The cam machines with 'double' cams make it simpler for me to understand the concept- one control for side-to-side, one for fwd-rev feed. Once that is under variable control, you can make the machine do just about anything.
Keep your 'eyes peeled' for an early Elna-locally (I've bought a couple, total for both was $60) - they are very nice IMO- real precision machinery aside from the rubber drive wheel- which you already know about with your Kenmore..
Info at
http://needlebar.org/main/elna/
The earlier cam Elnas need a cam (I forget the #, but 'mine' didn't come with it) to do zig-zag. Models a bit later (my beige one) in the Supermatic/Super line would do zigzag without an accessory cam, and many other stitches (most decorative) with the appropriate cam.
Of course, there were Singer machines that used cams as well, AFAIK.
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