What is the size of the lips of the chemical drums? We are unsure on whether the 4" radius includes the lip or not? 
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I measured a 1 qt paint can I found in my garage, and it was 4 1/4" across the top rim.
I looked for, but couldn't find the spec for the field tolerance, but typically it is at least 1/4"

There was a QnA on this with the usual expected answer, something to the effect that cans may vary, so design your robot to work with any similar size.
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