It only takes one DIY project gone wrong to realise what a big mistake it can be to use Tnuts to secure a driver. Or you might be stubborn like me and it might take several incidents before you give up on Tnuts forever!
Now I'll admit, I still have uses for them. If you need to secure castors to dollies that carry heavy loads, Tnuts are ideal. But for mounting drivers, there is a huge potential problem.
To illustrate, here's a problem Tnut removed from a subwoofer:
What happened?
In this example, the Tnut became dislodged. When you insert the hex head screw, it can easily push the Tnut out. In this example, I had actually drilled two different sizes of hole into the MDF to prevent this from happening. It didn't matter. It happened anyway. The weakness of Tnuts is that they resist twisting fairly well, but if they are pushed out by the screw, you're in trouble.
Now, you might be thinking you can just unscrew the driver. Not so. Now you have one screw that can't be easily undone.
Next you try with a spanner to pull the head outwards, so that Tnut prongs will bite back in. Often what actually happens at this point is not what you're hoping. It simply cuts a circular trench and the Tnut digs in and keeps spinning, like a car that is now bogged in mud. Sometimes you have to cannibalise the cabinet to gain access to the Tnut. You might be thinking a hand to hold the Tnut is enough. Again, not so! A tight grip with pliers is required. The Tnut can also jam.
The alternative?
The better choice is also in the photo - threaded inserts. Just like Tnuts, they are installed on the rear side of the baffle, on the surface opposite the head. They are easiest to insert prior to gluing up the baffle to the cabinet. You'll need an allen key or hex head bit in the cordless driver.
What makes them better?
In short, they don't tend to fall out. A Tnut resists rotation much more than it resists being pushed out by the screw on assembly. A threaded insert adequately resists rotation but more importantly, it has a thread which bites in to the hole in which it is inserted. The thread means it won't come out unless you bash it out with a hammer.
Still not convinced to ditch Tnuts?
Ok a few more horror stories. Sure, I'll play the clown in the interests of public education.
I was working on a prototype build for a custom church organ speaker. Yes, it happened. Tnut came loose and it prevented me from removing the driver. I was able to get some access inside the box. To hold the Tnut so I could unscrew it, I had to make up a quick jig. It was quite some time ago, so I'm not sure if it took 3 hours or just felt like 3 hours.
In another episode, I was helping a client assemble a kit where I supplied drivers and flat pack and he had built it. In this case, it was a long trip and that's when you don't want anything to go wrong. I had a calibration job to get done - the last thing I want is a Tnut coming loose. Yes - it happened!
There are many more stories like this. I'd like to say I learn quickly. In reality, it's often through mistakes.