A thread charts for Unified National Fine 2B threads, or a UNF 2B thread chart for short, provide information regarding the measurements necessary for drilling and tapping hole. A UNF 2B thread chart tell you the drill size that you should use for the tapped hole, as well as define the thread limits for that size hole. Furthermore, a UNF 2B thread chart can be used to determine the engagement length of threads within a given material, as well the translation of the drawing of the bolt to the actual size of the drill bit that will create that tapped hole.
UNF stands for Unified National Fine. The “fine” in this name indicate that there will be more threads along a given inch of the bolt than would be present on a coarse bolt of the same diameter. Thus, fine bolts will allow for the threads to provide more of an area of contact with thin-walled or soft materials.
How to Use a UNF 2B Thread Chart
The 2B refer to the tolerance class of the internal thread; class 2B threads are the standard for most production environments. The 2B class allows for plated bolts to fit within the threads, but ensures that the bolts are still strong enough to resist the load that those bolts will experience. UNF 2B thread charts indicate that a tap drill size will create a thread that is 75% of the total diameter of the bolt.
Using 75% threads allow for the tap to cut the threads cleanly and efficient; 100% threads require significantly more torque to cut, which can lead to broken taps. Furthermore, 50% threads are more easier to tap with a tap, but offer less strength to the tapped joint. The engagement length of threads is the length to which the bolt must be inserted into the material to achieve the desired strength of the bolt.
The engagement length for a bolt within a material will differ based off the material itself; stripped threads is caused by the parent material failing, not the bolt. Thus, steel bolts will have an engagement length of 1 time the bolt’s diameter. Aluminum and cast iron will have an engagement length of 1.5 times the bolt’s diameter due to its lower strength compared to steel.
Finally, plastics have an engagement length of 2 times the bolt’s diameter due to their even lower strength or bolts may require a thread insert. Depending upon the project that you are creating, you may have to choose between using fine threads compared than coarse threads. Fine threads are better for resisting loosening from vibration.
Coarse threads are better for those looking to assemble a product quick and efficiently. Thus, if the part that will contain the tapped hole is thin or will experience movement or vibration, fine threads should be chosen. If, however, the project is focused upon speed, then coarse threads should be used.
If the material is too soft for the tap to be used, or if the threads have already stripped, thread insert may be used to remedy these issues. Thread inserts come in a variety of forms; wire style inserts, like Helicoils, can be inserted into the material to create a steel thread within the hole. Solid inserts are used where high resistance to torque are required.
Rivet nuts are used in thin metals where only one side of the metal part can be accessed. Though not listed on the thread charts for UNF 2B threads, these thread inserts depend upon the 2B thread geometry that is defined on the thread chart. Depending upon the material that will be tapped, there are changes that must be made in the tapping technique that is used.
If tapping steel, spiral-flute taps and sulfurized oil should be used. If tapping aluminum, spiral-point taps and a light lubricant should be used to allow chips to exit the material. Stainless steel taps require low tap speed and frequent clearing of the cut steel.
Finally, because cast iron contains graphite, lubricant can be avoided for tapping operation. Though not included on a UNF 2B thread chart, the information on the chart regarding the proper size of the tap will enable these tapping technique to be followed successfully. Thread gauging is a technique for ensuring that a tapped thread is of the correct size.
A GO gauge for threads must be able to pass through the entire length of the tapped hole. However, a NOGO gauge will not enter the tapped hole more than two revolutions. These two gauges will ensure that the tapped hole is of the 2B limits for the bolt; no micrometer are required.
Overall, a UNF 2B thread chart is valuable in that it removes the guesswork involved in tapping hole. The engagement length, lubrication requirements and need for thread inserts will still have to be determined for each project; however, the thread chart ensures that the size of the tapped hole will be correct.
