I have the trailer remixed. I nixed the idea of zero fill. Instead I’m going with 3mm walls and 6% grid fill.
The tractor is remixed too. I considered several ideas for the king pin. Finally, I decided to create a hole in the back end of the tractor that can be printed without supports and with the tractor on its side on the printbed. To make the king pin as strong as possible I elected to cut it in half and print the flat sides on the printbed. In order to test this arrangement I created a test block, which is basically a section out of the back of the tractor. This way I can print the test block and the 2 halves of the king pin, assemble the king pin, and then check the fit and strength quickly.
Here’s a few photos of the result:
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Here are the 2 pieces: test block on the left and king pin on the right. Looking at the back end of the king pin you can see 1 of the 2 mortices that lock into tenons inside the hole.
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Look closely inside the hole and you can see 1 of the 2 tenons that lock the king pin in place.
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The king pin just started into the test block hole.
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King pin fully inserted into the test block.
I think the concept works well. Some adjustments are needed. The fit is too tight. I will shrink the 6 walls of the king pin by 0.1mm to start. I may also reduce the size of the tenons a bit.