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
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.
- Female standoffs have threaded holes on both ends, either fixed-depth, or all the way through for shorter ones generally 1/2" or less.
- 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.
- 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.