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Step 9: Joining Parallel Plates: Using Standoffs and Spacers

These next few sections will concentrate on ways to bind parallel plates together. Here, "plate" is used very broadly - resources commonly available to makers in "plate" form include wide barstock and premade square/rectangular tube and angle extrusions, besides actual cut plates. I suppose a better name for these sections might be "Joining Parallel Surfaces".

Continuing on our journey away from digital fab exclusive styles, we arrive upon one of my favorite (but admittedly, underused) methods which is already very popular with hobby robotics kit manufacturers, but seems to be still relatively unknown in the DIY domain, the plate-and-standoff.

Standoff vs. Spacer

I'm going to guess that these two words are some of the most confused in all of engineering. They describe very outwardly similar components that are both Little Round Things. The lead image shows the difference between a standoff and a spacer: the former has blind (or through) threaded holes for mating with a screw, and the latter has a thru-hole for using with an external bolt and thread.

Types of Standoffs

As McMaster might show you, there are about a billion (scientific estimate) types of standoffs and spacers. However, they're basically expressed as 3 high level categories, depending on what has the thread on a stud or in a hole.
  1. Female standoffs have threaded holes on both ends, either fixed-depth, or all the way through for shorter ones generally 1/2" or less.
  2. Male-Female standoffs have a threaded hole on one end and a threaded stud on the other. This lets them stack together. These are classical "PCB standoffs" used to hold boards inside enclosures.
  3. Male-Male standoffs have 2 threaded studs and are intended to join 2 plates together with nuts on the other side. These are less commonly seen in non-electronics applications since the thread on the ends means only a narrow range of material thicknesses can be bound. The nut and remnant thread on the other side might also be undesirable.
Other types like swage-in (shove into a predrilled hole, similar to an insert nut) are available too, but they are also less frequently dealt with.

Making or Buying Standoffs and Spacers

These days, little standoffs can be found at almost any hardware store or online robotics retailer, such as ServoCity, which is probably one of the best resources for small fasteners and attachment bits in recent memory. Larger ones can be found on industrial supplier websites like McMaster, linked above.

Making your own standoffs is more difficult as it implies access to a lathe to create the concentric center threads, unless you really don't care about the concentricity, in which case be my guest with a drill press! However, keep reading anyway. You may have luck finding sizes of aluminum tube which can be threaded by modifying the original hole; however, as aluminum tube is generally manufacturered with thin walls, this could result in a weaker standoff.

However, making your own spacers is substantially easier because of the same reason. For example, common 1/16" wall, 1/4" OD tubing can be used to clear #4-40 screws. My favorite is using 0.12" wall, 0.5" OD tubing to make 1/4"-20 clearance standoffs (a resulting ID of 0.26 to 0.27", perfect for the job) for my larger robots and vehicles.

Using Standoffsand Spacers

The primary function of these little round things is to contribute to a device's structure. If the standoff is essentially as rigid as the materials being fastened, then it functions in role similar to the flanges and webs expounded upon in Step 7. Basically, the more locations you force your materials to move together, the more they will transfer and share loading forces and the more rigid they will be.

In the coming chapters I'll also expound upon how you can use a standoff or spacer as an axle by putting bearings in the rotating member, like a wheel. Doing this is termed "dead" or "fixed" axle and gives the benefits of standoffs helping to stiffen the structure while carrying a load.

The example pictures ought to explain the idea of standoffs fairly well. Remember that in using standoffs, you fasten a material to it using a screw that threads into the standoff, whereas in a spacer, it is assumed you have a bolt running through the material and the spacer and fastened with a nut on the other side. There are some slight but important implications about using bolts with spacers that warrants its own discussion. For instance, standoffs cannot be preloaded, or pre-tightened such that forces under a certain magnitude have little effect on the structure. These will be addressed in the next section.
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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