Rookie seeking help with pre-annodize finishes (bead-blasting vs. chemical etching)

I am trying to learn more about bead-blasting, polishing, and/or chemicall etching 6061 aluminum, and what I can expect from each in terms of the work involved and the final result.

Example -- I have several pieces of 0.25" thick 6061 that were plasma cut. The largest is smaller than a square foot in total area and measures less than 12" on it's longest side. They were cut from plate and have scratches and mars that will be visible after anodizing if nothing is done to prep the surface. I will be anodizing several at a time in my small line.

My #1 priority is minimizing the work involved in hiding the scratches. Having a matte vs gloss finish is not critical for my application, I just need a workflow that will produce a uniform finish AND that hides preexisting scratches.

I have read that chemical etching using NaOH solution provides a matte finish, but I don't know if it goes deep enough to etch away the scratches.

I've already paid for some bead-blasting services in the past and it did a great job of meeting both goals, BUT I understand it can be more difficult to make it uniform and is also difficult to get a deep black anodize with a bead-blasted part.

I have never polished aluminum but I'm not opposed to it if a uniform finish can be achieved more quickly than either bead blasting or chemical etching. My assumption was that polishing would be the most time consuming so I've focused on the other two, but correct me if I'm wrong.

Again, matte vs gloss isn't important, but the time required to hide the surface imperfections is.

Thanks for your help.

You might want to look into vibratory deburring/polishing. There are several threads about it here and a nice summary here: http://www.cnccookbook.com/CCVibeDeburr.htm

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Originally Posted by rowbare

You might want to look into vibratory deburring/polishing. There are several threads about it here and a nice summary here: http://www.cnccookbook.com/CCVibeDeburr.htm

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That sounds good if I can find a tumbler large enough for my pieces, and affordably. Are there any good classifieds where you can find them used?

After some quick searching I don't know if this will be a good option for me. My largest piece is about 12.6" on the longest side and it takes a fairly large and expensive tumbler to hold something that long.

Bead blasting is going to be your easiest/fastest. Depending on how deep the scratches are you may want to do some light sanding first, try using around a 300grit. When you blast just go over and back, if you spend too much time in one area you will get a blotchy ano finish. As far as blasted parts not looking as dark, that depends on how much blasting you do to the part, the lighter he blasing you can get away with and still cover you scratches the darker your part will look.

I have also gotten away with just sanding in the past. 300grit, 600grit, 800grit wet sanding gives almost a semi-gloss finish. Easy finish, but if you have a blasting cabinet, I would go that route.

Dan

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Originally Posted by Danodizing

Bead blasting is going to be your easiest/fastest. Depending on how deep the scratches are you may want to do some light sanding first, try using around a 300grit. When you blast just go over and back, if you spend too much time in one area you will get a blotchy ano finish. As far as blasted parts not looking as dark, that depends on how much blasting you do to the part, the lighter he blasing you can get away with and still cover you scratches the darker your part will look.

I have also gotten away with just sanding in the past. 300grit, 600grit, 800grit wet sanding gives almost a semi-gloss finish. Easy finish, but if you have a blasting cabinet, I would go that route.

Dan

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Thanks, sanding was going to be my next question actually. I do not own a blasting cabinet yet, but I do own a good drill press and some hand-drills, both corded and cordless. I also have a random-orbit sander, but I'm not sure what kind of sand paper is best to use wet with metal, or if they make it in pads for a random orbit sander? What power tool did you use to sand with the increasing grits, or did you sand by hand?

You should be able to pick up wet/dry paper at any hardware store. I use a rubber sanding block and do it by hand. You will have more control over the finish by hand. As you already mentioned anodizing does not cover markings. It would serve much purpose to replace scratches with sanding marks.

Good point, but I would favor an evenly etched surface (i.e. one that looks uniform and intentional) over descrete scratches that obviously are not intentional.

I have read some articles that are in favor of using a random orbit sander for applying a uniform surface on aluminum because the random sanding doesn't leave a swirling pattern. I already knew this worked well with wood even across grain because it's my sander of choice for woodworking, but it makes sense it would also provide a uniform quasi-brushed appearance to metal as well. Since I"m already equipped for this I will start experimenting.

I’m curious on how the finish turned out using your sander.

I have also the same problem as IMHOTEP. I blast my aluminum parts with glass beads of size 75-150 microns and then get black anodized. The material is 6351 grade. The coating is not uniform throughout on the part but blotchy. I can not understand what mistake I am doing. I was told that before blasting I should get the parts degreased by washing them with hot water and detergent. I do not know if it is O.K. I am not degreasing the surface before blasting but straight away blasting the parts after machining. I want to try it but would like to know what is the suitable degreasing media. I would also like to know how the random-orbit sander is used on aluminum. Is there any of its usage video on you-tube? Thanks for your help.

Blasting aluminum before anodizing is really tricky. Turn your air pressure way down and use as even coverage as you can. The type of glass bead makes a difference as well. The best results are parts done in a vibratory finisher IMHO. I really only got lucky on a couple of pieces and just settle on a sanded finish.

Derek

Thanks Derek. I would like to have more information on vibratory finisher IMHO. I make 100+ pieces per day in odd sizes up to 48 sq inch, the largest size is 12 inch. So time/cycle is very important. I need finish all the pieces the same day.

Naresh

Hi! I make on CNC/VMC about 100+ parts per day in odd sizes up to 48 sq inch, the largest dimension is 12 inch, from 6351 grade aluminum. These are to be black anodized. As for anodizing, the part should have a clean surface, I need them finish or say remove the machining marks. As I understand, there are three options. 1) Vibratory Tumbler 2) Glass Bead Blasting 3) Manually Sanding or using Random Orbital Sander. I am confused which process should I go for that is fast. I want to finish all the pieces the same day. So time/cycle is very important. Big parts, I use flat bed belt sanders and for small parts I do bead blasting. Edges round up in bed sanders and anodizing is blotchy with bead blasting. I might be making some mistake somewhere. I want to know the preferred bead size and whether the part need be degreased (cleaned of oil/lubricant) before blasting. What are the suitable Random Orbital Sander and the sanding wheel for aluminum?
Naresh

One nice feature of mass finishing is the labor is a lot less than hand finishing. I'm no expert at vibratory finishing but here is what I've noticed. I have triangle shaped ceramics that leave the aluminum fairly bright. I have abrasive impregnated plastic rods cut at an angle that do a really good job of burr removal and leave a fairly dull surface.
I would contact a supplier of media and tell them the finish you're looking for. The will steer you in the right direction.

Derek

Just be careful how long you tumble parts. We used to have a whole lot of color variation, finally tracked it down to the guys leaving parts in the tumbler too long. It will sort of shot pein the surface, so the dye wouldn't penetrate. Put a timer on the tumblers and everything from then on was perfect.

Thanks Derek, for your help.

Thanks underthetire. What is the cycle time for tumbling?