How to Build your Everything Really Really Fast

A few years ago, I wrote a short document on methods for rapidly fabricating elements of mechanical systems entitled How to Build Your Robot Really Really Fast. It was catered towards students in MIT's 2.007 introductory design and manufacturing class for which I was a lab assistant at the time. The basic premise of the document was ways to build the structure and framework of a robot quickly using the tools available in the class, such as basic 'garage' tools like drill presses, saws, and sanders, as well as rapid prototyping and digital fabrication tools like abrasive waterjet cutters and laser cutters, weighing the tradeoffs of 'build it now' versus 'design it now and have the machine make it later'. At the time, it was a compilation of my own experiences with those tools up to that point, and so its scope was fairly limited.

However, times have changed, and so have my experiences and views on the applicability of the methods presented in the document. New ones have been tried, and old ones have been refined. With access to the aforementioned digital fabrication processes by more makers and students proceeding at a ever-expanding pace, I decided it was perhaps time to rewrite the document in a fashion that made it more generally accessible to mechanical project builders.

And because I was sick of getting questions asking about why my t-nuts are no longer flat-bottomed. If the answer interests you, then keep reading!

Organization

The underlying message will be techniques used in design for assembly. Now, strictly speaking, I use the term it in a much different context than the manufacturing industry's usage. But I believe the intents are the same: to design parts which are easy or quick to put together into the final product without complicated assembly steps. While for Sony it might have meant making all the parts of the Walkman insert and mate vertically, for hobbyists and "one-off" makers, this means trying to reduce the amount of hand-filing and fitting and drilling things in place, making "one way parts" which do not function if oriented incorrectly, etc. Common problems that many project builders run into.

So, this Instructable will be organized into several larger sections that address categories of challenges. For example, attaching parallel plates or making pinned joints. From there, there will be pages as necessary to demonstrate specific methods and parts usage techniques. I'll try to include content that spans the spectrum of tool accessibility - from simple garage tools to a full RP facility including laser cutters and waterjet cutters. On each page, I'll try to discuss a little about the recommended tools.

Periodically, in the sections, I'll link to a resource that is useful on its own. For example, I'll most likely link to Professor Alexander Slocum's Fundamentals of Design many times - it really is a treatise on the principles underlying mechanical engineering, focusing on machine and mechanism design. It's unproductive, then, for me to merely repeat his words. Other sides like roymech.co.uk are historical favorite go-tos for me, and will also be linked profusely.

The methods and examples presented will be primarily conceptual in nature, because they are generalizable to assemblies of different scales. I'll include generous amounts of finite element simulations of structures and components in order to show the concept isolated by itself. As with all of my writings, math and formal analysis is only brought up when needed to cement a concept or is critical to preventing massive systemic failure. Your mechanical engineering and manufacturing professors will likely be disappointed.

Caveats

By no means is this going to be comprehensive overview of all design and assembly techniques. That's practically impossible, and I believe also counterproductive.   Part of the joy of engineering and building & making is the discovery of your own "style", the compilation of your own set of favorite techniques for approaching a problem. Inevitably, you will come up with a new custom solution to a problem. Hence, trying to list exhaustively how to mate thing A to thing B will artificially limit the search space of solutions, and make it very easy to 'pick one, copy, and paste' without understanding why a certain action is needed.

It is also not intended as a totally fresh introduction to mechanical engineering. That is, the question "what is a screw?" will not be answered. I am assuming that you have at least a passing familiarity with engineering terms like bolts, screw, axles, washers, nuts, and some knowledge of what machining processes do such as turning and milling. If you don't, well, perhaps the substantial links and resources presented will change that!

All documents of this format will inevitably be clouded by the author's style or flavor, and I make no pretensions to the contrary. The methods and parts used will be reflective of what I've done personally and what I've seen done by others in my local peer cloud, and the pictures and diagrams will probably be from my own past projects or those of my peers. It's not my intention to make sure all of these become widespread, but more information and knowledge transfer is preferable, in my opinion, to less.

It's important to note that practicians of classic 3D subtractive machining will probably not gain much from this Instructable. In my opinion, 3D machining (e.g. milling, turning, manual or CNC) is an entire means of building on its own, since it has very high equipment capital costs and associated learning curves. 2D production techniques are still substantially easier for people to gain access to, or hire out for lesser cost than having a machine shop. So, this will not be a "how to machine" guide.

Step 1: General Lessons and Themes

Before I begin the laundry list, there are some high-level points I want to make. These are issues to keep in mind as you adapt the concepts to your own design.

Right angles and in-plane angles are really easy.

If your project is free of design constraints enough that the outer appearance does not play significantly into functionality, then you'll benefit more than if it needs to be pretty and sellable. Most of these methods are really good attaching square things to other square things. It's relatively easy to check for straightness and squareness; not so easy for making sure two parts are mating at a specific angle.

There's also a difference between in-plane angles and compound, that is out of plane and rotated, angles. Because much of this document is founded on planar structures and mechanisms (think anything you can do without lifting your hand off the table), there will be significantly more content on making those types of joints.

3D angles involve at least one frame member or structural element which has an acute angle or bevel angle cut into it. With generally 2D fabrication methods, this is much harder to achieve. There are ways of getting around this, such as approximating a 3D angle using 2D layers, but broadly speaking if there are compound angles in your design, custom legwork and 3D machining might be the only practical solution.

Speaking of constraints...

Constraining things properly is hard, but essential.

What I mean in this case is physical, mechanical constraints. All physical objects (that exist in 3 dimensions, anyway) have 6 degrees of freedom, and the goal of making a successful structure or mechanism is to eliminate all of the ones we don't want. This involves the use of pin joints, planar/face mates, and fasteners strategically such that nothing is just flopping around unsupported.

A related concept is the "structural loop", which concentrates specifically on those floppy unsupported parts. It's the path through which forces are reacted against in the device. Essentially, if your device was made of a very poorly cooked, rubbery Jello, what would move the most? And can you add elements that don't interfere with the function of the design to make it less movable?

Hopefully by the end of reading through this document you will have a better understanding of how critical constraining parts in directions which optimally load the material is to creating a device which isn't misaligned and floppy. If I can't beat it into you, then surely Fundamentals can.

No Mostly-Tightened Nuts!

One hallmark of a "newbie" build is the amount of screws that have to be tightened a very specific amount, or nuts and bolts that have to be left very slightly loose. Any deviation results in a floppy arm or slanted wheel, or just total lockup of the mechanism in question. This means your device is always teetering on the edge of being too bent and wubby to function - any unexpected loads will probably cause total disappointment.

Bolts and screws work, fundamentally, by creating compression forces between the parts they are holding together. The compression forces, commonly called preload, determine to a degree how stiff the joint is because immense friction is created at the part interface thanks to those compression forces. The basic idea is that the preload force must be overcome before the structure will even begin thinking of maybe shifting, just a tiny bit. Hence, properly designed machine structures are predictably stiff in their operating regimes. And, if your parts are otherwise constrained, or even overconstrained, excursions outside of its design load can even be tolerated without failure.

My mission is to deter you from creating such abominations by hammering it in from the beginning that all your fasteners have to be tight. A large part of this document will be dedicated specifically to how to constrain rotating members and pin joints as a result.