Quality Scan: Milling Can Improve Thread Quality
Thread milling is generally looked at as an alternative to tapping, especially within the mold and die industry. But it also offers users an additional ability to reliably make precision holes, even in hardened materials.
Not so long ago, a threaded hole in a mold or die would be tapped with an oversized tap before hardening to compensate for the distortion that can occur. In some cases, the tap would even have to go through the hole again to clean the thread after hardening. When the material is received in the hardened state, it's almost impossible to tap a hole without breaking the tap. Thread milling alleviates this problem by allowing you to consistently achieve precision threads with more accurate thread location, even with parts that have been hardened.
In addition, during the production of tapered threads for sealing, which is very common in the mold and die industry, tapping tends to tear threads. Taps also break more easily because of the tapered hole. Thread milling makes a high-quality precision tapered thread that in some cases creates a seal so tight that pipe sealant or Teflon tape is not needed for the threaded connection. Additionally, when making tapered threads using a thread mill, the user can repeatably achieve the same start point for the thread time after time.
Inserting eye bolts for transporting oversize molds is another example in the die and mold industry where users should consider thread milling. Traditionally, manufacturers tapped the holes for inserting these bolts, because tapping seemed the simplest, quickest way to obtain threaded holes. But due to the forces required to push large taps through properly sized holes-especially in hard materials-operators tended to drill the hole oversize to make it easier to tap the part.
Undercutting the full thread depth in this way can lead to a lack of full-thread engagement. In a worst-case scenario, when an eye bolt is inserted into a hole with weakened threads, the bolt may pull out when lifting the mold, potentially causing damage to the mold and injuring operators.
Thread milling provides a better alternative to tapping in this application because there is no need to drill oversize holes. In fact, to get 100% thread depth in thread milling you first drill an undersized hole. It's true that when threading holes deeper than about 1.5× diam, tapping is often faster than thread milling, which may require multiple passes. The programmability and repeatability of milling has a direct impact on the quality of threads, however, and that's where a safety issue comes into play.
Another consideration to bear in mind is that taps can also break inside a hole, and removing the broken tap can consume hours of production time. Thread mills not only break less often, but they also only take about 10 min to replace. Moreover, the work stays in the machine, eliminating the risk of introducing locating errors that would be inherent when reclamping it in the machine after the broken tap is removed.
Some resistance to thread milling has occurred due to the sophistication of machine programming. Although thread milling uses the helical interpolation function that is standard on most CNCs, many operators are intimidated by the complexity of the helical pattern that the spindle follows to produce the thread. To simplify programming, some, but not all, CNC manufacturers now offer macros for thread milling with their products.
Some cutting tool companies also offer free thread-milling software. For example, our company's Wizard program simply requires that the operator enter the type of thread, diameter, depth, and group of work materials being cut. The software then generates machine code automatically. It also recommends the best cutter for the job and selects the optimum cutting parameters for it.
Overall, thread milling improves quality and increases tool life-and it can be more economical than tapping.
This article was first published in the January 2006 edition of Manufacturing Engineering magazine.