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How a Sewing Machine Really Works

I once had a student who came to me in class and said, “I’m having problems with my machine. It breaks threads when I start to sew.” I suggested that maybe she was turning the hand-wheel backward to remove the needle from the fabric. She said, “Absolutely not. I never, ever do that!” The next week she came in and said, “I still don’t think I did that, but since you told me, I haven’t broken any threads.”

That is the best example I can give to explain what I believe causes the majority of home-sewing frustrations: what I call mind-set. We aren’t always aware of our unconscious habits. Our mind-set is what causes them.

We all work from a basic knowledge at whatever we do. Much of it's habit, or unconscious. Once we establish a mind-set on false information, however, we have no firm base on which to build real understanding. It’s like trying to create a recipe using the wrong ingredients: nothing works and you don’t know why. That is why mind-sets can be dangerous. They prevent us from discovering the true source of our problems.

For example, the first time we learned to sew, we visualized how the stitch was formed by using a needle and thread and making the stitch by hand. Then came the sewing machine. We still maintained an image of how it functioned from our earlier experience. When it didn’t work this way, it became the antagonist and sewing became a frustrating experience.

To think of a machine’s functioning in terms of hand sewing is the single most erroneous mind-set we can have. To understand how the machine makes a stitch, and to introduce the foundation of the guide, I want you to sit down at your sewing machine and perform the only exercise I will ask of you. The reason for the exercise will become clear as you read on.

Thread the needle and bobbin with alight color thread, as if you are about to begin sewing. Take a felt-tip pen and mark the upper thread about five inches above the eye of the needle. Slowly sew a scrap of material at straight stitch and watch what happens to the mark. You will see that it slides repeatedly through the eye, up and down—about 40 to 60 times, depending on the stitch length— before it's actually sewn into the fabric (Fig. 1.1).

The needle moves all the way down to its lowest point. As it comes back up again, the fabric closes around it. If you could see inside the machine, you would see a bubble or loop of thread forming as the needle rises (Fig. 1.2). The hook then comes past and spreads the thread over the bobbin casing to form the knot that holds the fabric together. That’s why when you try to remove your fabric from the sewing machine, some times it doesn’t pull out freely. The needle is out, but the thread is still wrapped around the bobbin. The stitch is finished and the fabric will always pull out easily when the uptake is at its uppermost point.

Upper Threading

To understand how the machine functions, we will trace the process by which the thread travels through it, en counters resistance or tension, and ends by making stitches to create your garments or crafts.

Fig. 1.2

The Escapements

The spool of thread sits on a spool holder. Whether the spindle is horizontal or vertical has relatively little to do with stitch quality.

After the thread leaves the spool, it travels through an “escapement”— meaning that it passes through and escapes out again. All escapements put some measure of resistance or drag against the thread, a pull measured in tiny increments called gram weights.

Because a machine is set up ac cording to gram weights, and since each escapement adds its portion of resistance to the total pull, it's critical to thread the machine through each one, without exception. If you miss even one, either intentionally or accidentally, you can throw off the tension and your stitches won’t form correctly.

Contrary to belief, tension isn't that little adjustable knob or dial on the front of the machine. Tension is the resistance against the thread as it travels through the machine. It is the interrelationship of all the layers that combines to form a stitch: thread, needle, presser foot, fabric.

The next escapement the thread travels through is what I will hereafter refer to as the upper “tension mechanism” or “tension assembly,” that knob or dial with the numbers or “plus and minus” indicators.

Your best rule of thumb is the higher the number or greater degree of “plus,” the tighter the tension, and vice versa.

The Separating Disks: The Tension Assembly

Inside the tension assembly, the thread will pass through shiny separating plates or disks. Refer to your manual if you aren’t sure of the proper direction for threading. If you are using two threads, check for an extra center. Make sure the threads pass on either side of the disk, if your machine features that option. These metal pieces are what press together and hold onto the thread with more or less firmness, depending on the mechanism’s setting. Only when the presser foot lifter is completely lowered do these disks come together and engage the tension on the thread (Fig. 1.3).

Fig. 1.3: Separating plate squeezes against thread: Side view; Separating plate; Thread; Tension mechanism

There are no best settings for all tension assemblies. Each machine is set up by a mechanic and tension depends on many factors, including his or her skill. The same amount of tension, or gram weight of pull, could therefore be achieved on the same brand machine at “3” or at “5”. It depends on an internal set ting, which can always be re-adjusted by a mechanic.

The separating plates hold the thread taut by means of a spring. One of the major differences in machines is in the quality of that spring. It can be a single-action spring that holds the thread, or it can be geared, so that it holds more securely and consistently with a thicker, more durable spring made of stronger metal.

The advantage of a geared spring is that you can change the amount of stress by finer, more exact degrees. This allows for a greater degree of control and more frequent adjustments with out fear of wearing out or slipping, or compressing, the spring.

If your machine was manufactured more than twenty-five years ago, you may have been advised to leave the presser foot lifter down when you aren’t sewing. That was because the spring was not as sturdy as those made today. Leaving the spring in continual contraction eventually compresses it permanently, which affects the tension and may result in a costly repair. Don’t leave the presser foot down when not in use on today’s machines.

“Universal” or “automatic” tension means that the machine uses a spring system that doesn’t have to be adjusted every time you change threads and fabrics.

After leaving the tension assembly, the thread usually travels through one escapement before it enters the uptake. This is the part you see going up and down on every machine, the part that will hit you between the eyebrows if you lean forward far enough.

