I found a very comprehensive troubleshooting chart in a service manual for industrial machines and edited it for home machines and home sewers. I will publish it here in installments because it is just too long to do all at once. Each installment will address a specific problem so you don't have to read through the whole document to find a solution to yours. Not all steps apply to all machines (if a bobbin case latch is mentioned and you have a top-loader, skip that step). If you have a newer, electronic machine, I can't help you if these steps don't fix the problem.
The steps do not need to be followed in sequence and not in order of the most likely to least likely.
Pick the easiest and cheapest solutions first and work your way to the more complicated and expensive ones.
If you notice errors or can suggest additional causes and solutions, please let me know so I can edit this post and keep it complete and accurate.
This is the installment for "Upper (Needle) Thread Breaks", one of the most common problems.
1.
Cause: The tension of the upper thread is too tight.
Solution: Loosen the tension of the upper thread by turning the Tension Nut counter-clockwise.
2.
Cause: The machine is not threaded correctly.
Solution: Check whether the upper thread runs correctly from the spool of thread to the Needle.
3.
Cause: The upper thread is of poor quality. The thread may be too weak or it may be rotten due to improper storage and does not withstand the tension imposed on it. The thread may also be too hard and not possess the necessary elasticity.
Solution: Loosen the tension of the upper thread by turning the Tension Nut counter-clockwise. If possible, replace the inferior thread with one of good quality
4.
Cause: The upper thread is of irregular thickness and has small knots which cannot pass through the eye of the needle.
Solution: Replace the inferior thread with one of good quality.
5.
Cause: The upper thread has wrapped itself around the spool pin on top of the machine.
Solution: Unwind the thread from the spool pin. Eventually, place a felt washer underneath the spool of thread. If necessary, remove some of the thread from the full spool.
6.
Cause: The upper thread is held back somewhere on its way from the spool of thread to the needle.
Solution: Examine the string of thread from the spool to the needle and eliminate any interference with the free motion of the thread.
7.
Cause: The upper thread curls around itself before entering the tension disks.
Solution: Place a felt washer underneath the spool of thread. If necessary, remove some of the thread from the full spool.
8.
Cause: The eye of the needle is rough or sharp, or the grooves in the needle are too shallow.
Solution: Insert a new needle.
9.
Cause: The needle is not straight.
Solution: Insert a new needle.
10.
Cause:The needle is too fine for the upper thread and for the fabric being sewn.
Solution: Consult the Needle and Thread chart in the manual and insert a needle of the correct size.
11.
Cause: The upper thread is too coarse for the size of needle that is being used.
Solution: Consult the Needle and Thread chart in the manual and select a thread of the correct thickness.
12.
Cause: The needle is not inserted correctly in the machine. The needle grooves do not line up correctly.
Solution: Insert the needle so that the short groove faces the point of the hook.
13.
Cause: The needle is threaded incorrectly.
Solution: Thread the needle so that the thread passes through the eye of the needle from the long groove to the short groove of the needle.
14.
Cause: The wrong type of needle is inserted.
Solution: Consult the Instruction Manual and replace needle with the one recommended for the material being sewn.
15.
Cause: The needle is bent and rubs against the stitch hole in the needle plate.
Solution: Check whether the needle is bent and replace it, if necessary with a new needle.
16.
Cause: The operator pulls the fabric in the direction of sewing. The Feed Dog cuts the upper thread.
Solution: The fabric must not be pulled during sewing. The machine alone must feed the fabric.
17.
Cause: The machine is started at full speed.
Solution: Start the machine at low speed and increase speed gradually.
18.
Cause: The machine is started with the thread take-up not in its highest position.
Solution: Start the machine with the thread take-up lever in its highest position.
19.
Cause: The bobbin is bent and cannot revolve freely in the bobbin case.
Solution: Replace the defective bobbin with a new one.
20.
Cause: Bobbin is wound too full.
Solution: Remove some of the thread from the bobbin.
21.
Cause: Thread is wound unevenly around the bobbin.
