Instructables

How to Build your Everything Really Really Fast

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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.


 
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dbc12181 year ago
Yet another incredibly informative and well written instructable, nice job! Love the FEA’s and especially enjoyed your notes on set screws. My goto sources are always McMaster and ServoCity. Similar to your RoyMech site I’ve used http://www.gizmology.net/ for reference many times.
teamtestbot (author)  dbc12181 year ago
I knew I was forgetting something! Gizmology has been added to the end - I may sprinkle relevant links in the middle too.
sparhawk79 months ago
http://web.mit.edu/2.75/fundamentals/FUNdaMENTALS.html is the correct link now, it's a great resourc, especially for offline use. THANK YOU FOR THIS INSTRUCTABLE!
donkeyknee1 year ago
nice one
mgainer1 year ago
Re using one part to template tthe other, "dimpling" -- mention transfer punches here?
teamtestbot (author)  mgainer1 year ago
Probably worth it. I was definitely in "slummin' high school" mode then, when we didn't have a set of center punches much less transfer punches! I'll look to adding it in.
Wow that was incredibly comprehensive! Are there still robot combat competitions going on?
teamtestbot (author)  JamesRPatrick1 year ago
Hell yeah. Primarily small weight classes and these days grassroots-level and builder run. The big event is RoboGames: http://robogames.net/index.php and Combots: http://combots.net/, and on the east coast, NERC: http://www.nerc.us/

Various other local clubs and organizations exist also. A current listing of events is on buildersdb: http://buildersdb.com/
Jayefuu1 year ago
This might be my new favourite Instructable. Great info!
Ganhaar1 year ago
Thanks for sharing that.
One suggestion to add for using set screws in transmitting torque on shafts (only works if the shaft and hub are the same material and ends flush) is to use the set screw as a key - drill and thread the keyway parallel to shaft on the joint between the shaft and hub.
Tomdf1 year ago
Wow, that is some invaluable mechanical design info. I thoroughly enjoyed the read and I feel like I just took an engineering class, an incredibly fun one. Seriously, amazing detail, thanks a bunch!

Simply out of pure curiosity, why weren't taped holes used over T-slots more often? I'm guessing that it didn't fit the 2D fab theme of the class?

PS: Working in that shop must have been like a dream come true, I'm olive green with envy :)
teamtestbot (author)  Tomdf1 year ago
Purely as a matter of convenience. The t-nutted holes are not nearly as strong as a properly drilled and tapped hole due to the number of inside square edges. It's a matter of recognizing when the structural loads in the device can be borne by material-on-material interference (the slots and tabs) and then having the fasteners (t-slots) only be there to keep it all together. There are far more instances when drilling and tapping is stronger than using t-nuts.
Excellent! Thank you!

Suggestion for an addition: how do you align parallel guide rails on which bushings will slide? Also, how do you keep things sliding freely when temperatures change?
Specifically, in my 3D printer project I have an aluminum carriage supported by 4 bronze bushings that slide on guide rails. The print-bed is bolted to the carriage and is heated. As the print bed heats up, it expands, applying force to the bolts that stand it off the carriage, which in turn bend the carriage, which in turn misaligns the bushings.
teamtestbot (author)  Mark Rehorst1 year ago
The design you describe is a classic "overconstrained slide". It's very sensitive to change in the center distance (gap) between the two rails regardless of what you do to the bushings.

Generally you have only 3 bushings - two on one axis to constrain it against planar motion (up/down, left/right) and against tilting/pitching, which are two rotations. And one on the other to make sure it does not pivot on the first axis (rolling). The result is only one motion possible (along the rail). Four bushings adds another constraint which is technically unnecessary, and for it to not impede the motion of the slide, they all have to act perfectly in line and on the same axes. Any misalignment of the rails or of the bushings, then would seize up the slide, as you've noticed.

The solution is usually to use 3 bushings - two on one rail, one on the other, and also 'float' the 3rd bushing on a mount which is compliant to misalignment in the center distance. For small applications it's sufficient to just use one of those rubber-mounted self-aligning bushings.

In addition, your issue seems to involve flexing of the entire carriage structure which can bind up the two-bushing side too, unless they are also self-aligning. Short of isolating the hot build bed from the carriage, perhaps one or more of the bushings on the two-bushing rail should be also flexible types. It's less rigorous machine design but also a practical solution.
teamtestbot (author) 1 year ago
Sup everyone,

Feel free to chat amongst thyselves and ask questions. Interesting discussions could very well get folded into the document for everyone to reference.
pawelski1 year ago
Well done.
monkeywork1 year ago
That was great! Now can you come over and help me build my Spencer Aircar?