Hi to all and pardon by bad english.
I have a problem with path and pattern.

I have a path that describe a circle and i need that this circle must be repeated over a line. Like this image.


Now i'm not understand why for have one circle yes and one circle not i must write this value in the spacing 42,5 <-- (what is it?)

The circle misure 12 mm and i think the right way is write 24mm of space but isn't correct. I must write a strange value in the space (42,5 of what?)

Another problem is that when i modified the path, i not have more the circle but this circle is modified like this image.


Have you an idea how i can resolve this problem?
I want that the circle remains circle and not must be another shape.
Thanks.

You can use the Scatter extension for this: http://tavmjong.free.fr/INKSCAPE/MANUAL ... ns-Scatter

Or you may try converting the circle to a custom marker -they are assigned to an 1 px wide stroke in their true scale.
Can be handy for the graphical look; add it to a path with no fill, stroke with 0 alpha value. However it may not be a solution if you need it for cnc/cutter machine.

I need this right solution for cutting cnc so it must be accurate to the tenth of a millimeter. So they have to build everything from a CAD software. Right?

Wouldn't say it is impossible but what you described in the first post is not how it works.
There you showed a pattern along path live path effect aproach.
That may rely on the bounding box size -stroked paths measure more than filled with no stroke-
and the lpe uses px values instead of mm -42,5 px is at 90 dpi, which equals 11,9944444 mm.
If you split the core path to straight segments then there wouldn't be any distortion.
Though then the spacing may not appear as expected.

In general the spacing of the object is not so trivial, could you elaborate on it?
Like, if the line breaks as on your image, at the node in the middle the sum of the distances between the two pattern's centre is correct measured on the core path but the distance between the two circle's origin is different from that.

'Scatter' will just copy the circles along the path. They will be exactly the same ones, and spacing can be adjusted.