Totally agreed. That’s why NMW printed parts are as short as possible and BEEEFY
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I think this is a universal truth that should be respected across every industry, not just our very niche little bicycle framebilding bubble… But more often than not it isn’t, and that’s where countless very expensive dust collectors on some middle management desks come from 
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I’m curious to read what you’ve found. Fatigue is an important consideration and I want to learn as much as I can before spending more time and money on prototyping and testing.
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@scharencycles I’ll do my best to put some information together over the weekend (I promise this time). Sorry the follow-up has taken so long, we’re about 6 weeks from the first snow in the mountains where I live so it is crunch time for my first frame build 
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Spencer from EOS was at MADE and was kind enough to share some insight into the industrial 3D printing world. Our convo led me to some more avenues of exploration
I don’t want to misquote, so don’t take these notes as the rule of law. Most of this was also regarding 3D-printed titanium:
- Fatigue is a big concern for additive metal parts
- Aerospace does fatigue testing, but they don’t share the data because it’s expensive and proprietary to their process
- Surface finish has a big correlation to fatigue life
– so much so that companies will go to lengths to smooth out the interior of their prints - Thinner walls are more effected by surface imperfections
Again, don’t quote me on it, but certainly more food for thought!
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Alright, I sat down to do a write-up on this subject over the weekend and was tracking toward a post that was >3000 words long, so I decided to call it quits for the time being. It is a challenging topic to address adequately, in my opinion, because it involves technical aspects as well as issues of safety and responsibility. So, for now, here’s a few bullet points summarizing key considerations for designing additive parts. Many of these have already been suggested, but they’re worth re-emphasizing:
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Unless something has gone horribly wrong, bike frames and parts fail due to fatigue. So, while you can use the yield strength, ultimate strength, and elongation provided in material datasheets to make basic comparisons between materials, they’re not going to give you much insight into fatigue performance. As a general rule, it is a good idea to use alloys in their intended applications. In the case of MS-1, I’d be reluctant to use a tool steel in a structural application unless I could generate data showing that its performance was equal to or better than something like SS 17-4, for example.
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The fatigue performance for additive metals generally exhibits significant anisotropy (direction dependence), with the lowest strengths corresponding to loading across build planes (trying to pull layers of the print apart). Unfortunately for tubular bike parts, this corresponds to the direction that maximizes printability and space efficiency on the build plate, and minimizes distortion, so it is what we have to work with in most situations. Regardless of print orientation, the fatigue performance of additive metals is worse than their conventional counterparts.
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The fatigue performance for additive metals also exhibits significant variability, which makes it hard to determine the correct design allowable and make comparisons between materials. This variability is also hard to characterize, especially across suppliers, because there is not a standard set of print parameters or a standard heat treatment that everyone uses. This variability is due to defects which result from the printing process (i.e. they are inherent to the technology and are unavoidable), and the particular sensitivity of fatigue performance to stress concentrations.
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In my opinion, the variable fatigue performance exhibited by additive metals poses a challenge to people who are testing their parts – unless a part or frame passes the required ISO test with flying colors (for example, it makes it to 1,000,000 cycles when only 200,000 are required, not sure of the exact numbers in these specs) I would not feel comfortable counting that as a pass. Testing is definitively a good thing, so I’m not arguing against its usefulness, but if you’re testing a low number of parts made from material with properties that vary more than the standard metals used in the industry, different success criteria are justified.
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I am also dubious about the efficacy of internal lattice structures. If the print orientation is optimized for the outside wall of a part, the internal lattice (or gyroid – applies to all infill patterns) will inevitably print in more extreme orientations in certain locations and increase the likelihood of generating a problematic defect. Infill structures also increase the unfinished surface area of the part, and likely create small stress concentrations where they intersect the walls. If these structures dramatically increase the part’s stiffness they may be worthwhile, but if I were designing a part I’d be inclined to add the extra material to the wall thickness instead. The same goes for internal ribs – internal ribs are most effective at reacting bending loads, but bike parts experience combined loading (bending, torsion, shear, tension and compression) and adding material to the wall instead will benefit the structure’s ability to react these other loads, too.
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I’d like to see more people experiment with vibratory tumblers to finish their printed parts. Tumbling media that works on hard metals like titanium and stainless steel is available for purchase, so it should be an effective way to smooth surface irregularities. Other than material selection, surface roughness is the only variable affecting fatigue performance that builders have control over, so it is worth putting some effort into improving this area. Printed titanium will benefit most, but it is worthwhile for stainless steel parts, too.
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I would be hesitant to stray from print fabs that have an established reputation in the cycling world unless I was able to work with a place that also serves more advanced industry. I credit Bastion, Prova, and Sturdy Cycles with bringing additive manufacturing to the drop-bar bike world, and each of them started by using Ram3D. Somehow Tom Sturdy has gotten comfortable enough with Ram3D’s processes to print entire forks, handlebars, and stems, but I don’t necessarily think this is an example that should be followed.
