Why Stitches Skip and Fabric Puckers: How to improve stitch quality of your sewing machine

Most sewers, even very accomplished ones, don’t really understand how sewing machines work, and it’s not because our machines have become so elaborate. I’m talking about how a stitch is formed; what hap pens when the needle goes up and down and the feed dogs move? In fact what hap pens is the same on every sort of machine, from treadles to the computer-controlled wonders. Once you have a clear idea of what’s going on, I think you’ll find that you can make better use of your machine, keeping ordinary tasks trouble-free and unmysterious, and even letting the ma chine handle much of your easing, gathering, and basting.

btp-16.jpg Making a stitch: 1. At start and end of a stitch, needle and uptake are fully raised. Both move down as a stitch begins. 2. As needle starts to emerge from the fabric, uptake is fully dropped, slackening the thread, which forms a loop at the needles eye. The hook catches the loop to swing it around the bobbin casing. 3. As thread loops around the casing, the needle leaves the fabric and feed dogs move the fabric the length of the stitch. As the loop clears the casing, the uptake takes up the slack Thread, and the process starts over. 4. Blue marker on thread has moved the length atone stitch.

Balanced tension, straight stitch: Tension is balanced when the knot isn't more visible from either side of the fabric.

btp-17-1.jpg Balanced tension, zigzag stitch: Adjust top tension so that the top thread is just visible from the wrong side.

Standard presser feet: The straight stitch foot holds The fabric firmly and close to the needle, for the best stitch quality. The all-purpose foot has a slot so the wide s but the needle can make bottom is smooth. The satin-stitch foot has a slot for wide stitches and a groove on the bottom to allow for the build-up of stitches.

Making Thread Loops

Although machine stitches attempt to duplicate hand stitches, there’s no comparison between your handmade single-thread stitch and a machine-made two-thread stitch. The drawings below trace the machine process.

Thread movement—The first thing to realize about a machine-made stitch is how much thread movement is involved. If you follow the path of the little blue dot on the thread in the drawings just mentioned, you’ll notice how it moves from one side of the needle hole to the other and back with every stitch. In fact, depending on the length of your stitch, that marked section of thread will slide back and forth through the needle and the fabric as often as 60 times before coming to rest in a stitch.

Before we examine why, I think you can see right away why you should choose your thread for smoothness and strength, and carefully match it to the size of your needle. I never skimp on quality thread; I choose all-cotton when I want my seams to look great, or long-staple polyester when the strength of the seam is more important than its looks. Unless you’re into decorative stitchery, I suggest you settle on a few brands of thread that work for you and are easy to get, and stick to them. There’s much more to say about thread, and I plan to devote an entire article to it in an upcoming issue.

Clearing the bobbin—The reason the thread moves so much is that the top thread must loop completely around the bobbin each time a stitch is formed, which requires about 2 in. to 4 in. of loose thread. Where does the loop come from? Let’s re turn to the drawings on p. 16 and look more closely.

Notice how the movements of the needle, the uptake (the moving arm you thread through at the top of every machine), and the shuttle race (the mechanism surrounding the bobbin and holding it in place) are carefully synchronized to create a thread loop and move it around the bobbin. If for any reason the little thread loop (see drawing 2) fails to form, or the hook on the shuttle race fails to catch it, you get a skipped stitch. Drawing 3 should explain why trying to remove the fabric before the uptake is completely raised won’t work, even if the needle is all the way Up: The loop is still wrapped around the bobbin casing.

Setting tensions

Setting and adjusting top and bottom tensions isn’t hard, and when you’ve got the confidence to do it, you’ll get much better results from your machine. Machines with “Universal” or “Automatic” tensions have superb tension mechanisms that permit a wide range of variables without adjustment, but their settings can be, and sometimes need to be, overridden. Sewers who want to be in control should feel confident to do so.

Bobbin tension—The bobbin thread merely has to stay put while the top thread goes around it, so its tension mechanism is simple: a screw holding a spring-steel tension plate against the thread is all it takes. It’s usually better to set this for your most usual combination of thread and fabric and forget it, except as described below, be cause you can weaken the spring with too much adjusting. If you use lots of different or heavy threads in the bobbin, get another case and mark it as the looser one.

When you do want or need to adjust bob bin tension, start with cautious turns of the set screw, making tiny shifts to the right to tighten, or to the left to loosen; a quarter turn is a major change, so restrain yourself. Test the results, and fine-tune with the top tension setting.

