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Pretty minor - but it puzzles me. All desk pencil sharpeners I've encountered have a spring triggered "feeding arm" (see image where the arm is extended).

Extended pencil sharpener

What's its purpose? I can sharpen a pencil without extending it, and after extending, why do I need the spring mechanism? It never triggers while there's a pencil inside.

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  • 4
    "It never triggers while there's a pencil inside." then it's not working properly.
    – Tetsujin
    May 29 at 16:36
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    The arm is supposed to push the pencil towards the knife so you don't have to apply pressure manually and risk pushing too hard.
    – Wolff
    May 29 at 17:38
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    And also to keep the pencil properly aligned so you don’t sharpen it at an odd angle.
    – Janus Bahs Jacquet
    May 29 at 17:42
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    Keeping the pencil straight obviously, but I didn't even know such a thing existed :)))
    – Lucian
    May 29 at 22:09
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In retrospect, I don't know how I missed it, but here are the details for anyone else wondering:

  1. Pulling the feeding arm extends the spring. When the arm is pulled all the way back, it's locked.

  2. At this point, rotating the sharpener crank (without a pencil) will release the lock, and cause the arm to spring back all the way in.

  3. But instead we keep the arm locked, and secure a pencil inside. When we rotate the crank, the arm (which now 'carries' the pencil with it) doesn't spring all the way in because the pencil end hits the blade - which is laid out diagonally (thus creating the desired cone shape when rotating). The spring power is used to push the pencil towards the diagonal blade while it's rotating.

  4. A secondary goal of the arm is to keep longer pencils leveled to create a symmetric sharpening.

(Thanks @wolff & @Janus!).

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  • 1
    Have you tried out what happens if you keep turning the handle? If you did, you could observe that the entire structure in question is to keep pulling the pencil inwards as the sharpening process progresses.
    – Levente
    Jun 1 at 23:06

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