The Upholstery Forum

Just a general observation about bobbin sizes and wondering if theres a reason they're so small compared to the top thread spool.
Sewing uses an equal amount of both threads but the top thread is heaps longer than the bobbin thread, which makes it necessary to refill the bobbin quite often.

Bob

Just haven't figured out how to make a lock stitch straight off of the spool i guess. Prewound bobbins get more thread on them compared to what you would wind on your machine and they're not too crazy expensive

Good question. Never thought about that. Adam12 is probably correct.

A lock stitch requires the top thread gets looped under the bottom thread. The only way to do that for the hook to pull the top thread all the way around the bobbin. That's why all lock stitch machines have a take up arm that pull the slack out of the loop that went around the bobbin and snugs it up tight. Also, the bigger the bobbin the more the top thread gets run back and forth through the eye of the needle every time the hook opens and the take up pulls up the slack. All that extra travel wears the thread and takes time for the machine to make those motions. So they pick a bobbin size that a reasonable compromise between thread capacity and top thread wear.

this video shows an oscillating hook in action

A better question. How do the CNC machines run non stop?

Chain stitch machines do not use a bobbin, nor do overlock machines, but they are generally not used for upholstery work, or other heavy sewing with the exception (that I know of) are bag sealing machines which use a chainstitch so they can be opened easily.