Step 2: Magical Finger Joints: Joining Plates at Right Angles

You might have noticed that pretty much everything shown at the beginning had little slots and tabs in it. This has become a popular method of making 3D structures from 2D plates, spurred on by the digital fab movement starting some time in the 2000s. The name for the joint style is called "finger joint" after the woodworking technique from which it was derived.

These joints are advantageous to make because they positively locate features, to within the tolerances of the material and process, anyway. This is because the tabs must necessarily align and fit into the slots.

Additionally, they create structures which react to loads through the material. Finger jointed structures tend to rely on fasteners only to hold the structure together from expanding outwards i.e. unseating the finger joints. Otherwise, loads are directly transmitted through the fingers.

Prudent design is still necessary to ensure that the fingered edges are not loaded along the thickness axis, in which they are weakest, i.e. flapping using the finger joint as a hinge. A finite element analysis simulation is shown in image 6 - notice how significant stress builds up in the finger joints when the plates are bent. This will be discussed along with methods of preventing it.

Open (Underconstrained) Finger Joints

The simplest method of joining perpendicular plates with finger joints. This isn't so much a joint as an alignment feature, without anything else (e.g. fasteners or welding) to keep the joint together. The joint is only strong in the direction of the edge, where the fingers are loaded in compression. This type of joint, especially with no backup, is vulnerable to bending Think opening up a stiff book.

Closed (Fully Constrained) Finger Joints

These joints have one part with fingers and the other with fully closed slots. More strictly, it can be interpreted as a type of mortise joint. The fully enveloping slot captures the fingered piece well in all 6 degrees of freedom, if fastened with screws, but suffers from the same "edge hinging" bending vulnerability without additional support.

These are more difficult to make correctly because material thickness tolerances can impact whether or not the slots fit significantly. This is discussed in more detail in Step 5, tolerancing.

Regular Patterns

There exist two popular 'schools of thought' when it comes to how many finger joints to use. One of them is what I term 'sparse' finger joints, in which a single joint consists of two slots and one fastening hole. That pattern itself is patterned several times, usually at least three - one on each end of the material, and one to hold down the center.

The other is what I call "edge stitching" in which the entire edge has a regular zig-zag pattern of fingers and mating slots. The distance between the 'peaks and valleys' is constant, and repeated for as long as possible. However, unless the part dimensions are a multiple of the slot width, there may be irregularities at the ends.

For example, 0.5" wide slots and tabs work well with a 2.5" (or, really any x.5") part width. If the part were instead wider, then the outermost two slots and mating tabs get increasingly wider. The same principle works in metric part lengths. For 12mm slots to be patterned regularly, the parts must be an odd number times 12mm. The extra lengths generally aren't design problems, but for aesthetics, such as a "closing the box" design, it may be important. More on this subject is found in Step 6, making boxes.

Direct Welding

Notice that there's been no discussion so far on how to join the actual edges. Later on, I'll introduce methods of attaching the plates to each other with fasteners, but I do want to discuss welding.

While these joints have historically been the domain of plastics and wood, there are now an increasing number of project which use finger joints as alignment features in steel or aluminum with the intent of welding the joint closed. Welding is perhaps the strongest if done well and is also the least "bulky" method. This has been used to success on fabricated steel structures, such as giant hexapod legs.

In aluminum, TIG welding must be used, or alternatively, a zinc-aluminum braze. The former creates a strong, nearly homogenous weld, while the latter is more of a surface bond similar to regular brazing. However, the aluminum brazing alloy tends to dissolve into the joint, increasing its strength, but not over a properly TIG welded joint.


Also falling under the no-fasteners joining methods, adhesives can also be used effectively with finger joints. Most plastics, for instance, can be glued with a chemical cement, epoxy, or superglue (cyanoacrylate).

Cementing is particular well suited to plastics such as acrylic, PVC, and polycarbonate because the solvents tend to be very thin, seeping into the tight joints between slot and tab. Plastic cement, as opposed to "glue", is made primarily of monomers of the plastic embedded in a solvent - it actually melts the joint and fuses it again as one piece.

Wood also responds well to gluing, though my experience in this is limited to standard yellow PVA glue and thick CA glue only; woodworking is not one of my strengths.

Finite Tool Diameters

It's often easy to model waterjet and laser-cut pieces as having infinitely sharp square corners because the tool kerfs are usually very small (0.01" or less for lasers, and usually 0.03 to 0.04" for waterjets). It is wholly possible to use these finger joint techniques with a CNC router, also a popular 2D fabrication tool. Because the tool radiii are very large, features called "corner passes" are often added.

This is what it sounds like. The routing bit or endmill literally passes the corner, keeps cutting for a little while, then backs up and begins to cut perpendicularly. This extra travel ensures that the radised portion of the cut is not interfering with the finger of the mating piece. The corner pass is generally no more than 1 tool radius and can even be less in flexible, compliant materials like wood. The resulting slot would be more constricted at the corners, needing more force to assemble.
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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