Years ago, I worked for a conveyor company and I learned a few metal working skills. One of the more useful skills was threading pre-drilled holes. Threading blind or through holes is called tapping. With this technique, you can have a threaded hole where ever it may be needed. You can use bolts, screws, and threaded rods to fasten metal, wood or plastic to the object with the threaded hole.
The way it works, is you drill a hole with recommended size of drill bit and then you usually tap by hand. Typically, you turn the tap in the hole about a half turn clockwise then backing off a quarter turn to clear chips generated. It can be a slow process but it is worth the effort and patience.
Usually when I finish the initial tapping, I completely remove tap from threaded hole and reinsert it to clean up the shavings and chips. Then I test with a bolt of the same diameter and thread count.
The photo above shows me testing my angle iron bracket with a 2 1/2″ long 3/8-16 thread hex bolt. I have completed four of these now an am ready to assemble my table. That is the reason I enjoy tapping despite its labor intensive qualities. Not only are you able to create strong cold connections in metal, but you can make your own personalized hardware for your next project.