Hardware, Enclosure, B_shell.scad, LED has Line-Line Fit, Bad For Assembly #496

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1. ForrestErickson commented on Apr 26, 2026

   

   ForrestErickson

   on Apr 26, 2026

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   In the file It appears that the LED diameter is set to exactly 5mm. The same as the LED.

   Here is the parameter for the 5 alarm LEDs diameter. The Power LED is set elsewhere.

   

   This diameter is a MAGIC NUMBER and should not be.
2. ForrestErickson

   assigned

   Yhya99

   on Apr 26, 2026
3. ForrestErickson commented on Apr 26, 2026

   

   ForrestErickson

   on Apr 26, 2026

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   @nk25719
   Here is the AI explanation of why "line-to-line" fit is bad.

   A line-to-line fit (or "line fit") should generally be avoided in mechanical design because it is unstable, highly sensitive to manufacturing tolerances, and difficult to assemble. A line-to-line fit technically means the maximum material condition (MMC) of the shaft and the hole are exactly the same size, resulting in either a "zero-gap" (clearance) or a "zero-interference" (interference) state. [1, 2, 3]
   Here is why this is problematic:

   • Sensitivity to Tolerances (The "Almost" Effect): Manufacturing produces parts within a range of tolerance. If the shaft is at its maximum tolerance and the hole is at its minimum, the parts will not fit together. Because a line-to-line fit has no inherent "buffer" or "gap" built in, tiny inaccuracies in machining lead to interference rather than a smooth fit.
   • Difficult or Destructive Assembly: Because there is zero intentional clearance, assembly often requires forcing the parts together, which can mar surfaces, create debris, and lead to difficult or destructive disassembly.
   • Uncertainty in Final Fit: With a line-to-line fit, you cannot guarantee the outcome. Parts might be loose (vibration) or tight (press fit) on a part-to-part basis, causing unpredictable performance.
   • Thermal Expansion Issues: Because the fit is already so tight, minor changes in temperature can make the parts expand or contract, turning a "perfect" line-to-line fit into a jammed assembly or a loose one.
   • High Manufacturing Costs: To maintain a true line-to-line fit across many parts, extremely tight tolerances are required. This exponentially increases machining, inspection, and tooling costs. [1, 4, 5, 6, 7]

   Instead of a line-to-line fit, designers should typically use a clearance fit for ease of assembly and movement, or an interference fit (when intentional, controlled interference is required for a permanent joint). [1, 8, 9, 10, 11]

   AI responses may include mistakes.

   [1] https://prototek.com/article/a-guide-to-tolerance-and-fit-in-mechanical-design/
   [2] https://prototek.com/article/a-guide-to-tolerance-and-fit-in-mechanical-design/
   [3] https://www.linkedin.com/pulse/how-understand-machining-drawings-st-cncmachining-ltd-upz0e
   [4] https://tarkka.co/2019/02/20/fits-and-tolerances-how-to-design-stuff-that-fits-together/
   [5] https://rapidaxis.com/blog/beginners-guide-to-picking-the-right-fit-in-engineering/
   [6] https://xometry.pro/en-eu/articles/limits-fits-guide/
   [7] https://accendoreliability.com/importance-fit-tolerance-clearance/
   [8] https://www.fictiv.com/articles/engineering-fits-clearance-transition-interference
   [9] https://www.fictiv.com/articles/engineering-fits-clearance-transition-interference
   [10] https://www.studysmarter.co.uk/explanations/engineering/design-engineering/limits-and-fits/
   [11] https://restoric.co.uk/2025/05/16/understanding-tolerances-and-fits-how-restoric-delivers-precision-in-every-manufacturing-drawing/
4. nk25719

   added this to the Enclosure, 28mm SPK Release milestone on May 3, 2026
5. Yhya99

   added

   type: bugSomething isn't workingSomething isn't working

   on Jul 17, 2026
6. Yhya99

   added the

   Bug

   issue type on Jul 17, 2026
7. Yhya99

   linked a pull request that will close this issueincrease the led hole diameter (CLOSES #496) #588on Jul 17, 2026
8. ForrestErickson

   closed this as completedin #588on Jul 29, 2026