Professor Joshua Pearce writes about how you can use your 3D printer to create functional replacement parts.
Although your 3D printer can certainly be used to make models and toys (and save a lot of money doing it), desktop 3D printers can now be used to make actual functional replacement parts for complex machines that demand mechanical integrity of the parts.
In order to investigate the potential, my 3D printing research group teamed up with John Gershenson’s research group that specializes in the design and mechanical engineering of bicycles. We--Nagendra Tanikella, Ben Savonen, John Gershenson and I--made a careful study of the use of open source 3D printers to fabricate widely used Black Mamba bicycle components. Specifically, we chose to start tests with pedals that fail often and have clear standards namely the CEN (European Committee for Standardization) standards for racing bicycles for 1) static strength, 2) impact, and 3) dynamic durability. First I will explain the tests and then how to avoid having to do them yourself.
We made a CAD model of a replacement pedal using parametric open source FreeCAD to enable future customization. You can download both the CAD and STL files from Youmagine. Then we printed it with PLA, a biodegradable and recyclable bioplastic, because it is the most common 3D printable material. It also has good strength and reasonably low costs (see this paper) for further information.
Static strength test
The CEN static strength test for bicycle pedals requires that the pedal be subjected to a 1500 N vertical downward force as shown below. The test is satisfied if there are no fractures present.
We tested to a compression load of 3,000 N applied uniformly on the pedal, double the prescribed amount to clearly test for exceeding the standard. Nothing happened, our pedals clear the first hurdle!
The CEN impact test for bicycle pedals requires that a mass of 15 kg be dropped on the pedal from a height of 400 mm, 60 mm from the mounting face, as shown below.
The test is satisfied if there are no fractures as you can see the test made a minor dent but no major damage was sustained. It passed test #2!
Dynamic durability test
The CEN dynamic durability test for bicycle pedals requires that the spindle is spun at 100 rev/min for 100,000 revolutions while the pedal has a mass of 65 kg suspended by a spring. This test is intended to simulate a real-world bicycle with a person standing on the pedals. In this case, testing was designed to surpass the CEN standard in more realistic conditions. The pedal was attached to a bicycle and tested directly rather than with a testing rig. We went 200,000 revolutions where the person’s weight (75kg) was carried by the pedals alone (again double the standard). It passed test #3!
Our humble 3D printed pedal is now good enough for European racking bikes...but wait it is actually better!
Print your own to ride faster and save money
The 3D printed pedals are significantly lighter (almost a third of the mass) than the stock pedals used on the Black Mamba. This provides a performance enhancement without even forcing you to shave your legs! Better yet, it cuts the cost of replacement pedals if raw pellets PLA or recycled materials (such as ABS, PET or nylon) are used as the filament. Therefore 3D printing bicycle parts is both technically possible, but can also reduce bicycle costs for everyone.
DIY trick to get lab results without the lab
This is nice, but you probably do not want to redo such experiments for every part you want to replace on your own bike - or on anything else you might make with your 3D printer. To avoid this work follow these simple guidelines:
- Identify a study that looked at the parts you are interested in, like this study: Viability of Distributed Manufacturing of Bicycle Components with 3D printing CEN standardized Polylactic Acid Pedal Testing for the bike pedal. More are showing up all the time.
- Identify the mechanical requirements for the 3D printing material. For a good open access list of most commonly available tensile strengths of common materials see this study: Tensile Strength of Commercial Polymer Materials for Fused Filament Fabrication 3D Printing.
- Print the object in the correct material and required infill to get the mechanical properties you need for your application.
- Then inspect the exterior of the print for sub-optimal layers from under extrusion like in the image below. If under extruded fix your printer and try again.
- Lastly, in order to determine if there has been any under-extrusion in the interior that you can’t see, weigh the part. A digital food scale has acceptable precision and accuracy for most FFF prints. This mass is compared to the theoretical value using the densities from this table for the material and the volume of the object.
This works because the study above found a nice linear relationship between the ideal mass of the printed part and the maximum stress samples could take. If your part is off of the ideal, you can look at that study to see how far off. You can then decide if you need to then reprint before obtaining a high probability of having the properties you need. If you look closely at the mechanical studies that have been done on an assortment of FFF printers, you will note the open source printers can have stronger prints than proprietary FFF and FFF machines. This is mostly due to settings limitations of the proprietary systems. But be aware that there is a range and the properties of your parts will depend a lot on your machine and the settings you use. In general printing at the high end of the extruder temperature range for your material will result in a higher strength. You can figure out where your print is most likely to fall on the strength range using the weighing technique and comparing your mass to the ideal.