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What is a 400 grit finish?


Q. My company manufactures stainless steel pumps for the food and drug industry that currently meet 3A surface finish requirements and have 3A certification. The exterior of our pumps are polished to a #8 finish (4 Ra or best effort) for aesthetics, while the interior meets the required 150 grit or 32 Ra finish. I am "desperately" looking for a reference chart for polished s.s. which compares the polished finish # with the equivalent Ra measurement and grit finish. We have a potential large order for an end user in Japan that is wondering if our pumps meet their JIS standard of a 400 grit finish. However, I do not have any reference material, other then that supplied under the 3A certification, which is vague. Nothing I have tells me what a 400 grit finish is equivalent too? Are you aware of any technical publications or material that would have the information I am looking for. Please forward any info that would help us. Thank you.

Ken Smith
pump mfgr. - Grand Terrace, California


A. Ken, This is a complicated question...
400 grit of SiC has a particle size of 20-23 microns,
400 grit B4C has a particle size of 16.3 microns and
400 grit diamond has a particle size of 40 microns.
320 grit finish for polished steel is 4-16 microinches while a
320 grit finish is 10-32 microinch. I would say a
400 grit is from 16 to 20 microinches.

Donald M. Mattox
Society of Vacuum Coaters
Albuquerque, New Mexico

Ed. note:
            Mr. Mattox is
            the author of -->


A. Ken & Don, I spoke with my associates. There are some many different standards in the world. A 400 grit finish in one part of the world is better than a 600 grit finish in another part of the world. We actually encounter this! I sometimes wonder if the polishing compound is the difference. The best case would be to ask the customer to send you a sample and have it analyzed in a lab. This is probably the best way to match Ra requirements. IMHO Mike

Michael Liu Taylor
   specialty stainless steel distributor

Dallas, Texas

(2004) -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Dear Sir ,

We are a Indian based company & in the line of pharma equipment manufacturing. All our products are finished to 4b & 2b. One of our clients asked for 200 grit surface finish. Can you help us by providing the co-relation between 2b, 4b, & Grit. I mean 180 Grit is 2b or 4b.

Ravi Samant
pharma equipment design - Mumbai , (MS) India

September 15, 2008

Q. Dear sir
Ours is Dairy industry going for expansion. And all our S.S equipment finishing should be minimum 200 Grit. But we are desperate to know how to measure the Polished surface, or is there some kind of meters available in the market to tell us up to what standards the surface is polished. Or is there any indirect method to know the quality of the finished surface.
Please Advice

With Regards,

Chandan Patil
Project leader - Pune, Maharashtra

Can ultrasonic cleaning give 400-grit finish?

June 21, 2013

Q. Hi,
I am working in quotations and I have a drawing that requires a 400 grit surface finish on a SUS 304 Hollow Tube? Can ultrasonic cleaning get the job done?

Please advise.

Henry Wilfred
- Malaysia

June 29, 2013

A. Hi Henry. To my knowledge ultrasonic cleaning is only a liquid process that removes dirt. To get a grit finish you need to move metal, so you must do some sort of mechanical finishing (sanding or blasting). Are you looking for the 400 grit surface on the interior of the tubing? If so, please provide the diameter and length, and hopefully a reader can make a suggestion for that situation. Thanks.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

What Ra is a #250 buff?

September 9, 2014

Q. Sir,

My client wants to do buffing their 3 inch seamless pipe by length 3 meters. Their required buffing is #250, what is the equivalent in Ra?

Thank you and waiting for your prompt reply.


bienvenido baysa jr
- johor bahru, Malaysia

September 2014

A. Hi Bien. Assuming that "#250" refers to grit size, and I'm not sure it does, two readers have told us that there are multiple grit standards, so it's difficult to relate to Ra; on top of that, there is Ra in µm and in µin.

Letter 9842, "Ra surface finish vs. grit conversion chart", suggests that a USA grit size of 240 is equivalent to an Ra of about 0.45 µm, while a UK grit size of 240 is equivalent to an Ra of about 1.5 µm.

But with this as background, I think you'd better ask your customer what Ra they want :-)

Luck & Regards,

Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

June 5, 2015

A. Hello,

My experience voting on sanitary standards at 3A, and years of engineering experience I have learned that...


ISO No. = N5-N4
RA (µm) = .23
RA (µin)= 9 USA
CLA(µin)= 8
RMS(µin)= 10
Rt (µm)2= 1.3

so your answer would be a 9Ra finish = 400 GRIT


The progressive process of metal polishing and grit identifiers are not so straight forward.

Medium grit belts of 100-120 grit belt can remove 1/32-1/64 of material but they may not produce a 120 grit finish.

The type of sander that you use along with speed and feed adjustments will effect surface finish. You could use a 3-headed sander of 100-120-150 and not always produce a perfect 32Ra finish.

This is why I dislike using GRIT as reference, And of course let's not forget the grain standard.

Sanitary equipment engineer - boligbrook Illinois, USA

November 9, 2016

Q. Hi, our client has asked us to give a BUFF#200 finish to SS304 tubing, supposedly this is a Japanese standard, and I'm looking everywhere for something that is equivalent in America (I've found that this standard is in between a 320 grit and a 400 grit), also, can someone explain to me if this process means I need to polish first with a smaller grit then move up to the one I need?, help is greatly appreciated.

Maximiliano Gonzalez
- Monterrey, Mexico

November 2016

A. Hi Maximiliano. After you have enough confidence to guess what the customer might want, and you finish the tubing accordingly ... if the customer then tells you: "No, that's not what I wanted" -- what then?

I say you can't take possible customer slang like "Buff #200, supposedly Japanese" and put a fine point on it. I think you need to tell your client that you are unsure what they want and need clarification. But that's just me, a former finishing consultant retained quite a few times to mediate costly disputes over stuff like this :-)

Good luck, and Regards,

pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

February 5, 2017

A. Hi Maximiliano,
You are correct! Buff#200 is a Japanese callout for 12 micro-inches Ra. Data: Buff#200 is equiv. to 42 µinch Rmax. Buff#200 is also equiv. to approx. 13-14 µinch RMS. In metric terms: 0.3 µm Ra. 1.5 µm Rmax.

blake kneedler
Blake Kneedler
Feather Hollow Eng.
Stockton, California

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