Well, I'm baaaaaack! And I'm basically here to close out this thread - although I will start a new one on a slightly different topic.
Anyway, you'll remember that I decided I was inept and causing all sorts of problems with a program that has more bells and whistles than I should be playing with. So, off I went to try another program, Cubify, and it crashed just trying to import the project I'd saved in TurboCAD, which didn't bode well. So, I tried redrawing the whole thing in Cubify. But, since I don't where the tools are that I want to use it was seriously frustrating. And, I kept thinking "You know where it is in TurboCAD, so why are you doing this???" After a couple of days banging my head against a new
wall I came back to TurboCAD. And, I've had some success so wanted to close out this thread with what I found.
What I did was to start deleting things in the previous project file and found gobs and gobs of extraneous "stuff". Once I got down to the essentials I redrew the plate, chamfered it, placed hundreds of new cylinders that overlapped the surfaces, and subtracted them. And, it worked perfectly! Stoked, I redrew the lower plate, the one that caused the most recent problems, and sure enough it skimmed over on the corners when certain holes were created with the hole tool. So I created cylinders that overlapped the surfaces and got the same results when the cylinders in the problem areas were deleted.
Given that, and not wanting to completely start over, I copied and pasted the wireframe of just that collar and cylinders into a new file - and it skimmed over when I subtracted the cylinders from the plate. But, on a whim I took the .1875" thickness to .150" and the skimming went away! So, I put it back to .1875" and the skimming didn't reappear. Yippeee! But, what would happen if I tried that in the original file? Well, over there the skimming didn't leave when the thickness was reduced. So, I copied the "good" wireframe into the "bad" file and ...... the skimming appeared immediately. Further, reducing this one to .15" and taking it back to .1875" didn't help either. So, apparently I have something in that file that is haunting it, like Henry said. I don't know what it is, but I can live with what I have as I have two files that I'm pretty sure will work on a 3D printer - one for the original plate and one for the new, lower plate.
So, I don't see a need to discover what the problem was with the original file. (My mentor in programming said not to go figure out why a program does what it does. When you fix it just move on.) But, I do want to thank y'all for the help as I wouldn't have gotten this far w/o you. And, if you'll take me back then I'll start a new thread with more, but different, questions.