Ra surface finish conversion chart

I am looking for a conversion chart from English to metric for Ra surface finish values.

One (1) microinch equals 0.025 micrometer.

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,

Thanks for posting the chart. Can you tell us the source of this chart. Also, what about stainless steel mirror surfaces that has a Ra of 1 (ONE) microinch? What are the corresponding US Grit and UK Grit numbers?

Thanks,

Michael Liu Taylor    specialty stainless steel distributor Dallas, Texas

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

Andrew Provan
Bovis Lend Lease Pharmaceutical - Manchester

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

March 31, 2008 I would like to know the equivalent finish of root of 32 Gaurav Dodia Production Engineer - Pune, Maharashtra, India March 31, 2008 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, Ted Mooney, P.E. RET finishing.com Brick, New Jersey April 1, 2008 Hello, Thanks for the chart, it really got me rolling with one of my current projects. Do you have any information regarding the difference between valley's in steel = x Ra versus peaks on the surface = x Ra? thanks Dan Sperfslage - West Bend Wisconsin

Does someone have a chart of Ra32=US grit?
thanks

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,

Ted Mooney, P.E. RET finishing.com Brick, New Jersey

Hi

I'm looking for an example of a 100 grit (number 3) finish for stainless steel. Can someone upload a picture and draw some equivalent for me?

Thanks,
Angela

Hi, Angela
Surface finishes are 3-dimensional, so a picture will not do much good. Rather, you can buy a "comparator"; this is a small piece of sheet metal looking something like a bingo board, finished with 100 grit, and with other grit sizes above and below for comparison.

Good luck.

Regards,

Ted Mooney, P.E. RET finishing.com Brick, New Jersey

I am working on quoting a machining part that is currently being made in Europe.
The problem is the print info. is all in another language. There is a symbol for a microfinish that the line goes up and then straight right and under the line there is RZ 25.I have been looking all over the internet with no success. Does anyone have any ideas on what it means or where else to look.

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.

February 28, 2012

Q. What is the surface roughness achieved by using 320 and 400 grit abrasive belts on steel and chrome?