Top tension—The top thread traces an elaborate path from spool to needle; it’s important to realize that each step along the way contributes to the top tension, not just the setting on the tension dial. Tension is the total resistance against the thread as it travels through the machine from spool to needle. The dial merely fine- tunes the resistance in order to control the top thread’s longer movement, so if your tension’s way off, check the whole path.

Inside the tension mechanism are separating plates between which the thread passes. Newer machines have three plates, so there’s a separate space for each of two threads. Lowering the presser foot engages the top tension mechanism by pressing the plates together; the tension dial controls the amount of pressure that gets applied. The proper tension is whatever setting balances the bobbin tension so that the knots that hold top and bottom threads together in a stitch are equally spaced between the top and bottom of the fabric layers, as shown in the familiar drawing above left, and so that there’s no seam puckering. For most situations, dial up a zigzag stitch and adjust it to look like the one in the drawing above center; this is the easiest way to see the balance between the threads.

The fabric you’re stitching is a major contributor to this happy balance. As you sew, the top thread is attached to the tension path at one end, and to the just-made stitch on the other. To make the next stitch, the machine will pull the thread it needs from whichever end offers the least resistance.

If the fabric is soft, pulling thread from the last stitch could be easier, resulting in puckers. You can shorten the stitch length, so that there’s less fabric to give between each stitch, and you can loosen top and bottom tensions equally so the resistance from the spool end is less. If you loosen just the top tension, the knots will fall to the bobbin side of the stitch, which looks better than puckering, but weakens the stitch. Sewing on thin, soft fabrics like charmeuse, chiffon, crepe de chine, Swiss batiste, and voile is about the only situation for which I’d recommend that you ad just your bobbin tensions away from your preferred “normal.”

Tension settings for specialized stitches: The tension dial can also be used to make some sewing activities easier. When you need to gather, but the exact amount of gathering isn’t critical, try increasing the top tension and setting the stitch length to maximum. The machine will hold back the thread, so that the feed dogs pull tile single layer of fabric farther than the amount of thread tile machine has allowed. What’s the result? Deep, regularly spaced puckers, or gathers. Experiment with different set tings; you won’t hurt the machine. I prefer this to using a gathering foot, which can cause skipped stitches. To baste without puckers, reduce the tension to half tile normal setting. In addition to no puckers, the knots will all fall to the back, and you’ll be able to pull the bobbin thread out easily to remove the basting. When you’re satin stitching to appliqué or to make button holes, reduce the thread tension slightly so the bobbin thread isn’t seen, and to lessen the puckering.

Presser foot basics

As needle and thread move up and down through the fabric, the fabric naturally tries to move with it, and if the presser foot doesn’t hold the fabric securely, that’s just what happens. If the fabric moves up with the needle, the little loop shown in drawing 2 doesn’t form, the hook misses it, and the stitch gets skipped. This is why it’s hard to stitch so close to a fabric edge that the presser foot can’t grab both sides; the fabric flutters up and down with the needle. Whenever possible, sew at least 1/2 in. from the edge and then trim away. This is also why you need to hold fabric tightly in a hoop and press it down against the throat plate if you’re sewing without a presser foot, for embroidery or darning.

Most people evaluate a presser foot by looking at the top of the foot, but the part of the foot that has the most contact with the fabric is the bottom. If you compare the most-used sewing feet, the straight-stitch, zigzag, or “all-purpose,” foot and the satin-stitch foot, you’ll see that a satin-stitch foot has a grooved bottom to allow the mound of thread created by the tight zigzag to move underneath without restriction, while the straight-stitch/zigzag foot is flat, in order to hold the material firmly and evenly across the whole width of the feed dogs. If you use the satin-stitch foot for normal sewing, you may get skipped stitches because the groove can allow fabric fluttering when there’s no thread build-up.

There is one instance where the satin- stitch foot is appropriate for straight-stitch sewing. Soft, spongy fabrics like fleece get slightly spread out under the pressure of the foot, and this can cause stretched, wavy seams. If you switch to the satin-stitch foot, the groove will allow the fabric to keep its shape without stretching; the fabric is thick enough to prevent fluttering.