Solution: Replace the bobbin with one wound correctly.
22.
Cause: The lower end of the needle bar is bent. The needle strikes the presser foot, the throat plate, the feed dog, or the hook.
Solution: Replace the needle bar with one that is straight.
23.
Cause: The throat plate is not held firmly in place and shifts around. The needle strikes the side of the stitch hole in the throat plate.
Solution: Tighten firmly the screws holding the throat plate to the machine.
24.
Cause: The feed dog is not held firmly in place and shifts around. The needle strikes the side of the stitch hole in the feed dog.
Solution: Tighten firmly the screws holding the feed dog to the machine.
25.
Cause: The needle is not pushed up entirely in the needle bar or in the needle clamp.
Solution: Push the needle up entirely in the needle bar. Tighten firmly the screw that holds the needle.
26.
Cause: The needle is not held securely in the needle bar. The needle shifts slightly up and down.
Solution: Make sure the needle is pushed up entirely in the needle bar. Tighten firmly the screw that holds the needle.
27.
Cause: The bobbin case latch does not close entirely due to accumulation of dirt and lint underneath. The upper thread is prevented from sliding freely around the bobbin case and breaks.
Solution: Remove dirt and lint under the latch. Clean the entire bobbin case thoroughly with a small brush dipped in kerosene.
28.
Cause: Lint or dirt has accumulated in the circular groove on top of the center post of the bobbin case holder (shuttle). The center post is the part upon which the bobbin case is placed. Due to the accumulation of dirt or lint, the bobbin case latch does not close entirely; the upper thread is prevented from sliding freely around the bobbin case and breaks.
Solution: Remove the dirt or lint from the circular groove of the center post. Clean bobbin case holder thoroughly with a small brush dipped in kerosene.
29.
Cause: The feed dog is timed too early. Excessive tension of the upper thread is required to eliminate the small loops of thread which appear on the fabric. The excessive tension causes the upper thread to break.
Solution: See whether loops of thread appear on the underside of the fabric. If increased tension of the upper thread does not eliminate these loops, adjust timing of the feed dog.
30.
Cause: The thread take-up spring is not adjusted correctly and does not take up the slack of the upper thread. The needle pierces and tears the upper thread.
Solution: Adjust the take-up spring. There must be no thread under the point of the needle when the needle is about to enter the fabric.
31.
Cause: The thread take-up spring is bent out of shape or broken.
Solution: Bend the deformed spring back to its proper shape, if possible. Replace a badly defective or broken spring with a new one.
32.
Cause: The loop of the upper thread twists around itself below the throat plate, forming two small loops. The point of the hook enters both loops, causing the upper thread to break.
Solution: The point of the hook is too close to the needle. Adjust the clearance between the needle and the point of the hook so that the point of the hook is as close as possible to the needle without touching it.
33.
Cause: The hook has burrs, rough spots, or sharp edges that cut the upper thread. The point of the hook may be blunt or damaged.
Solution: All portions of the hook that come in contact with the upper thread must be perfectly smooth. Smooth the defective portions of the hook with an oilstone or fine emery cloth, then polish with crocus cloth or on a buffing wheel. If the point of the hook is damaged to a minor degree, sharpen it with a fine emery stone and polish with crocus cloth or on a buffing wheel. The material must not be taken off the side of the hook point that faces the needle. If the hook point is defective beyond repair, replace the old hook with a new one.
34.
Cause: The head of the screw used for adjusting the tension of the bobbin case protrudes too far beyond the tension spring. This causes the upper thread to catch and break.
Solution: Adjust the screw so that the upper thread can slide smoothly over it. If necessary, remove some material from the head of the screw by grinding or filing. Polish the head of the screw after this correction. If a new screw can be obtained, replace the faulty screw with a new one.
35.
Cause: The top surface of the bobbin case is damaged, or has burrs or rough spots preventing the upper thread from sliding smoothly around the bobbin case.
