I've wound new colors onto partially filled bobbins even with my computerized machine as well as my FW. Did it with my old White mechanical machine. None of the machines ever complained. The new color just empties the same way the initial does on the first bobbin fill.

I do it all of the time because of a shortage of bobbins. I have never had a problem.

I have at least a dozen bobbins for each of the machines I use regularly (3).....so always have a few empty for colored threads...wind what I may need, then use rest for piecing...the majority of my bobbins are all wound with white thread.

I buy vintage machines, it's amazing the number of different thread colors I've pulled off of bobbins. I always wonder why someone whould leave 3 - 4 feet of one color and then wind another over it (and do it several more times).

It also attests that very, very few ever end up using that leftover thread once they wind on top of it.

I have wound over a partially full bobbin in the past on both old mechanical and new computerized machines with no problems. Now I have so many bobbins that I don't. I just have a big plastic box of partially wound bobbins that I'm trying to use up :D

I'm amazed at the variety of answers... so many viewpoints!

I always unwind the remaining thread before I fill a new bobbin. Also, I bought more bobbins :)

I used to wind thread on top of thread. But, when you come to the end of the top layer of thread, there's a tangle -- no big deal, right? Then I became a sewing machine mechanic. Now, I know what a tangle of thread can do to a sewing machine. When thread tangles, it pulls the needle into the hook and damages it. Tangles can also pull the needle into the needle plate, break the needle, and even knock the timing out. So I never wind thread on top of thread any more. It's just a little thing I do to take care of my sewing machines. I pull the thread off and put it in a jar, and it adds a spot of color to my sewing room. I currently work with about 30 bobbins -- 10 for polyester threads and 20 for cotton threads.

This is the reason why I keep my empty NEB pre-wound bobbins. They're great for just needing a little thread for a mending job. Right now I'm using the leftover embroidery thread to piece with. I'm working on some PP sections so usually very little stitching on each piece so using up the thread is a quicky way to empty them completely. I find at the end of the NEB bobbins the thread seems to get caught up so rather it happen while I'm piecing than when I'm working on an embroidery project.

I've never done that. It never occurred to me. However, with my newer machine, it automatically stops exactly where the bobbin runs out and I put a new one in and it picks up right where it left off. I throw the thread that's on an old bobbin on my older machines out if I don't have any empty ones available. But now I have so many different types of thread and sewing machines that I'm having to keep the bobbins for the different machines separate...and then I have to keep the type of thread on the bobbin separate too. So I just have bobbin cases next to each machine so I don't get confused. I can tell the type of thread just by looking at it.