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Thread-milling tool tip; technique eliminates imperfect threads at end of cut.

Popularity of the almost forgotten thread-milling process is undergoing a dramatic resurgence. Emergence of a new technique that eliminates imperfect threads at either end of the machined surface promises to further enhance thread milling's use. all conventional threading processes such as tapping, die chasing, rolling, and single-point flycutting leave an incomplete thread (or fin) at both the beginning and end of the threaded section, Figure 1.

Because this incomplete thread is so thin, it's easily deformed as it is engaged during rundown. After being deformed, it can gouge its mating part and, possibly, lock up the threaded assembly before rundown is complete, often forcing both mating parts to be scrapped. Sometimes the imperfect thread is removed in a secondary end-milling or turning operation; however, this involves significant machining, handling, and setup costs.

This is a particular problem in high production industries, such as automotive and off-highway equipment, where the need for rapid assembly leads to high run-down forces, either manually or with automated equipment, and in other industries, such as aerospace, where quality is so critical that imperfect threads or excessive run-down torque are grounds for rejection.

With the thread-milling process, this imperfect thread can easily be removed by adding a flat protruding section on the end of the cutter. This effectively end mills the troublemaker down to the approximate root diameter. The operation takes place at the same time the thread is cut, so no additional cycle time is required. Also, there is normally no additional cost to add the end-milling section to the tool.

The imperfect thread may be removed at either one or both ends of the threaded section. Figure 2. If the thread is removed from both ends of the part, tool length--and particularly distance between the two protruding sections--must match the part. This eliminates one of the inherent advantages of thread milling, i.e., being able to machine a wide range of thread depths with a single tool. It is still possible, however, to machine a variety of parts with a thread depth equal to or less than the original part if one-end imperfect thread removal is acceptable on short-depth parts.

If the part will always be assembled in the same orientation, it's normally necessary only to remove the thread on one end. This makes it possible to specify a cutter that's longer than the longest threaded section likely to be encountered, thus maintaining the advantage of being able to thread parts of any depth.

Programming a thread-milling operation for removing an imperfect thread hardly differs from programming conventional thread milling. Imagine three circles--one concentric with the hole to be threaded, with a diameter equal to the hole diameter minus the cutter diameter; the other two tangent to this circle, with diameters about one fourth the hole diameter to be threaded.

The spindle is centered, moved tangent to one of the smaller circles and ramped into the cut by moving it along the circumference of the circle representing the cutter centerline. In normal thread milling, the cutter travels an arc of slightly more than 360 degrees. For imperfect-thread removal, this additional travel is more critical because it determines the amount of thread removed. Normally, the additional travel should be about 20 degrees.

Thread milling primer

A quick review of the essentials of the thread-milling process is in order. Each tooth of a thread-milling cutter is shaped like the thread form it produces. The teeth are arranged in parallel rows without any spiral lead. The cutter rotates at high speed, while its axis slowly moves around the part in a planetary arc of just over 360 degrees.

As the spindle axis moves around the part, it advances one pitch in the axial direction. This is commonly referred to as helical interpolation, and is easily programmable on almost all CNC machines.

The most important advantage of thread milling is it allows machining many types of threads on a machining center that would otherwise have to be performed by an expensive secondary operation. Examples are large diameter threads, threads with critical tolerances, off-standard forms, etc. This is because thread-milling cutters require low horsepower, produce accurate threads, and are versatile. A single cutter can produce ID and OD threads of any length and diameter at least 20 percent greater than the tool diameter.

Other advantages include: lower tool cost, because the cutters are considerably smaller in diameter than the threads produced; longer tool life, because significantly more regrinds can be achieved than with taps or die heads; superior thread accuracy, because the machine, rather than the tool, controls the lead, and it can be easily adjusted; and superior surface finish, because cutting speeds are higher, and feed rates lower, than other threading methods.

For more information about thread milling, circle E53.
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Copyright 1985 Gale, Cengage Learning. All rights reserved.

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Author:English, Gale
Publication:Tooling & Production
Date:Apr 1, 1985
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