Letter 5024

Mirror Finishes 

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I am currently design a jig for a vision system. Because of the nature of the product and space constraints, I had to make my mirrors from machined parts rather than using commercial available ones. I had specified a mirror finish on some surfaces of my parts. Unfortunately, the local vendors have not been able to meet my specifications of an optical grade reflective surface.

Just like to know the following:

1) Are there any recommendations for the type of material I can used to machine my parts to increase the reflectivity?

2) What are the recommended finishing that can give me the desired results? Will electro polishing + Chrome Plating do it? Thanks a lot

Kevin Tan
- Penang, Malaysia

Kevin, What type of machined parts are these? We do CMP mirror polished stainless steel sheets and sanitary fittings. Ra is below 1 microinch on sheet products while Ra is between 5-7 microinch on sanitary fittings. Substrate is stainless steel without coatings. Reflectivity # I'm told would be better with coatings. Cheers Mike

Michael Liu Taylor    specialty stainless steel distributor Dallas, Texas

Kevin:

A great deal depends on the size, shape and metallurgy of the parts you are working on. Very low Ra finishes can be developed on a variety of metals utilizing successively finer abrasive materials in sequence.

If your parts are random or non-flat in shape using a loose abrasive method such as centrifugal barrel polishing can produce low single digit Ra microinch surfaces with high reflectivity and image fidelity. These kinds of process may not be appropriate if the surfaces you are trying to work have larger broad flat areas. If this is the case you might want to consider precision lapping.

David A. Davidson
- Bartlett

I don't think chrome is used much as an optical surface because of its high absorptivity. Silver plating or ideally rhodium plating would be better, or aluminum sputtering; or you might make the mirror from chemically polished or electropolished aluminum.

 
Ted Mooney, P.E.
Finishing.com Inc. - Brick, NJ

There are lots of site that I have visited that give step by step instruction on how too.... Don't we all feel that in this community we should have a similar page where all the contributor input their expertise. Finishing companies usually require large and repetitive orders leaving the small batch finishers in the quandary. Any suggestions.

Thanks,

Haider Bilgrami
Insi - Singapore

We have more than 12^⁴⁵ thousand topics on line here, Mr. Bilgrami, and we'll be happy to take any thread as deep as you want. You start :-)

But our experience is that, no matter how much willingness to share, the web is simply not a substitute for books, because a topic cannot be presented in orderly fashion without the years of planning that book writing involves. Just cogently writing the book is almost always a far bigger job for the author than accumulating the information.

Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

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