This one I picked up on Freecycle was beyond my ability. It was a cheap, off brand plastic "special" so I had nothing invested in it. It ran, but wouldn't stitch so I think the timing was fubar"ed". I started just taking it apart for fun. My son, seeing this, asked if he could do it. Sure!!
So after loosening a bowlful of screws, taking umpteen greasy gears apart and destroying just a couple of plastic parts to get the switches out, we had this:
When all was said and done, we tossed a bag full of gears and all the screws in a bag, and kept the motor. That will come in handy for a future science project, yet to be named. The motor works, has 2 speeds, and a light on a separate switch. Science will never be so much fun, :)