Threading (manufacturing)

The method for any one application is chosen based on constraints—time, money, degree of precision needed (or not needed), what equipment is already available, what equipment purchases could be justified based on resulting unit price of the threaded part (which depends on how many parts are planned), etc.

Unlike drill bits, hand taps do not automatically remove the chips they create.

Therefore, in manual thread cutting, normal wrench usage is to cut the threads 1/2 to 2/3 of a turn (180 to 240 degree rotation), then reverse the tap for about 1/6 of a turn (60 degrees) until the chips are broken by the back edges of the cutters.

The tool moves linearly while the precise rotation of the workpiece determines the lead of the thread.

In external thread cutting, the piece can either be held in a chuck or mounted between two centers.

[6] The coordination of various machine elements including leadscrew, slide rest, and change gears was the technological advance that allowed the invention of the screw-cutting lathe, which was the origin of single-point threading as we know it today.

On CNC machines, the process is quick and easy (relative to manual control) due to the machine's ability to constantly track the relationship of the tool position and spindle position (called "spindle synchronization").

With the widespread dissemination of affordable, fast, precise CNC, it became much more common, and today internal and external threads are often milled even on work where they would formerly have been cut with taps, die heads, or single-pointing.

[10] Additionally, for large, awkward workpieces (such as a fire hydrant casting), it is simply easier to let the workpiece sit stationary on a table while all needed machining operations are performed on it with rotating tools, as opposed to rigging it up for rotation around the axis of each set of threads (that is, for the "arms" and "mouth" of the hydrant).

In one variant of form-milling, the single-form cutter is tilted to the helix angle of the thread and then fed radially into the blank.

A similar variant using a multiple-form cutter exists, in which the process completes the thread in one revolution around the blank.

This lack of a lead-in chamfer allows the threads to be formed within one pitch length of the bottom of a blind hole.

The disadvantage is that the process is limited to hole depth no greater than three times the diameter of the tool.

To a casual observer (without slow motion), it looks rather similar to traditional tapping but with faster movement into and out of the hole.

Then the blank is slowly rotated through approximately 1.5 turns while axially advancing through one pitch per revolution.

Common centerless thread grinding production rates are 60 to 70 pieces per minute for a 0.5 in (13 mm) long set screw.

This is a toolroom practice when the highest precision is required, rarely employed except for the leadscrews or ballscrews of high-end machine tools.

[23] Of course, this fact highlights the importance of the moldmakers getting the mold just right (in preparation for millions of cycles, usually at high speed).

Cast threads in metal parts may be finished by machining, or may be left in the as-cast state.

For parts where the extra precision and surface finish is not strictly necessary, the machining is forgone in order to achieve a lower cost.

For most additive technologies, it has not been long since they emerged from the laboratory end of their historical development, but further commercialization is picking up speed.

To date, most additive methods tend to produce a rough surface finish and tend to be restricted in the material properties that they can produce, and thus their earliest commercial victories have been in parts for which those restrictions were acceptable.

Good examples of threaded parts produced with additive manufacturing are found in the dental implant and bone screw fields, where selective laser sintering and selective laser melting have produced threaded titanium implants.

Inspection of the finished screw threads can be achieved in various ways, with the expense of the method tailored to the requirements of the product application.

Even industrial radiography (including industrial CT scanning) can be used, for example, to inspect internal thread geometry in the way that an optical comparator can inspect external thread geometry.

Ball-shaped micrometer anvils can be used in similar fashion (same trigonometric relationship, less cumbersome to use).

Digital calipers and micrometers can send each measurement (data point) as it occurs to storage or software through an interface (such as USB or RS-232), in which case the table lookup is done in an automated way, and quality assurance and quality control can be achieved using statistical process control.

Therefore, a comprehensive discussion is beyond the scope of this article; but much historical information is available in related articles, including: The first patent for the cold rolling of screw threads was issued in 1836 to William Keane of Monroe, N.Y.[24][25] However, the dies for rolling the threads onto the screw blanks were made of cast iron, which is brittle, so the machine was not successful.

The process languished until 1867, when Harvey J. Harwood of Utica, New York filed a patent for the cold-rolling of threads on wood screws.

[26] Further efforts to cold-roll threads on screws followed,[27] but none seemed to meet with much success until Hayward Augustus Harvey (1824–1893) of Orange, N.J. filed his patents of 1880 and 1881.