These are tips on fixing problems with NURBS models.

• If you create a surface but it isn't visible in viewports, click Flip Normals. Flip Normals is available on the surface's creation rollout, or at the Surface sub-object level on the Surface Common rollout.
• If you create a blend surface and it looks like a bow tie, use Flip End 1 or Flip End 2 to correct the twist.
• If a CV curve gives you unexpected or incorrect results, try reparameterizing it. Click Reparam. at the Curve sub-object level. This button is on the CV Curve rollout. In the Reparameterize dialog, choose Chord Length reparameterization.

  If the curve still gives you trouble, try rebuilding it. The Rebuild button is on the same rollout.

• If a blend between a surface and a curve gives you unexpected or incorrect results, try reparameterizing the parent surface. Click Reparam. at the Surface sub-object level. This button is on the CV Surface rollout. In the Reparameterize dialog, choose Chord Length reparameterization.
• If you see a seam in a shaded viewport, render the viewport first before you try to fix the seam. What you see in viewports might not be what you get in a render, and the viewport shader is less accurate than the production renderer. Seams in viewports can also result from different surface approximation settings for the viewport and the renderer, so check these as well.
• If you see gaps between faces in the rendered model, increase the Merge value for the renderer in the surface approximation settings.
• Sometimes gaps between faces appear after you convert a NURBS model into a mesh. (For example, by using Mesh Select.) If this happens, increase the Merge value for the renderer in the surface approximation settings.
• If you see odd twists in a 1-rail or 2-rail sweep, add more cross sections at the areas of change in the surface. For example, if your rail looks like a box with rounded corners, placing cross sections at the corners helps to control the shape of the sweep. On the other hand, you don't need more cross sections for a rail shaped like an 'S', because the curvature is more constant.
• If a U loft or UV loft doubles back on itself unexpectedly, make sure that all the curves are going in the same direction. Click Reverse to change a curve's direction. Use the Start Point spinner to align the curve's initial points.