Construction lines on a curved surface

[jmstovall]

Got another one that I think is straightfoward, I’m just lacking knowledge.

I’d like to place an object dead center on the “bottom” of this object:

But I can’t get a sketch to “stick;” it’s not allowing me to put a plane on a curved surface so that I can draw construction lines to guide me.

For context, the goal is to put a simple block on the bottom of the cylinder that aligns to a cutout on a base (not shown) to make a traincar for my kid.

Thanks in advance!

[Lout]

One way would be to add a construction plane to the end of your cylinder, rotate 90° and then lower to the base of the cylinder

[jmstovall]

Yup, that’s what I went with. Thanks for the trick!