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Hi.There is a very high risk of kickback rotating your workpiece on the blade.I appreciate that you have said "it's dangerous" but it's more than that... It's an extremely poor idea... You're inviting the piece to be thrown and your hand pulled into the blade.I don't think you should be instructing people to do this... If you want to make bowls get a lathe!
Someone will lose some fingers with this.I would suggest also, don't crosscut on the fence. When using the fence the waste end should be the opposite side from the blade. The way you are pushing the workpiece across the blade to reference off the fence will lead to it kicking back eventually and possibly pulling you hand into the blade.
Asking for an accident IMO.For the cost of the circular saw and the plywood you could pickup an inexpensive used table saw with a guard, splitter, etc... Not to mention the issues with fence alignment, accuracy, stability... lack of dust collection... Anyone considering doing this should think about the risks very carefully.