MAN PRO – EML4325 Study Guide E PDF

MAN PRO – EML4325 - STUDY GUIDE© Guide E Swaging → Thread Rolling There is a variation on FORGING that has been developed to be more suitable for round or circular parts. This method is called SWAGING, and it us...

MAN PRO – EML4325 - STUDY GUIDE© Guide E Swaging → Thread Rolling There is a variation on FORGING that has been developed to be more suitable for round or circular parts. This method is called SWAGING, and it uses multiple dies that close on the part along radial paths, rather than a single pair closing along a vertical path as with forging. Although this process is routinely performed on an industrial scale using large machines, it can also be done on a small scale using a HAND TOOL. This is the common approach used in attaching fittings to steel cable or wire rope. The SWAGING TOOL above is like an oversized pair of pliers that uses a compound lever action to generate large forces between the jaws (dies). Hydraulic SWAGING TOOLS that use a pumping action are also available for deforming ferrous metals. These tools are used in conjunction with a FERRULE to create a loop on a cable end. The cable is initially looped through the oval hole in the ferrule which is then crushed between the jaws of the SWAGING TOOL. The jaws have semicircular cutouts on each side (with different sizes to accommodate various standard ferrules) which effectively create circular die cavities. Although the initial crushing (deformation) only occurs along a single path as the jaws come together, the overall procedure involves rotating the ferrule through 90° before crushing for a second time. Multiple deformation strokes along different radial lines means that this method is indeed SWAGING, although it is performed in several stages rather than all at once. Industrial scale SWAGING, such as that on the left, uses multiple dies (four in this case) that all move towards the center simultaneously. This mode of action generates circumferential grain flow in circular parts. ©Dr. Stuart Wilkinson, Mechanical Engineering Department, University of South Florida SWAGING, as with FORGING, is performed in cases where superior strength is required, which is especially important when minimal metal is available to resist the loads. One such example product is the RATCHET SOCKET WRENCH SET. This common product is designed to assist in the tightening or loosening of hexagon-shaped threaded fasteners (bolts/nuts). The SOCKETS themselves come in a variety of sizes to accommodate standard (English or Metric) bolt/nut dimensions. They are circular on the exterior, but invariably have a rather thin wall, which is necessary when dealing with a row of closely spaced bolts/nuts. Given the application (high torque) and configuration, strength is paramount. The SOCKETS are hollow but suddenly transition from a hexagonal hole at one end (to slip over the bolt/nut) to a square hole at the other end (for attaching to the ratchet drive lug). These holes should not be cut since this would sever the grain flow and compromise strength, and must therefore be formed as part of the SWAGING process. SWAGING is therefore performed around a MANDREL. A MANDREL is a generic term in engineering which refers to “a metal tool around which other metal is formed (or shaped)” The special MANDREL required for the SOCKET WRENCH simply mirrors the internal cavity in the socket itself, which means that the mandrel is hexagonal at one end and square at the other. The metal used for swaging the socket must first be OPEN DIE FORGED to create a WROUGHT structure, and then formed into a tubular shape with internal and external dimensions slightly larger than the final product. The tube is heated to make it pliable and then placed at the center of the open SWAGING dies. The MANDREL is then fed into the center of the hot tube prior to the simultaneous forcible closing of the dies along radial paths. The material is thus pushed down onto the mandrel, while the external circular dimensions are reduced. ©Dr. Stuart Wilkinson, Mechanical Engineering Department, University of South Florida The SWAGING dies are next withdrawn, which allows the workpiece to be removed from between them. The MANDREL is then pulled out (hex end first), revealing a thin-walled circular component with the required internal configuration. No thin web of flash is generated, but the workpiece did get longer as the diameter was reduced. This length extension is effectively the flash material, which then needs to be trimmed off. The grain flow closely follows the internal and external boundaries, thereby imparting superior strength to the SOCKET. It is not uncommon to SWAGE long lengths of material, but this is not practical to do with a single stroke because the dies would have to be prohibitively long. To do this a process called ROTARY SWAGING (see below) must be used. In this case the dies incorporate CAMS at their far ends, which are actuated by a ring of cam followers (pressure rollers in the picture) located around the periphery. Rotation of the ring will result in the dies (anvils in picture) simultaneously being driven inwards and withdrawn outwards in a radial reciprocating motion. By progressively feeding the workpiece through the center of the dies it is possible to SWAGE long workpieces in steps from one end to the other. This very same method is used by the firearms industry to create what they call “Cold Hammer Forged Barrels” for long guns, although it is simply ROTARY SWAGING. In this case a twisted-grooved MANDREL is used in the center of the barrel blank to leave an impression for RIFLING inside the bore. The