This (and its counterpart on the other side) is the most important part of the whole project. Getting this part wrong, or not accurate to a millimeter means your two torus halves won't connect and won't transition one into the other. I can not stress enough the importance of getting this step right.

Using your template mark the high point of the inner curve. That would be at the top of the "leg" of our template, but only when the template is held at a perfect right angle to the plane of the bowl. That point is the "high" point of the curve, or the top of the curve, which we will create.

Measure and mark a second line at a distance equal to the radius of our half circular template. That will be the confine of our would be curve, and it also marks the low point of our curve. Now that we know the point to which we need to curve to, we can use our parting tool to clean out some material in a groove OUTSIDE of that line (toward the center of the form). Take special care not to part into the wood to more than half the width of our curve, as this means that when you will be cutting the same curve from the other side, you will part the bowl before you are done. So if our curve is 6.5 cm wide (like on my bowl), I don't part the wood here to a depth of more than 3.25 cm, I will part it to 3 cm, and so I'll know that when I will finish this curve, and the one from the other side, I will have a 0.5 cm membrane right at the center of the form still holding the bowl to the center of the blank which will remain on the lathe after I part the bowl off.

Using your spindle gouge (or bowl gouge) cut the curve away, using the template to check and see that the curve you are cutting completes the outside curve (by sliding it from the outside in along the curve). Take it slow, one fraction of a curve at a time, especially if this is the first time you are cutting this type of curve, or if you never made a perfect sphere,which will be a perfect exercise for this kind of project.



Finish up this side by sanding the face of the curve to bland it into the outside curve, and this time sand the face and outside to a finish grit (I sanded it down to 1000, and then applied pressure with shavings, burnishing the wood).