As I mentioned earlier, I bought this gadget from Grainger to solve my oversized hole problem. The list price at the counter was $17.50 for a bag of five, but because I was wearing a state ID--voila! The price fell to $13.26 per bag. (Of course, I needed six because I was trying to fix two tripod sets, so now I have four left over.)
Here's what it looks like:
Since I am giving them some media attention for this, I suspect that they won't mind if I steal a small amount of bandwidth to show it to you.
It is really quite elegant how well it works, and amazingly enough, there is something not made in China. This comes from Hungary--a country that in the past has only sold me incandescent lightbulbs and cosmetically-challenged Walther PP clones.
The interior is 1/2"-13 threaded (the same size as the caster stems); the exterior is 3/4"-16 threads. I bought a 11/16" drill to enlarge the 1/2" holes in the acetal, then tapped the holes to be 3/4" deep, so that the 5/8" length thread inserts would be slightly below the surface of the plastic. If you are one of those weird sorts who memorizes tap charts (normal people memorize the periodic table of the elements instead)--yes, 11/16" is not the right drill size for tapping steel, but for aluminum and plastic, you use a slightly smaller drill bit, because plastic and aluminum are more flexible than steel, and need a bit more material to make a tight fit that holds under stress.
Then I turned in the threaded insert. Once it was below the surface, if you have the special $40 tool for turning these in, you press those little square pins down, and they effectively lock the exterior threads in place. Because the friction between the exterior threads and the plastic was far higher than between the caster stem and the interior threads, they really weren't necessary, so I just pulled them out. Only the magnetic forces exerted by UFOs at close range are likely to cause a problem for this.
Problem solved. Even with the four leftover, this was cheaper than throwing away these six pieces, because of both material costs and labor, and it meant that I will be able to ship to customers tomorrow morning. I honestly don't feel bad about this solution--this is at least as solid a solution as the correct 1/2"-13 threaded hole in the acetal.
I have long wondered if I could use something similar to solve another annoying problem. I use two different types of casters right now for the ScopeRoller products: one is an inexpensive caster with limited features, which uses a 3/8"-16 threaded stem. The more expensive caster with a better lock on it uses a 1/2"-13 threaded stem. When I am making these assemblies, I can't complete the hole where the caster goes in until I receive orders--and the two different types of casters are not available with the same thread stem.
My readers before have suggested using a threaded insert like this, but it does not appear that there is one that has a 1/2"-13 exterior and a 3/8"-16 interior thread. (A little arithmetic will tell you why.) But there is one that adapts 5/16"-18 to 1/2"-13 exterior threads, and I think the cheaper caster is available with the 5/16"-18 threaded stem. The threaded inserts are about $1.60 a piece, but it means that I could manufacture everything with the 1/2"-13 threaded holes, and use the threaded inserts with the cheaper casters, which are already highly profitable units anyway. (Just checked: the less expensive casters are available with 5/16"-18 threaded stems. When the current supply runs out, we're switching.)
UPDATE: Ooooh! Even better! Grainger sells a 25 pack of the 5-16"-18 to 1/2"-13 threaded inserts for what turns out to be less than a dollar each. This makes the whole prospect that much more attractive. And they don't have the little locking columns; you turn them in with a screwdriver blade. It is unfortunate that this model doesn't exist for the size that I was having to use earlier today. If I order a 100 pack, it comes to about $0.45 each.