Sandblasting as an aluminum finish

[dberndt]

I'm just a HSM'er with a small basement shop, and I do work just for myself basically. Alot of my parts are small aluminum bits and pieces that are show pieces or for display/looks and I dont always want to spend what seems like 100s of hours sanding the surfaces to get a decent polish or pre-anodize surface.

So having no experience at all with bead blasting or sand blasting I'm wondering what they're capable of in terms of part cleanup. I'm looking to remove rough file marks, mill scale, light to medium/light scratches. Is this practical, what kind of results/surface finish could I expect from some sort of a blasting setup.

Also, since I dont have a decent compressor I might outsource this work if anyone does it relatively cheaply, anyone have any suggestions as to what sort of shops would have the equipment and the willinness to blast a few misc bits for a reasonable price? What sort of time does it take to do a part, measured in time per square foot or something?

[Evan]

Sorry, but sand blasting isn't an easy way out for a finish on aluminum. You could tumble them and that will produce an interesting finish. I sand blast various aluminum parts with fine river sand before anodizing black. But before I sand blast I polish them. The sand blast produces a fine matte finish and after anodizing black it comes out flat black. These are parts used in things like my telescope where I don't want reflections.

If you are only doing little bits then even a small compressor will do for short bursts. I don't use a cabinet, just do it in my unpaved driveway.

Oh yeah, if you are going to anodize after sand blasting count the surface area as double the geometric area.



[This message has been edited by Evan (edited 08-11-2005).]

[ERBenoit]

As Evan stated, if you are looking for shine, bead blasting won't do it. At work I ocassionally bead (glass 70-140 mesh) blast aluminum parts. The parts come out with a dull grey finish. They are almost like the grey background around your post on the board, maybe a bit lighter. They may also appear to be slightly shiny (reflective), depending upon how you hold them in relation to a light source. Blasting will remove the file marks, scratches etc., however the parts will not shine. I would suppose different blasting media and different grits will have differing results on specific materials. Bead blasted parts will have a different appearance after anodizing when compared to those that have not been bead blasted. You may want to try walnut shells or corn cob in a vibratory tumbler, or as a blasting media.

http://www.mcmaster.com/

Enter "Abrasive blasting media" in the search box for a comparison on blasting medias.

[Mcgyver]

what type of finish are you looking for? brushed? polished? how much automation etc.

If you are after a polished look, there are various tumblers/polishers or vibratory machines - but i think the media can be expensive. The vibratory tubs sound neat, set the part in the media and come back in day and its polished! (or something like that – never done it but its intriguing)

the easiest and cheapest way (hence what I do) is a drum sander in the drill press. Use different grits to get where you want and use a small table – better than freehand as the �brushing’ is then uniform in direction. If for some reason you want them polished, you could keep going to over 10,000 grit but you’d have a mirror before. Steel or AL finishes well this way.

The drum sander idea is good for a final brush look, but its tough to beat the humble file for taking out tool marks. Back it with some worn emery for a final finish on one of’s. This leaves an appropriate finish for tooling or model engineer stuff imo. Use progressively finer files will tooling marks out quicker than abrasives and you have a better chance of not making lumps (all the surfaces and corners rounded off). If filing isn’t working, maybe you’re not using a fine enough cut?

[tonydacrow]

Try baking soda as a blasting medium. I've not tried it yet on aluminum, but it is very soft and should leave as nice a finish as is possible with blasting.

Has anyone yet tried this? If so, what was your expirence? If not, dberndt, you may want try it on a small, unnecessary part and report back. . .

[Randy]

For a while my dentist used a baking soda blast for polishing teeth. He's gone back to the traditional rotary tool & polishing paste, though. I don't remember the specific reason, but it was some mundane practical reason, rather than lack of performance or problems experienced by patients. I thought it worked pretty well.

[BillH]

Hmm, Well watching American Chopper, I saw him do something I thought looked pretty cool.
He took an aluminum part that was already polished and applied a decal design around it. He then sand blasted the part and removed the decal. WHere the decal was, it stayed a beutiul polish finish, everything else was dulled, looked pretty cool.

[QSIMDO]

<font face="Verdana, Arial" size="2">Originally posted by tonydacrow:
Try baking soda as a blasting medium. I've not tried it yet on aluminum, but it is very soft and should leave as nice a finish as is possible with blasting.

Has anyone yet tried this? If so, what was your expirence? If not, dberndt, you may want try it on a small, unnecessary part and report back. . .</font>

Do a Google on soda blasting, you'll want to get in the business yourself!

It degreases, cleans and creates a like-new finish on even complete assemblies (bearings installed, etc.) and it's environmentally friendly.
Used a lot in chemical, food and some medical industries.

I may try it for a retirement gig!

[wierdscience]

I worked for a company where we preped literaly 1,000s of small parts for anodizing.They ranged anywhere from 3/4" square to 2x2x36".

The processes we used depended on the material condition.We used floor model 6x48 sanders with a fine mist of water and ivory soap running on the belt,the soap provided lube for the sanding action without adding oil or solvent to the polish grain two things you don't want.
If the material had a decent mill finish on it with few scratches we just glass beaded it with 180-240 mesh glass beads,followed up by walnut hulls.This left a very even satin finish that worked great for clear anodized parts.

The parts we did that were colored we used stainless steel wool and the ivory soap/water mix.

We never did any anodizing in house,we always sent it out,but it was lots cheaper if we did the prep work.

[gizmo2]

my experience is bead blasted aluminum looks great, but is every bit as fragile as a high polish. The least little bump and it leaves a shiny spot, just the reverse problem. Only reblasting will fix it.

[Your Old Dog]

My basic experiance with aluminum is be extremely careful when working with it because you could spend more time in finishing then in actually making the parts. I have baltic birch plywood vice jaws for working with soft materials. If you have heavy tool marks I find that draw filing works reasonably well.

I also like to buff it on a hard felt wheel and then strike it back carefully with Scoth Bright pads. The Grey ones seem to work the best for me. Just becarefull and take deliberate directional swipes with the pad and it will strike back the finish. this finish isn't very durable as oxidation will set in rather quickly.

It looks like hell when chrome shops buff the hell out of imperfections and tool marks on aluminum parts as the have a "weeping" appearance. I'd rather look at the tool mark the weep caused by over buffing.

There is a guy near me that does anodising and it's much cheaper then I would have imagined. You might look for one locally and get some prices. They do insist on knowing what kind of aluminum your working with as it dramatically affects the color of the finish.

[bob_s]

To achieve a nice matte finish on aluminum parts try immersing the part in a sodium hydroxide (lye soap) solution. Ordinary household bleach will also work, but it is much more corrosive and persistence time in the solution will be much shorter.

-bob

[dberndt]

I guess I should add that the finish doesnt have to be polished, even just a matte finish would be fine. The major goal is making the surface extremely consistent and free of irregularities before anodizing.

[Evan]

If you blast the parts to a matte finish and then anodize you may be initially disappointed in how they come out of the seal bath. I wipe the parts with lanolin and then wipe it off as much as possible with flannel. This leaves a smooth even flat finish.

[dberndt]

you wipe with lanolin after anodizing is what you're saying? My understanding of lanolin is that its a grease from animal fur? Similar to a wax, so what makes it special in comparison to other greases/waxes?