Array on cylindrical surface.

Hi all. I am a new be to AUTOCAD and do some self-study on how to handle Mechanical Desktop 4. Now I have got trouble in a part dwg. I want to draw a bicycle
rim, hub and spoke. When I come to making all holes for spoke in the rim I am lost. There must be a way to array 36 hole on the cylindrical surface of the rim? Every alternative I have tried give the error: "Selected axis is not perpendicular to the current sketch plane" Any advise is appreciated.
Thanks in advance Bjorn
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