The authoritative public forum
  for Metal Finishing since 1989
  

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Diamond Polishing Stainless Steel to Optical Precision

Q. I am polishing optical surfaces on to (17% chrome, 4% nickel) stainless steel. I have a problem with achieving a mirror finish.

The surface is initially cut on a high precision CNC centre lathe which gives a finish of about 0.10 microns rms (PRa) and about 1-2 microns total (PRt), over 6

I am looking for a mirror finish better than 0.015 microns PRa and 0.5 microns PRt.

The polishing starts with a 25 micron (rough) diamond paste and a soft lap. This removes lathe lines and blends the peripheral (non-optical) area. I then progress to 6 micron (medium) diamond paste with a hard lap. This recovers the optical surface and removes the worst of the scratches caused by the 25 micron polish. The scratches that remain are then removed using 0.35 (fine) micron paste, first with a soft lap and then a hard lap.

The problem I have is that the optical performance degrades if I polish for long enough to remove the scratches from the previous processes. I am also getting pitting, it would appear, from the 6 micron and finishing polishes.

Is it the case that I am more likely to introduce pitting by using a finer polish?

Will a finer polish remove more material (logic says no, but it has been suggested to me that it will)?

Am I right in using a diamond polishing compound given the chemical affinity between stainless and diamond?

Jim Curtis
- U.K.
1996


1996

A. Diamonds under certain conditions, water and pressure react with Iron to form Iron Carbides. In fact many diamonds are "cut" this way with a spinning wet iron wheel. These process-formed iron carbides may be what is ruining your final polishing operation as they are larger than your polishing compound and harder than your stainless steel. To solve your fine finishing problem, try another superabrasive, made from boron nitride, or a synthetic ruby type. A switch to a non water type lubricant/carrier like olive oil or pure glycol may also solve this problem. Place a magnet in your used slurry and see what you pick up, analysis of this material can help you solve this problem.

The Pitting is the "Big Clue" that you have a chemical reaction going on...

Gordon Smith

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Restoring an A-1 mirror finish by hand with light scratches

Q. I am working to restore an A-1 mirror finish to a stainless steel sculpture. I usually work on aluminum and painted surfaces. I was able to take out the small scratches in the surface with 2400 micromesh, and then worked my way up to 12000 micromesh, and then used Metall polish, p37, p100, and then polychrome polish for the final polish. I used a round polisher with foam pads. This has left the surface shiny at first glance, but it is not optically clear and many small scratches can be seen in it. I've read through many of your posts, if I use felt bobs on a drill with diamond paste, will I be able to get the mirror finish back?

Lucinda Davis
Sculptor/restorer - Chattanooga, Tennessee, USA
August 28, 2016