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Pitting


#1

I have been trying to mirror-finish some 2x3 1/2
inch 14g copper pieces. First with 400 paper,
then 600 and all to way to 4/0. Then black emory
through white diamond. This is about the time
deep little marks appear in the copper. The
lines appear to be in the axis of the polishing
wheel. Being a large piece it is easy to get it
really hot.

I suspect the heat generated during the white
diamond phase is pitting the metal.

Has anyone else experienced this problem?

Bill


#2

Try cleaning the metal w/steam and ultrasonic after you sand, perhaps that
may help the pitting, then look w a loupe.

Rael


#3

Bill:
Sometimes when a metal is polished to a very high finish, excessive
polishing times and/or pressures on the wheel will cause the base metal to
pull out of the matrix causing “polishing pits”. Minimizing time and
pressure will minimize these pits.

Lee


#4

Lee,
Thanks for the directions. I will repolish
the copper pieces with less pressure
and keep the heat down as well.
Bill


#5

Hi Bill & Lee, I’m sorry to butt in, but…there are no such things as
polishing pits. There is such a thing as very slight porosity, which is a
casting problem. It is true that you can “minimize” this with a light
handed polishing technique. People used to polishing just sheet, wire and
rolled stock will need to learn this when dealing with cast metal,
especially sterling silver. I teach a workshop called Designing For
Casting that deals with the nature of casting, what happens to the metal
during the process, and how to design problems out of the piece before
making the model, thereby realizing the advantages of reproduction by
casting without adding finishing problems (extra sprues, porosity) J.A.


#6

Well, I can see this may get more interesting as time goes on.

John, the metal that is giving me problems is 14 g sheet copper.
Since I bought it at the scrap yard, I don’t know what kind of
copper it is. I would guess it is a cold rolled oxygen free material.
Other kinds of copper are pretty scarce. It exhibits no porosity.

After all the papers are used (600 through 4/0),
I use white diamond then red rouge. The piece is 2 x 31/2.
It is real easy to get the piece too hot to handle.
It seems to pit if I bear down on the plate
and let it heat up.

To remove the pits I’m back at the 600 paper routine again.

Have a nice weekend all.

Bill


#7

Hello William, If you are polishing the copper with a buff on an electric
motor, either slow down the motor or use a smaller buff. Either of these
will reduce the heat, which is [I think] causing the pits. Good luck. Tom
Arnold


#8

To remove the pits I’m back at the 600 paper routine again.

This may be of no use, but worth a shot.

I have seen something similar to this if there is moisture on the metal,
buff or hands of the polisher. If the pits are very round and deep, this
is probably not the problem though. Linda