1.2512 unf rh2b thread forming tap8/30/2023 ![]() ![]() The sample was flipped over and the process was repeated with the undamaged threads on the opposite end.īoth formed and cut threads in correctly sized holes held until loads reached 35,000 lb. To test strip-out strength, we inserted a 0.5-in.-long, grade 8 steel, ANSI 5/8-18 setscrew into each tapped hole and pressed it out with a steel rod while recording the required force. rings were cut from the end of each specimen, and thread profiles were examined in cross section. Forming taps need larger holes because the roots of the tap extrude material inward to form thread crowns.īefore testing thread strip-out strength, 0.25-in. This represents a diameter increase of 8% for the cutting taps and 3.4% for the forming tap. ![]() for a forming tap for 75% engagement - up to 0.617 in. Tap-hole diameters varied from the ANSI B1.1 minimum recommended diameter - 0.5708 in. Spiral taps, on the other hand, have angled lead-in flutes that guide continuous cuttings up and out the back of the hole. Plug taps have little or no lead-in and are used primarily for tapping blind holes. Both spiral and plug-type cutting taps were used, in addition to a thread-forming tap. ![]() We precision bored 5/8-in.-18 UNF tap holes into the ID of the tube stock to create 1.75-in.-long specimens. The basic material properties were 91-ksi yield strength, 99.7-ksi tensile strength, and 27% elongation before break. AK steels have finer grains and see less work hardening during plastic deformation. The “AK” stands for “aluminum killed,” meaning aluminum helped remove oxygen that causes bubbling in the molten steel while ingots formed. The tubes were ASTM A513/SAE 1026AK steel - mild plain carbon steel with 0.26% carbon and a maximum of 1% manganese. We fabricated specimens from tube stock with 1-in. To test this theory, we bored and tapped holes in tube stock, varying thread-forming techniques and diameters, then tested them. This includes light alloys and steels with tensile strengths up to 174 ksi and hardnesses below 35 to 40 on the Rockwell C scale. The prevailing wisdom says formed threads are the way to go for strength if the material is soft enough for cold forming. in diameter may be more susceptible to incorrect dimensions because they are fabricated by boring or turning, not with standardized drill bits. The process is generally taken for granted since drill and tap geometries are standardized and repeatable. Whether threads are cut or formed, fabricating them takes two steps: drilling or boring a tap hole and cutting or forming the threads. But internal threads are a different story, especially when internal-thread material is weaker than the bulk material or thread dimensions are incorrect. In fact, the first three threads have been shown to take up 81% of the axial force. ![]() It’s well known that the external threads of properly fabricated bolts are stronger than the bolt itself. Recent tests show there are some nuances to the strength of internal threads. Once the hole diameter and thread type are set, there’s little need to consider the strip-out strength of the internal threads, right? Wrong. Specifying internal screw threads isn’t the most thrilling task for engineers. Simulia, University of Tulsa, “The nuts and bolts of specifying fastener torque,” Machine Design, July 9, 1998, /MDFastenerTorque
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