A T-nut is used to create a secure removable threaded connection in wood or aluminum, solving the problem of a bolt spinning or pulling out. Without some sort of resistance, a bolt will simply pull out or spin. With a T-nut, the material become the anchor. Picking the correct size and type for your application can make all the difference between a loose joint and a tight one.
It is a tight joint. The infographic neatly sorts these options into families. Wood-based versions include pronged types that sits above while other aluminum extrusion-slash-slotted drop-ins or hammer-in types ride inside. Router- and mill-friendly machine-table version accommodate high clamping force.
How to Choose the Right T-Nut
Within each family, the method of resisting rotational force is different (often determined by how it’s installed), so generally speaking you’ll choose the family based off what best suits your material first. Then there’s the question of frequency of changeover. There is also a difference between having three prongs and four prongs within the pronged family. Small jigs and light fixtures can get away with three, but if the joint will be subject to repeated vibration or torque, then four prongs spreads the load better.
The flange diameter increases as the thread size increases, as seen on the chart. Flange diameter is your indication of amount of bearing surface on the wood face. So the larger the flange, the less likely the nut are going to pull through under tension. However, because the flange is larger you’ll need a larger surface area upon which to seat the flange cleanly.
The result also depends upon how it’s installed. When the barrel is drilled for clearance, the nut will not rock in place. The prongs seat lightly by tapping, but pulling them home with a bolt from the other side is better still. It pull the prongs directly into wood’s grain without splitting the board. Even thin stock, which you might risk hammering, works well with this method.
The same applies to material selection. For indoor use, zinc plating will suffice, it’s cheap. If assembly will be exposed to the elements or even just moisture, spend the extra money for stainless steel. If you are using a machine table with high clamping forces, get the hardened steel version, any deformation will ruin repeatability.
The same concept holds true as you move into aluminum extrusions. Your T-nut series depend on the slot width of your profile, while thread size is the factor that affects speed for hammer-in vs drop in variety of styles. After determining the profile, just find the nut to suit the slot and expect it to hold the apropriate load.
But picking the right tool results in a joint that performs as designed and the fastener becomes invisible, or background noise. By removing the guessing game from size selection, you can concentrate on your work without fighting against hardware designed for a load it was never intended to handle in the first place. You should of picked the right one earlier.
