Author Topic: Anatomy of an overlocked seam.  (Read 1412 times)

hutch--

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Anatomy of an overlocked seam.
« on: March 19, 2016, 09:05:39 PM »
An overlocker is a particularly useful machine that is used by a wide range of tailors, home sewers, large scale producers and in particular people who make stretchwear. The basic logic is you lay 2 layers of fabric face to face, pin or bast them together and either mark a line OR use the edge as a guide to cut and stitch the two layers together with a strong and flexible join. While there are variations, the most common on modern domestic overlockers is a 4 thread machine that has 2 needles and 2 looper threads. Depending on the seam type and the garment's application, you can remove either of the needles and only run 1 needle so that you get a three thread overlocked seam. Depending on which needle you remove you can get either a wide overlocked seam or a narrow overlocked seam which in conjunction with the normal seam width adjustment, gives you considerable flexibility in the seam you choose to make.

There is another adjustment on most modern domestic overlockers which is called "differential feed" and its purpose is to compensate for the tendency of stretch fabrics to stretch as they are being fed through the overlocker. This at its worst gives a stretched wavy edge which is unsuitable for most garments. By adjusting the differential feed up from the neutral position (usually 0 on most machines) you alter the feeding ratio to reduce the stretching effect. On a low stretch fabric you will only need a small amount of differential feed, on a high stretch fabric you will need to use a higher setting to compensate for the stretching while overlocking. On non-stretch fabrics you usually do not use any differential feed.

This image shows both sides of a 4 thread overlocked seam, the top side is the side you see as you are feeding the 2 layers of fabric through the overlocker, the underside is the side you cannot see. On the top side you can clearly see both needle threads and reaching out to the edge cut by the overlocker you can see the top looper thread. On the bottom side you see a different shaped stitch, you can see the outer needle stitch but the inner needle thread if you are using both needles grabs the lower looper thread and pinches it together to form a Y shape. Now the advantage of running the extra needle thread is it provides a stronger and more secure overlocked seam and where strength is important, it has an advantage.

Now something that does take a little patience is getting the top and bottom looper thread tensions correct. If the top looper has a lot higher tension that the lower looper thread it pulls the edge of the seam onto the top side and vice versa, a higher tension on the bottom looper thread pulls the edge around to the under side. You usually start with both needle threads set at the same tension, start with the looper threads at the same tension and tweak the looper tensions until you get the looper edge at the fabric edge. Most machine come with a book or a CD that explains adjustments of this type, it is worth the effort to read them and test out the results.

It is fair to say that no overlocker was ever a joy to thread. Some are better than others but they all involve feeding 3 or 4 threads through a complicated set of arms and holes and they do need to be loaded correctly. Almost exclusively overlockers come with a threading diagram that is under the front cover. Also the manual will usually tell you what order to put the threads in. Most will put the needle threads in first, followed by the top looper thread and lastly by the bottom looper thread. On most machines you can lift the foot and also remove it so you have more room to see what you are doing.

Do yourself a favour and get two decent pairs of long tweezers as it saves you trying to feed threads in impossible places by hand and allows you to pass the bottom looper thread from one side to another using the two pairs of tweezers. Something you should take notice of is how you position the 3 or 4 threads once you have them threaded through the various parts of the machine. Many require that once you have the threads loaded you route them out under the foot position. Once you have done this you put the foot back on, drop the foot back to the normal position and test out your settings.

If you have done it correctly, you will see the threads chain as the machine runs. You are then ready to start overlocking fabric together.

Now if you you are a bit canny with looking for a bargain, often you see a new overlocker for sale on eBay that says it has never been used. Now in many instances this is because someone throught it would be easy to set up and get going but when they tried to set it up, they were intimidated by the complexity and eventually gave up. Often you can pick up a very good machine that has not been used for a reasonable price.

The particular overlocker used to make the example in the photo above is a Bernina 800DL which was bought new about 15 years ago and it has been a very good machine that does almost everything well. Some of the lower cost machines are lighter in their construction and better suit finer work on lighter fabrics. The strength of the machine will also dictate what thread you use with it. A reasonably strong machine can routinely use #75 thread for the needles and #120 for the two loopers. On a lighter finer machine, you can use combinations like #120 for the needle threads and a thread type like COATS GRAMAX which is an untwisted polyester thread for making fine stretchwear.
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