I’m making a yoyo, but I’m not exactly sure how the threads (for the axle…?) are put.
Should I make a hole for the place for threads?
Typically threads like this are created by essentially carving out the thread pattern into the wall of a pre-existing hole. The size of the hole is dependent on the size of the screw/bolt (in this case the axle). Therefore, you should definitely have a hole where the axle will go, but you need to decide on an axle size first and then make an appropriate size hole. The length is essentially up to you, but on a yoyo should allow at least I would think a couple of mm of axle to prevent the usual stresses and strains put on the yoyo from tearing out the axle and stripping the thread. Once you have a hole, the thread is added using a ‘tap’, which have a variety of stats, including threads per inch (mm), tip shape and width. Again the threads per inch value needs to be matched to your axle of choice. In my experience of threads (not yoyo based), a higher value of threads per inch provides a more stable fixture, but is slightly easier to strip based on improper screwing together. The axle can be created from a rod of aluminum/steel using a die, but is more simply a standard component which can be purchased from a hardware store. Again, these are sold based on diameter and # of threads per inch. Typically axles are of the same material as the yoyo. Some companies provide axles made of a harder metal than the yoyo, however, while there are benefits to this, for example it is harder to deform the axle, the difference in hardnesses between the metals means you can be more prone to stripping the thread on the softer metal. I should add that none of the problems of thread stripping exist if one is careful!
The process of tapping should probably be done using a lathe or a tower drill unit (maybe other fixed equipment too) to prevent the threads being created off-axis. This will be a huge source of vibe if this is done incorrectly. Tapping can be performed by hand, but is imprecise.
As it turns out the wiki page is a moderately useful read:
Pm’d 