I recommend that you add a straight- stitch-only foot, like the one in the right- hand drawing on the previous page, to your collection. You’ll get the best possible straight seams, because the fabric will be held as securely as possible around the needle. As long as you remember not to switch to zigzag without changing feet, you’ll love it. If you can’t find one, or you switch to zigzag often, try adjusting your needle position all the way to the right or left so that the zigzag slot will be providing support on three sides. Remember to change your seam-width markings to reflect the new needle position.

Shifting fabric

While the needle and thread are swinging up and down, the feed dogs are waiting for just the right moment to make their move. They can’t move at all while the needle’s in the fabric because that would pull the needle off course. Once the fabric starts moving, however, a race begins between the layers of fabric; and the bottom layer al ways wins. The feed dogs have a better grasp on the bottom layer, so they push it a tiny bit farther than the top layer with each stitch. At the same time the presser foot pushes against the top layer and the cumulative effect is the bottom layer coming out shorter than the upper one.

This movement is called shift; it’s in creased by a number of factors. When more than two layers are being sewn, as in machine quilting, the upper and lower layers have less contact with each other and are more likely to move unequally. Any in crease in drag on the top layer, such as let ting your hands rest on the fabric, letting the fabric hang off the sewing surface, or sewing on extra-thick fabrics, will increase the shift. Soft, spongy, or stretchy fabrics, like sweater knits and hand-wovens, offer a lot of resistance against the foot. Very slick or slippery fabrics don't hold onto each other, so the bottom layer moves without taking the next layer with it.

To prevent or equalize the shift, try these techniques: Never sew more than two layers together at one time. If three or more layers are necessary, sew pairs first and then join the sets. With your fingers on both sides of the foot, stretch the fabric equally to either side and allow the fabric to feed steadily and evenly. Use a walking foot, available for most machines from local dealers; whether it’s built-in or a separate attachment, this foot duplicates (over a smaller area) the motion of the feed dogs on the top layer. It’s especially helpful on quilts. Every six inches, or more often as necessary, lift the foot and smooth back the fabric bubble that's created by the foot, shoving the top layer for ward. This helps a great deal on knit fabrics. You can also hold just the bottom layer, let ting the top rest free as you guide them both under the needle. The increased drag on the bottom will equalize the movement of the layers. Hand basting will eliminate shift completely, but pin basting will merely slow it down. If it’s important to eliminate shift, take the time to baste.

You can also use this shift tendency to your advantage. Whenever one side of a seam must be eased in against the other, put the longer side against the feed dogs and allow the machine to do the easing.

Fewer and fewer puckers

Of all sewing machine irritations, puckered seams are the most frequent and the most likely to be fixable by the knowledgeable home sewer. In addition to what’s already been described, here are the most common reasons for puckers: The “knot” where the threads cross on a machine-sewn seam be longs within the fabric, but if the thread is thicker than the fabric, the knot is so bulky that it displaces the remaining fabric. The cumulative effect is a bumpy seam that appears puckered. Standard-weight cotton thread can cause puckers when it's initially sewn in, but the natural fiber will often press flat; a test will determine if this is the case. For very fine fabrics like crepe de chine or cotton batiste use extra-fine cotton machine-embroidery thread. The weight is similar to these fabrics and it will make smooth seams, especially when combined with a short stitch length.

Stitching straight seams through elastic can stretch it out of shape. The problem is those knots again; they take up more room than just thread would. That’s why elastic is chain-stitched in ready-to-wear garments; there’s no knot to displace the elastic. Today there are specially-made elastics with channels between the elasticized rows for machine-stitching with a traditional sewing machine without stretching. If you can’t find this elastic, use a zigzag stitch to spread the knot pattern and prevent stretching.

Bias or knit seams always seem to pucker, but the true problem isn't puckering, it’s stretching fabric. When the seam is sewn it’s lying flat in the machine. As soon as the garment is hung, the bias or knit drapes and the seam doesn't . To prevent this problem use a baby zigzag stitch. This is a width 1, length 1½ (20 stitches per inch), normal zigzag. This stitch will give and drape with the rest of the garment for a soft, smooth seam. This also works well on very long seams like princess seams on floor-length dresses.

The quality of your stitching can be improved by beginning your seams meticulously. Hold onto the two thread ends as you begin a seam. Gentle pressure toward the back will help you start without a pucker. It’s important to press gradually on the foot control; starting too fast is like skid ding tires on the pavement. The feed dogs can’t get a grip on the fabric and the bottom layer becomes distorted.