by Jacob Bakovsky, Application Engineer, GoEngineer

The marketing department challenged me to create a fun, 3D printed train for kids. I wanted to make something that was easy to print and assemble, a functional train that would go around a floor or table. Plus, I wanted a train that didn’t require a track, one that could be immediately ready for action.

Here’s what I designed:

Prototype X1: Initial design
My initial design (Figure 2) was similar to trains I found online in that it required the user to print multiple, various parts and assemble them.

The number of parts, especially small parts, seemed excessive, let alone the agony of fighting the tray to get the 3D printed support material off the smaller pieces. Keeping track of them in the support tank would be another challenge. Plus, the intricacy of fitting all the printed parts would require some trial and error for tolerance fits on the design end of things.

It wasn’t an appealing route for me to consider.

Most of the GrabCAD and Thingiverse models required assembly, so I decided to try designing it myself in SOLIDWORKS and then printing it using our Stratasys F370 FDM machine.

Prototype X2
I wanted to simplify the train so it would be ready to go after removing it from the support tank. The 3D printing experts at GoEngineer recommended I give the wheel axels a 0.02” clearance (Figure 3). I placed a 1 mm clearance between the housing and wheel so the entire thing could print as one assembly. (We figured a little slop in the fit wouldn’t hurt the performance, and the support would break off and allow the wheels to rotate once removed from the tank.)

To ensure the wheels would stay round, I oriented the train for printing as shown in Figure 4. The front of the train has excessive geometry, and the entire thing does not need to be filled solid, so setting the print fill style to Sparse-Double Dense saved me a few cubic inches of material. I was worried the snap-fit would not work if the train was printed on its side, so I printed second coal bed train to see if it would fit.

Post print X2

After printing prototype X2 (Figure 5) I discovered the snap-fit works great (Figure 6) and can allow the second coal train car to rotate freely. However, the wheels did not budge. A considerable amount of support material seemed to be lodged inside the front of the train’s main housing and the pieces wouldn’t move, even after a few days in the tank.

Prototype X3
Since prototype X2 wheels didn’t move and the goal was to minimize the number of parts and amount of hardware, I printed a movable hinge to minimize the assembly needed and to have moving parts ready once prototype X3 was removed from the support tank. Because the snap-fit worked so well, and movable hinges were easier to print as one, I built in a snap-fit and movable hinge on the bottom of each train car. This also eliminated the problem of having support material lodged inside the axle.

I intended that the prototype be assembled in a single step, so the user could also glue the snap-fit hole to prevent the bottom portion from dislodging. Using about a 3mm tolerance for the hinge diameter (Figure 7), I designed the hinge in multibody and assembled it in SOLIDWORKS assembly mode with a hinge mate.

I created the bottom of the train by slicing the center of the existing wheel holes to sandwich the axels in place with the snap-pin in the front, as shown in Figure 8.

Using the tray for the Stratasys F370 I printed a test with just the front train to see if the concept would work (Figure 9). I used SOLIDWORKS Composer to quickly generate the exploded view and the digger tool to capture a zoomed-in image of the snap-fit pin.

Post print X3
After a successful print I was confident that prototype X3 worked and a user could have a functional train in only three assembly steps. The movable hinge works perfectly once the support is removed and makes it easy to slide the wheel and axle assemblies into place. The user flips the train upside down, places the axels in the grooves, snaps the bottom shut, and snaps the two linkages in place for the large wheels. The snap-fit is incredibly tight and works so well there is no need for glue. The assembly instructions are detailed in the production section below.

However, there is one design flaw that became obvious when looking at the assembled train.

The bulge in the middle large wheel assembly sticks out too far and impedes the linkage, so a minor change to prototype X3 was eliminating the bulge shown in Figure 10. Since the front train was so successful, I printed the remaining train and associated wheels. The details for the print estimate for the rest of the train are shown in Figure 11.

Production A
The final production design, updated and rendered from SOLIDWORKS Visualize, is shown in Figure 12.

The printed version is shown in Figure 13.

Production assembly instructions
To quickly generate instructions on assembling the train I created views in SOLIDWORKS Composer, exported multiple views, and then inserted and linked the views here (Figures 14 through 16).

 

Repeat steps 1 and 2 from the instructions above to assemble the rest of the train.

Additional considerations:

• I used GrabCAD Print to process the 3D printing for the Stratasys F370.
• I printed using ASA and 0.010” slice height for all prints.
• To save on material for the three cars, switch the printing style to Sparse-Double Dense to save on material for the main cars (Front, Coal and Caboose Train).
• The axles for the wheels are delicate: When you pry the wheel and axle assembly off the printing tray, grab it from the bottom, not the middle, or you will snap the axle.
• The rod for the hinge in the back is thin, and the snap-fit is strong. You can pry it off and reuse, but you will most likely snap off the hinge rod (this happened to me). The tolerances are tight enough that the hinge can be snapped in place even with the broken hinge rod and hold without needing additonal glue, tools, or hardware.
• One entire train will consume roughly an entire 96 in^3 spool of material.
• To ensure a proper fit, make sure all the support is out of the snap-fit holes and extrusions.

If you’d like the files to print the train yourself, you can download them from this GrabCAD site. Good luck, have fun, and let me know how it goes!

GoEngineer
www.goengineer.com