acme

acme thread form  

Acme leadscrews have a 27 degree thread angle compared to a 60 degree for normal bolt threads. The large flat on the crest of the thread gives it a "square" appearance, but it is not the same as a square thread. They are used for linear motion because they don't have so much wedging/locking action of a typical bolt which contributes to friction.

tapping acme threads  

An acme tap can cost from $40 to more than you want to know. Commercial acme nuts are also expensive, and often of poor quality. Because of this, many hobbyists tap their acme nuts by turning a tapered stub of acme threaded rod on the lathe, then gouging out flutes with a grinder or milling machine. This results in a functional tap, and when tapping in soft materials like delrin or HDPE it can give satisfactory results. However, the inital zero-backlash condition is mostly due to the spring action of torn "fuzz" sticking to the flanks of the threads, not because it has been cut perfectly. After some time this fuzz will go away and you will be left with about 1mm of backlash for a 1/2-10 thread.

anti-backlash nuts  

You can buy anti-backlash acme nuts made from delrin from http://dumpstercnc.com

This might give you the idea that you can just tap a nut with a piece of threaded rod, split it axially, and squeeze it to take out the backlash, but it isn't so! When you tapped the thread it faithfully copied the whole thread profile, including the thread flat. Normal vee-thread taps have sharp threads so when you compress them to take out backlash the thread flanks touch first, since there is no flat on the nut threads. But with a homemade acme nut the thread profile includes a sizable flat portion, which will touch first. The result is that squeezing the thread radially just causes friction, and doesn't reduce any backlash.