ÁREAS NATURALES PROTEGIDAS UGA 11 Sinaloa Norte

The parameters influencing the quality of the sheared billet can be divided into workpiece- tooling and shear-related parameters. Machine- and tooling-related parameters, such as a weak frame design or inconsistent blade alignment, re- duce the shearing quality and should be avoided [Breitling et al., 1997].

There are two ways to shear billets: A shear- ing tool may be mounted in a mechanical press, or, alternatively, a regular hydraulic billet shear, solely designed for shearing, may be used. The second option is more desirable, because it pro- vides more accuracy and productivity. The latest shear designs incorporate the following features:

● Rugged frame construction and precise guid- ance of all moving parts in order to eliminate deflection under load

● Adjustable hydraulic billet support and bar holder in order to minimize billet bending

Fig. 13.6 Stock volume monitoring system [Breitling et al., 1997]

● Fast blade clearance adjustment to reduce the lead time for new setups

● Hydraulic knife clamps for fast blade changes

● A tiltable shear base that can be used for in- clining the bar when blanking soft materials

● A high shearing speed and shear rate in order to improve billet quality and process pro- ductivity

● Automated billet quality control (for exam- ple, continuous stock volume monitoring) The last point becomes increasingly important, because it is not sufficient to control the billet weight and geometrical accuracy only manually and intermittently. Modern shearing machines use a stock volume monitoring system, which ensures the maintenance of a constant billet weight despite changes in bar diameter.

The system measures the bar diameter, by means of laser sensors, which is then sent to a programmable logic controller (PLC) that com- putes the adjustments and moves the back gage accordingly (Fig. 13.6). The billet is then sheared and ready for further processing.

Most conventional shears are mechanical and their operation is based on the eccentric slide principle, as in mechanical forging presses. The holddown mechanism is necessary for obtaining good sheared surfaces. It operates mechanically, through an additional linkage from the eccentric, or hydraulically. The use of an outboard support also improves the quality of the billets. In that device, the billet is supported during the entire operation.

A radically different design, a high-velocity rotary-type shear, is seen schematically in Fig. 13.7. In this machine, the energy is provided by a flywheel that carries an open-type moving blade. According to tooling arrangement, one or two billets per revolution can be obtained. With this shear, production rates of 300 billets/min are feasible. In the design seen in Fig. 13.8, the ma- terial to be sheared is confined by a close-fitting closed blade and held against a stop by an axial

Fig. 13.8 Schematic of a shear with axial load to improve shear quality [Altan et al., 1973]

Fig. 13.7 High-velocity rotary-type shear. (a) Tooling for one billet per revolution. (b) Tooling for two billets per revolution [Altan et al., 1973]

load. The axial load ensures squareness and in- hibits crack propagation.

In general, any metal that can be machined can be sheared, but power requirements increase as the strength of the work metal increases. Fur- ther, blade design is more critical and blade life decreases as the strength of the work metal in- creases. Equipment is available for shearing round, hexagonal, or octagonal bars up to 6 in. (152 mm) in diameter or thickness, rectangular bars and billets up to 3⳯ 12 in. (75 ⳯ 305 mm) in cross section, and angles up to 8⳯ 8 ⳯ 11_⁄

2

in. (203⳯ 203 ⳯ 38 mm).

Cutoff-type shearing machines are used for cutting round, square, flat, or special-shaped bars into blanks or slugs. This process can be performed on a machine specifically designed for slug cutoff, or it can be performed using a box-type shearing die in conjunction with a press [Wick et al., 1984].

One manufacturer of cutoff machines utilizes a double-cutting principle to shear the blanks or slugs. The dies (Fig. 13.9) are actuated with short strokes by two flywheel-cam assemblies

that rotate at a constant speed. The capacity of the machine is a 21

⁄2 in. (63.5 mm) diam bar

having a maximum length of 36 in. (914 mm). This method is fast, efficient, and economical when billets in large quantities are required. Some machines are capable of maintaining the length to withinⳲ0.005 in. (Ⳳ0.13 mm) as well as maintaining square cuts and ends that are free of burrs, distortion, and rollover. Production can be as high as 150 pieces per minute.

REFERENCES

[Altan et al., 1973]: Altan, T., Boulger, F., Becker, J., Akgerman, N., Henning, H., Forg- ing Equipment, Materials, and Practices, Metal and Ceramics Information Center, Bat- telle Columbus Laboratories, HB03, p 4–7. [Breitling et al., 1997]: Breitling, J., Chernaus-

kas, V., Taupin, E., Altan, T., “Precision Shearing of Billets—Special Equipment and Process Simulation,” Journal of Materials Fig. 13.9 Double-cutting principle [Wick et al., 1984]

Processing Technology, Vol. 71, 1997, p 119– 125.

[Camille et al., 1998]: Santiago-Vega, C., Vas- quez, V., Altan, T., “Simulation of Bar Shear- ing Process,” ERC/NSM-97-27, Engineering Research Center for Net Shape Manufactur- ing.

[Duvari et al., 2003]: Duvari, S., Isbir, S., Ngaile, G., Altan, T., “Optimization of Tool Design in Hot Shearing of Billets for Forg- ing,” ERC/NSM-03-R-09, Engineering Re- search Center for Net Shape Manufacturing. [Schuler, 1998]: Schuler, H., Hoffman, H.,

Frontzek, H., Metal Forging Handbook, Schu- ler Group, Springer, Goppingen, Germany, p 457–459.

[Wick et al., 1984]: Wick, C., Benedict, J.T., Veilleux, R., Tool and Manufacturing Engi- neers Handbook, Society of Manufacturing Engineers, Dearborn, MI, p 11-1–11-21.

SELECTED REFERENCES

● [ASM International, 1999]: Davis, J.R., Ed., Forming and Forging, Vol 14, ASM Handbook, p 714–719.

● [Geleji et al., 1967]: Geleji, A., Forge

Equipment, Rolling Mills, and Accessories (in English), Akademiai Kiado, Budapest, p 168.

● [Stotmann, 1968]: Stotmann, W., “Evolu- tion of Machines and Automation in the Drop Forging Industry,” Metal Forming, May, p 136.

WEB SITES

● www.bemcor.com, Bemcor, Inc.

● www.ficep.it, Ficep Corp.