Study this case carefully!

Bobbin cases are rather tricky devices, believe it or not.  They are complex and they account for fully one half of the tension you are looking for in your sewing.

Modern day late model machines have bobbin cases that are very delicate and need adjusting by trained professionals so this article won't address them at all.

However, bobbin cases that look like these are  somewhat adjustable but you are warned that dealing with them is something you ought to approach with both eyes open.

This article is really designed to give you basic information, not to give you a step by step repair manual.

 The case on the left is commonly referred to as an "Apollo" bobbin case and is usually found in midrange Singer machines although it is also found in other machines.  The one on the right is a very common one and is found in a huge number of machines but its complexity can range from low to high.

Now, one of the features of this case is on the inside.  Turn it upside down and you'll see a little clip.  When you open the lever on the front side then this clip moves and grabs your bobbin which keeps it from falling out onto the floor while you finish putting everything back into the machine.  Try moving the front lever as if you were taking the case out of the machine and you'll see the clip moving.  Who would've thought?

On the side of the case are two screws.  It'd be best if you didn't fool with either There is a flat piece of metal on the side (called a spring" which is held in place by one of the screws and if you loosen it then you've started a problem.  Examine this area and you can tell which screw is this one.  Generally it is the smaller one.  The other screw adjusts the tension and slight loosening and tightening of it will have dramatic results on the machine's ability to produce correct tension.  Some machines are so delicate in this area that a a quarter turn is all that is needed.  So if you decided to play around with this adjustment, be very careful...it is not recommended.

Another problem with this spring is that thread, fluff, and odds and ends such as elephant tusks can get under it.  This reduces the amount of tension the bobbin case is providing and things like that must be removed, Brave souls might use a sewing needle to examine underneath but they have to be rightly careful !

The "apollo" type bobbin case is for drop in bobbins. and you probably will never have it out of your machine because it is held in place by screws and brackets.  And it is held in exact place:  no room for fudging so don't ever take it out.  When you drop the bobbin into it you pull the thread around and the thread automatically goes into the tension spring which you really can't see without a long look. 

Another problem with the apollo is that  if it works itself loose  the needle can hit it and being it is plastic it gets pretty well damaged and generally has to be replaced: an expensive proposition.

On the whole, then, it is not recommended that you fool around with your bobbin case cause it has many different parts that have to be set carefully and positions that need to be carefully monitored.  But, it's your machine!