Solution: All portions of the bobbin case that come in contact with the upper thread must be perfectly smooth. Smooth the defective portions of the bobbin case with an oilstone or fine emery cloth, then polish with crocus cloth or on a buffing wheel.
36.
Cause: The small coil spring inside the bobbin case which presses against the bobbin case latch is either broken or has become weak, compressed, or clogged with dirt and lint. The bobbin case latch no longer closes entirely and catches the thread.
Solution: Remove the latch and latch lever from the bobbin case, then remove the spring with a needle. Clean the spring and stretch it somewhat, then replace in the bobbin case. If the spring is broken, badly deformed, or too weak, replace it with a new spring.
37.
Cause: The bobbin case holder (shuttle) may have rough spots or sharp edges that cut the thread.
Solution: Smoothen the defective parts with an oilstone or fine emery cloth, and then polish with crocus cloth or on a buffing wheel.
38.
Cause: The hook or bobbin case may be misshapen and not permit the thread to slide freely around these parts.
Solution: Remove the slide plate from the machine, turn the balance wheel by hand , and observe how the upper thread slides around the hook or bobbin case. Take those parts out of the machine and with fine emery cloth, remove any excess material from those portions where the upper thread gets caught. Polish these portions with crocus cloth or on a buffing wheel. If not successful, replace the defective part.
39.
Cause: The hook is not adjusted (timed) correctly in relation to the needle.
Solution: Adjust the machine so that the point of the hook enters the loop of the upper thread at the correct time. For timing, follow this general rule: The needle must have risen about 3/32 of an inch from its lowest position at the moment the point of the hook is just at the center of the rising needle. At this moment, the point of the hook must be about 1/16 of an inch above the eye of the needle.
(Do not attempt without instructions for hook timing. Use this solution to determine if hook is out of time and then either adjust in accordance with instructions in a service manual or take to an experienced technician.)
40.
Cause: Some of the parts along which the upper thread passes (thread guides, tension discs, thread take-up spring, take-up lever, etc.) have become rough or sharp and possess deep, sharp grooves.
Solution: With fine emery cloth or emery cord remove all rough or sharp portions and grooves. If these parts are too badly worn, replace them with new parts.
41.
Cause: The top edge and inside of the stitch hole in the feed dog have become rough, sharp, or chipped. This may be due to wear or broken needles.
Solution: Smooth the stitch hole with fine emery cord. If the damage is excessive, replace the feed dog with a new one.
42.
Cause: The needle is too close to the edge of the slot (or stitch hole) in the presser foot and rubs against the presser foot.
Solution: Check the straightness of the needle and replace it if necessary. If the presser foot is not correctly aligned, loosen the screw that holds the presser bar in position; turn the presser bar until the needle goes through the center of the slot (or hole in the presser foot; tighten the screw firmly.
43.
Cause: The needle is too close to the hook and rubs against the point of the hook.
Solution: Check the straightness of the needle and replace it if necessary. If the new needle still touches the point of the hook, adjust the clearance between needle and hook by moving the hook somewhat from the needle. The point of the hook should be as close as possible to the needle without touching it.
44.
Cause: The portions of the presser foot close to which the needle passes are rough, sharp, or chipped.
Solution: Remove the presser foot and smooth the defective portions with fine emery cloth. Polish them afterward with crocus cloth.
45.
Cause: The balance wheel revolves in the wrong direction.
Solution: The feed dog must move the fabric away from the operator. Observe the feed dog and change direction of rotation so that the above requirement is met.
(Mostly applies to treadle machines that can spin either direction)
46.
Cause: Sometimes the fabric being sewn causes breakage of the upper thread.
Solution: Experiment with various grades of thread and with different thread tensions.
47.
Cause: A needle thread of the wrong twist is used, causing the thread to unravel or break
Solution: Use needle thread with twist recommended for the machine in use. Left twist thread is required for single needle machines.
(Probably only applies to industrial machines)
Happy Hunting! Ed
Ed's Vintage Sewing Machine Shop
