Tapping a hole is the process of creating threads on the sides of a drilled hole to enable screwing in a bolt or machine screw. This instructable will focus on primarily the use of taper taps, although there will be some mentioning of bottoming taps.
Step 1: Drill the Hole
Prior to drilling the hole you will eventually tap, determine what size you need to drill for the bolt you are trying to screw in. Your hole will be smaller than the diameter of the bolt, because the threads tapped will increase the diameter. The chart above matches tap, or bolt, sizes up with their corresponding drill bit size.
Once you know what size hole you need to drill, go ahead and drill it in the designated location. Be conscious of how deep the hole is if it does not go all of the way through. If you are using a taper tap, you may need to drill a deeper hole to account for the part of the tap at the end that doesn't create complete threads. It is important to be conscious of how much depth will actually be able to be tapped and secured with whatever fastener you are using.
Step 2: Tapping the Hole
Now that you have a drilled hole, use the tap of the correct size and a tap handle to thread the hole.
To begin tapping, rotate the taper bit clockwise until you feel resistance from the threads cutting into the material. From there, for every full rotation clockwise, make a half rotation counterclockwise. This will break off any chips that have been created and clear the space for the cutting teeth of the tap to keep cutting. Taps are very brittle, so accumulation of these chips or any other extra stress on the tap could cause it to break in your material.
Make sure to pay attention to any abnormal resistance or binding you feel as tapping, and when in doubt, back the tap out a few threads and retry. You can also use lubricant of some sort to make tapping the hole easier.
Once you have used the taper tap to go either all the way through the hole, or as deep as a blind hole will allow, you can use a bottom tap (if available) to complete the threading closer to the bottom of the hole.
Step 3: Cleaning and Testing the Hole
After you finish tapping the hole, remove the tap and clean out the chips from both the tap and your material. Compressed air can be very helpful for removing chips. You will also want to clean off any lubricant you used in the process.
Once the hole and material is cleaned, test the threads of the hole with the bolt or machine screw you plan on using. If there is a lot of resistance and the bolt does not screw in easily, run the tap through the hole again. It should be easier this time because the threads are already formed. However, it is important to make sure the threads begin in the same place within the hole to avoid cross threading.
You might also want to consider using lock-tite when you have completed everything and are securing the bolt in the material. Lock-tite will make sure that the bolt doesn't come loose.