A quality sewing machine repair course will cover these kinds of repair in detail, but the basics of repairing a flawed hook are as follows.
A flawed hook is a common problem that affects tensions, stitch quality, and even timing. The development of burrs on the hook's point can make sewing very frustrating. Flaws on the hook require sewing machine repair immediately.
While the hook does not look like a needle, it functions in much the same way as the needle.
The needle draws the upper thread down through the hole in the needle plate. It accomplishes this by having the upper thread threaded through the eye of the needle. The thread slides tightly into the groove on the front of the needle so prevent it from snagging. As the needle reaches its lowest position it begins moving back up. The thread it is drawing is now loose and forms a loop on the back of the needle. The needle has a cut out on its back called a scarf. The scarf accentuates this loop.
The hook has no eye, but it does have a point.
At just the right time the point of the hook slides behind the needle in the middle of the scarf. The point enters the thread loop. It slides between the needle and the thread. The needle continues to move upward and out of the bobbin area. The hook continues to move drawing the thread around the bobbin. The upper thread and bobbin thread wrap together to form a stitch.
The point of the hook, however, must be sharp and free of nicks, scratches, and burrs. If the hook develops rough spots, scratches, or nicks in its surface, it can snag the thread and mess up the stitch. The most common issue is the development of burrs on the tip of the hook point. A burr is an irregularly formed flaw in the surface of the point.
You can identify these flaws by gently drawing your finger over the surface of the hook near the point. Feel for irregularities. If it has a burr, you may feel like there is little ball on the end of the point. There may a chip, or other distortion. You may also see these by use of a light and magnifier.
These flaws must be removed. Use of emery cloth, sandpaper, or a small file are commonly necessary. In extreme cases you may need to use a dremel tool. In all cases, the surface and point of the hook must be smooth.
If the damage to the hook is so extreme that it can not be buffed our smoothed, replace it.
To replace the hook assembly, will require the services of a professional sewing machine repair technician or completion of a sewing machine repair course.