Any machine that says it is at a perfect quarter inch from the get-go may not actually be, and it’s a good idea to check when you’re using one that’s not your own, anyway. Or, if you switch from your home machine to your travel machine, the settings will likely be slightly different. The idea behind the scant quarter inch is that you shave off a few widths of thread from your seam allowance to allow for the thickness of the thread you’re sewing with, to make a true and perfect quarter-inch seam.
When I was first learning how to make my seams nice and quarter-inchy about 10 years ago, I found it exceptionally annoying to get it just right. I didn’t like the idea of having to sew and resew test swatches moving the needle each time until my three 2 1/2″ strips were a perfect 6 1/2″, and figured there had to be a quicker way. As I thought about it, I figured that there were lots of things that had a perfect 1/4″ already that I could use as a guage–like graph paper, the lines on an index card, or college-rule paper. I got out my grid ruler and checked, and, sure enough, these lines were exactly 1/4″ (your milage may vary, so always check with your ruler beforehand).
Once I had a piece of paper that had the exact lines I needed, I lined up one line with the edge of my 1/4″ foot, where the fabric would butt up against it, and lowered my needle. If the machine was *really* at a 1/4″, then the needle should hit that line exactly, right? Well, it depends. I have a built-in walking foot for my machine and a standard 1/4″ foot, and the necessary needle position is different for both of them. SIGNIFICANTLY. My standard foot is something like 3.7 and my built-in walking foot is 5.5!
Once I lowered my needle, and raised it up again, I was able to see exactly where the needle fell with respect to that line.
If it fell to the left of the line, I clearly had a “fat” quarter inch, and needed to move the needle position to the right. If it was right on the line, then I generally still move it to the right, position by position, until I get what I felt was a “scant” quarter inch, which is usually just a hair or two to the right of the line. At this point, I am ready to test with a fabric swatch to make sure that I get a perfect seam.
The nice thing about this method is that, if you save your little piece of paper, you can transfer it from machine to machine, and compare; this is really useful if you’re piecing on two machines because you can REALLY introduce huge wavy bubbles into your quilts piecing on different machines. Ask me how I know.
I know there’s a tool out there you can buy to do just this, and some machines have a fancy “scant” function, but in all the machines I have used, not one of them has ever been set up exactly how I’d like it, and I really don’t like buying crap I don’t need and will probably misplace and never find when I need it and then lose precious time looking for it. If I can do it perfectly fine with something I already have, then that’s great. Also, different threads are thicker than others, and you need to adjust for that, sometimes–I’ve got a little notebook where I make a note of what threads require which needle postions, for when I am feeling super obsessive. Which, to be honest, hasn’t happened since I made it, but hey! I know how to do it!
And, if you don’t have a 1/4″ foot because you left it at home (who… me?) You can use this method to align it with the edge of whatever foot you DO have, or whatever line on your throat plate you can use, and adjust your needle position as necessary.