"Nesting or Rat's Nesting" on the bottom side of Bernina sewing machines (CB hook type) is generally the result of very loose top tension. If the problem is found in newer sewing machines such as machines made after 1986 (serial numbers beginning with 26xx xxxx), may have an older Bernina bobbin case in the machine. The problem with the older bobbin cases is that the rounded projection of the latch on the left side of the casing will catch thread, preventing easy thread escapement during the take-up lever cycling resulting in loops on the bottom side and or thread breakage, etc..
Note:....Timings and modified latch geometry's were applied to later models to allow for good thread escapement. Older Bernina cases cannot be used on newer machines but newer cases can be used on all CB hook Bernina machines.
Additionally, Chinese copies of the Bernina/Cerliani OEM bobbin are rampant and can cause sewing issues. Always ask for OEM Bernina cases.
Note the following trouble shooting logic:
1. The center tension disk that is visible through the thread slot in the top cover of most machines must be positioned exactly in the middle of the top cover slot. By loosening the fixing screws on the top tension mechanism, one can adjust the disk position to center.
2. It would be a very rare situation for the top tension to change whatsoever from the (red center marking) factory setting. The most likely cause of loose top tension under the normal center mark position, would be accumulated lint, thread, or a sticky substance between the disks. This would prevent the tension compression disks from applying an even tension on the sewing yarn (thread). Often this can be blown out with compressed air when the presser foot lever is lifted in the up position. In some cases, soft drinks and other sticky or corrosive liquids can find their way between the disks. In this case, the tension assembly must be removed, disassembled, and the disks cleaned and polished.
3. Check the facial surfaces of the bobbin case for corrosion, and sticky substances that prevents the thread from easily sliding over the bobbin case.
4. Check that the spindle of the hook body is not bent or warped, causing the bobbin case to wobble, thus creating an intermittent pressure on the hook race cover at the moment that the thread is trying to escape over the bobbin case finger. (Replace the hook body)
5. Inspect the undercut on the hook body spindle for thread wrappings and knots. This condition can cause the bobbin case to press hard against the hook race cover, hence hindered thread escapement .
6. Exceedingly tight bottom tension can cause some problems as well. ( If you don't know how to adjust Bernina bobbin cases to factory settings, please ask).
7. Hook timing, (loop lift) can be out enough to affect thread escapement. This is typical of newer machines made after 1986 since Bernina no longer made fixed timed machines like the earlier Swiss made models. The later made models have timing components that can slip under stressed machine mechanics.