05-04-2012, 11:55 AM
Deformative or transformative methods
Deformative or transformative methods.doc (Size: 107 KB / Downloads: 33)
practice as of 1914.
Thread forming and thread rolling are processes for forming screw threads, with the former referring to creating internal threads and the latter external threads. In both of these processes threads are formed into a blank by pressing a shaped die against the blank, in a process similar to knurling. These processes are used for large production runs because typical production rates are around one piece per second. Forming and rolling produce no swarf and less material is required because the blank size starts smaller than a blank required for cutting threads; there is typically a 15 to 20% material savings in the blank, by weight.[15] A rolled thread can often be easily recognized because the thread has a larger diameter than the blank rod from which it has been made; however, necks and undercuts can be cut or rolled onto blanks with threads that are not rolled. Also, the end of the screw usually looks a bit different from the end of a cut-thread screw.[3]
Materials are limited to ductile materials because the threads are cold formed. However, this increases the thread's yield strength, surface finish, hardness, and wear resistance.[15] Also, materials with good deformation characteristics are necessary for rolling; these materials include softer (more ductile) metals and exclude brittle materials, such as cast iron. Tolerances are typically ±0.001 in. (±0.025 mm), but tolerances as tight as ±0.0006 in (±0.015 mm) are achievable. Surface finishes range from 6 to 32 micro-inches.[16]
There are four main types of thread rolling, named after the configuration of the dies: flat dies, two-die cylindrical, three-die cylindrical, and planetary dies. The flat die system has two flat dies, the bottom one is held stationary and the other slides. The blank is placed on one end of the stationary die and then the moving die slides over the blank, which causes the blank to roll between the two dies forming the threads. Before the moving die reaches the end of its stroke the blank rolls off the stationary die in a finished form. The two-die cylindrical process is used to produce threads up to 6 in (150 mm) in diameter and 20 in (510 mm) in length. There are two types of three-die processes; the first has the three dies move radially out from the center to let the blank enter the dies and then closes and rotates to roll the threads. This type of process is commonly employed on turret lathes and screw machines. The second type takes the form of a self-opening die head. This type is more common than the former, but is limited by not being able form the last 1.5 to 2 threads against shoulders. Planetary dies are use to mass produce threads up to 1 in (25 mm) in diameter.[4][15]
Thread forming is performed using a fluteless tap, or roll tap,[17] which closely resembles a cutting tap without the flutes. There are lobes periodically spaces around the tap that actually do the thread forming as the tap is advanced into a properly sized hole. Since the tap does not produce chips, there is no need to periodically back out the tap to clear away chips, which, in a cutting tap, can jam and break the tap. Thus thread forming is particularly suited to tapping blind holes, which are tougher to tap with a cutting tap due to the chip build-up in the hole. Note that the tap drill size differs from that used for a cutting tap and that an accurate hole size is required because a slightly undersized hole can break the tap. Proper lubrication is essential because of the frictional forces involved, therefore a lubricating oil is used instead of cutting oil.[2][4]
When considering the blank diameter tolerance, a change in blank diameter will affect the major diameter by an approximate ratio of 3 to 1. Production rates are usually three to five times faster than thread cutting
Thread casting and molding.
In casting and molding the threads are directly formed by the geometry of the mold cavity in the mold or die. When the material freezes in the mold, it retains the shape after the mold is removed. The material is heated to a liquid, or mixed with a liquid that will either dry or cure (such as plaster or cement). Alternately, the material may be forced into a mold as a powder and compressed into a solid, as with graphite.
Although the first thoughts that come to mind for most machinists regarding threading are of thread cutting processes (such as tapping, single-pointing, or helical milling), Smid points out that, when plastic bottles for food, beverages, personal care products, and other consumer products are considered, it is actually plastic molding that is the principal method (by sheer volume) of thread generation in manufacturing today.[18] 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. (The same can be said of cast gear teeth.) Whether or not to bother with the additional expense of a machining operation depends on the application. For parts where the extra precision and surface finish is not strictly necessary (although it might be nice), the machining is forgone in order to achieve a lower cost. With sand casting parts this means a rather rough finish; but with molded plastic or die-cast metal, the threads can be very nice indeed straight from the mold or die. A common example of molded plastic threads is on soda (pop) bottles. A common example of die-cast threads is on cable glands (connectors/fittings)