The 15 Bobbin Case Insert (TM-505) does not work correctly for the Singer 15’s, especially if you are trying to measure thicker threads like a Tex 69.  The Angle where the finger rests needs to be rotated about 20 degrees clockwise.   The Bobbin Cases on the Singer 15’s turn in the opposite direction of most other Class 15 Bobbins with the Vertical Bobbins (like the White 742).  So, to use the TM-505 Insert  in the Towa, the thread has to feed out the right side, not the left.  This means that you have to run the thread under then Towa and over to the top left Pulley, then to the Center Pulley.  But then, you have to pull the thread to the right so that it is perpendicular to that Center Pulley.  And even then, you get a reading that is about 10-20 grams too high.  I know this is true because I confirm all my Towa readings with the Tajima Tension gauge. 

 

The Standard L Bobbin Insert (TM-501) works best with the 15 Cases.  You just let the Bobbin Case freely rotate so that the Finger is in the bottom left position, which is where it will stop spinning.  Then, make sure your Thread is draped against that Finger, and then feed as normal.