![]() This was a generic programming description. The thread now complete, the tool disengages by executing another small arcing motion back to its starting point before withdrawing from the hole. The rotating thread mill then describes one complete circle while simultaneously moving upward (in the Z-axis) by an amount equal to the thread pitch-in our 1/4"-20 example, this would be 0.050", or one complete turn. Regardless, it gives programmers great latitude in their machining approach, the types of material they can machine, and even the size of the threaded hole-a single 20-pitch thread mill can produce any size thread, for instance, provided it has 20 threads per inch and does not exceed the tool’s maximum cutting depth.Ī thread milling operation starts by driving the cutter down the center of a drilled hole at a fairly rapid rate, then using a small arcing motion to move the tool radially into the workpiece until reaching the required diameter. Unlike tapping, which can be performed on practically any machine tool or even by hand, thread milling is only possible on a CNC machining center, Y-axis equipped mill-turn center, or multitasking lathe. Note the term “programming approach” a moment ago. What's different is that a thread mill's flutes are a mirror image of the thread form itself, generating it as they pass. It works like any milling cutter, removing material radially, along its peripheral edges. Unlike cut and form taps, though, it’s far more flexible in terms of feedrates, thread size, and programming approach. However, form taps are limited to ductile materials like aluminum, stainless steel, and superalloys, and should not be used with cast iron or hardened steels.Ī thread mill also requires a pre-drilled hole. These follow the same basic rules as cut taps insofar as feedrate and tool geometry, but require a slightly larger pilot hole to allow for the displaced metal. Instead, they displace material, much like the thread-rolling process mentioned in the introduction. ![]() There are also forming or roll taps, which produce no chips at all. Spiral point taps tend to drive chips forward, whereas those with spiral flutes direct them upwards, out of the hole. Plug taps are generally used to thread “through-holes” while bottom taps are as their name describes, able to produce threads up to the very bottom (almost) of blind holes. Just as there are many different types of threads, so does a wide variety of taps exist. The two caveats to this are as follows: there must be a hole slightly larger than the thread’s minor diameter (taps are not drill bits and only cut on the tool's outer edges), and the feedrate must be precisely equal to the thread’s pitch-a 1/4"-20 tap, for example, must advance 0.05" per revolution (or 20 threads per inch) to produce a good thread. Taps work much like any other rotary tool, in that they’re gripped in a chuck, collet, or special “floating” toolholder (more on this shortly) and then driven into the workpiece at a specific feedrate. These grooves are used to conduct chips up and out of the hole during machining, just as the sharp edges on the end and periphery are used to cut the threads. As noted, threading is complex stuff, so let’s eat the threading elephant one bite at a time.Īt first glance, a tap looks quite similar to a bolt or screw, albeit one with grooves running down the sides. Nor will we cover thread milling’s alter ego, single-point threading, a process commonly used to produce internal and external threads but which can only be performed on appropriately equipped lathes. ![]() There’s no room to talk about thread rolling, chasing, whirling, or other threading methods, all of which are limited to external fasteners-screws, in other words. What we will discuss are the two primary approaches to making internal screw threads- tapping and thread milling-and when to use one over the other. Coarse and fine, left or right, tolerance classes, form, angle, and pitch-we won't attempt to explore the near-encyclopedic amount of screw thread terminology or all the many types of threads in use today, except to say that most have a 60-degree form and fall under the metric or Imperial measurement system. Threads and threading is a complex subject. Threaded fasteners hold the world together. So would bicycles, computers, refrigerators, and virtually every other modern electromechanical device. Since the dawn of electricity, though, we've used pumps to move fluids, which would be impossible or at least very difficult and expensive to manufacture without threaded fasteners. Historians will tell you that the screw thread was invented more than two millennia ago and was originally a means to bring water to thirsty crops and people.
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