There’s nothing like knowledge to defeat fear and I know for sure that many sewers are actually scared by learning how to use a serger.
Let’s explore one of its most interesting features: the differential feed!
But, first: what are the feed dogs?
Feed dogs are those thin spiky metal bars you can find right below your presser feet.
They move the fabric toward the back of your sewing machine and/or serger, under the needle, determining the exact length of your stitches.
They can also move the other way, doing what we call “backstitch” (although I wouldn’t suggest you to backstitch on your serger!).
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The differential feed system
Looking at your serger needle plate, you'll notice two independent sets of feed dogs: one front, the other one rear.
Each one has an individual feed mechanism: they will be able to feed the fabric at a different ratio (but you can also make them move at the same speed, if that's what you need!).
The differential feed knob/lever has a numbered scale, usually going from 0.6 (or 0.7, like mine) to 2, where:
- From 0.6 to 1 (negative differential feed) the front feed dog doesn't move as much as the rear feed dog, causing the fabric to stretch under the presser foot to offset the puckering;
- 1 (sometimes labeled as N - neutral differential feed) both feed dogs walks at the same speed;
- From 1 to 2 (positive differential feed) the front feed dog moves more than the rear feed dog, amassing fabric under the presser foot (when set at 2 it actually makes twice as many rotations of the rear one!)
To help visual people like me, most sergers comes with two little graphics next to the numbered scale, meaning stretched (0.6) and gathered (2) fabric.
If you're sewing a medium weight woven fabric, 99% of the times your differential feed will work just fine set on 1.
If you sew other kinds of fabric, you may need to adjust the differential feed to achieve a pucker-free or waver-free seam, laying flat.
Note: Always try on scraps to decide the ideal settings, paying attention to keep grains, bias and stretch directions consistent with your project!
- If you're sewing a sheer, lightweight or slippery fabric, you may experience puckering.
How to fix it:
Decrease your differential feed ratio until you get a flat seam.
- If you're sewing knit, stretch, or even woven fabric cut on bias, you may stumble upon a stretched wavering seam.
How to fix it:
Increase your differential feed ratio until you get a flat seam.
Differential feed: Creative uses
Differential feed >1
Since using settings above 1 will cause your front feed dog going faster than the rear one, you can easily create gathering with a 4 thread overlock, just increasing needles' tension.
Experiment on scraps to see how much you are able to gather your fabric:
- If it's not enough ruffled, pull the right needle thread to increase gathering;
- If it's too full, just pull the fabric to make it looser.
Another practical way to use positive differential feed, is using it to ease in: think to fit a sleeve head into an armscye or sewing a rounded hem.
Remember you need to put the fabric to be eased in against the feed (to learn more good tips like this one, check Janet Pray's "Sew Better, Sew Faster" classes!)
Differential feed <1
Since using settings below 1 will cause your front feed dog going slower than the rear one, you can easily create a lettuce edge on a stretch knit (use narrow hem settings and learn more with Angela Wolf's "Creative Serging Class"), creating that fun wavy look!
If you're in the market for a serger, I would suggest you buy one with this feature, but what if your old/inherited/second-hand one doesn't have a differential feed and you're experiencing unwanted wavering on knits (or puckering on sheers)?
You can mimic the differential feed with your right hand in front of the presser foot, gently pushing (positive differential feed) or holding (negative differential feed) the fabric while it feeds through the needle, just as you would do with a regular sewing machine.
If all else fails: the stitch length and the foot pressure can influence the waviness/puckering of the fabric.
Play with those settings until you get the best result, while you're testing your settings!