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If I had limited ability to do useful stress analysis or testing of my parts, I would try to move my design as far in the direction of “goodness” as possible. So, I would do as much as possible to eliminate stress concentrations (small radii, large changes in part thickness or cross section over short lengths, etc.) and look to aluminum frames for inspiration when it comes to wall thickness and geometry. There’s a lot of nuance and detail to this discussion, which I may have time to develop further over the winter, but I can’t make any promises for now.
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Finally, in my opinion, ride testing is useful but not good enough for parts that have catastrophic failure modes like stems, handlebars, and forks. The insidious thing about fatigue failures is that they happen suddenly and usually after long periods of time. So, if you do 500 miles on a prototype and call it good, you can really only sign off on your product for 500 miles of riding. The ISO tests are not perfect, but they do a decent job of solving this problem on timescales that are useful for product development cycles, and generally ensure that riders are safe and businesses retain good reputations.
Anyway, again, mostly reiterating what others have said but hopefully that is helpful!
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Also, in case anyone thinks all this concern about fatigue performance is unwarranted, Mythos, the component spin-off of Metron Additive Engineering (the people responsible for printing the Pinarello Filippo Ganna used to break the hour record last year), only offers a two year warranty on their printed stems. Two years!! For a stem! I’ve been mountain biking for a long time, and I’ve seen just about every component break at some point except a stem. I’m sure there are examples out there, but it’s not exactly a component that is top-of-mind when it comes to liability. This is despite Mythos testing the stems to the relevant ISO standard and making superlative claims about their stiffness in the marketing material.
EDIT: It looks like stem warranty periods are a bit all over the place regardless of construction method, so maybe I’m being a bit negative. Still, a classic example of a hammer looking for a nail imo.
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Excellent! I could not agree more! Thanks for taking the time to write that up. I hope people can take the time to read it.
I totally agree that fatigue is the biggest question. I know of a few handmade bikes with both 3D-printed stainless and 3D-printed titanium weldments that have passed ISO testing (withholding the names because I don’t know if they want me to reveal them).
But to your point, with 3D printing, it’s not a 1 to 1 extrapolation to all designs and prints. The bikes that passed those tests would only validate that specific design, lattice, print orientation, and printer.
I think the fact that a few handmade bikes with additive parts have passed ISO is good news for the future. The printed parts and designs may already be good enough. I have seen a few high-end titanium frame manufacturers in Asia start experimenting with printed parts this year. Since they make thousands of frames per year and have testing equipment, I am sure they are extra conservative when adopting the technology. I would love to chat with them to get their experiences so far.
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This is an excellent post, thank you for taking the time to do it. 3D printing is here to stay, and should only get better, as all technological advancements do. Paying close attention to the pros and cons in printing will help to manufacture higher quality components.
We have a good selection of vibratory tumblers, and run them almost daily. If anyone out there wants to add their parts to some of ours, we’'ll do it no charge to see what they look like. (Test subject pays shipping both ways). Respond to this or DM me if interested.
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Adding some thoughts to the topic. Peter Verdone has broken some 3D printed parts on his bikes and thankfully walked away. He graciously documents his design process and failures as well as success. Here are 2 places where he had failures.
I don’t have personal experience with 3D printed steel or titanium, but I am super interested in the future implications and trying it out for myself one day.
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You got it @Daniel_Y!
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Further highlights the need to get parts from reputable businesses using well maintained machines. Much like castings, an AM part can look good on the outside but be full of big ol’ holes!
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Excellent write up and 100% agree with everything you’ve comented on.
I held off using 3DP because I was wary of it but now that I have had some made, put them into use and done my research I am more comfrotable to keep venturing down that path. As it’ keeps being repeated, fatigue is the enemy. Smart design that looks after loads and load paths and not trying to cut weight is the best way forward with these parts. In reality they make up a small part of the total frame so adding material/wall thickness is not really a penalty to the bike.
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Yeah, I’m also cautiously optimistic about the future. Parts with good geometry generally seem to survive which is encouraging. Hopefully that continues to be the case as more suppliers come on board and there’s a greater number of designs in the wild. If the Asian factories start using AM parts in their frames that should give us a decent feel for the margin in designs and how well they can be produced at scale.
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This might be worthy of its own thread, but does anyone have connections to ISO 4210 testing? I’m interested in making a printed fork crown and that would provide at least some peace of mind.
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My team works with ACT in Long Beach, Ca pretty frequently. IIRC they do the ISO testing at that facility.
They’ve been great with guidance, etc.
Feel free to DM me. I can do an introduction to the guy we work with.
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I loosely know a few people at EFBE in Germany, and afaik they’re always up for testing novelty stuff…
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I am intimately familiar with ISO4210 and CPSC1512 testing standards, if you or anyone are interested in learning specifics about the testing protocols, feel free to send me a message!
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Anyone care to share how they are finishing/polishing their 3d printed parts? I have a bike almost done with some 3d printed lugs that my client wants nice and shiny. Have been using my belt sander (similar to the famous Brody one) which works great but small corners I still have to by hand, but maybe there are other clever tricks? Frame will be powdercoated.
current state:
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Use finer grades of sandpaper with a square timber dowel backing till you get to 1600ish then hit with a metal polish.
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