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.

Types of thread manufacturing process

Note the term “programming approach” a moment ago. 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. 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.

Thread cutting operation on lathe machine pdf

This last bullet—flexibility—is also thread milling’s Achilles heel. The programming is admittedly more complex, which is why some shops have steered clear of it. But given the large number of online programming calculators and widespread CAM support for thread milling, there’s no reason to avoid it, especially considering its greater flexibility and thread quality.

A 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. 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. The thread now complete, the tool disengages by executing another small arcing motion back to its starting point before withdrawing from the hole.

Whatever method makes the most sense for your application, be sure to follow best practices. Always use the appropriate toolholder. For machines without a rigid tapping function, a tension-compression holder or self-reversing tapping head will be needed, otherwise a synchronous tap chuck like this one from Kennametal should be used. Thread mills are best gripped in a high-quality milling chuck, whether hydraulic, mechanical, or shrink-fit holder. Avoid side-lock holders and collet chucks.

Machining threadssizes

The combination of design elements on Kennametal high-performance solid carbide thread mills offers a range of benefits to improve overall thread quality and tool production. Short, easily evacuated chips generate less heat and friction, so there is a lower risk of damage to threading. Also, the superior carbide grades make threading easier and machining times shorter.

Kennametal spiral flute taps offer high performance for blind-hole applications in steel & steel alloys, stainless steel, cast iron, nickel- & cobalt-based alloys, titanium & titanium alloys, aluminum, and hard steel.

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. 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. So would bicycles, computers, refrigerators, and virtually every other modern electromechanical device. Threaded fasteners hold the world together. Literally.

Thread cutting pdf

Types ofmachining threads

There are also forming or roll taps, which produce no chips at all. Instead, they displace material, much like the thread-rolling process mentioned in the introduction. 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. 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.

A thread mill also requires a pre-drilled hole. Unlike cut and form taps, though, it’s far more flexible in terms of feedrates, thread size, and programming approach. It works like any milling cutter, removing material radially, along its peripheral edges. What's different is that a thread mill's flutes are a mirror image of the thread form itself, generating it as they pass.

Threads and threading is a complex subject. 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.

Thread manufacturing process pdf

This was a generic programming description. If the material is tough like Inconel or titanium, multiple passes or so-called “spring passes” might be needed. Different arcing strategies can be used, and since the feedrate is not dependent on the thread pitch as with tapping, programmers are free to adjust their strategies based on machining conditions and the type of tool used—the example just given described a typical path of a “full profile” thread mill. However, some manufacturers offer “single-plane” thread mills, which must trace the entire thread from top to bottom, circling round and round until reaching full depth.

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. 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. 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. As noted, threading is complex stuff, so let’s eat the threading elephant one bite at a time.

Threading process in lathe machine

And if through-the-tool coolant is available (an increasingly common feature with thread mills), clean, high-pressure cutting fluid is preferred, whereas for tapping, it might be necessary to stop the machine and squirt a little wax or specialty fluid into the hole to prevent seizing. Whatever the approach, be sure to weigh your options and do the math. With the exception of high-volume applications, we've seen that thread milling often comes out on top, although tapping continues to have its place. When in doubt, give the experts at Kennametal a call. We know threading.

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Just as there are many different types of threads, so does a wide variety of taps exist. 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. Spiral point taps tend to drive chips forward, whereas those with spiral flutes direct them upwards, out of the hole.

The tool's travel distance during a single spindle revolution is referred to as the feed rate. It is referred to as the cutter feeding speed. It is expressed in terms of turning distance per revolution.

Cncmachining threads

At first glance, a tap looks quite similar to a bolt or screw, albeit one with grooves running down the sides. 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. 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.

As with so much else in the manufacturing world, the answer to “which approach is best?” depends on numerous factors. Tapping and thread milling each have distinct pros and cons, and the choice of one over the other comes down to production quantity, material hardness or toughness, available machine tool power, accuracy requirements, and personal preference. Here are some things to consider as you weigh the two options:

According to the tool geometry and application settings, the Chip Thinning factors ensure that the actual Feed per Tooth [Fz] will retain the required Chip Load.