I made a 1mm wide circle on one layer with an inside diameter of 9mm, then another 1mm wide circle inside of it with the outside diameter of 9mm. They line up correctly in the four quadrants but at any other angle the circles do not line up. It looks to be about 0.05mm difference. My question is how do I get the two circles to exactly line up so that the inside diameter of one matches the outside diameter of the other?

This was an experiment because I'm making a circular circuit board and have been trying to place components and wires in a polar fashion and they don't seem to be lining up. I've been using the command line with the polar function to place things. I've also tried lining it up in one of the four quadrants, selecting everything as a group, and rotating everything to the angle I'd like. It's still off about 0.05mm. As small a difference as this is, I'd like to know why it isn't exact and why, when I place something in one of the four quadrants, it is exact.

 

If you haven't yet, turn Snap To Grid OFF when placing polar-referenced components.

You might be against the resolution limits of Eagle in mm. Try setting the grid to 'mic'rons.

Keep in mind that Eagle uses the CENTER of the traces as a reference. If you want a 9mm OD circle with 1mm trace width, the circle 'diameter' has to be 8mm from circle center to the mid-width of the circular trace. Adding 1/2 width all around yields 9mm OD.

I did a circular board a few Eagle versions ago and used AutoCad to locate the radial components' and traces' coordinates then placed them using absolute XY read off the .DWG (the Eagle facilities in that version were lacking in some of these areas.) Worked fine but once laid down, pushing circular traces around the the board was problematic.

EDIT: I just did this on a scrap project. With the grid set to 1mm / .1mm, I dropped a circle (without regard to position or diameter) onto the top layer then used INFO to set the origin, trace width and diameter. Did the same on the bottom layer, 1mm less in diameter. The outer edge of the smaller circle matched the inner edge of the bigger one perfectly. I placed a track on the top layer to monitor rotation then used group rotate to slew it around (make group, left click-hold and drag). Worked fine and at big zooms, the circles remained concentric.

 

I originally was using Rhino to get my x,y coordinates. Transferring those numbers into Eagle alerted me that something was off a bit.

Before posting here, I did a ton of experiments to see if maybe I was doing something wrong. The last experiment I tried was placing a circle in a circle and this was really telling. In theory, the inner diameter of one circle should line up exactly with the outer diameter of the other circle. What gets me is that they do line up exactly, but only in the quadrants.

 

Here are the details. If anyone out there wants to just input these numbers in Eagle, you can see what I mean. It doesn't seem to matter what the grid is set to (mm or inches).

Inner circle -Width 1
-Position (10.5 10.5)
-Radius 9
Outer circle -Width 1
-Position (10.5 10.5)
-Radius 10

 

So I did it again using the same values (width 1, radii 3 and 4) with the same result. Then I used 8 and 9 as radii and got some flat spots in the onscreen rendering that caused gaps between the two so it looks like some Eagle thing. I don't know exactly why that is, maybe even number of polys on one and odd number on the other when drawing so you might be able to set the polygon drawing to a finer scale i.e. increase the number of segments used to draw curves/circles.

Looking at the Gerber output (with GC-Prevue) showed the two polygons used to approximate the circle in copper. With a finer polygon scale in the PCB, you'll get better circles at the expense of larger files and more plot time.

Beyond that, I can't add much more only that I've never given it much thought beyond the defaults and have had no issues when fabbing the PCBs.

EDIT: Tried it with your values from #4 (after originally posting this) and I get the same thing as you. I think its an Eagle
artifact in drawing the circles on the screen.

 

Thanks, John.

I guess I just wanted to know if it was me, if I was doing something wrong. I think I'll just go with the "close enough" coordinates. (I hate doing that, I'm an exact kind of girl.)

 

In layer setup for the top and bottom layers, make the fill style an outline (top left box) for each layer. You'll see that the edges of the circles now line up perfectly.

(I hate doing that, I'm an exact kind of girl.)

I've been married to that kind of girl for going on 30 years. Its a good way to be.

 

Oh my, that worked. I would have never figured that out.

I wonder why that happens with the fill style and not the outline style. I guess it could be because I'm using version 5.11 which is 32-bit. There is a newer version that's 64-bit.