Design
For my initial design, I imagined a simple ring-shaped stopper with a hole on either side of the ring where the needle would pierce through. My designs evolved into a more detailed solid object—one stopper shaped like a bear head and the other stopper shaped like a cat head (inspired by me and my best friend). The object is 3cm by 2cm to give it a good enough size to be manipulated, and the edges are smooth so it’s comfortable to hold. I initially designed the hole to be exactly 5mm in diameter, though that didn’t work—more on that later.
Most knitting needle stoppers are made from silicone and they cap the needle, rather than having the needle go through, but since this would be made from PLA I didn’t want my knitting needle tip to be hitting a hard plastic surface, so I designed the hole to go all the way through.
My points of evaluation were the following:
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Usability: The stopper can physically fit onto the needle without being forced, and also does not slide around by gravity (i.e. if the needle is tilted around, the stopper won’t slide).
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Desirability: The stoppers are attractive and cute (subjective to testers)
Prototype
From my initial designs, I began with a very simple paper circle prototype. This design actually worked well enough that for a week I used it as an actual stopper, though this helped me realize that the stopper can’t be hollow otherwise it gets squished. After that, I designed the bear and cat stoppers.
I designed my knitting needle stoppers in Solidworks. This was a fun experience because I had taken a Solidworks class nearly three years ago and I thought I’d forgotten everything, but a ton of knowledge came back to me as I started designing.
Since my size 8 knitting needles are manufactured to be 5.0mm, my first design had a hole radius of 2.5mm (diameter 5.00mm), but the needle didn’t fit inside. I eventually found out this was a combination of two factors: (1) using a caliper, I found that the needle’s diameter actually ranges from 5.02 to 4.98mm, and (2) the 3D printed supports inside the hole leave a small amount of residue which narrows the hole size (thank you Brock for helping me realize this!).
Through trial and error, I printed five tests: 2.5mm, 2.55mm, 2.65mm (twice; one fit and one didn't for some reason), and 2.75mm. At 2.65mm I tried forcing a fit by jamming a chopstick into the hole to widen it, but it was too much effort and didn’t really seem to help, and then when I increased the radius to 2.75mm I unfortunately decided to run my final full print with it (which took 3 hours) only to find it was too loose. At the very least, this allowed me to narrow down my guess to 2.70mm, which worked perfectly!
Analysis
I user tested and received feedback from three peers and a friend. In a test prior to my 2.70mm radius print, reviewers agreed that the 2.65mm radius fit was too tight (it took way too much effort to pull the stopper off the needle). One reviewer said that the soft edges from the fillets are comfortable to hold.
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In terms of my usability evaluation criteria, my 2.70mm radius prototype succeeds because it’s a perfect “smooth fit” —it goes onto the needle smoothly and doesn’t slide around if I tilt the needles around.
Desirability is harder to measure since it’s more subjective, but I’m proud to say that my reviewers all said unprompted that the designs were cute. One reviewer even said that they could see these stoppers as decorative trinkets.
Reflection
For next time, I'll give much more leeway to fits in 3D printing and take time knowing that I might have to print lots of guesses. I grew pretty frustrated each time I incremented the radius and it still wouldn't work---I still can't really believe I printed this project six times. Knowing this is a possibility will help me adjust my expectations and be okay with this as a learning process.
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I’m sincerely really happy with this project! It taught me a lot about 3D printing and an object creation process (it’s so incredible to create a physical object from an idea in my mind). I'm also so glad I got a reason to pick up Solidworks again, and now I've recovered a lot of my 3D modeling skills.
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I learned how to knit this summer and I've grown to love knitting, so I'm proud of myself for creating these tools to support something that brings me a lot of joy.