Modeling a bottle cap
Regardless of the task at hand, the importance of good reference material can never be stressed enough. So, the only responsible thing to do in this scenario is to visit the closest pub for the ungrateful and cruel duty of gathering reference material.
Once you’re done (and hopefully sobered up a bit) start XSI and from the Get > Primitive > Curves menu choose Circle. You will need 4 points to create the shape of each tooth and you will need 21 teeth, so set the Radius to 1.25 and the Subdivisions to 84. Rotate it 90 degrees along the X-axis, press [Ctrl]+[D] to duplicate the circle and move it down 0.15 along the Y-axis. Now press [T] to set the selection filter to points and select every 4th point along the entire circle. Once selected, scale them about 1.06 along the local x and y axis’s so they’re just protruding the edges of the first circle. Duplicate the new circle and move it down another 0.1 units along the Y-axis. Press [T] to select the points again and scale them about 1.06 again. Click the Select button in the Main command panel and choose Invert Using Filter. Scale the new selection of points to about 0.98. Duplicate the circle again and move it down about 0.15 units along the Y-axis. Press [T] to select the points again and scale them about 1.01. Click the Select button and choose Invert Using Filter and scale the new selection 1.03. These are all very small values, but then again the teeth on the bottle cap aren’t big either so even the smallest adjustment will make a big difference. To create the top of the cap, select the first circle (the one which is perfectly round). Duplicate the circle, scale it down to about 0.93 and move it upwards 0.35 along the Y-axis. Create another duplicate, scale it down to 0.65 and move it upwards another 0.3 units. Create the the last duplicate and scale it to 0 (zero) and move it upwards another 0.05 units.
From the Create > Poly.Mesh menu choose Loft. Now pick the curve at the very bottom, then the one just above, etc till all the curves are selected. Right click to end the picking session. In the Loft PPG change the U and V span to 4 and you’re done.
By adding a bend deformer with low vales you can avoid the otherwise unnatural perfect look. Or you can use high values to actually fold the cap.