Sanding
Sanding is
done to make a surface more consistent or to remove machining marks. There are a variety of sandpapers ranging
from a large grit (low number i.e. 60) to small grit (i.e. 600). The two main types of sandpapers used in the
PRL are wet and dry. Both oil and water
can be used with wet when manually sanding.
There are tons of sanding techniques depending on the attended shape -
talk to a TA. A good rule of thumb for
sanding: if you are sanding for more than 5 minutes without much progress, move
to a coarser grit. Then work up with
finer grits, to remove sanding lines.
Brushing,
Polishing, Grinding, & Buffing
Brushing is primarily used for deburring and
removing rust and scale (after welding).
Polishing is a wearing down
process using abrasives or buffs. It is
more aggressive on surfaces than brushing and leaves defined lines on the
surface. Chemical polishing and
electro-polishing will not leave lines.
Grinding is hardcore wheel
sanding for metal. There are die
grinders in the foundry, but be careful - material comes off quickly. Do not
use the grinding wheels on Al (ruins wheels). Buffing is a final
surface finishing process that produces a smooth, bright surface finish with
minimal lines.
Anodizing
Anodizing is
electrolytic treatment of metals that forms a thin coating on the surface of
aluminum through controlled oxidation. Anodizing can be regular and hard;
voltage and current are turned up for hard leaving a stronger surface. Addition of color to regular or hard weakens
hardness; clear (grayish) and black are the strongest. Depending on the type of anodizing and the
part’s intended use, anodizing will add a thickness anywhere between
0.0002”-0.001”*. Don’t polish aluminum before it is anodized
unless you want a splotchy finish.
Powder Coating
Powder coating
is a dry painting process in which powder particles are applied directly to the
surface to be coated without solvents or water. The process starts with a preparatory phosphate steam bath, the
object is suspended for the electrostatic application of powder, and then baked
in oven at 350°F. Whereas anodizing
seems transparent with the aluminum visible through the finishing coat, powder
coating is more like applying a layer of paint. Powder coating adds a thickness of 0.002”* and comes in lots of colors.
Plating
Plating adds a
thin coat (~0.0003”*) of another
metal (i.e. zinc, gold, silver, nickel, etc.) to the surface of your part. Your workpieces are cleaned, put into
ionized plating metal solution and circuit is completed to add the
material. Electroless plating (Ni) leaves a smoother more even finish and
better hardness.
Spray Paint
For metals,
there are lots of anti-rust spray paints in multiple colors (A good brand:
Hammerite) - read the label. ABS can be
painted, but Delrin can not. Primer
might be useful for less porous materials.
Some primers can be sanded - check the label. Make sure your part is clean before painting. Also - BE PATIENT! Allow each coat to dry - better many light coats then a few
uneven ones. To avoid hot spots, start
and finish the spray off of your piece.
Patina
Patina
produces a surface finish produced by oxidizing the surface of a metal. A variety of colors and textures can be
obtained by applying a coat of metal paint and then oxidizing it with the
oxidizing agent. Practice before
applying to your part!
Special finishing
There are many
other processes and treatments that are available (Teflon coating, heat
treatments, etc.). Talk to PRL staff and
check the phone book.
* Finishing
thicknesses depend on vendor and application.
If you need to know a thickness for part design - CALL THE VENDOR - do
not depend absolutely on the numbers given in this handout!
Ver 1.0 10/97 Krista Donaldson