FlexiDinos Chompy the flexi Spinosaurus print in place articulated dinosaur

It's in the jaws

When I ran a poll for my club members this spring, I had no idea what I was getting myself into. I thought, ‘What could be so difficult about getting gears to work?‘ Everything! Try googling 'how to construct a gear' and you'll only find descriptions of how to use the automated gear creator in Fusion 360.

It took a trip to my favourite café and a stroke of luck to strike up a conversation with another customer there who had studied Maschinenbau twenty years ago. When I asked him if he could explain it to me, he said that it was too long ago and there was no way he could remember. However, he suggested that I search for 'Abrollkurve', which describes the shape of a gear's teeths sides. This was the magic word. I found this video, and everything fell into place. So, thank you to Stefan for pointing it out!

After starting over four times, I lost count of how many versions there were. The version published here is 4.98, but I had over 300 iterations of the third attempt before deciding that I couldn't work with that file anymore as it had become too confusing. 

This was also a reason why it took so long, as I needed to take some breaks  from the model because it wasn't fun to work on anymore. The design also felt quite wrong in the previous versions, and the Spinosaurus with all its weird angles is difficult to position. So, I had to choose between geared joints and a huge sail. I opted for the gears as I had wanted to try them for a long time, and the Spinosaurus, being the biggest predator of all time, gave me another advantage by providing some extra space to work with (where it makes sense my models are 1:100 scale to fit each other's size). If you want a sail, check Karli out.

Print Instructions

I achieved the best results when printing the Spinosaurus in PLA. The results with PETG were not so satisfying, as this model has very steep overhangs (over 50°). With my settings, it tended to sag, especially at the sail, which resulted in some unsightly irregularities on its surface.

The gears are very difficult to get right, and if the tolerances are slightly inaccurate, they tend to wedge in certain places. The good news is that this improves over time as you play with the dinosaur and grind everything into place. Also, there is a tendency for the wedging not to occur if you only pull the upper jaw; the lower jaw is dragged down by gravity and usually moves along.

When your settings are dialed in, the jaws tend to loosen quite easily. To avoid breaking something, I'd recommend gently pulling the upper and lower front teeth away from each other when opening the mouth for the first time.

Which tolerance for the geared jaws?

I have included a test file for the three different tolerances. You should print the three heads of the test file first, try the gears, and then decide which tolerance to use for the full model. The tolerance is embossed on each head on the plate side. When you have them printed, check how the jaws work and choose the according model to print. 

I even got very different results with a spool of PLA two days after unpacking it, compared to when it was freshly unpacked.

As always, please let me know if you have any problems printing the model. I will try to find a solution as soon as possible.

Happy printing!