Part of what makes most metal fasteners so cheap is the vast quantities in which they are made. Look for a fastener that is not a high demand item--and the prices get really shocking.
My son was helping me fill an order today, and because I had left a 1/2" drill bit in the drill press, he used that to drill the holes that needed to be tapped for 1/2"-13 threads. I should have pulled the drill out when I was done with it; it is close enough in size to the .4375" drill bit that we use for that purpose that I can see why he assumed that I had put the right drill bit in place. Of course, you can still tap the hole, but the threads it cuts are like a Hollywood false front movie set. They look like threads, but they won't hold anything.
I have two options: make new parts (delaying shipping orders to customers), or enlarge the hole, and use one of various thread inserts. (I won't consider any solution that involves filling the hole with fiberglass and starting over. That's cheesy, and might fail at some unexpected time in the future.) If you don't know what a threaded insert is: it is a metal fastener that contains interior threads to accept whatever bolt you need, and are either threaded on the outside, or are a press fit into an existing hole.
The exterior threaded form are used for situations where an existing threaded hole in metal has been damaged or otherwise been so worn that it is now loose. (Think of what happens if you cross-thread a spark plug hole in an engine block, or several hundred thousand turns of a bolt in a hole has finally worn out the threads in the hole.) The press fit is used in wood and plastic, or other soft materials.
But the prices of these threaded inserts are sufficiently high (like $5 a piece) that it suddenly makes sense to make throw away these parts with the oversized holes. (This item might do the job, at about $1.16 each, but I can't tell whether you need a $200 tool to install it, or you can just screw it in with pliers.) Unless, of course, my very knowledgeable readers can suggest some likely source of something that is like a threaded insert with 1/2"-13 interior threads at least 3/4" to 1" long, and more like a dollar or two. Even if I have to order it because no one has the parts in Boise, I could make new parts, and retrofit these six pieces with threaded inserts when they arrive.
UPDATE: Or maybe this item from Grainger, which is down the street from where I work. It's $17.50 for a pack of five, which comes to more than $3 per insert, and the external thread is 3/4"-16, and I have one of those taps. Of course, it requires an 11/16" drill bit (at least for tapping 3/4"-16 in plastic). You may be wondering: if I have a 3/4"-16 tap, why not a drill bit that goes with it? Because the 3/4"-16 tap I have only used with the lathe, where I can use the boring bit to make the .6875" hole. This particular part can't be drilled on the lathe, and I don't have a boring head for the vertical mill. I could use the 5/8" Forstner bit to drill the hole, but that is still 1/16" too small. Grainger charges an arm and a leg for 11/16" drill bits, but I suspect that I can get one at a reasonable price elsewhere. Perhaps Industrial Hardware in Garden City has the threaded inserts and the drill bit.
UPDATE 2: I purchased the Coil-Sert Thread Repair Inserts this morning--12 of them cost a bit more than 18 dollars. There was an entire kit, including the tap and installation tool, but the tap was 1/2"-13, and I already have one of those. What has me a bit mystified is that the instructions from Coil-Sert indicate that you use a 17/32" drill, and the 1/2"-13 tap, then screw this in. I don't see how this is quite possible, because the internal diameter of the inserts is 1/2". A 1/2"-13 tap should just flop around inside a 17/32" hole.
UPDATE 3: Coil-Sert called me back. It turns out that it isn't a 1/2"-13 tap, but an STI 1/2"-13 tap, which is oversized. I think I'll return this, and buy the Grainger part instead.