and service. By doing so, simple problems can be fixed  with little to no downtime or repair bills. The information  on troubleshooting provides you with the basic knowledge  needed to keep your machines running.  [Figure 6-67]  Figure 6-68  shows a close-up of the head only. Only those  parts, which the rigger must deal with on a regular basis in  order to operate and maintain the machine, are shown. For  those individuals who wish to become more involved in the  machine, a thorough study of the operator’s manual and parts  manual is encouraged. The following numbers correspond  with the part descriptions in  Figure 6-68 . Sewing Machine Model Comparison Make Single needle  light duty  drop feed Single needle medium duty needle feed Single needle compound feed Two needle feed Zigzag medium duty Harness machine heavy duty Harness machine extra heavy duty Bar tack Singer Consew Juki Brother Mitsubishi Pfaff Bernina 31-15 292R DLN-415 DB-130 — 111W151 N/A B-791 DY-340-12 — 111W155 206RB-4 LU-563 LU2-400 — 112W116 333RB-1 LH-515 LT2-220 — 17W15 199R-2A LZ-780-11 138 217 68 or 69 class N/A 3334 — 7-33 733-R2 — 97-10 N/A — Figure 6-66.  Sewing machine model comparison. 6-17 1.  Bed—base of the machine. 2.  Arm—upper casing of the machine. 3.  Uprise—upright part of the machine that joins the base  and the arm. 4.  Faceplate—cover that protects the needle bar and  presser bar mechanisms. 5.  Balance wheel—pulley assembly that drives the  machine via the motor and belt. 6.  Reverse lever—mechanism that, when depressed,  reverses the sewing operation of the machine. 22. Feed dog—feeds the material through the machine  from the underside. 21. Slide plate—covers the area of the bed to the left of the  feed dog and provides access to the bobbin assembly. 20. Needle plate—surrounds the feed dog and protects  the material during the movement process. 19. Presser foot—holds the material in place while the feed  dog moves the material forward for the next stitch. 18. Presser foot bar—holds the presser foot in place to  hold pressure on the material. 17. Needle bar—holds the needle and carries the upper  thread downward through the material to where the  stitch is formed. 16. Thread take-up lever—provides for slack in the needle  thread after the stitch is formed and pulls the correct  amount of thread from the spool for the next stitch. 15. Presser bar tension nut—regulates the pressure of the  presser foot on the material. 14. Tension discs—provide tension on the upper thread. 13. Thread controller spring—provides for the correct  amount of slack in the needle thread when the needle  is descending so that the needle does not cut the thread. 12. Upper tension regulating thumbscrew—regulates  pressure of the tension discs on the thread. 11. Right arm thread guide—provides thread guidance from  the upper tension assembly to the thread take-up lever. 10. Thread take-up cover—covers the thread take-up lever  and protects the operator. 9.  Thread retainer—provides direct guidance for the  thread to the upper tension assembly. 8.  Pre-tension thread guide—assembly that provides initial  thread tension and thread straightening before the thread  reaches the main upper thread tension assembly. 7.  Stitch regulator—adjustor that controls the length of  the stitch. The larger the number, the longer the stitch;  the smaller the number, the shorter the stitch. Figure 6-65.  Pfaff 3334.