The Uptake

The uptake measures off portions of thread. If you watch the thread as you sew, you will see that the feed off the spool isn't continuous and smooth, but piecemeal and somewhat random. The uptake’s other job is to pull the thread up from the bobbin area. After leaving the uptake, the thread passes through other escapements, the number varying among machine brands. The one closest to the needle is important not to miss because it keeps the thread from fluttering, and causing skipped stitches.

Last but hardly least, the thread passes through the eye of the needle.

Lower Threading

The unseen parts below the throat plate complete the introduction to how a sewing machine makes a stitch.

The Shuttle, Race, and Hook

The “shuttle” defines the entire lower mechanism. The “race” is the part that moves around the bobbin casing. In a rotary machine it revolves in one circular direction; in an oscillator type, it rocks back and forth in semicircles. In either case, on the race is the “hook,” which is a metal projection that picks up the incoming thread from the needle and spreads it over the bobbin casing to form the knot (Fig. 1.4).

Fig. 1.4

Lower Tension Mechanism

The bottom tension is a spring-steel metal plate that presses against the casing that holds the bobbin in place. It is usually regulated by a tiny screw in the bobbin case or casing area. Its job is to anchor the lower thread in position so that the upper thread can make a stitch around it. That is also why the bottom mechanism isn't as critical as the up per in terms of setting; it does very little in the process of stitch formation.

Tightening the screw presses the plate more firmly against the casing, which makes the thread harder to pull out (Fig. 1.5). For firmer tension, tighten the screw (turn it to the right); for less pull, loosen it (to the left).

Fig. 1.5: Bobbin tension plate presses against casing as screw is tightened.

The direction that the bobbin thread winds into the casing area isn't universal on all machines. Check your manual. Then make an arrow with an indelible marker to remind you of the proper direction.

A bobbin case can wear out. If it just keeps getting looser, even though you’ve tightened it, use Loctite on the set screw, following manufacturer’s instructions. Loctite is available in hardware stores. Better yet, buy a new bobbin case.

How a Machine Makes a Stitch

To learn how a machine functions and to lay the foundation for trouble free sewing, I will first discuss the two troublesome T’s—Timing and Tensions.


Timing is something that you truly can't set up or repair yourself. I will explain what it's so that you can be come a more informed consumer, but you really need an expert to do this repair.

Timing is critical, and bad habits can cause it to go out. Three functions must happen in sequence for a ma chine to make its stitch: the machine must move fabric, the needle must enter the fabric, and a knot must be formed. This is how it happens:

The feed dogs come up to move the fabric while the needle is out; in fact, the fabric can only move while the needle is out. The needle must not reenter the fabric until the feed dogs have dropped. As the needle starts back up again and before it leaves the fabric, a loop of thread is formed. The hook then comes by, picks up this loop, and spreads it over the bobbin case or casing area. The hook’s timing must be coordinated with the needle’s upswing. The actual knot is made only when the needle is out of the fabric and the fabric is being moved forward by the feed dogs. The ending and beginning of each stitch is when the uptake is at its highest point (Fig. 1.6).

Fig. 1.6: Beginning and end of a stitch

If any part of this sequence is out of order, a stitch can't be formed. The main result of an improperly timed ma chine is a failure to produce stitches, period. The machine will not sew at all. (Other reasons for the machine’s not forming a knot will be discussed later.)

If your machine goes out of time, please know that it's usually your fault, not the machine’s. Only a poor-quality or damaged machine will go out of time.

Note: f you know that your sewing machine goes out of timing often, it may have been tooled from soft, poor-quality metal or plastic. If this happens, ask your repairmen to use Loctite on the set screws. It hardens up the area enough to hold a lock in place, but it loosens if cranked hard enough.


Perhaps the most common com plaint heard in a repair shop is “the tension is off.” More often than not, such a problem is caused by the ma chine’s operator, not by the machine. What’s more, it's a problem that you can solve yourself. Tensions rarely go out by themselves, no matter how in expensive the machine. With the help of this guide, you will learn that it's more likely an improper combination of needle, thread, and presser foot.

A definition of tension bears repeating and expanding. It isn't complicated, although at first it may seem overwhelming. It is crucial to the understanding of stitch formation and how that relates to proper machine functioning:

Tension is the resistance against the thread throughout the machine. It isn't only a function of the adjustments on the knob or dial at the front of the machine.

Without exception, two separating plates or disks comprise the upper tension assembly, even on the earliest machines. The lower tension mechanism is either a bobbin case (which may be located in various places) or a bobbin casing.

The needle comes down and enters the fabric. As it makes its next stitch, it's still attached to the previous one. The source of the thread for the next knot can either be from the spool by means of the uptake, or the needle can steal from the last stitch in the fabric. The machine’s goal is to find the medium between the two, but it will always follow the line of least resistance if the two choices aren’t equal. If the fabric is soft, for example, a machine at nor mal tension will pull from the last stitch, rather than from the spool, because it doesn’t have to pull as hard or from as far away—in other words, the line of least resistance.

Fabric can act to stabilize the thread so that the machine can pull enough down from the spool to give the desired stitch length. But if the fabric isn't firm enough, the thread will yank it back up. The softer the fabric, the more likely it will gather up. So you get puckers.

Adjusting the overall tension in the machine will result in knots forming evenly between the two layers of fabric on a straight stitch. What you want to achieve is equal tension, not equal pull.

The lower thread comes a short distance straight up from the bobbin, whereas the upper thread has a winding and longer route down from the spool. There is more drag on the upper thread because it must be pulled back up by the needle and the uptake, forming the knot.

Don’t be concerned if this section isn't clear yet. All this information is new to you and it will take some time to sink into your reality. Continue on and refer back to this section if necessary.

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Monday, 2009-04-20 21:07