Barry Kelly wrote:

The REVOLVE{2} will always close the polygon (first to last point).
So of you want the surface back to the rotation axis you need to add a point with a 0 (zero) y-position.

i.e.

<code class="hljs">PUT 0, 0.000, 2, shellSurface,
 0, 0.5*diskWidth, 2, shellSurface,
 0.5*diskHeight, 0.5*diskWidth + lipWidth, 2, shellSurface,
 diskHeight, 0.5*diskWidth, 2, shellSurface,
 diskHeight, 0.000, 2, shellSurface

ADDZ 1
 ROTY -90
 REVOLVE{2} NSP/4, 0, 360, 4*1 + 8*1 + 16*1 + 32*1 + 64*0 + 128*0 + 256*0, shellSurface, GET(NSP)
 DEL 2 

Barry.

That worked perfectly, thank you Barry