parameter flips direction after setting to zero ??

Hello all

I am developing a parametric model of a precast stair using Inventor 2013. I have set a parameter call "rake" to define the rake distance of the stair (the distance the nosing of each tread protrudes over the tread below - usually about 25 mm) This is initially set to 25 mm in the sketch.

If there is no rake, I set this to 0 mm, and this gives a perfectly horizontal & vertical stair profile.

If I then set this back to 25 mm, Inventor flips (reverses) the direction and the profile produced is incorrect (it is like a negative rake angle).

I guess it is because when I set the rake to 0, the two points are directly in line vertically, and when I set it to 25 mm again it pushes them 25 mm apart horizontally, but in the wrong direction.

Is there any way to constrain these parameters or points such the distance is measured only in the desired direction, or to stop one point from going beyond the vertical position of another point.

Any help on this matter would be very much appreciated.

Many thanks in advance.

I don't have time to find a previous discussion or work up a solution, but as I recall someone else had a similar problem that was solved by using a radius parameter on a construction circle (don't get confused on this - as I recall the radius didn't go to zero - it worked the other way around).