Ra surface finish vs. grit conversion chart

A. One (1) microinch equals 0.025 micrometer.

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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

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

A. Depending on several variables not only type of material but also type of media such as alumina, silicon carbide, organics, glass beads, etc., (the heavier and harder etch deeper), the air pressure, nozzle type, distance of blast, etc. you can have a ballpark of 190-320 microinches Ra with alumina mesh 24, from 26-32 with alumina 240, from 10-12 with glass beads No.13. This comes from standards of the plastics industry.

Bye,

A. Talk about differences. Wow, there are a lot of problems here. Not only do metals effect the finish, type of blast media, size, and pressure all have an effect on the final surface finish, plus the nozzle I.D. size and shape besides the human factor that involves the position of the nozzle angle and distance to or in relationship to the part. Other than that, no problem. Then again, most manufacturers of equipment do normally have some kind of test results that you can refer too.

A. Read your intentions and thought you better re-consider again. Your grinding results are going to be a 2 fold factor. One, will be the results of the abrasive grit of the grinding material and the second will be the human factor which is also the pressure factor. A skilled operator can do amazing things with very coarse systems almost as well as can be obtained with very fine materials.

A. Having clarified your problem I would only like to add that this is such an ample or complex situation that you'd be better off asking "the older guys" that have performed such operations for years. Pour their experience and your personal input in your standard for the rookies to follow. I think that's the best you can do.

A. Another factor I didn't see considered is the manufacturer. I have found the grit to vary on a disk or sheet from one manufacturer to the next. I would think you would have to contact a given Company and then probably use that material consistently.

A. Surface finish on parts depends on wheel grit size, but not only on it. It is practically impossible to establish stable correlation between Ra and wheel grit size. Surface finish will be also affected by part material, machine rigidity, coolant type, all the grinding parameters (plunge/traverse speeds, wheel/work surface speeds, dressing techniques, etc.) A situation is possible when a coarser wheel produces a finer finish in one application than produced by a finer wheel in another application. If wheel grit sizes need to be restricted then the whole combination of grinding techniques and parameters must be restricted also. Otherwise, production may be unnecessarily over-limited, or not limited enough, if grit size limits only are set.

Regards,

Standard

Grit   Ra in    Ra            RMS in    RMS

150    27-32    0.68-0.80     30-35     0.76-0.89

180    16-23    0.46-0.58     20-25     0.51-0.64

240    14-18    0.34-0.46     15-20     0.38-0.51

320     8-10    0.21-0.25      9-11     0.23-0.28

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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. If needed you could consult a drafting standards book like French's Engineering Drawing ⇦ on eBay or Amazon [affil links] for a fuller understanding of this kind of drafting note. Good luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi, Dow. An 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.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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A. Hi, Angela
Surface finishes are 3-dimensional, so a picture may 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,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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.

Ed. note: Thank you, Vitaly, and 'correlation' is the right word! Ra and Rz do not measure the same thing, so 'conversion' would be the wrong word  🙂

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're already there.

A. Hmmm. Saw cut could be very rough; therefore, that might not be 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 given us a chart showing Ra of 70µin for 80-grit stainless steel. Let us know what you need beyond that. Can you further explain your situation? Thanks!

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Hi Scott. He is on Linked-In, and apparently is currently with exmoorpharma.com, but we'd rather not have readers contact him if he requested that people desist, so thanks for any update if you've tried contacting him.

Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Q. 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!

Ed. note: ... better yet, since this is where the question is posed, please <send it here for posting>  🙂