Page 28 Modeling a Tank: Thingers

Hit BlendSrf and select the edge of the turret and the edge of the sensor. What you will find out is that the edge of the sensor surface is split in half. What you need to do is escape from the BlendSrf command and then use the MergeEdge command on the back edge of the sensor. Hit BlendSrf and change the 'Degree' option to 'Quintic' and from the right view drag the seam points to the top of the sensor. If the BlendSrf doesn't work change the absolute tolerances to .1 or more. Result shown below.

With the surfaces unjoined the seams where the surfaces meet will show when rendered. Select all the turret parts shown highlighted below and hit JoinSrf. Rhino will only join surface parts that are capable of being joined.

Select all the same objects and ChangeLayer them to the turret layer.