mandrel is ultimately withdrawn by combined pulling and twisting motions. The resulting barrel has a grain flow that follows the contours of the barrel, bore and rifling grooves, and is thus very strong. ©Dr. Stuart Wilkinson, Mechanical Engineering Department, University of South Florida Many common products are SWAGED, although it may not be obvious. The product shown (R) is a Philips Head Screwdriver. These require swaging for strength since simply cutting grooves into the end of a metal bar would undoubtedly result in rounding off the screwdriver upon encountering a reluctant crosshead screw. SWAGING in this case required no mandrel since the screwdriver shaft is solid and not hollow. It should be noted that FORGING/SWAGING produces discrete components, and does so with dies that travel linearly. Conversely, ROLLING is a process that produces long lengths of stock material and does so with dies (rollers) that rotate. There are some manufacturing processes that don’t fit into either of the above definitions, and are generally referred to as ROLL-FORGING. In the two examples of ROLL-FORGING shown above, discrete products are being created between rotating dies (rollers). To achieve this, the rollers include an evolving shape attached around their perimeter. Metal placed between the rollers will be subject to progressive deformation as the rollers turn. Each component shape is fully evolved and completed through a single turn of the rollers. Another example of Roll-Forging is important enough to get its own name, which is SKEW ROLLING. This is the way that steel balls are made for use in bearings. It basically consists of two conical rollers with integrated helical grooves of semicircular cross-section. The two rollers are placed on independent offset shafts which are set at an angle (skewed) with respect to each other. Simply stabbing a heated rod of stock between the rollers will result in metered pieces of material being bitten off and rolled along the helical grooves. This creates smooth flash-less balls with a perfect internal grain flow everywhere parallel to the boundaries. ©Dr. Stuart Wilkinson, Mechanical Engineering Department, University of South Florida ROLLING is a process that produces very long lengths of stock material using dies in the form of ROLLERS. It is more practical and cost effective to keep this process running non-stop. This requires that it be fed with metal by a process called CONTINOUS CASTING where molten metal is formed into SLABS which then enter into the roller stages. If these rollers are smooth and featureless the result is PLATE or SHEET stock. The rollers not only form the SLAB into a PLATE/SHEET configuration, but also crush the CAST STRUCTURE and turn it into a WROUGHT STRUCTURE. Additionally the smaller grains are constrained to flow between the rollers resulting in them being parallel to the plate boundaries. ROLLING of this type must be done at temperatures above recrystallization to avoid micro cracking, and so this process is called HOT ROLLING. The great advantage of rolling is that it produces stock with forged-like strength, but in such quantity as to be relatively inexpensive. Creating such material requires multiple steps, which are performed in facilities known as ROLLING MILLS. These buildings can be up to ¼ mile long since the processing must include progressive THICKNESS REDUCTION stages along with REHEAT stages, COILING stages and TENSIONING stages; all in a row. ©Dr. Stuart Wilkinson, Mechanical Engineering Department, University of South Florida Progressively reducing the thickness of the plate/sheet is performed using roller stages, where each stage has a slightly smaller gap between rollers than the stage before. These roller stages need not consist of just two rollers (such as the 2-High shown below), but can be arranged into other configurations. The choice of roller arrangement depends on metal ductility (forces necessary), surface finish required, throughput (velocity), etc. The final result of all this processing is HOT ROLLED PLATE or SHEET, with the difference between PLATE or SHEET being defined respectively as whether it must be stacked flat or can be coiled. In the case of steels, this material has excellent material properties, but is not very attractive to look at. It generally has a black/grey coloration as a direct result of BLACK IRON OXIDE, otherwise called MILL SCALE that forms on the surface at the elevated temperatures involved. In fact the product has to remain at such temperatures for long periods during the multi-stages of rolling and would develop an excessively thick MILL SCALE it is wasn’t for the active steps taken to remove the bulk of it during processing. However, MILL SCALE will always reform if hot enough, and some gets crushed into the hot pliable surface by the rollers, resulting in a similar cornflakes on putty surface finish similar to that seen on FORGED parts. MILL SCALE is somewhat protective of the underlying metal and will hinder corrosion. ©Dr. Stuart Wilkinson, Mechanical Engineering Department, University of South Florida For situations where superior surface finish and dimensional accuracy are desired, HOT ROLLED STEEL can be subjected to additional processing steps that result in the creation of COLD ROLLED STEEL. This involves ANNEALING (softening heat treatment) the hot rolled steel, followed by PICKLING where the mill scale is stripped off. The material is than fed through a set of TEMPER ROLLERS (at room temperature) which reduces the thickness by just a few thousandths of an inch. This is sufficient to smooth the surface finish, create precise