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john.yu
632
Jan 26, 2018
Hi everyone! For those of you who don’t know me, I’m a mechanical engineer here at Massdrop, and I help manage development and production of many of the products we manufacture.
TL;DR We are preparing to implement a new pocket clip design into production. The re-designed clip is easier to use and has a higher yield strength, so it should be an improvement on all fronts.
Following the first shipment of the Massdrop x Ferrum Forge Falcon knives, a few customers voiced concerns about the pocket clip permanently bending and deforming, often due to the clip getting caught on an object when the user is navigating the environment of daily life. We wanted to take a deeper look into this and see what the root cause of these issues were, and understand if there is the opportunity to make this part perform better.
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Since a pocket clip is essentially a cantilever beam with force applied to the free end of the beam, root cause of the failures are plastic [permanent] deformation clip along the length of the beam. The first version of the clip features a design of varying thickness and a relatively short thin section. The thinnest portion of the beam is the first deflect when force is applied, thus the design only allows a relatively short portion of the clip to flex under load; this situation forms a stress concentration at the thin section of the part. Once the force applied exceeds the yield strength of the material, the clip will deform permanently.
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After a few rounds of design iteration, review, and prototyping (a joint effort with the teams at Ferrum Forge and WE Knife) we arrived at a final design.
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The new design extends the thin region of the pocket clip to allow for more deflection before the material yields and permanently deforms. To ensure the new design is a quantifiable improvement, we sent some samples of each design (keeping the material of the clips the same, so the only variable is geometry) to a test lab to perform cantilevered force testing. The test data is plotted below:
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This plot shows the load (x-axis) vs. strain (y-axis). Strain is measure of the amount the part is deforming under load. The following areas of the graphs are significant:
  • The linear portion of the lines show the parts deforming elastically, meaning if the applied force is removed, the part will return to its unloaded state. For the best reliability performance of this part, we want to ensure the maximum expected loads fall within this region of the force response curve.
  • The point where the lines transition from linear response to non-linear/chaotic response is the condition where the material is yielding and transitioning to plastic deformation. The further this point is along the X-axis, the higher load the part can withstand before it deforms permanently. With this in mind, the following conclusions can be drawn:
  • The new clip design yields at a higher load than the old design.
  • New clip 70N = 15.73 lbf = 7.13 kgf.
  • Old clip 60 N = 13.48 lbf = 6.12 kgf
  • The slopes of the linear regions of the lines indicate the incremental amount of force required to open the clip. In this case, the steeper the slope, the more the clip will deflect when the same amount of force is applied. For our part, this means:
  • The new clip has a steeper slope than the old clip, which means it is easier to open.
  • Another way to interpret the differences in strain rates is: given a set thickness that the clip must be opened (e.g. the thickness of a pocket), the new clip design requires less force to open, and therefore has a bigger margin of safety from its yield strength.

We hope you enjoyed this glimpse into some of the work we do to ensure our products are the best they can be. It’s incredibly rewarding for me as an engineer with a focus in user-centered product design to be able to take in such direct feedback and translate them into meaningful product improvements.
TomKau
15
Jan 31, 2018
john.yuHi John/MD, I gotta say, the proactive approach to improving this design collectively & departmentally is commendable. Was the T/R&D done by all parties? Or conducted by MD then approved by FF & WE? If by all, would more data than the graph be available to check out? I only ask because am finding it difficult to interpret the statements to the graph. (I'm not an engineer!) As an example, the point at which plastic deformation may occur on the new design looks to begin at around 42Nm with Compressive Strain approx 33%, not the 70Nm stated. By Yield I'm imagining a bend/break situation, so yes the new design takes more force to become broken but appears to begin showing signs of stress around 40-42Nm? Whereas the old clip looks to sustain more force (58Nm approx) before bend/break failure? How, without changing materials, can the Compressive Strain % increase while the part thickness decrease? Understandably the elongation would increase the area affected by stress concentration, as represented with a linear form, but ultimately the breaking point remains the same. Is the 15-25% increase in Compressive Strain accurate under these constraints? Also, is Compressive Strain (%) meaning Compressive Strength or Tensile Strength for this representation?
