Surface Flatness

[FITIN IRISH]

I just received a drawing with the note,

 

"Surface D must be flat within one helium light band or less" has anyone come across this??

[FITIN IRISH]

Ok. I just figured it out.

[3R!< W3NG3R]

just curious, but what is it?

[FITIN IRISH]

Eric,

It's a method of measuring flatness. One helium light band equals 3 microns-.0000116 inches. Seal faces(apparently) are normally lapped to within three helium light bands of flatness.

 

Never came across this before and I am 52. That's what I love about this-always learning

[Kevangel]

Does this have to do with optical flat gages that you can buy to check flatness? Never used one but saw one in a catalog, I think

[FITIN IRISH]

Yes. This is what I got.

 

The optical flat - which is a piece of transparent glass that has itself been lapped and polished on one side - is placed on the lapped surface. The monochromatic light is then shone down through the glass. The light will pass through the glass and reflect off the workpiece. As the light reflects in the gap between the workpiece and the polished surface of the glass, the light will interfere with itself creating light and dark fringes. Each fringe - or band - represents a change of 1HLB in the width of the gap between the glass and the workpiece. The light bands display a contour map of the surface of the workpiece and can be readily interpreted for flatness.

[Quickmike]

I thought 1 micron = .000040 in english.

[FITIN IRISH]

It's .00003937 Multiply by 3 =.0001179

[3R!< W3NG3R]

3 microns = .003mm

.003mm / 25.4 = .0001179

 

I like the 25.4 number as it is eisier to remember. Then I just multiply or divide using the same number.

 

The optical flatness test is cool. Love learning new things.

[calnet42]

25.4 is also more accurate than .03937.

Believe it or not!

[3R!< W3NG3R]

It is more accurate because its either 25.4 or .03937007874015748031496062992126........

 

of course, .03937 is close enough for almost everyone.