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I'd like to build a scale model catapult, and I'm building a tensioning/ratchet system similar to what is shown in this video:

https://www.youtube.com/watch?v=yi4p8ZR4n28

The ratchet gear in the video has a square keyway and the rotating bar is cut square as well, giving a high strength joint.

Unfortunately I don't have a mill or a lathe, so mechanically fastening my gear to my shaft would be difficult. I'm wondering if a bronze-on-bronze shrink fit would be strong enough to hold the torsion force. Or whether a brazed joint would be enough. Given that this is a scale model using torsion ropes, I don't really know how much force that would be either.

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    $\begingroup$ nothing stopping you from using both interference and a key $\endgroup$
    – Tiger Guy
    Oct 1, 2021 at 19:11
  • $\begingroup$ A close fit with high-strength loctite (example) is surprisingly strong - if it's keyed it might be good enough $\endgroup$
    – Pete W
    Oct 1, 2021 at 21:19

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A shrink fit is possible, but do you have the mill or lathe to make the appropriate dimensions and then sufficient heating and/or cooling?

Liquid nitrogen is a possibility, but a household freezer is about -18 deg C and a household oven about 280 deg C, which means a delta T of 300.

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  • $\begingroup$ Dry ice may be available in some grocery stores. A keyway is a poor method to transmit torque , shrink fit is preferred . Hot water is convenient to expand one member. $\endgroup$ Oct 1, 2021 at 18:12
  • $\begingroup$ @blacksmith37 I did not mention a keyway. $\endgroup$
    – Solar Mike
    Oct 1, 2021 at 18:29
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    $\begingroup$ kudos on needing to machine an interference fit $\endgroup$
    – Tiger Guy
    Oct 1, 2021 at 19:12
  • $\begingroup$ The OP specifically said they don’t have a mill or lathe. $\endgroup$
    – Eric S
    Oct 2, 2021 at 13:01
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As you don't have a mill or a lathe, I don't see much options here. Brazing is a very strong joint if done properly (nearly as good as welding).

You could also drill, tools permitting, a round keyway or keyways.

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