Hi YHWH_777,

> but it puts the curve on the z axis plane and not on the
> surface of the sphere.

The way that arc drawing command works is that it orients itself to the z axis of the construction plane.

The construction plane by default is set to be aligned with the world z axis plane, but you can alter the construction plane to the orientation you need first before drawing the arc to control the arc's orientation.

You'll need 3 reference points to set the construction plane - one at the origin of the sphere, and then the 2 points on the sphere.

Now hide the sphere, because it can easily get in your way with the following steps trying to snap on to it in a couple of ways that you don't want.

Then run the View > CPlane command to alter the cplane - set the origin point of the cplane at the sphere's origin point. Then grab then grab the x axis line of the axis triad widget, hold down the mouse button and drag and release it over the first point - now the cplane has its x axis aligned to that point. Now drag the y axis line to the other point - this will rotate around the xaxis line so that you will be at the proper orientation that you want. Now right-click to tell it that you're done manipulating the axis widget and want to place the cplane. Then you can draw an arc as you wanted. Afterwards, right click on the View > CPlane command to reset back to the default cplane.

It looks like this:




You want to hide the sphere because otherwise things will try to snap on to the sphere's arc seam edge when you snap to the center point there, and the orientation picker will try to align itself to that plane of that arc edge.

- Michael