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Step 8: Nutstrip, Uber-nuts, and Corner Blocks

But what if I don't have access to a laser cutter or waterjet, or can't hire it out? Don't worry, these next few sections are well-suited to garage builders, and much of it was how Yours Truly was able to build consistent robots for years.

Without a freeform 2D fabrication process, the square edges of finger joints are extremely hard and time consuming to make. Usually, it's not worthwhile at all because other methods of fastening exist.

Nutstrip

It is fairly common for builders to attach 2 plates at right angles, contacting eachother (a Tee joint). Most often, this is done with an angle stock in aluminum or steel. If the stock is thick enough, end tapping, or drilling straight into the thickness of the material and cutting threads into it, is another common tactic.

Another method which can be versatile is known as "nutstrip". Typically it takes the form of an extruded barstock, for known square sides and size standardization, drilled and tapped at regular intervals. In the past, they were DIY items - literally made by hobbyists on mills and drill presses, but these days they can be purchased stock on sites like Kitbots or ServoCity, among others. However, if you can obtain small barstock economically, they are still easily made on drill presses.

Nutstrip is the most helpful when the plates being joined are of roughly the same thickness as the nut's square dimensions. In this case, it can offer increased rigidity over using a thinner angle-stock-based right angle joint. In cases where the stock being fastened is very thin, then the angle stock is generally of a greater surface area and hence stiffer. However, watch out - the rest of the structure can be floppy!

Uber-Nut

A nickname for a single-row-tapped nutstrip that takes the place of individual nuts. The highly increased surface area of an "uber nut" compared to multiple discrete nuts makes the joint stiffer. This is helpful in situations where the mounting substrate is very flexible (e.g. metal fastened onto plastic), or fiddling with individual nuts is difficult, such as a confined space.

These can easily be made from drilled and threaded bar stock.

Corner Blocks

An offshoot of the nutstrip is a corner block or endcap, tapped on many faces and used to secure a modular assembly to another. It's very commonly found on generic drive base robot kits because of its easily reconfigurable nature.

The corner block is usually made from a much thicker material into which threads can be cut on (usually) at least 4 out of 6 faces of a prismatic stock. Different situations might necessitate other thread configurations.

Besides offering high stiffness to otherwise floppy corners, designing with a regular bolt pattern makes for easy disassembly and swapping of modules. The ultimate example of this is perhaps the optical table, also called the optical breadboard, for which myriad accessories and mounting kits for various implements exist across manufacturers.
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.
I knew I was forgetting something! Gizmology has been added to the end - I may sprinkle relevant links in the middle too.
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!
<a href="http://web.mit.edu/2.75/fundamentals/FUNdaMENTALS.html" rel="nofollow">http://web.mit.edu/2.75/fundamentals/FUNdaMENTALS.html</a>&nbsp;whoops.&nbsp;
nice one
Re using one part to template tthe other, &quot;dimpling&quot; -- mention transfer punches here?
Probably worth it. I was definitely in &quot;slummin' high school&quot; 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?
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/ <br> <br>Various other local clubs and organizations exist also. A current listing of events is on buildersdb: http://buildersdb.com/
This might be my new favourite Instructable. Great info!
Thanks for sharing that. <br>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.
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! <br> <br>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? <br> <br>PS: Working in that shop must have been like a dream come true, I'm olive green with envy :)
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! <br> <br>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? <br>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.
The design you describe is a classic &quot;overconstrained slide&quot;. It's very sensitive to change in the center distance (gap) between the two rails regardless of what you do to the bushings. <br> <br>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. <br> <br>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. <br> <br>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.
Sup everyone, <br> <br>Feel free to chat amongst thyselves and ask questions. Interesting discussions could very well get folded into the document for everyone to reference.
Well done.
That was great! Now can you come over and help me build my Spencer Aircar?

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