Welcome to Section View! A (mostly) weekly newsletter focused on prototyping and R&D for mechanical and electrical engineers. We’ll be sharing our favorite tools and techniques for designing and building better prototypes, along with an equal dose of inspiration and a pinch of silliness. We hope you learn a little something with each issue while having some fun. Subscribe here.

(Digital) Tool of the week

This week's tool of the week is FEMM.  It’s a free Windows finite element solver for 2D and axisymmetric magnetic, electrostatic, heat flow, and current flow problems with graphical pre- and post-processors.

At first glance it looks a bit like being transported back to Windows 95, but once you get past the interface this little finite element package is super handy. I’ve used it primarily for magnetics problems related to hall sensor triggering, and for ballpark answers to heat transfer problems.  Import a DXF, apply material properties and boundary conditions, auto-mesh and solve. Highly recommended.

For those of you looking to spruce up your Finite Element skills and adopt the method for problems other than stress analysis, you may find our FEA engineering guide helpful to build your analysis confidence.

Practical Finite Element Analysis
Understanding the fundamental math, guidance for building trustworthy models and interpreting the results.

Prototyping Tips

It's just holemaking, how hard could it be?  If you’ve ever tried to finish a precision bore with a reamer you know it can be a nerve wracking process. Your part is nearly finished and you’ve got several days of project timeline at stake if it gets damaged. I made all kinds of mistakes before I learned how reamers are actually supposed to work. I didn’t even know what part of the reamer actually did the cutting (as stupid as that sounds). Eventually I learned that feeds and speeds are absolutely critical for preventing chatter, surface finish problems, or bell-mouthed holes. I find the Hannibal Carbide reaming guide to be a super helpful resource when I’m in the shop. Also their reamer selection guide is a great primer on tool geometry for the uninitiated.

Designer’s Corner

Stay Flexible.  I’ve been thinking a lot about the shortcomings of modern CAD/CAE design tools and one area that particularly stands out to me is flexure design. Most engineers have heard the “everything is a spring” refrain from a coworker before, but I find it particularly hard to internalize when I’m designing in CAD. Compliance in a mechanism is something that is so often only considered after the design phase when parts are actually manufactured and assemblies come together. I find CAD further reinforces this kind of thinking by unconsciously locking me in a “rigid body mode” of design. So in an effort to broaden our collective toolkit as design engineers we’ll be looking at flexures.

The original Other Mill is perhaps my favorite example of the simplicity and elegance of using compliance as a design feature.  The y-axis stage of this tiny CNC machine rides on two linear rails mounted to the HDPE plastic frame. To prevent over-constraint and binding of the machine table, one of the linear shafts is mounted using a beam style flexure that allows deflection in the machine x-axis direction.

I love that their solution to a potentially complex manufacturing tolerance problem is to simply remove enough material for the second linear shaft to deflect as needed.  Brilliant.

For more mechanism inspiration you might like this flexure demo board from Amy Makes Stuff.

I had never seen the butterfly rotational flexure before, and there are some nice detent mechanisms as well. Her github page has a BOM for a flexure based gripper inspired by the Festo MultiChoice Gripper.

The incredible Dan Gelbart has a great summary of the advantages of flexures that is worth internalizing. When designed properly, flexures require no lubrication, have no wear, no backlash, and incredibly low cost relative to performance. The backlash advantage alone means these mechanisms are particularly well suited to precision motion applications like x-y stages. Waterjet, laser cutting and EDM processes all enable complex and precise flexure mechanisms essentially for free.

If you are looking for a more rigorous treatment of flexures, there’s plenty of academic and commercial literature to scratch that itch. Bal-tec has an approachable primer on flexure types that's worth a skim at least. I found this thesis on flexure design performance that has a more thorough taxonomy of joint types and kinematic implications. Dr. Larry Howell from BYU wrote the book on compliant mechanisms, and is featured heavily in this excellent Veritasium feature on their practical application in aerospace and other fields.

Fun Stuff

1983N13 is a McMaster part number… for Swedish Fish.  Thank me later.

Here’s a few more of my favorite obscure offerings as well: a railcar derailer, hot chocolate, gatorade concentrate, and a bike with tires that won’t pop. Silliness aside, having some working knowledge of the McMaster catalog can really help your rapid prototyping skills.  Ian from Root Ventures has a cool take on this in his Designing For McMaster Carr (DFMC) medium post. If you are looking to bolster your DFMC skillset you might like this more serious look at some of the awesome things available on McMaster by Justin Ketterer

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