Diamond Polishing Aluminum
Proven step-by-step instructions for polishing aluminum and useful and accurate pointers for those applications where a diamond finish is not required.
One of the most difficult jobs for any polisher is getting a high diamond finish on aluminum. Achieving a good shine in soft gummy material that constantly clogs stones and easily takes deep scratches from sandpaper is enough to challenge anyone. Brushing and buffing can easily lead to grooves and lines in the finished surface and all the while the polisher still has to be responsible for holding tolerances on sizes.
This article will review proven step-by-step instructions for polishing aluminum, which if carefully followed will yield professional diamond finishes every time. This article also will provide useful and accurate pointers for those applications where a diamond finish is not required.
Common Problems with Polishing Aluminum
The most common problem associated with polishing aluminum is the deep scratches that plague the finishes. Three things primarily cause deep scratches in aluminum:
1) The wrong type of abrasive being used.
2) Too much downward pressure from the polisher's hand.
3) Using too long a stroke while stoning.
Aluminum is primarily stoned by hand and easily turns to powder as it is stoned. This powder constantly clogs the pores of the stone and then builds up to "score" or "tear" the aluminum. Efforts to leave stones and use sandpaper yield somewhat better results. This method is the most commonly used today, but even stoning with sandpaper can leave a long, tough road of deep scratches, which refuse to diamond out properly.
The answer to the first problem is to use newly developed stones that are specifically made for all nonferrous metals (e.g., aluminum, copper and brass). If used with mineral spirits as the lubricant, these stones will never tear the metal! Even using these new stones while purposely trying to cause rips and tears won't cause a tear. Check with your favorite stone vendor for samples of these newly developed stones for non-ferrous metals.
The secret to getting a good diamond finish is to first have a fine scratch stone finish. The way to achieve this finish is to lighten up on the downward pressure as you stone (Second Problem). At first, you push down as hard as you need to in order to get rid of deep existing scratches, but then this time you go over the entire area with the same stone and a much lighter pressure. You will take out your own scratches! If a 320-grit stone put the scratches in, then take that same 320-grit stone and go over it again with a lighter pressure and take out the earlier scratches.
Next move up to a 400- or 600-grit stone. Test them to see what you like and pay close attention to the depth of all of the scratches at all times - look at them with a magnifier, compare them to each other, but do not put in any deeper scratches than what you already have. Always use a finer scratch to take out a deeper one and lighten up on your downward pressure until the very end. You are basically "massaging" the grit in between the stone and metal. Don't even let the stone touch the metal. Only massage the fine grit that is in between the stone and the metal. Pick small areas and see how quickly you excel at taking the deep scratches and turning them into fine scratches.
Stoning properly is not easy to learn and it may take forever to do, but with practice you will eventually become quite good. Practice on small areas until you perfect this technique of lightening up on the downward pressure.
Keep all of the strokes (Third Problem) down to two or three inches in length - because longer strokes tend to make deep scratches regardless of what type of abrasive you use and create deeper scratches going in the wrong direction. Now it's time to diamond.
Assuming that your work is wide enough, start by putting a one-inch cup brush into your rotary hand piece. If your stone finish is truly good enough, a diamond compound in the nine-micron range will work like magic; however, don't feel bad if the nine-micron diamond won't take out the stoning marks and you have to switch to the coarser 15-micron diamond. This just means that you need more practice at creating a fine stone finish.
Experiment with different bristle lengths. The longer the bristle, the longer it takes to cut. The smoother the finish in the metal and the shorter the bristles, the faster it will cut - but you run the risk of damaging the flat-ness of the job by cutting shapes and grooves into the aluminum. Find a good balance.
Cool the brush only with denatured alcohol. Mineral spirits will make the brush cut much too fast - ruining the job. Be careful not to brush out deep scratches, because you will never end up with a decent flat surface - you'll end up with a ruined wavy surface. Flatness comes only from a stone; over-brushing will only ruin the flatness. Wood sticks cut to shape can be used in a profiler machine with diamond to lap corners and sidewalls, if necessary. Once brushing is complete, it's time to buff.
After a good brushing job, it's time to do a soft buff, but before you begin it's critical to take a good, long look at exactly how deep the brush marks are. They are probably only .0002 of an inch deep or less, so when you start with the buff, press down with less pressure. Be sure to use plenty of denatured alcohol to lubricate the buff and use a number three-micron diamond compound.
It's best to have the buff loaded (fully impregnated) with diamond compound before you begin because at this point the buff is far coarser than the finish and only by having a fully impregnated buff will the point of contact be finer than the polish. With a fully impregnated buff only the diamond is touching the surface and not the buff that's holding it.
Continue buffing until all of the brush marks have been replaced by the finest buffing finish you can get.
Rubbing Out by Hand
The final step is optional, depending on how good a diamond finish you need. Start by folding a piece of facial tissue into a square - six or 11 layers thick - impregnate it with number three yellow diamond compound, lubricate it with denatured alcohol and gently rub out any buff marks remaining. Be careful to use only two- to three-inch strokes by hand, so as not to cause scratches in the finish. Clean everything with mineral spirits first, then again with alcohol and you're finished.
This method works on all types of aluminum and once you learn it you will understand how easy it is to do. Keep practicing on small areas or blocks of scrap until you see for yourself the "scratch range" you need to be in for the next step to work well.
The third in a series of articles on heat treatment, this month's offering looks at the heat treatment procedure, its principles and why heat treatment is necessary for steel.
Plating aluminum molds for longer production life.
Schools for mold polishing do not exist and most veteran polishers don't want to teach the subject, so here are a few simple polishing guidelines.