Tapping is a process that require a high degree of precision when selecting the right tapping tool for the job. If the tool that is used to tap into a workpiece is too large for the workpiece, the resulting bolt will have loose thread and will wobble within the workpiece. To avoid this problem, the size of the drill bit that is used to create the pilot hole must be the correct size for the tap that you will use afterwards.
This critical relationship between the drill and the tap can be found on a chart that provide the diameters of drill bits that are appropriate for specific size of taps. Using the proper drill bit size will allow for seventy-five percent of the threads to be engaged with the workpiece. Seventy-five percent engagement of the threads is the industry standard for tapping processes because it creates a maximum strength of the threaded workpiece without the risk that the tap will break.
How to Tap a Hole Correctly
The type of hole that must be tapped will determine the type of tapping tool that should be used. For through holes, a taper tap is the type of tap that is used. These taps has long leads that allow them to find the center of the workpiece to start the tapping process.
For blind holes, a sequence of different taps must be used. A taper tap or plug tap will be used to start the tapping process into the blind hole. Once the blind hole has been tapped to a depth that allows for the threads to be established, a bottoming tap will be used to complete the tapping process in the blind hole.
A bottoming tap has a short chamfer on its edge that allows the bottoming tap to cut the threads in the blind hole as close to the bottom of the blind hole as possible. Taps have different designs to push the metal chips in different directions out of the workpiece. Spiral flute taps are also referred to as machine taps because they work in the same way as a screw to remove the metal chips created during the tapping process out of the workpiece.
These types of taps are useful in materials like aluminum because aluminum has a tendency to clog the holes created in the metal by the tapping tool. Gun taps is another type of tap that is specifically designed to push the metal chips created during the tapping process forward into the workpiece. Gun taps are only useful in through holes because if they are used in blind holes, the metal chips will get stuck in the blind hole and the gun tap will break.
The type of metal that is tapped into requires specific speed to tap and specific lubricants to aid in the tapping process. For metals like steel and stainless steel, the metal will work harden if tapped too much. If stainless steel is tapped without lubricant, the stainless steel metal will gall and seize the tap.
In this case, heavy tapping pastes or extreme pressure oils should be used when tapping into stainless steel. For metals like aluminum, the metal is more forgiving of tapping but can clog the holes created in the metal by the tapping tool. For this reason, a light lubricant like kerosene should be used when tapping into aluminum metal.
For metals like cast iron, the metal contains graphite that acts as a lubricant for the tapping process. For this reason, compressed air instead of lubricants should be used when tapping into cast iron metal. It is important to monitor the torque that the tap applies to the workpiece when tapping with a hand drill.
When tapping, the tap should bite into the metal of the workpiece. If too much torque is applied, you should reverse the tap a quarter turn to break the metal chip before continuing tapping into the workpiece. The back and forth tapping of the tap into the workpiece ensures that the tap does not break during the tapping process.
Before tapping into a workpiece with a tap, certain preparations must be done to ensure the tap stays centered in the workpiece. A center punch or spot drill mark should be made in the workpiece to ensure that the tap starts in the proper location in the workpiece. A countersink should also be tapped at the top of the workpiece where the tap will enter the workpiece.
The drill bit, lubricant, and speed should all be matched with the metal that will be tapped into to ensure that the tapping process is successfull. You should of checked the tool size first so the process is more comfortabley handled. Dont forget that the drill bit size is more important then the tap size for a good fit.
Its vital to use the right equipment for the jobs you do. One of the most common mistakes is using a drill bit that is too small and then the tap will break. Making sure the tap is centered is also a key part of the process.
