Radius vs Diameter: The eternal battle

An important feature of the thing I’m building now is a tripod fitting on the base. I’ve decided that the best way to implement this is to get an adapter that converts a 3/8ths inch hole into a 1/4 one. If I make the right sized hole in the case I can just screw the adapter into the hole and it will provide the socket that I want.

I carefully converted 3/8ths of an inch into millimetres, added the hole to the design and printed it. However, as you can see above the hole is much too big. Twice as much too big. I’d used the value as a radius, not a diameter. The thing that amazes me is that I managed to check the diagram, look at the enormous hole and then say to myself “Yep, that looks like 3/8ths of an inch”.

Oh well.