pcbradin = 2.5; // inside radius of cktboard
pcbradout = 5.0; // outside radius of cktboard
pcbthin = 1/16;  // thickness of circuit board
pcbthick = 1/8;  // thickness of the subtracted cylinder
render (convexity=100) difference()
{
color("red");
echo ("first",pcbthin, pcbthick);
color("red") cylinder(h=pcbthin,r=pcbradout,$fn=100,true);
echo ("second",pcbthin, pcbthick);
color("blue") cylinder(h=pcbthick ,r=pcbradin,$fn=100,true);
}
color("blue") cube([pcbthick,pcbradin,5], true);

It's a first pass into creating some 3D presentations. But it's also my
first time with the code.

I have a printed circuit board that needs to be circular with a hole in
the center. Gerber files I have handled, but I need to mount the thing
using ball bearings.

I'm failing to get a decent picture when I run the above which displays
the circuit board with a 1 "inch" thickness that just won't go down to
the 1/16 inch that's the real thickness.

I'm trying to subtract a smaller cylinder from one with an undesired
center. Without the "render" command the cylinders look OK and have the
requested colors but the smallest dimension I see is still one inch.

With the render as above all I get is an annulus with the right sized
hole but 1 inch thick. None of the colors, except for the test "cube" at
the bottom, are recognized.

Please hold my hand. I'm willing to admit I came from FORTRAN without a
date code but I like perl and ubuntu is like the old days.

Another thing: OpenSCAD displays and exports in mm. To switch to inch you
have to introduce a factor:

inch = 25.4;

pcbradin = 2.5inch; // inside radius of cktboard
pcbradout = 5.0inch; // outside radius of cktboard
pcbthin = 1/16inch;  // thickness of circuit board
pcbthick = 1/8inch;  // thickness of the subtracted cylinder
//render (convexity=1)
difference()
{
color("red");
echo ("first",pcbthin, pcbthick);
color("red") cylinder(h=pcbthin,r=pcbradout,$fn=100, center = true);
echo ("second",pcbthin, pcbthick);
color("blue") cylinder(h=pcbthick+1 ,r=pcbradin,$fn=100,center=true);
}
color("blue") cube([pcbthick,pcbradin,5*inch], center=true);

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You are perfectly right. As almost all tools downstream will interpret units
as mm, it's worth mentioning it, to prevent unexpected results.

And if you remind  this discussion
http://forum.openscad.org/strange-polygon-behaviour-in-small-scale-tp17390.html
it is not advisable to use very small values in OpenSCAD, because there is
some unexpected rounding "magic" going on behind the scenes. So it is not a
good idea to use inches for a design that is going to be specified in
fractions of inches.

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My experience is, you can mix named and unnamed parameters. But you have to
take care.
The unnamed are counted by place. You can redefine a parameter multiple
times. The last definition will win.

test(10, c=20);
test(a=10, 20);
test(10, a=20, a=30, a=40);

module test(a = 1, b = 2, c=3) echo(a,b,c);

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