HI @stephensashley
I made a quick video using your drawing file to show this process.
I am using AutoCAD for Mac 2018.2 on Mojave. First I show how you can select the small down arrow to the bottom right of the BOX to show the other 3DSolids types.
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Then, I type SUBTRACT (I could have used the same small down arrow for the third icon to get to the SUBTRACT icon), I first pick the box to subtract from and then I select the cylinder.
I hope this helps.
Please select the Accept as Solution button if my post solves your issue or answers your question.
John Vellek
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