A little while ago I got the idea to model up a dump truck for my nephew who really loves trucks. I chose the Caterpillar 777f, as it is absolutely gigantic and looks pretty cool. Since I recently started using OnShape when it came out with it’s beta a few months back, I decided this would be a perfect project to test the capabilities of OnShape.
I started off using a sales PDF and scaled everything 1:12 by inputting the dimensions as inches instead of feet. In order to make the truck printable on my Makerfarm Prusa i3, I simplified the geometry and split up components into multiple parts that I glued together later. I’ve made it a public project in OnShape so feel free to check it out if you’re a user. One of the cool things about OnShape is that it allows you to easily export to STL right in your browser. After drawing and assembling all of the parts, some of them will need to be reoriented. I did this by copying them into a new assembly, converting that into a parasolid, converting the parasolid to an OnShape part with the “Flatten” option checked. This process allows you to separate the geometry to a new file and then make modifications/translations to allow for export to an STL for printing. Below are some screen captures of the different components:
Next it was time to print. Printing all of the components probably took around 30 hours spread over 6 print jobs.
With the parts printed, I then needed to make the axles. I found a really cool product at a local Ace Hardware from K&S Engineering. They have a number of different tubes that seem to be readily available online. I used the #9309 stock profile. It was a 36″ long aluminum tube with 0.035″ wall thickness, making the inner diameter around 0.116″. The standard drill for a #6-32 thread in aluminum is a #36 drill bit at 0.1065″ so while it won’t give you a perfectly structural thread, it did the job well for this application. I was able to cut the tube with a hack saw and thread it quickly by putting the tube in the chuck of my cordless drill. Additionally, all of the holes had to be drilled out with a drill bit. The aluminum rod seemed to come in at 0.189″ instead of a dead even 3/16″. So, I used a #10 drill bit at 0.1935″ to make sure that there was enough clearance for the wheels to freely spin, but not so much that they wobbled.
Next, I had to bend one of the hand rail parts. On the truck, the left side hand rail has a bend in it. Instead of printing the part with a bend and then needing support material, I decided to print the part flat, and then bend it with a heat gun. At a thickness of only 0.080″, the PLA part heated up very quickly and became very pliable. I bent it to the correct angle with a simple jig that I designed to hold the rail and then allow me to bend it once the plastic was pliable. The bending process took about 5 minutes and gave me a much cleaner part than if I had printed it with support material.
With all of the parts ready, I used 5 minute epoxy to secure everything together. On parts that get assembled together, I modeled in an offset of the mating surfaces by 0.010″ to allow for printing tolerances. I’ve found that this allows me to make parts the fit snugly with a bit of shaving/scraping using a razor blade. It took about 30 minutes to put together.
You can download the part files on Thingiverse… here are some pictures of the finished truck. If anyone ends up making one, I’d love to hear from you! Enjoy!