Different class and grades of surface finish? Is it different?

Go to

www.mcmaster.com

search for surface finish comparator. You'll find several.

Check out the Standard Handbook for Mechanical Engineers or the Machinist handbook. Diana

Hi all,

What kind of surface finish do you mean?

I can think quickly in two types:

1. Surface finish of raw materials for further processing, like painting. This are described in SSPC STEEL STRUCTURES PAINTING COUNCIL Guide To Visual Standard No. I, Guide To Pictorial Surface Preparation Standards For Painting Steel Surfaces.
2. Surface texture achieved or required by manufacturing processes (machining)

This are described in some other type of standards like the following:

ISO 1302:2002 Geometrical product specifications (GPS). Indication of surface texture in technical product documentation

ISO 4287:2000 Geometrical product specification (GPS). Surface texture. Profile method. Terms, definitions and surface texture parameters

ISO 4288:1998 Geometrical product specifications - Surface texture: profile method - Rules and procedures for assessment of surface texture.

Best regards

The Machinery's Handbook is the classic source of information of this type ... well worth the small investment. And if you pick up an Edition that is not the latest, you can save a few bucks more.

But in the end, you are trying to understand a tactile issue, by reading about it. That is similar, but not quite as bad as, trying to explain a certain colo(u)r, in words.

The best way to truly get a grasp of this subject is to buy a Surface Finish Comparator, as TVP45 mentioned. But I'll elaborate. This is a small thin plate, sizes range from credit card size to 8"x11", that has small squares that give actual samples of each surface grade and finish. You can see it, feel it, etc. It's an indispensable gizmo if surface finish knowledge is of concern to you. They can get quite expensive if you want a sample of every combination. But a comparator that has 12 to 20 examples will cover the range pretty well. The better ones will have examples of each grade and how they look and feel when made by different machines, i.e. lathe, end mill, etc.

They can be made from plastic or metal. I think they are great!

McMaster Carr has a good selection of them.

Actually the Machinery's handbook info is abit out of step with the specs that we are currently seeing. Most jobs today are measured in Rz and the US system is "Ra or AA" depending on how old the equipment in the shop is.

Here is my standard explanation on why you can't directly translate RMS into Rz as well as a great source for info.

"Ra does not differentiate between peaks and valleys. it is the average deviation from a mean line. It's highest and best use these days would be to monitor a process over time for gradual changes in the surface finish due to changes in cutting tool wear. Instruments measuring Ra are typically more repeatable than those measuring other parameters because they are less sensitive to individual peaks and valleys.

Rz averages the mean peak to valley heights over 5 lengths.

For applications where the surface integrity is critical, R max the maximum peak to valley height within one cutoff is the best determinant.

Using both Rz and Rmax together gives you an excellent tool for monitoring variation of the surface finish in your process. SImilar values of both indicate a process that is in control generating a consistent surface finish. Significant differences between these two indicates a surface defect in an otherwise consistent surface (ie scratch or groove.)

In the old days people would multiply Ra by 1.11 to get RMS, and then someone with a beginning Stats course figured RMS= Rz cuz they are both averages. Thousands of rejected parts was their tuition... They ain't the same.

www.predev.com has a great guide on their site. You will need to register, its worth it!

milo

Hi Milo,

Wish u a great day.

I'm confused which is the latest one to show surface finish ?

Whether it's by showing 'N' value i.e, N6, N9, N11 etc. or by showing RMS value directly in the drawing .

Would anyone like to solve this problem ?

Thanks.