Understanding Thread Tension
Whenever you switch from your standard sewing thread to another thread, first thread your sewing machine and test your setup to see if you can get away with a tension-dial-only, temporary adjustment. If that doesn't work, get out your second bobbin case, and start moving the screw in quarter-turns to loosen or tighten it, as your sample dictates. Typically, when you use a lighter-than-normal thread for both needle and bobbin, the tensions will stay balanced, even though they're both lighter. This is often just what you need to avoid puckering lightweight fabrics, so no adjustment may be necessary. A heavier thread in top and bottom will increase both tensions, and you'll probably need to set a lighter tension to accommodate heavier fabrics.
Don't touch that dial!
So many things can affect the tension that it's worthwhile to run through the following checklist in the order given before you reach for the tension regulator:
• Incorrectly threaded machine: Incorrect threading is responsible for more "tension" problems than any other factor. Did you use all thread guides? Did you thread with the presser foot down, thus keeping the thread from slipping fully between tension discs? Is thread unwinding freely from the spool, or catching on the spool's slash? Are you using a bobbin as a spool (which can interfere with the thread flow)? Is the bobbin inserted correctly?
• Incorrectly filled bobbin: Remove any thread on the bobbin be-fore you wind on new thread. Wind the bobbin following the machine instructions, so it's evenly wound at the proper tension. Remove any thread from the outside of the bobbin. Wind at a consistent, slow or medium speed, especially with polyester and nylon threads, to keep them from stretching; they relax in your seam, causing puckers.
• Dirty machine: Lint and thread ends lodged between the tension discs, under the throat plate, or around the bobbin case and bobbin, increase the resistance and restrict the thread flow. "Floss" between the tension discs with a lightweight, lint-free cloth, and check in the bobbin area for thread ends and lint.
• Damaged machine parts: Bent needles and bobbins, and rough or damaged surfaces on the needle eyes, thread guides, tension discs, take-up lever, throat plate, presser foot, bobbin case, and in the bobbin area can all cause problems. If you drop a metal bobbin on a hard floor, throw it away, even if it looks fine; the smallest damage can distort tension. Avoid damage to the bobbin-tension spring by cutting the thread close to the case before removing the bobbin. Raise the presser foot before removing thread from the upper tension.
• Needles, threads, and fabrics: Different thread sizes and types on top and in the bobbin can throw off basic tension settings. A needle that's too large or small for the thread can also unbalance your stitches, because the size of the hole adds to or reduces the total top tension. If you find that you're getting puckers on organza, chiffon, jersey, lace, or blouse-weight silks or polyesters, try changing to a straight-stitch foot and needle plate, and shorten the stitch length to 1.75 mm (15 sts/in.), before you reach for those tension dials.
Claire Shaeffer, author of Couture Sewing Techniques (The Taunton Press, 1993), writes about all levels of sewing in Palm Springs, California.