Surface finishes N1 thru N12 and their equivalents in Ra

A. Hi Awais. We generally are more concerned that a surface is smooth enough to meet our needs, rather than that it is rough enough to meet our needs. For this reason, when we see 'grades' in charts like the one on this page and in ISO 1302, the issue becomes whether a given finish is smooth enough to qualify for a given grade.
Since an Ra 7.4-7.8 is rougher than an Ra of 6.3, such a surface does not qualify as an N9, but only as an N10.

Luck & Regards,

A. Grade Numbers Older drawings may use roughness grade numbers to indicate Ra values. The following table is given in ISO 1302 [info about this spec at Amazon; and at Techstreet]

Q. Bill,

Great chart! Is this a new ISO standard for all metal surfaces?

Thanks,

Hi,

I like your table. However, I believe there's a small mistake. The metric value corresponding to a 250 micro-inch surface roughness/finish should be 6.4 micrometers, rather than 8.3.

Best regards,

A. Hi, Henry. You can see that the chart tells you N6 = Ra of 32 microinches. So your test device which measures Ra should work.

If you search the site for "Ra vs Rz" you will see that while they tend to generally track each other, they measure different things and cannot "be converted".

Sort of like taller people tend, on average, to weigh more than shorter people but you cannot convert people's height to their weight. If you want to 'convert' from one to the other you need large safety factors in each direction, which is very wasteful. An average person of five foot height might weigh 100 pounds but that does mean that a 120 pound person is taller than five foot -- they might even be shorter :-)

Thus the same truism is repeated on one surface metrology site after another after another: Do not attempt to "convert" or "correlate" -- If you want to know the Ra, measure the Ra; if you want to know the Rz, measure the Rz! :-)

Regards,

A. Hi, Bruno. That is the symbol for micrometers, so it would be a '63 micron finish'. Good luck.

Regards,

A. Hi, Waqas. In the chart above, rougher finishes are at the top and smoother finishes are at the bottom, so N5 is a smoother finish than N10.

Regards,

Apologies, Hector, but I'm not familiar with "MPc" in this context, nor with "12S" in this context, nor with what you even mean by "convert to 118 Ra". Sorry.

Regards,

Hi Dennis. My guess is that it means 'Unless Otherwise Stated (all surfaces to be finished to) 125 Arithmetic Average'. But when we Google something and don't come up with a definitive answer, it usually means the phrase doesn't have a universally accepted meaning ... so we'd better talk to the customer or a contract dispute could be brewing. I doubt that it means 'Utterly Outrageous Standard 125 As Amended', but we shouldn't guess :-)

Regards,

A. Hi Madhusdan. What sort of components, what production volume, and what material of construction? Typically, bulk finishing (tumbling, vibratory finishing, centrifugal barrel finishing, etc.) is the least expensive way to a smooth finish because it doesn't require individual handling.

Regards,

A. Hi again. Sorry, I am not experienced with machining graphite, but a quick read said that it can be molded from different grain size, which affects the smoothness of cut, and also that it tends to fracture off, leaving pits if milled from the outside in.
With metal, a bandsaw cut is usually rough, and the faster the cut the rougher. I can only guess that you may be able to cut graphite with a bandsaw to an N8 finish but not an N6 -- but guessing is all I can do :-)
Hopefully someone experienced with machining graphite and measuring the smoothness will chime in. Good luck.

Regards,

Yes to Ted's answer. You can get a N9 finish for sure, with a bandsaw, N8 if you are using a good blade with fine teeth.
N6 finish will need to be machined.

A. Hi Melissa. It seems that ISO 1302 only goes from N1 to N12.

Luck & Regards,