thickness dimensions with tight tolerance, and work harden the surface layers. The resulting COLD ROLLED STEEL is a more refined product, but more expensive. You can also use HOT or COLD ROLLING to create BARSTOCK (1) rather than plate/sheet. This simply involves using grooved (3) rollers (2) rather than plain ones. ©Dr. Stuart Wilkinson, Mechanical Engineering Department, University of South Florida SHAPE or PROFILE ROLLING also uses grooved rollers (2 & 4) to create STRUCTURAL STOCK from cast “bloom” (1), but can include a variety of additional rollers (3 & 5) of various sizes, shapes and orientations to form the required cross-sections (for I, angle and channel stock, etc.) ROLLING is not immune to manufacturing defects, although these are less common and varied compared to those associated with CASTING. Generally these flaws are related to heating issues, too aggressive thickness reduction steps, or deficient rigidity/concentricity of the roller systems. ALLIGATORING is perhaps the most interesting of these rolling defects, since the rolled product develops a configuration that resembles the open jaws of an alligator. It is caused essentially by an adhesion problem as material is first fed into a set of rollers. The upper and lower surfaces can become attached to opposite rollers and be peeled apart as the halves partially wrap around each roller. Large shear stresses develop down the center of the workpiece which allows the cracking to proceed. ALLIGATORING is not always obvious since subsequent stages can slam the jaws shut, leaving an invisible central crack. It is standard practice in rolling mills to saw a foot or so off of the ©Dr. Stuart Wilkinson, Mechanical Engineering Department, University of South Florida ends of rolled product to remove the possibility of hidden alligator cracks. However, as a purchaser of rolled stock it is advisable to inspect the ends of the delivered material. BLACK OXIDE (MILL SCALE) present on the ends of stock would suggest an end piece that has not been cut, and it may behoove one to cut the ends off as a precautionary measure. Placing structural stock into service when it contains an alligator crack could result is failure and possible injury or death. Not all rolled material is created in long lengths, or in multiple stages arranged in a row. In the case of RING ROLLING the goal is to create a large circular workpiece using rollers. This is most likely performed in a rolling mill, but at just one location, with the roller positions adjusted to develop the final form. Another smaller scale rolling related process is SPLINE ROLLING. This can be done with linear style dies or with more traditional rollers, similar in many respects to ROLL-FORGING. SPLINES are widely used to concentrically attach shafts together for torque transmission, and require proper grain flow for strength. ©Dr. Stuart Wilkinson, Mechanical Engineering Department, University of South Florida BOLTS are a critical part of assembled structures, and must by necessity be strong. To this end the heads of bolts are created using HEADING or ORBITAL FORGING techniques to ensure proper grain flow. The threads must also be similarly strong, and so THREAD ROLLING is used to achieve this. Similar to spline rolling, THREAD ROLLING can be done with rollers which have helical grooving, or via linear dies with angled grooves. Whichever approach is adopted, the process produces no chips, and simply redistributes material. This guarantees a grain flow that follows the contours of the threads. The importance of this is shown below in a comparison between CUT and ROLLED THREADS. ©Dr. Stuart Wilkinson, Mechanical Engineering Department, University of South Florida CUTTING will always produce smooth, sharp precise threads, and will result in a major diameter EQUAL to the shank. Such threads are inherently weak because the grain flow no longer follows the thread boundaries. By contrast, ROLLED threads are very strong because the grain has been forced to follow the thread contours. Rolled threads are more rounded and result in a major diameter that is LARGER than the shank diameter. This latter phenomenon occurs because rolling doesn’t remove material, and so metal from the roots of the threads is piled up to create the crests of the threads. It is not always convenient to have the major thread diameter of ROLLED bolts larger than the shank. Bolts made this way tend to rattle around in plates where clearance holes for the threads have been drilled. Therefore it is common practice to reduce the diameter of that portion of a rolled bolt which is to later accept the threads. The goal is to reduce the shank diameter such that the major thread diameter will equal the unthreaded shank once thread rolling is complete. In this respect, most quality rolled bolts will be indistinguishable from a cut equivalent. CUT THREADS offer accuracy and precision, but at the expense of strength. It is, however, possible to have “the best of both worlds” with what are known as PRECISION BOLTS. In this case the bolt is made via HEADING or ORBITAL FORGING and with THREAD ROLLING, and is thus very strong. This rolled bolt is then machined (cut) over all surfaces to create precise dimensions. It is perfectly acceptable to machine a rolled or forged part provided the cuts follow the preexisting grain flow. You cannot, however, cut a standard rolled bolt in this way or it will become undersized. Precision bolts are deliberately created oversized at first so that the correct dimensions are achieved only after machining. ©Dr. Stuart Wilkinson, Mechanical Engineering Department, University of South Florida