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Thread Milling

Screw threads, both external and internal, can also be cut efficiently and economically by milling. This is done on a milling machine, so-called thread-milling cutters being used for this purpose. These cutters may be of either the single or the multiribbed type, according to the kind of thread required and the design of the milling machine employed.

An innovative six-cutting corner system of a thread turning insert promises revolutionary gains in productivity as well as halving tooling costs, said the MACH 2008 exhibitor. A couple of weeks before, we featured the news release Thread milling tool works in 62 HRc metals from Vargus Tooling UK: Thread milling tool can produce threads as small as 2mm (M2 x 0.4 - No 2-56 UN) in hard materials up to 62 HRc, said a manufacturer, where other tools normally break.

Thread milling has become quite popular in recent years as an alternative to tapping or other forms of threading. Thread mills look similar to taps, but function entirely different. Taps feed into the part at the rate of the lead of the thread, utilizing the chamfer and first full thread beyond the chamber to cut and enlarge the thread to finished size.

On the other hand, a thread mill has no chamfer. The mill is inserted into the hole along the axis of the spindle; deep enough to produce the full thread depth required. The controller moves the thread mill out to the hole diameter until the threads cut into the sidewalls of the hole. The thread mill then moves in a 360 circular motion until it is back to it starting position.

During this circular motion the thread mill must be lifted toward the top of the hole or moved along the Z-axis of the machine one thread pitch or lead to produce a thread. This lifting movement in conjunction with the circular motion is called helical interpolation. Machines must have a helical interpolation program capability to utilize a thread mill. After the 360 rotation, the tool returns to the center of the hole and extracted from the part.

A long screw thread of coarse pitch can suitably be cut by means of a disc-shaped single cutter (Fig. 1). The machine used for this work somewhat resembles a lathe, but instead of a saddle there is a carriage supporting a cutter head in which the cutter is mounted. The cutter is inclined to produce the correct helix angle of the thread. The feed (longitudinal motion) of the carriage and the rotation of the workpiece are interlinked by means of a lead screw and gearing so as to obtain the correct pitch of the thread being milled.

The long-thread milling technique with a disc-shaped cutter can also be used for internal threading (Fig.2) if the hole is of sufficiently large diameter to admit the cutter. In short-thread milling the tool is a multiribbed cutter (or multi cutter) of the ring or shell type (Figs.3 and 4) or of the taper-shank type (not illustrated). The ribs on the cutter have the shape of the screw-thread profile they have to cut. The cutter is usually as long as, or longer than, the required length of the threading on the workpiece. Generally the latter performs only a little more than one revolution, the axial motion being only little more than one pitch. The axis of the cutter is usually parallel to the axis of the workpiece, but in some cases tilting may be necessary to produce the helix angle of the thread.

Another method of producing screw threads is by rolling under pressure. The part to be threaded is rotated and brought into contact with rollers that have the required profile and pitch of the thread. This is not a cutting method and involves no removal of metal, the thread being formed by plastic deformation. Not only is there a saving in metal, but the rolling operation causes cold working and thus improves the mechanical properties of the thread.

The method is especially suitable for soft metals, such as aluminum, which are difficult to screw-cut with a smooth metal finish. It can be done on a lathe with the aid of a thread-rolling head (Fig.6), which is equipped with three ribbed rollers that can move in and out radially. The rollers open out automatically at the end of the operation. A variant of this process consists in rolling the thread between grooved flat plates on a thread-rolling machine (Fig.5)

Thread mills are most commonly found in solid carbide, with either straight or helical flutes. They are also available in indexable style with carbide inserts, or premium high-speed steel (powder metal) with helical flutes. Ideally all thread mills are coated with TiN, TiCN, or TiAlN depending upon the application. Indexables are typically for sizes " or larger, and accommodate a variety of threads per inch by replacing the insert. Solid carbide is generally for production threading and for materials up to 62 Rc. Powder metal HSS thread mills are recommended for materials softer than 30/32 Rc, less rigid setups, interrupted bores and machines that have a limited speed capability.