Ra surface finish vs. grit conversion chart

A. One (1) microinch equals 0.025 micrometer.

A. Danforth,

Here is a copy of the table that we use for stainless steel surface finishes in the pharmaceutical industry. This is included on a fact sheet of useful information for process engineering. If you want a copy, please send me an email, and I will reply with a copy for you.

Surface Finish of Stainless Steel, Ra vs US & UK Grit size.

US Grit   UK Grit   Ra    Ra 
ref       Ref.      µm    µ inch
          120       3     125
          180       2      85
80                  1.65   70
          240       1.50   50
          320       0.75   30
180                 0.62   25
240                 0.45   18
          500       0.40   15
320                 0.25   10

Andrew Provan
Bovis Lend Lease Pharmaceutical - Manchester, UK

Excellent! I am still getting one or two enquiries every week on this, despite this posting being some 6 years old.
Please continue to send me emails, and I will respond with either a PDF version of this chart and other useful information...or if you would like a plastic laminated version, let me know your address, and I will put it in the post for you.
Cheers
Andrew

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Ed. note: Potential advertisers might take a hint :-)

A. Hi, Gaurav. It's not exactly a square root sign, it's a finish mark sign, which looks half like a check mark and half like a square root symbol. Lacking any other notes it probably means 32 microinch RMS, I think. You could consult a drafting standards book like French's Engineering Drawing for a fuller understanding of this kind of drafting note if interested. Good luck.

Regards,

A. Hi, Dow. The approximate answer is already on the page. Ra25 = U.S. Grit 180 and Ra70 = U.S. Grit 80. You can interpolate to a reasonably close number I think. Good luck.

Regards,

A. Hi, Angela
Surface finishes are 3-dimensional, so a picture will not do much good. Rather, you can buy a "comparator"; this is a sheet of electroformed metal which is divided up into like a bingo board, with one area finished to 100 grit, and with other grit sizes above and below for comparison.

Good luck.

Regards,

A. Hello Tim,

Have you already found the information about Rz parameter?
This one is typical for old designs based on GOST 2789-73 "Surface roughness. Parameters and characteristics".
Following this standard, Rz is the height of roughness by ten points. On a base length one can find 5 profile maximums (highest lugs) and 5 profile minimums (deepest cavities). Rz is 1/5 of the sum of these.
There are following correlations between Ra and Rz depending on the processing method:
for turning: Rz = 5Ra;
for grinding: Rz = 5.5Ra;
for polishing: Rz = 4Ra.

A. Before anyone can answer your question, we need to know what you are starting with. Most mill stock metals are finished to between 32-36 RMS, so unless you did some machining, you are already there.

A. Hmmm. Saw cut could be very rough; therefore, that is still not enough info to give you a one step correction. My guess is that you might want to start out with a 60 grit belt. You will end up with either an 80-90. Maybe 120 as the finest.

A. Hi Kevin. Andrew has already given us a chart showing Ra of 70µin for 80-grit stainless steel. I'm not quite understanding what you are looking for beyond that. Can you explain your situation? Thanks.

Regards,

Hi Scott. The implication of your posting is a bit vague. Are you implying that the posted e-mail address now bounces, or he asked you to stop bothering him? He is on Linked-In, and apparently is currently with exmoorpharma.com, but I don't want to contact him or have others contact him if he requested that people desist :-)
Thanks for any update.

Regards,

Ted,
When I emailed I got an error screen that said the email doesn't exist. I just figured that since it was so old that maybe things had changed.
I am still trying to locate a copy of that chart though and any help you can provide will be greatly appreciated.
Thank you,

Q. This thread is quite old, and Andrew Provan is no longer reachable. But evidently there is an ongoing need for this information. If anyone has the document, could they please post it to Eng-Tips.com? Metals & Metallurgy section.

Thanks!

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Ed. note: ... or just e-mail it to