What is the threading path for a singer t&s 640?

Most Singer manuals are available here: https://www.singer.com/support . Searching just for 14 gave two pages of manuals with the manual for the 14U46B and 14U46 on the second page. The current link to the manual is https://res.cloudinary.com/singer-sewing/image/upload/fl_attachment/Singer-Website-Library/outdated_product/SINGER_14U46B_and_14U46_Serger_Overlock.pdf . The threading depends on your type of stitch. See page 9 for the steps common to all threading (raise the presser foot, change the thread tension). See page 11 (page 13 of the PDF) for the left looper path. The right and left needle threading is shown on page 12. If you are using a four-thread stitch, see page 15 for the right looper thread path.

I hope this helps. Check YouTube for videos on preparing a Singer serger. Those are often more useful than the manual's pictures.

Cindy Wells

Install brand new needles.
Rethread completely from the beginning & verify the thread paths are correct for each.

If it is still not working, then the machine may have been knocked out of time. Search for "sewing machine timing" or take it for service.

Usually, a thread nest or bird nest, is a result of an upper thread issue: misthreading, thread not seated in the tension disk, upper tension too loose. Remove the upper thread from the machine. ALWAYS RAISE the presser foot and rethread from the beginning. Verify the thread path is correct. Set the upper tension to the midway point between high & low number. Then retest.

try this link.
sewing machine computer stuck Google Search
or this one.
My needle is only sewing in place the feed dog is Google Search

Make sure to use a brand new needle.
ALWAYS RAISE the presser foot when threading.
Unthread and re-thread from the beginning.
Verify the thread path is correct.
Set the upper tension to midway between High & Low numbers.
Verify the bobbin is loaded and threaded correctly in the bobbin case.
Begin sewing slowly and check the result. The upper tension can be tweaked so the upper & bobbin threads meet in the middle of the fabric.

If it is still not working properly, the machine may be out of time and require service.

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flashing lights, beeping sound are error signs its posiibly the upper tension path is not threaded correctly. Check that the transparent thread path located at the top of the machine is threaded correctly.
Hold thread in both hands and slip under transparent thread guide. While holding the thread securely, pull the thread towards you and then slip it back and forth like you were flossing. You should hear or feel two clicks as the thread slips past the thread guides.

here are 2 links for a manual, that should have the information
inside, one is a downloadable pdf copy the other is a hardcopy

http://www.sewingpartsonline.com/instruction-manual-singer-touch-and-sew-630-and-640-download.aspx

http://www.sewingpartsonline.com/instruction-manual-singer-630-touch-and-sew.aspx

Sounds like what is called bird nests or thread nests. The top thread is not seated completely in the tension disk.

Remove the top thread completely from the machine.
ALWAYS RAISE the presser foot and rethread from the beginning. Verify the thread path is correct.

Now, retest your machine.

web site www.sewusa was gread threading diagrams

"Birdnesting" occurs when there is no tension on the needle thread to pull the bobbin thread. This is easily corrected by rethreading the machine, following the threading path carefully and making sure the thread is fully engaged in the tension mechanism.