What do you call these, anyway?
There's not an industry standard for these things, and "t-nut" is just my shorthand name. Strictly speaking, "t-nut" or "tee nut" refers to a type of pointy nut you insert into wood to create a strong threaded hole. Alternatively, it goes into the T-slots of a machine table to anchor workpieces, vises, etc.
Many names have been proposed. Slotted-insert nut is one common name, because "insert nut" itself is already a type of nut. Crossed-T nut describes the shape of what you slide the nut into. Yes, I've heard them called Jesus Nuts. Captive slot nuts. Slotted nuts.
Regardless of what they are called, they are used to simulate a tapped hole in the edge of a workpiece by creating a slot into which you slide a machine screw nut.
It should be obvious why these are often used between two finger joints.By itself, the nut can easily deform away and burst the two narrow steps holding it in place under a tensile (pulling-out) load. However, if it is surrounded by finger joints, tensile loading forces will push the finger joints into their slots harder. The tensile strength, then, is generally only limited by the pull-out strength of the screws.
T-nuts and Constraint
Based on the previous statement, it can be seen why a slot and tabbed structure backed by T-nuts can actually be very strong. However, it's important that the nuts be used in multiple planes on each joint and that the joints have proper bracing and gusseting to avoid "opening like a book".
The first image, a machine base by Daniel Fourie, clearly shows an open-finger-joint gusset, but with t-nuts facing into all of the planar surfaces such that the corner is very well constrained.
Flat-bottomed vs. Crossed
The first style of t-nut I used years ago was a parallel discovery. I realized while designing slots in a part to be waterjet-machined that I could widen the bottom of the slot, drop a nut in it, and have a fake tapped hole. This was a very exciting discovery that I used initially, and is in fact forever recorded in history in the How to Build Your Robot Really Really Fast.
However, later research led to me finding that this was in fact a common thing already. And that everyone elses was better: the fully crossed nut.
The reason flat-bottomed nuts are not as strong is because of the potential for the fastener to bottom out at the end of the slot. Screws are made with a length tolerance usually on the order of a hundredth of an inch (0.01", .25mm or so) or more. If the screw hits the bottom of the slot, it will "tighten" the nut against the opposing wall of the slot. But the rest of the screw, then, is without tension. If you design a flat bottomed T-nut to account for the longest screws, then you risk not engaging enough thread in the nut, again creating a weaker scenario. Imagine my disappointment when I discovered I was not the smartest person to have ever lived.
A fully crossed nut, as shown in the 1st and 3rd image, gives some leeway for screw length. The nut can be positioned within the known good lengths of screw thread, while the very tip is made longer than the worst-made screw. Taken to the extreme, the tip can extend even further so you stop caring what length of screw is used!
Critical Dimensions for your T-nuts
The fourth image (the one with actual numbers) is my usual CAD layout when putting in a t-nut. There are 5 critical dimensions, appropriately numbered.
- Fastening Length. I usually set this as the nominal length of screw to be used (e.g. 0.5"). This distance is measured from the top of the finger in a finger joint scenario, since screws are usually rated by the length under their heads.
- Thickness of Nut. This depends on the precise nut in use. Generally, this is a regular "finished machine screw nut", so standardized dimensions are available. (Is there such a thing as an unfinished nut?). Here's another table that includes very small screws. The example dimension is 0.095", just barely above the nominal thickness of a U.S. #4-40 nut (which is 3/32", 0.0938" thick). Why 0.095? Find out in the next section!
- Clearance Width of Screw. Again a table-lookup operation, this should be the clearance hole you'd normally drill to pass a screw through. A screw size chart or tap drill chart is invaluable here. The example dimension is 0.120", a reasonably loose fit for a #4 screw.
- Width of Nut. This is usually the width across flats of a hex nut. However, in some materials, the thickness is less than the point-to-point width of the same nut. If a flat surface is needed, then this width must be the width across points. You can find out this dimension with a little bit of geometry. The example width is 0.25" for a #4-40 nut.
- Screw Clearance Depth. This length should be greater than the sloppiest screw in your collection. I often go up to 0.03 (1/32") over.
Cautions
There's some design "nonoptimalities" you can easily corner yourself in if you misapply the Art of the T-nut.
Bottom of slot is too close to material edge
Shown in image 5, one of Chibikart's front bumper-splitter mounts, there's a nut very close to the bottom of a slot. The area past the nut to the left has very little meaningful strength. The rectangular edges of these finger joints and t-nuts are basically stress risers and places for cracks to form. It is essential that the bottom of the nut be far from the edge of the material as a result.
Just how far is a matter of how the structure will be loaded. In a situation like image 6 (the U shaped piece) where the plate is backed up by being interlocked into many other plates near by, generally one screw diameter is my safe accepted minimum depth. This is because you can assume the material itself takes most of the loads (assuming the tabs and slots are tightly fitting), and very little is actually transferred into the screw.
However, in a longer beam situation like image 7, the material can deform much more, to the point where the screw and nut are not just providing a tensile load to keep the tabs and slots mated - the nut can actually be loaded against the inner walls of its slot. Being a square, inside edge, this is a great stress riser case study.
One way to get around this is to make "camel humps" where the bottoms of the slots are such that the stresses "flow" around the nut smoothly. The size of the "hump" should make it such that there is at least 1/2 material thickness between the corner of the slot and its closest edge.
