reflective. If the aluminum is shiny before anodizing, I find that my sulfuric acid anodizing process tones it down.
IMG_3804

Here are the steps in my process:
1. When machining, arrange to take a very light cut (say 0.001) as the last pass. This is particularly important when using lightweight tools such as mine by Sherline. A smooth machined finish eliminates much sanding.
Most of the frame parts are 6061 aluminum. A problem with this alloy is that small chips get picked up by the end mill and welded back onto the surface that was just cut. These can be sanded away, but it is easier to just take a reverse direction (i.e., climb milling) pass over the part with zero tool advance.

2. If end plates or spool ends are to have lightening holes (as illustrated above), do not put these in until a later step in the process.

3. If machine marks are still visible, wet sand with 180 grit until they are gone. Round parts are sanded while turning on the lathe.

4. Wet sand with 320 grit, again using the lathe.

5. Now add the lightening holes.

6. Vibratory tumble 2 hours with medium cut plastic pyramids and water. My tumbler is small, so I do one reel as 2 or 3 loads.
Note 5 Oct 2012: In regard to my “Fixed Spindle” reels, the smaller reel can be done as 1 load. It is the rear end plate of the larger reel that has to go into the tumbler by itself. A larger tumbler would solve this problem.

7. Vibratory tumble 2 more hours with very fine cut plastic pyramids. (If you do not have a tumbler, steps 6 and 7 can be replaced by wet sanding with 600 and 1200 grit. Just be sure to remove sharp corners.)

8. Wet sand with 2000 grit, this time hand held.

9. Wet sand with 3M Trizact (3000 grit), hand held. (This is the state of the parts in the photo.)

10. Clean well and anodize.

Update 3 Oct 2012: Step 8 can be eliminated; sanding with Trizact alone is enough after the tumbling. I cut an 0.75 by 1.5 rectangle of Trizact, sand one reel, then throw the pad away because it has visibly lost its grit.

Update 25 Nov 2012: I have been using the Trizact pad because I could not find wet sandpaper any finer than 2000 grit. The Trizact pad is grit bonded onto foam, and wears out rapidly when used on metal. Recently I visited a Hobby Lobby store and found an “Ultra Fine Waterproof Finishing Kit” that has sandpaper of 320 through 12000 grit. This item #0110 of Alpha Abrasives. You can see and order all their products here. Like any wet sandpaper, this is more durable than Trizact. To be fair, Trizact was not really designed for this application.

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