Polishing Stainless Steel to a Mirror Finish

A. Hi Christopher. My knowledge is only book knowledge, mostly from doing hundreds of postings on this topic. So we'll have to wait for someone else to really answer your question, but I can repeat what I've heard...

First, download the Metal Finishing Guidebook immediately -->
It has chapters entitled "The Science of Scratches--Polishing & Buffing" and "Buffing Wheels & Equipment" -- but I doubt that it will be available forever. Second, never use a buffing wheel with a fine abrasive if it has previously been used with a rougher abrasive; there is just no way to know that no rougher grains remain. Third, the ever-fineness of abrasives and mirror finishes can probably go on forever -- some people say they've used india ink as their final abrasive :-)

Luck & Regards,

After reading through this thread I went back and used a floppy buffing wheel (without the sewing) and a very small amount of white compound.

Using the technique described by another poster whereby I did not press too hard, but very lightly ran the wheel over the piece, the piece looks much better and quite acceptable for what I wanted. I found it interesting that there was a 'sweet' spot between pressing too hard and pressing too lightly.

Yes, there are still extremely fine 'swirl marks' but they are no where near as bad as they were before and are much less noticeable.

Thanks for the book recommendation. I have downloaded it and uploaded it for others to easily access as I didn't like having to sign up to access it. The link is here: http://matrixfiles.com/pdf/Metal_Finishing_Handbook_2012.pdf

A. Sounds like a heat problem due to lack or little lubricant or rush. The correct sequence depends on what you or your customer define by mirror, the volume of parts, their weight and surface geometry among other variables. I can speak about low volume, relatively large and heavy parts with complex and selective surfaces to be polished to A-1 (mirror class defined by the Society of Plastics Engineers close to optically perfect with a total roughness and waviness value around 2 or less microinches). You need oil or mold maker stones down to 600. sandpaper down to 1500 or better. Then felt bobs and diamond paste down to 3 microns. A very clean environment (including hands, cloths and hair of the operator). And, above all, patience...a lot of patience. For large volume of small parts you'll have to ask a mass finisher. Good luck.

? What is size of your component? Problem maybe because of over heating.

A. Add to Guillermo's list, part size and if possible what surface finish you have prior to reworking

A. Large hardware stores or those specialized on abrasives will have the stones or just type "polishing mold maker stones" on a search engine. Several well known brands (Boride, DME and Gesswein) come to mind. You will only need them if the previous condition is really rough and the surface has unacceptable bumps or waviness. Otherwise you can use flat sandpaper and scotchbrite wheels mounted on rubber cups and high speed rotary motors (you need at least 10,000 feet per min). The felt bobs are pretty much like polishing wheels (they are also known as felt buffs) but are made of non-woven wool (vegetable or synthetic fibers are more prone to over-heat or scratch the surface). Keep in mind that SS has a very low thermal conductivity so it builds up heat quite fast. Use a light oil to thin your polishing compounds and pastes if they are too heavy. Good luck.

A. Reread your info. This statement "We started with a piece of raw 12 gage plate and have slowly lowered the grit size to produce a moderate mirror finish good enough to see yourself in.", indicates to me you are blasting these parts before trying to polish. DON'T DO THAT! You also sound like a smaller shop that doesn't have access to a large mass finishing machine, so your best process is probably to work these parts with a non-woven wheel or belt system.

A. The orange peel you're seeing is due to large grain size. If possible use material which has been cold worked and not annealed. The orange peel should go away.

A. O.K. I noticed you are using 4/0 paper, we don't use that number here. Could it be proportionally around 2000 according to the depth of lines left? How close are you after that paper to a reflecting surface? Are you getting scattered pits or are they throughout the surface. In what stage do they appear or when do they become visible? Sometimes, the polishing compounds tend to produce pits due to excessive pressure exerted trying to clean previous sandpaper lines, but if you look carefully you can trace them back and find out if they were already there or you caused them. Something else, can you get diamond paste 15 and some hair brushes? Give'em a chance at moderate speed.

A. Mirror polish finish as you described is not a problem in sheet form (4'x 10') up to 11 GA or 3.0 mm thick. Ra of 1 microinch even in 11 GA material has not been a problem. However, the raw material substrate must be from the best steel mills in the world to avoid problems with pits and pinholes.

A. Mass finishing equipment can work and produce mirror finishes on parts less than 12" square very effectively and efficiently; however, over that size it becomes a little more difficult. I'm not sure you can find enough specific info about this on any website.

A. Sorry about my confusion on size. Based upon that info, I would not recommend mass finishing. Much too expensive and questionable results.

A. Dear Lawrence,

Late response though I hope it reaches you.

I feel that 12 gauge is not such an issue. 301 is also not an issue.
Please note the following;

1. Is it Hot Rolled or Cold Rolled
2. What is the finish of the plate before you start any activity. It is a must to determine where you start!
3. Mirror finish on SS should be achieved at around 3000 Grit (If you want it absolutely impeccable) I know of people stopping at 2000 and being happy. But any thing below that would be really not a great idea.
4. Did you ever explore using Engineered abrasives. If not you must. Any problems with that, ask me.
5. What is the machine you use and the type of abrasive you use? Explain to me please.
6. Are you using any lubrication or cooling media, if yes I would also like to understand that too.

Lawrence, I think Mirrorization of Stainless Steel is not at all Difficult, just that it is Different.

If you have any query, it will be my pleasure to assist you.

Ciao!

I want to thank everyone for the help and advice you have given me. We are now able to produce the mirror polished finish that we were trying to obtain. I want to share the information that we found on our quest in hopes that it will help others obtain their goals.
1) One of the biggest things we learned is not to rush through the polishing steps. If you read the previous post you will hear this over and over. Follow their advice they are right!
2) Make sure every line from the previous operation has been taken out before you continue to the next step. If you go 90 degrees to the last operation you will be able to see the lines you have not taken out yet.
3) Don't put too much pressure on your tool. We found that too much downward pressure is counter productive. It does not get the lines out faster! It actually creates larger grain lines forcing you to back up a polishing step or two just to get the grain out (It also wears belt out prematurely).
We invited 3M to come out to our facility and they recommended a product line called the Trizact series. This is by far the best sanding belts we have found to date. We put it on a hand drum sander with an air pneumatic drum (Dynisher).

We are also using Nushine II made by Nuvite for the final polishing.

Here is our sequence:
1) Sand out all defects with a 90 deg hand sander using 180 grit disk pad.
2) Go over entire part with Trizact A45 using hand drum sander.
3) Go over entire part with Trizact A16 using hand drum sander. (90 deg to last cut)
4) Go over entire part with Trizact A6 using hand drum sander. (90 deg to last cut)
5) Go over entire part with Nuvite F9 polishing compound. (Make sure ALL defects are out.)
6) Finally go over entire part with Nuvite Grade A polishing compound.
That's It!

Thanks again for all the help.