Keen to learn more about the process! Thanks
john.yu
632
Jan 31, 2018
TomKau@TomKau Thanks for your interest in this discussion! We did this design change in close collaboration with FF and WE (lots of CAD files getting passed back and forth, design reviews, and prototype parts getting made).
One of the key things to keep in mind here when comparing graphs with stress/strain curves typically found in text books is our experimental setup is different. Usually those tests are pulling or compressing samples perfectly aligned with the central axis. This minimizes the amount of change in the direction the force is applied as the material deforms (this is vert important when trying to characterize material properties). The test setup for our clip samples is a cantilever bending/deformation setup, so the moment is actually constantly changing as the part deforms (thus the vector of the applied force is changing). This means the plotted lines are not perfectly linear while the test samples are deforming elastically. I've marked the points in the graph where the material is truly yielding.
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As to your question about the relation of part thickness, the thickness of a beam is the most important variable that affects stiffness of a beam (In a rectangular cross section, the equation is (b*h^3)/12). Thus decreasing the thickness will allow the part to bend more (increased Strain %). However because local stresses are higher when thickness decreases, it will break at a lower load so it's definitely a trade-off.
In our graph, "Compressive Strain" can be equated to Deflection, meaning it is not factoring material properties or other factors that affect Strength. (https://en.wikipedia.org/wiki/Deflection_(engineering)) This is OK because all of the samples are using the same material.
Hope this answers some of your questions!
TomKauGreat questions and thanks for your keen interest!
rob-okray
185
Apr 9, 2018
john.yuI fully support and commend you for this corrective action and follow up testing as a now medically sidelined qa/reliability lab tech, I appreciate the effort and attention to detail.
john.yu
632
Apr 9, 2018
rob-okrayThank you for the kind words!
Joeyz
0
May 15, 2018
john.yuI seem to have the old one. I received my falcon drop march/april but it is the old clip design..?.. How do i get the new one? Is it free from mass drop? Thank You, My email is: joeynovack@rocketmail.com
JonasHeineman
5987
Jun 12, 2018
JoeyzHi JoeyZ - Discussions are a good place to ask general questions, but not to get support requests addressed. The best way to get support is to submit a help ticket by clicking the "Contact Support" button below any order in the Transactions area of your member profile.
If you broke or bent yours, you can ask for a replacement. If you simply prefer the new style, it will be offered at a discounted price for all members who have the original version, and then it will be available at regular price to everyone else.
Gunnersmate2
1646
Sep 3, 2018
JonasHeinemanWhen will the new pocket clips be available? I have a first run falcon (serialized) that I LOVE!! I don't carry it daily and have not had any issues with the clip, so far. How about trying to organize a drop with pops custom clips on ETSY
Gunnersmate2
1646
Sep 15, 2018
john.yuHello john.yu I have a first production run falcon and just recently received a crux with a new designed pocket clip. The new design clip is awesome, it looks basically the same but has a completely different feel and action. Thank you for addressing our concerns. Makes me feel like a valued customer. Keep up the good work
john.yu
632
Sep 15, 2018
Gunnersmate2Thank you for the kind words...! Glad you like the new pocket clip design. We're still working on how to handle spare parts from a logistics/operations point of view, so we hope to have those available separate from the knives soon.
Gunnersmate2
1646
Sep 15, 2018
john.yuI ordered a pops custom clip off etsy for my falcon, not because I had any problems with the original clip, I just wanted a deep clip. I do have to say I will be carrying my crux with the new clip more because of the clip redesign.
Gunnersmate2
1646
Sep 15, 2018
john.yuSince you have intimate knowledge of ferrum forge knives can you tell me what the coating is on the standard grey titanium handles. The crux I just received sparkles in just the right light. I like it, just curious
Benjabooly
373
Sep 17, 2018
Gunnersmate2I believe it is a stonewash. It is beautiful but can be sensitive.
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