This was generated by Mathematica using the following code:
f[u_, v_] := {u, v, u^2 - v^2}; scale = 40; radius = 0.75; numPoints = 24; gridSteps = 10; curvesU = Table[scale*f[u, i], {i, -1, 1, 2/gridSteps}]; curvesV = Table[scale*f[j, v], {j, -1, 1, 2/gridSteps}]; tubesU = ParametricPlot3D[curvesU, {u, -1, 1}, PlotStyle -> Tube[radius, PlotPoints -> numPoints], PlotRange -> All]; tubesV = ParametricPlot3D[curvesV, {v, -1, 1}, PlotStyle -> Tube[radius, PlotPoints -> numPoints], PlotRange -> All]; corners = Graphics3D[Table[Sphere[scale f[i, j], radius], {i, -1, 1, 2}, {j, -1, 1, 2}], PlotPoints -> numPoints]; output = Show[tubesU, tubesV, corners] Export["MathematicaParametricSurface.stl", output]