Describe the stitching mechanism in a sewing machine?

Answer 1

Sewing machines use two spools of thread, unlike hand sewing, which uses one fixed-length piece. You can emulate the machine-stitching process by hand. Imagine you have a needle through which the thread from a large spool is threaded and a second, smaller spool of thread. Push the needle eye-end first through the fabric from above. Pull the loop of thread on the back side bigger, until you can pass the little spool through the loop. Now, from above pull the top thread tight to close the loop back up again. That is all a sewing machine does. The arm at the front that moves up and down a lot supplies the slack to make the bottom loop big enough, and then moves back up to take up all the slack, and to pull a tiny bit more thread off of the top spool through the tensioning mechanism. The hard-to-visualise bit is at the bottom, where the bottom spool is passed through the loop. The key to understanding it is that the spool and whatever carrier it is in "floats" in a space rather than actually being fastened to the rest of the mechanism. There is a gap round it on all sides (though sometimes an oddly shaped one) which allows the loop of top-thread to pass all the way round it. Typically a hook of some sort grabs the top-loop near the needle eye, and pulls the loop bigger while passing that loop round the outside of the bottom spool assembly. If you operate the machine by hand with the covers off you should be able to see this happening.