Sewers using sergers cut the time and work of sewing considerably, because these speciality sewing machines cut, bind and sew at the same time. Most sergers, or overlock machines, use two needles on top and a pair of loopers below the machine's cover to create loops that the needles close. Knives on a serger that do not cut may indicate improper installation, position or dull knives.
The top knives for sergers rest in front of and to the right of the innermost needle and cut the fabric as the feed dogs pull the fabric through the needles. Some overlock machines have bottom knives that sit below the feed dogs. Serger manufacturers make the sergers easy to access, since the knives create considerable lint as the cut fabrics, and it's typically easy to open the doors to the different parts of the overlock machine to either service it, change the knives or clean it.
The upper knives rest on an arm that has a spring inside it. Users can push the knife in to raise it out of the way when threading the right-hand needle. It's easy to forget to lower the knife and in its upward position, the knife won't cut. Unlike the needles and loopers, the knives on sergers do not move when sewing. The knife needs to be in its lowered position to cut the fabric.
All knives eventually dull and if the knives are not cutting evenly or at all, they might need replacing. You can take out the knives using a screwdriver and place them in a small envelope to take to the sewing machine repair shop, or transport the overlock machine for the repair staff to replace the knives for you. If you are replacing the knives yourself, write down the brand and model of your serger, since the repair shops use inventory numbers for part identification. The manual for your machine contains detailed information for changing the knives for your model.
Whenever you are working on your serger, even threading it, turn it off and unplug it. Sergers have many moving parts that can hurt you. If you need the light on the machine to remove the knives, leave the machine plugged in, but disconnect the foot pedal so you can't accidentally activate the machine while working on it. When installing the knives, use care when putting them in because new knives are sharp.