The class of the thread is a form of specify the tolerance for threads; the thread class determine the way in which the fastener will fit into the hole. The thread class of UNC 2B is applied to internal threads, and UNC 2B is the standard thread class for internal threads in everyday hardware. UNC 2B internal threads will accept external threads of size 2A, which are common on the majority of bolt.
The combination of internal threads of size UNC 2B with external threads of size 2A allow for the assembly of the components without the use of hand fitting or specialized tool. The form of the threads feature a 60-degree angle with rounded roots and rounded crests. This form of threads helps the components feature strength when the threads are in use.
UNC 2B Threads and Tapping Basics
Additionally, the 60-degree angle makes the threads easy to cut when manufacturers are making the components. The coarse threads are more likely to resist cross threading than the fine threads. Coarse threads are also capable of holding tension in most material.
Due to these feature of coarse threads, UNC 2B threads are used in the manufacture of machine frames and automotive engine block. Class 2B features no allowance for the threads. The fact that there is no allowance for the threads mean that the minimum pitch diameter of the threads is the same as the basic size of the threads.
The tolerance for Class 2B allow for the hole to be slightly larger than the basic size. This fact that Class 2B features no allowance for the threads allows for normal bolts to be manufactured without binding in the holes, and any excess play in the threads between the bolt and the hole. A standard bolt of size 2A will thread into a hole of size Class 2B without binding, and a standard bolt of size 2A will thread into a hole of size Class 2B without excess play.
The tap drill is the drill that is used to create the hole in the component prior to the application of the tapping tool to create the internal threads. The targeted engagement for taps is 75 percent of the total length of the tapped component. A tap that achieve 75 percent engagement is better than one that achieve 100 percent engagement because the 100 percent engagement may lead to breakage of the tap.
For aluminum, however, the engagement should be 50 to 60 percent because the metal is relatively soft and may lead to binding of the tap. If the drill bit used to create the hole before tapping is of the incorrect diameter the tap bit can be ruined. Using a drill bit that is too small force the tap to remove more material from the work piece than the tap bit is designed to remove.
Using a drill bit that is too large reduces the strength of the created internal tap. Using a reference grid for the size of the tap drill can avoid these problem. Such a grid can be used to find the 75 percent engagement mark for the threads.
Additionally, the grid indicates the limit of the pitch diameter of the tap. Using go and no go plug gauges can help ensure that the tapped hole is of the appropriate size. The go gauge will slide into the hole, but the no go gauge will not go further than a couple of turns into the hole.
This test can identify any problem before the component leaves the bench. The torque for each fastener depend upon the size of the fastener and the type of metal that is being used in the component. Small fasteners only require a few inch-pounds of torque to set the bolt.
Large fasteners, however, may require over one hundred inch-pounds of torque to properly set the bolt. The metal that is used will also impact the amount of torque that is used when tightening the bolts. Stainless steel will require more torque than mild steel.
Aluminum, however, will require less torque than mild steel. Blind holes and through holes require different method for the clearing of metal chips created during the tapping process. For blind holes, in which there is no place for chips to go after they are cut, spiral flute taps can be used to allow the chips to exit the blind hole.
For through holes, in which chips can exit the component during tapping, the tap can be backed out periodically during the tapping process. Cutting oil can be used when tapping. For components made of stainless steel, cutting oil is used to prevent galling of the threads.
For components made of cast iron, cutting oil can be avoided because the chips that are created from cast iron are brittle and will cleave apart from the component without the use of cutting oil. Some of the most common mistake made in tapping include not starting the tap square to the component, using the incorrect size drill bit, or forcing the tap bit with the component. To avoid these mistakes, a tap guide or a block with a drilled hole can be used to ensure that the start of the tap is true to the component.
Additionally, ensuring that the size of the drill bit matches the tap drill size chart will avoid any need for rework of the component. For the assembly of the component, the length of the engagement of the threads is one of the last variable to decide. For fasteners of steel into steel, the engagement length should be at least one times the diameter of the bolt.
For fasteners of steel into aluminum, the engagement length should be at least one and a half times the diameter of the bolt. For softer metals, inserts can be used to ensure the strength of the tapped fastener. The UNC 2B thread class does not impact the rules regarding the length of the engagement of the threads.
However, the UNC 2B thread class ensure that a standard bolt of size 2A will fit into the component with Class 2B threads once the engagement length is of the appropriate size.
