According to my sewing machine repairman husband, the most probable cause of the lower thread breaking is an improperly wound bobbin.  Regardless of where you wind the bobbin, inside the machine, on the top of the handwheel, or on the front side near the hand wheel, the basic "bobbin" rules apply. **Always start with an empty bobbin.  Never wind one color over another color.  ** Don't wind the bobbin so full that it would be tight and hard to insert into the bobbin case.  Most machines have an automatic "shut off" when the bobbin gets full, but if yours does not, be careful not to fill it too full.  ** Using bent bobbins can cause uneven stitching, and thread breakage.  Try rolling the bobbin on the table - if it wobbles, throw it away!  ** Always use the right shaped and type of bobbin for your machine.  Many machines will use "generic" bobbins - check your owner's manual.  ** And lastly, NEVER BUY CHEAP THREAD!  In most cases, cheap thread will cause more lint build-up in your machine, will cause thread breakage, as well as tension problems.  Just hold a piece of cheap thread up to the light and you can see all the "fuzz" along the edges.  Stick with the good quality thread, and your sewing projects will go much smoother.