How an Overlock Works (in progress)

A Gritzner 788 machine is used for this wiki. However, other machines work according to the same or very similar principles.

An overlock stitch requires at least one needle. Most machines have two. They are attached to the needle bar by means of a needle holder. The needle bar performs an up-and-down motion. Usually, the needle bar is tilted slightly toward the rear.

The upper looper performs a rather complex movement in two spatial directions.

The lower looper performs a pendulum movement around an axis.

The feed dog is a sharp-toothed bar that performs an elliptical movement. While the needle is in the fabric, the feed dog is below the needle plate. Once the needles are above the fabric, the bars move up and pull the fabric one stitch length forward.

The knife is usually designed as a movable upper knife, which is synchronized with the needle, and a fixed lower knife.

The needle plate has various openings for needles and the feed dog, and behind the needles, two prongs and a movable metal part, against which the interlaced threads are pulled tight. For a rolled hem, this part is pulled forward and removed; then the knotting takes place at the prongs.

For illustration, I define the position where the needles are at the highest point as 12 o'clock. Most machines, however, turn counter-clockwise. Therefore, in the illustration, the clock runs backwards.

xx o'clock: The needle pierces through the fabric and through the needle plate.

6 o'clock: The needle has reached the lowest point and is moving back up. As it does so, the thread forms a loop at the needle eye.

4:30: The lower looper passes through the loop with its tip and slowly pulls it to the right.

xx: The tips of the upper looper and lower looper meet. The upper looper enters the loop of the lower looper.

10 o'clock: The needle passes between the upper looper and the upper looper thread and takes the upper looper thread with it.

For stitch formation, it is important that the 3 components are positioned correctly relative to each other and move in synchronization (timing).

The high art of positioning is that the components that pass threads (lower looper - needle, upper looper - lower looper, upper looper - needle) should come as close as 0 - 0.05 mm to each other, but must not touch. Adjusting the geometry is a moderately difficult task that requires a lot of patience and understanding without precise service manuals and adjustment gauges.

All movements are performed via mechanical connections to the main drive shaft. By means of eccentric discs, the rotational movement of the shaft is converted into a translational movement. The eccentrics are generally attached to the round drive shaft with 2 screws. If these are loosened or unintentionally become loose, the synchronization (timing) changes. Work on synchronization is only for advanced users.

Of course, it depends on the model, but almost every overlock has the following adjustment options:

• Thread tension for each thread
• Stitch width
• Stitch length
• Option to choose between normal setting and rolled hem
• Adjustment for presser foot pressure
• Activating and deactivating the knife

Many machines also have a differential fabric feed, where the feed dog runs differently before and after the needles, and fabrics can be stretched or compressed.

• First, practice with a cheap fabric, try out various settings, and document the result.
• Open the cover of the loopers and turn it slowly by hand. Follow the interaction (see stitch formation) of the components. Take photos or videos. Measure important positions (height of the needle above the needle plate at the highest point, position of the loopers when the upper and lower loopers meet, position of the needles relative to the lower looper at the lowest point). There are adjustment instructions available on the web for very few models, not even for money. If you document all of this on your new, well-functioning machine, a looper adjustment—for example, after a crash—will be more successful.

• Cut the threads, pull them out, and re-thread everything completely in the manner specified by the manufacturer.
• Insert new needles.
• Thread the sewing thread and use a simple fabric with which the machine works best. Ideally, use different colors as I did in the photos.
• Open the flap and turn the handwheel slowly. Check if the loopers catch at the correct point (presumably at least one does not).
• Sleep on it one more time.
• Adjust (or have adjusted) the looper.

• Pull the power plug.
• Raise the presser foot.
• Remove the presser foot.
• Open the flap and cut all threads, if necessary multiple times.
• Turn the handwheel slightly forwards and backwards until a resistance is felt. This can often be immediately. Sometimes, however, the blockage can be resolved this way.
• Release the needle screw. Press the needles downwards, tilt them, and try to pull them out. Try the step with the handwheel again.
• Release the screw of the needle plate. Try to loosen the needle plate, or remove it if necessary. Try the step with the handwheel again.
• With pointed scissors, cut the fabric close to the blockage point.
• If the fabric could be removed, also remove all threads.
• Reattach the needle plate.
• Turn the handwheel and test whether the loopers run without blockage.
• Reattach the presser foot and insert a new (!) needle. Check for blockages again with the handwheel.
• If everything is good, re-thread and attempt to sew.

• Switch to on. Does the light work?
• If not, check if the socket has power and if the combo connector is plugged in correctly.
• If that does not help, insert an incandescent bulb (INCANDESCENT, not LED) whose resistance you have measured beforehand.
• Switch on and measure the resistance between pin and pin. If it corresponds to that of the light bulb, the cable/connector of the foot controller is defective. If it has high resistance, then the switch or the wiring is the issue (—> electrical specialist).
• Light works, but motor does not (when the pedal is pressed): The front flap, when closed, actuates a safety switch. This can be mechanically or electrically defective. Take out the needle and keep your fingers far away from all moving parts behind the flap. Briefly actuate the switch with a tool (keep pedal pressed). If the machine starts running, fix the mounting and closing mechanism (DO NOT permanently fix or bridge it).
• If that was not the cause, front flap closed, switch on. Measure the resistance between pin and pin. If the resistance is at Ohm, the motor is okay; the error then lies with the foot controller (Layperson: new foot controller; electrical specialist can perhaps check and repair the triac).