Richardson Seating uses Sigma brand T-Nuts. We got a deal from them where they would lend us their T-Nut riveter and in return we would agree to buy T-Nuts only from them. If we buy enough, we get the machine for free. We usually come close to getting the machine for free but not quite. The nice thing about using the machine for the T Nuts is that it splays the end open essentially riveting into the wood. It takes several hundreds pounds of pressure to pull out a single T-Nut. All of the T Nuts that we use are 1/4-20. This means that the diameter is 1/4 of an inch and there are 20 threads per inch. It is probably the most common size in US furniture manufacturing. In the past we also used Sta Fast which is the other large manufacturer of T Nuts. The circle that the worker is riveting will eventually be made into a round box and used inside our model 1957 or 1958 bar stool. We also attach most of our bucket style seats with T nuts. We attach those after the seat is made. You can see a bunch of built up wood seats right behind me waiting to get their riveted 1/4-20 T Nuts.