Stitch Types, Stitch Length and Tension Adjustment
What the exam tests
You must recognise common stitch classes, choose stitch length for a fabric and job, and balance upper and lower tension using a test sample.
Stitch types
The lockstitch (class 301) is the standard straight seam; two threads interlock inside the fabric. Zigzag (304) is used for knits, overcasting and buttonholes. Blind-hem stitches are almost invisible on the face. Overlock or overcast edges stop fraying. Decorative and stretch stitches are chosen on the stitch selector.
Stitch length
Length is set in millimetres. Use 2.5 mm for general seams, 1.5-2 mm for fine fabric and tight curves, 3-4 mm for basting and gathering, 3-3.5 mm for topstitching, and 10-12 stitches per inch for durable seams.
Tension balance
A balanced stitch has both threads meeting in the middle of the fabric, with no loops on either side. Too much upper tension pulls the bobbin thread to the top and puckers; too little leaves loops underneath. Set upper tension first with the dial, then, only if needed, turn the bobbin screw a quarter turn at a time.
Worked example
A cotton sample shows loops under the fabric. Reasoning: loops underneath mean the upper thread is loose, so increase the upper tension one number, or check that the thread sits in the discs. Test again. If the seam puckers with correct tension, the needle may be too large or the thread too heavy for the fabric.
Traps
Reaching for the bobbin screw first; testing on a single layer; confusing stitch length with tension; using a long stitch on thin fabric and causing seam grin; forgetting to reset tension after decorative stitching.