Thank you for the clarification Michael, I wasn't aware NURBs curves don't actually pass directly through control points.
That explains it.

Is there at least a way to force the flow tool to preserve position of control points in the Z axis?

Hi geekmidget,

re:
> Is there at least a way to force the flow tool to preserve position of control points in the Z axis?

Not for points internal to the curve.

But if you cut the curve into pieces, I think the start and end points of each piece would be preserved.

Also the fitting tolerance for flow has been tightened up in v5 so you may get better results there.

- Michael

I will try both ways, thanks!

Splitting the progression slope curve in 3 pieces didn't help much with precision, I will try later today with a radial array of 90 pieces and see if it makes a difference then. Flow tool in V5 behaved more or less the same as in V4, I didn't notice particularly better tolerances. Helix should have 32.5mm in diameter, but when selecting it dimension was between 32.46 and 32.4999, I guess small roundings per each interpolation point add up at least in this particular case.