So I made a second attempt at the tank top, and would you believe it? The straps came out too short this time.
This one is at least wearable. Sort of. And I am pleased that my technique has improved. The neck and arm bands, for example, look pretty good.
I'm going to put this tank top away for the present and work on things for which I have commercial patterns. I will come back to the tank top when I have a bit more experience with knits. It's not the most flattering style for me, given my relatively narrow shoulders and large hips, and I just can't bear to make it again right now.
I stitched the neck and arm bands with twin ballpoint needles. There's a hole in the top of the machine in which a second spool holder can be inserted. You run the thread to the second needle exactly the same way as you'd normally thread a single needle. Only the needle on the left pulls up the bobbine thread, but both threads must somehow loop around the bobbin, because they both form a chain of stitches. I wish my machine were completely transparent so I could see exactly what's going on under there.
I learned tonight that if you install the needle backwards the machine skips stitches, and the thread gets all wadded up around the bobbin case. I'm the sort of person who only reads instructions as a last resort. I'd been kinda wondering if there was any certain way the needle was supposed to be inserted. Now I know, having perused the instruction book that came with the machine. The flat part of the shaft goes at the back.
From time to time, I take the position of an observer, watching myself as I learn an unfamiliar task. It's fascinating to see the way my hands seem to learn by themselves, without conscious effort on the part of my brain. For example, I hemmed the tank top by folding a quarter inch over at the bottom edge, then folded over again an inch. I sewed a double row of stitches (using the twin needles) an inch from the bottom of the hem. I didn't mark the hemline, but I found that I knew exactly where to run the stitches, because my fingers could feel the edge of the hem through the front of the top. My hands are learning to exert just the right amount of push and pull on the fabric to get it to run smoothly over the feed dogs (why are they called dogs, I wonder ...)
So I guess I can call it a successful lesson, even though the straps came out too short.