Blind Hole Thread Chaser
By David Hawkins
The department chief of our welding shop asked for my help with a CNC plasma cutter. The problem was a stripped 1/4-28 threaded blind hole in a brass component. Someone had cross-threaded the mating part and destroyed the top two threads of the hole. If the hole were open on the bottom, I could, of course, run a tap through the other side. However, that was not an option since it was a blind hole and a permanent part of a larger assembly.
Since I couldn't do it the easy way, I made the tool seen in the photos. The split allows it to be squeezed together to fit into the hole past the bad threads, then it’s released in the section with the good threads. A paper clip with a flattened end was used to pry the halves apart while the screw was backed out through the ruined threads, straightening them in the process.
It's not pretty, but it only took about 20 minutes to make, two minutes to use, and saved me from having to buy a new torch head for a plasma cutter.
I know making this tool wasn't wasted effort, as I'm sure another student will eventually cross-thread that part again.