Sew a line of straight stitches and a line of zig-zag stitches to determine if your sewing machine tension is correct. The straight stitches should be even, and in alignment on both sides of the fabric. The zig-zag stitches should be perfect zig-zags and not look like Ys on either side. The ‘Y’ shape arises when the top thread is pulled to the underside of the fabric, or the lower thread is drawn to the top – depending on which side you see the Y.

We aim for the top thread to only be visible on the top of your fabric, and the lower thread to only be visible on the underside of the material. If there is any problem with your stitches then read on and let me help you sort it out.

Understanding tension

Your sewing machine tension is a straightforward thing to fix if you understand what tension is in the first place. Tension happens between two things. In this case, it is between the top and bottom threads.  For both threads, the tension is set by a tension mechanism. Let’s start by understanding what creates the tension on our machines.

Sewing Machine Tension Top thread tension

It is difficult to see the two discs which close on the top thread. They are pointed out in the diagram. Always thread your machine with the presser foot raised as this opens the upper thread tension discs, allowing the thread to sit nicely between them.  The thread is ‘clamped’ by the discs when the pressure foot is lowered.

Lower thread tension.

The lower thread winds around a bobbin. The bobbin is either dropped into the sewing machine directly or nestled into a bobbin-case.  The tension is set by dragging the lower thread through the thread guide.  The pictures show the different guides for top-loading and bobbin case sewing machines.  It is vital to the machine’s correct operation that the thread passes through this guide. The most common problem with tension in a sewing machine is not having the bottom thread passing through this guide.

Sewing Machine Tension     Sewing Machine Tension






Changing the tension on your machine

A tension dial sets the strength of the top tension. The dial usually is around, above or below your tension discs. The top tension is easy to change and for most domestic machines is set to about 4 or 5. Some larger machines will also have an ‘auto’ tension setting which is brilliant.

Changing the lower tension requires a small screw to be adjusted to tighten (close) or loosen (open) the metal around the bobbin thread. This mechanism doesn’t have a dial. It is tricky to tell how far you have changed the lower tension. As I am about to show you, it is scarce that you would need to change this lower thread tension.

What do I do if my tension is wrong?

I like to think of tension like a tug of war game. I have a top thread team and a lower thread team. When the tension is perfect, they are both pulling with the same balanced strength.

Sewing machine tension

If I see my lower thread on the top of my fabric, it can mean either that my top team are pulling too much or that the lower team have slackened off. In this scenario, I would first check that my bobbin thread was through the guide and recheck the tension. If needed, next, I would reduce down the top tension until my tensions were corrected.

If I see my top thread on my work’s underside, then it can mean either that my top team has slackened off or that my lower team are pulling too hard. In this scenario, I would check that my top thread was passing through the upper tension discs. To do this, I would rethread the top thread, with the presser foot raised and then recheck the tension. I would increase the top tension dial until the issue was resolved.

Resorting to the screwdriver

I only alter the screw in the bobbin area or on the bobin case when I have exhausted the process above. In all my years of sewing, you would rarely need to do this regularly.

Other problems that can cause stitches to look poor are

  • Top and lower threads being of different weights or thicknesses
  • Stitch length not being set correctly
  • Blunt needles

I hope this has helped you understand tension better.

Gill x