In order to create clean threads in metal, you must make three specific decisions before you begins to use a tap to create these threads in your metal. You must decide what size pilot hole you will use, what type of tap you will use based on the material you are using, and what depth you need for the threads in your application. Many problem can occur if you do not make these decisions prior to beginning to use a tap to drill threads into your metal components.
For instance, if you use a tap, the tap may snap within your metal component. Additionally, the fastener may spin freely within the drilled hole because you did not drill it to the correct size prior to inserting the fastener. By making the decision of what size pilot hole to use prior to beginning to drill threads with a tap, you can avoid these problems.
How to Tap Threads in Metal Correctly
The chart provided to you in this lesson displays the standard sizes for the threads and the diameter for the pilot hole that should be create. Use this chart to determine the drill size for the pilot hole so that you dont have to guess at the correct size for your metal component. Additionally, 75 percent of the threads should engage with the fastener.
Using 75 percent engagement will provide enough strength to the fastener without requiring the taps to cut at all of the metal for that component. If you use more than 75 percent engagement, you will remove excess metal that will not provide additional strength to the fastener. If you do not use 75 percent engagement, the threads will strip when you attempt to place a torque on the fastener.
Thus, you should use the number in the chart for your drill bit size. After you drill your pilot hole, you must also decide which type of tap to use. For instance, if you are drilling into a through hole, you can use a taper tap to begin your threads.
For through and blind hole of medium depth, you can use a plug tap. For blind holes that do not go through the entire component, a bottoming tap can be used. Additionally, if you are using a blind hole, a spiral flute tap will allow the metal chips to exit the blind hole rather than getting stuck in the tap.
Thus, by properly selecting the correct type of tap for your component, you will save time and not break your taps. Finally, the material that you are cutting with your taps will impact the rate at which you cut and the type of lubrication that should be used when cutting that metal. Aluminum will cut easy but can cause a tap to grab too much metal when the heat begins to build within the aluminum component.
Therefore, you should use kerosene or light oil in this case. For materials like mild steel, steady cutting oil will help to prevent the metal from work hardening. Stainless steel is a more difficult metal to cut because it will work harden quick if the tapping tool begins to rub against the metal instead of cutting it.
Thus, you should use cobalt bits for cutting stainless steel and the speed at which the tap rotates when cutting that metal should be slow. By ensuring that the correct bit, speed, and lubrication fluid are used for the workpiece, you will ensure that the threads created by the tap are clean rather than featuring torn or welded threads. The thread pitch is another factor that will impact the working of the tapped threads.
Coarse threads will go into the hole faster then fine threads and are more forgiving of any damage to the workpiece. Fine threads will provide more surface area of contact between the two threaded pieces. Fine threads are beneficial in applications that experience significant vibration or where significant clamp load is needed.
However, fine threads are more difficult to start and require precise alignment of the two pieces being tapped. Use coarse threads in applications where the tapped piece must be taken apart frequently. Use fine threads in instances where the tapped piece must remain tight and in place under heavy loads or with temperature change.
The depth of engagement is another factor to consider when tapping into a workpiece. Because steel is a very strong material, it can withstand a thread engagement of only approximately one diameter of the threaded hole. Aluminum has a lower shear strength than steel and requires more engagement of the threaded hole than does steel.
Plastic requires an even greater engagement of the threaded hole than aluminum. If the workpiece has a stripped hole or if the material is too weak to take the engagement of the tap, use a thread insert to restore the strength of the threaded hole. Some of the most common failures in the tapping process can be avoided by ensuring that common mistakes are not made.
One of the most common is drilling a hole of the wrong size. Another mistake is not using cutting fluid in the tapped workpiece. Yet another mistake is not backing the tap out of the workpiece with every turn.
If you do not back the tap out of the workpiece with every turn, metal chips will pack into the tap and cause it to jam in the workpiece. It is never recommended to force a tap that is jammed in a workpiece because of the risk of breaking the metal within the workpiece. The drill bit size can be calculated by the formula that states that the pitch of the thread must be subtracted from the major diameter of the workpiece.
This calculation is represented in the provided chart. Thus, if the size of the workpiece is not represented in the chart, you can subtract the pitch from the major diameter to find the correct drill size. A good thread is one that allows the fastener to remain securely within the tapped workpiece and one that permits for the fastener to be removed easily.
The chart, the choice of tap, the lubrication fluid that is used, and the depth of engagement are all provided to remove the guesswork involved in tapping into a workpiece. All of these factors leave the individual with the workpiece ready to have the correct fastener selected. You should of checked the material first to make sure you use the right bits.
There is alot of ways to mess up if you dont pay attention to the details of the moddern equipment. Its important to recieve the right tools for the job.
