I enjoy Facebook posts by my quilter-friends, and this little video clip was posted this morning. I found it to be a very good illustration of how a sewing machine forms a lock stitch between the action of the needle and hook. It show how the hook catches the needle thread and wraps it around the bobbin thread to make a stitch. Take a look at it now:
Have you ever wondered why you machine will skip stitches? Can you see how there must be a balance between the fabric, thread, needle and hook for the machine to form the perfect stitch? In most cases of skipping stitches, the problem can be easily solved by finding the right needle to match the thread and fabric types and maybe adjusting the machine's tension for the thread and/or fabrics being stitches.
Can you now imagine how a different kind or size of needle can change how the stitch is formed? Sewing machine needles are designed with different kinds of points for use with different kinds of fabrics and thread. Some fabrics stretch or the fabric may grab onto the thread as it passes through the fabric, not allowing enough of the thread to go into the bobbin area for the hook to catch the top thread to wrap it around the bobbin thread. So the needle must be adjusted to make accommodations for the kinds of thread and/or fabrics.
If the thread is thicker than normal, the needle and eye size must be adjusted to allow the thread to glide easily through the eye of the needle. Some threads have a tiny amount of stretch, for example: nylon monofilament thread. The needle must be designed to allow for the thread's 'bounce'. If the eye is placed slightly lower on the needle, the needle is then able to draw the thread slightly deeper into the fabric, allowing the thread to 'spring' back a bit, but still making it possible for the hook to catch the loop of thread that forms the stitch.
This video illustration certainly shows why your sewing machine will skip stitches if the machine's timing is even a tiny bit off, too. If the hook isn't able to catch the thread as the needle goes deep into the fabric layers at exactly the right moment, the stitch isn't formed and that stitch will be skipped.
The animation also helps explain how the balance between top and bobbin tension on the threads are important to make a perfectly formed stitch. If the top tension is too loose, the top thread is drawn into the fabric by the hook too far, and the thread remains on the bottom of the fabric sandwich making big loops there. If the top tension is too tight, the hook may not even be able to catch the thread, skipping stitches, or the too-tight top thread may pull the bobbin thread to the top of the fabric sandwich, leaving little bumps of bobbin thread on top of the fabric. Balance between the top and bobbin threads is important for perfect stitches to be formed by the hook and needle.
I hope you find this little instructional clip as interesting as I did. If you want or need more information about thread and fabric types, and the kind of needles that will work best with your particular combination, I would refer you to All-in-One Quilter's Reference Tool by Harriet Hargrave, Alex Anderson, Sharyn Craig, and Liz Aneloski and the section about needles and needle/thread compatibility.