# How would a CSWE design a marble roller coaster? (Part 1 of many) [VIDEO]

Article by Adam Bridgman, CSWE updated July 10, 2013

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A marble roller coaster should be easy to assembly and use as little material as possible.  If it can be made with very little material then it can be 3D printed at very low cost.

When designing these parts I will consider how they will have to be printed to try and minimize the amount of plastic and support material required.

Another requirement of the 3D printer is that the roller coaster must be modular, otherwise the printer envelope would control how big the roller coaster can get, and a lot of support material would get wasted, therefore the trickiest problem to solve is how to connect the modules together. My first thought was to use a snap, but unlimited by software and manufacturing process I believed I could design a perfect spherical snap that would minimize space, maximize holding force, look good, and show off the capabilities of our 3D printers.  Here is a mock-up of what I came up with.

The actual snap that I designed is no longer available because I was so embarrassed that I created such a thing I deleted it without leaving a trace.  Then I thought why not take advantage of the quick snap functionality built into SolidWorks.

As I went through a few iterations with this I created a calculator in Excel to calculate all the appropriate values such as stress and holding force.  As I minitiarized them for my roller coaster I realized that snaps probably are not the best choice of connection for my roller coaster segmenets.

The third and final choice I came up with was to make a custom connector with dowels and a tapped hole.  Our 3D printers are capable of extremely fine surface finishes with tight tolerances, hopefully tight enough to print our own bolts and threaded holes.

Creating a bolt with threads is actually a relatively simple process.  But the first step is to find the specification for the bolt, I found this at: http://theoreticalmachinist.com/Threads_UnifiedImperial.aspx

This led me gave me the following dimensions:

 Max Min Middle Major diameter 0.112 0.1075 0.10975 Minor diameter 0.0872 0.0804 0.0838 Over wires 0.1165 0.1147 0.1156 Pitch 0.02083 0.02083 0.02083 Root flat 0.0047 0.0026 0.00365

First it is necessary to create the helix, this can be done by creating a circle on the tip of the bolt.  Once the circle is created a helix can be created using the helix command. In this case the number of revolutions can be set to a value somewhere beyond the length required. This in general is not good practice and can be corrected by using an equation, but there is no option such as up to surface.

Once the helix is created a plane must be created normal to the end of it.  This can be done by selecting curve and the end point.  This plane will hold the tooth profile cut.

The sketch should look like it does in the image below.  A pierce relation is a useful command in this case to position the tooth cut profile on the helix.

Turn this into a swept cut and add a chamfer and this is the final output

Please watch this video for a demonstration of this technique