It might make the metric to english conversion a little less confusion if you show him where "40" comes from. If you show the mechanics behind your value he has a better chance of catching on.
40 is just a rough value from metric to english conversion.
Let's first look at the wording of the formula:
micro "meters" to micro "inches".
We all probably know that to convert millimeters to inches you divide millimeter values by 25.4. Ie.. 1mm = 1/25.4in or .03937.
And to go from inches to millimeters you go .03937x25.4 = 1mm.
You see how that .03937 looks alot like .04? Yup, .04 is just rounded just like 40 is rounded from 39.37. See where I'm going yet?
Now since the wording I pointed out in the beginning is "micrometers" and "microinches" you may see why our conversion is .03937 instead of 39.37.
The difference is micromillimeters and micrometers.
1 meter is 1000 millimeters.
The surface formula calculates micrometers to microinches, not micromillimeters to micro inches.
So to convert 3 micrometers to microinches here is what we do:
convert micrometers to micromillimeters. 3 micrometers = 3000 micromillimeters. 3 x 1000 = 3000.
next convert 3000 micromillimeters to microinches. 3000/25.4 = 118.11 microinches.
Using the correct formula instead of an abreviated formula or rounded number is much better. It's kinda like when machinists try to calculate sfpm using the value 3.82. Most never remember the formula because most don't know where 3.82 comes from. They don't understand the mechanics. So they can never do the math.
It depends on a method of surface finicing, milling, drilling, eroding or lathing Rz is the heigt of the tallest "bump" on surface Ra is the avarage heigt of all the bumps on surface. And as we all know there is no direct link between avarage and max. Allthoug if u know the method u can derive something from it using a sample of surface finicing.