Why the thread is broken when iam sewing at the tip of the needle?

Most likely, it has to do with incompatibility of the needle, thread, and fabric. The force of the needle penetrating the fabric multiple times is actually causing the thread to (wear &) fray and eventually break. There are needles with larger grooves that tend to protect the thread during stitching. Some may recommend a Top Stitch Needle (Home Machine Needles Superior Threads Why Topstitch? The Topstitch needle is the most common needle type used and recommended by professionals and educators. We recommend the Topstitch for piecing, quilting, embroidery, appliqu?, and construction. These needles are available in four sizes to accommodate all thread types, including metallic threads. The Topstitch needle has a larger eye and deeper groove. It is often referred as the MAGIC NEEDLE. It really does make a difference in your sewing experience.)
Why Use Topstitching Needle

Also, if the thread is a tight fit through the needle eye, it will also tear the thread. Make sure that the thread will slip easily through the eye. Go to a larger needle, instead of 90, try a 100.

Be sure to use a brand-new needle.

All About Needles

The Parts of Sewing Machine Needle

You may also consider changing the type of thread you are using. Go with a stronger thread, ie polyester, rather than cotton. Cotton will shred and break easier.

You will probably need to re-adjust the thread tension. If it is too tight, it will also break the thread. If you are using a heavier weight of thread, you will need to adjust both the upper and bobbin tension to accommodate the heavier thread.
Bobbin Tension How and Why Tension is Adjusted

If you sewing fabric that is sticky, ie vinyl, oilcloth, you should try to use a roller presser foot, a teflon presser foot, or a walking foot to help the fabric move with the feed dogs.

Sewing Heavy Fabric with NON Heavy Duty Sewing Machine SEWING IN GENERAL

Tips

Sewing through many thick layers of fabric Sactin

http://www.kollabora.com/q-a/sewing-denim-and-thick-fabric

14 Fantastic Sewing Tricks and Tips Making the World Cuter

How to Sew Upholstery DoItYourself com

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I have found this happens when I need to replace the needle. It might be slightly bent, or the tip broken. Needles are cheap, so when in doubt replace the needle!

check for sufficient thread clearance between bobbin case finger and locating slot. in other words it must be free to allow the thread to pass through and complete the stitch

Carmen, seem that you have carried out the necessary rectification, but could it be your rotary hook tip blunt/broken, if not most probably it needs needle/hook timing adjustment. And this will require your sewing machine technician to do it.

Do you have thread in your bobbin?
Is your needle fully inserted in the needle bar?
Is it the right needle for the machine?
Is your needle damaged, bent or dull?

Hello there! Make sure your pattern selector is compatible with the presser foot. Good idea to test with a straight stitch setting: Tension: 4. Width:4-5. Length 5 after you insert a new needle and re-thread machine. Your previous needle may have become bent when machine malfunctioned. Use fabric compatible with needle size. Always raise presser foot when threading! Then always lower presser foot before sewing. Remember to use blue tip scarf needles with synthetic fabric silk thread 50. Purster thread 50, mercerized cotton thread 50-60; #11 or #9 for fine or lightweight fabric with silk thread 50 or fine Pollyester thread; #18 leather needle for leather, #14 - 16 denim . Jimmy Sweep out shuttle carefully with small brush for broken threads and lint.

It is very possible that the timing is off, which often occurs with broken needles. A common symptom is that the needle will not pick up the bobbin thread. Missed stitches aren't necessarily timing related--be sure to install brand needles often and use the correct needle for the fabric & thread you are using. Using the wrong needle will often cause skipped stitches.

What to Do If Your Sewing Machine Is Skipping Stitches

10 reasons for skipped stitches

Here What to Do if Your Sewing Machine is Skipping Stitches

Basic Maintenance Tension Skipped Stitches

You can search for sewing machine timing or take it for service.

some sewing machines can not sew denim good,
here are some suggestions
since a needle was broken & replaced--if the needle has a flat side it goes to the back of the machine
make sure the needle is inserted correctly & pushed up inside of the needle holder & the needle screw tightened correctly
is the correct needle being used for the thread & fabric being sewed?
when sewing denim, a denim needle is needed, I usually purchase mine at walmart, I have 2 necchi sewing machines & the denim needles work just fine on my machines.

if none of the above suggestions help then the machine could be out of time, just because of a broken needle. only a repair shop has the correct tool to fix this problem

have you try changing the needle
sometimes the needle & thread are not compatable--here's how to check

Cut 6-8 inch piece of thread of the spool you are going to use for your project
Take the needle you are going to use for the project
& insert the thread thru the eye of the needle
& lift one end of the thread to a 45 degree angle
& if the needle is the right size for the thread it will slide down the thread
But if the needle hangs on the thread you need one size larger needle for the thread

Make sure the bobbin has a 3-5 inch thread tail
Then make sure the top thread has a 3-5 inch tail
& make sure the threads are always pulled to the back behind the needle area
happy sewing

I had many broken needles before I put my thread on a thread stand behind my machine. I found that the thread was winding around the thread post and causing a jam and thus, breaking my needles. This solution worked!