MCE 311-HW06-sol-fall2011

MCE 311-HW06-sol-fall2011 24.4 For the following application, identify one or more nontraditional machining processes that might be used, and present arguments to support your selection. Assume that either the part geometry or the work material (or both) preclude the use of conventional machining. The application is a blind-hole in the shape of the letter G in a 50 mm cube of steel. The overall size of the "G" is 25 by 19 mm, the depth of the hole is 3.8 mm and its width is 3 mm. Solution: Application: the letter “G” drilled to a depth of 3.8 mm in block of steel. Possible processes: ECM and EDM would be useful for pocketing operations such as this. 2 24.8 In an electrochemical machining operation, the frontal working area of the electrode is 15.6 cm . The applied current = 1500 amps, and the voltage = 12 volts. The material being cut is pure aluminum, whose specific removal rate is given in Table 24.1. (a) If the ECM process is 90 percent efficient, 3 determine the rate of metal removal in mm /hr. (b) If the resistivity of the electrolyte = 15.5 ohm-cm −2 determine the working gap. Solution: (a) From Table 24.1, C = 3.44 × 10 3 mm /A-min R = f A = (CI/A)(A) = CI MR r −2 R = CI = 3.44 × 10 MR 3 (1500) = 51.6 mm /min at 100% efficiency. 3 3 At 90% efficiency R MR = 51.6(0.90) = 46.44 mm /min = 2786.4 mm /hr. (b) I = EA/gr; Rearranging, g = EA/Ir = 12(15.6)/(1500 × 15.5) = 0.008 cm 3 24.14 A metal removal rate of 0.156 cm /min is achieved in a certain EDM operation on a pure iron workpart. What metal removal rate would be achieved on nickel in this EDM operation, if the same discharge current were used? The melting temperatures of iron and nickel are 1539°C and 1455°C, respectively. Solution: For iron, R MR 3 1.23 = 664 I/1539 3 = 664 I/8323 = 0.0798 I mm /s Given that R MR 3 = 0.156 cm /min 0.0798 × (0.1) I = 0.156 3 I = 0.156/(0.0798 × (0.1) × 60) = 32.58 A. 1.23 3 For = nickel, R MR = 664(32.58)/1455 = 21633.12/7768.22 = 2.18 mm /s 24.16 A wire EDM operation is to be performed on a slab of 1.9 cm-thick aluminum using a brass wire electrode whose diameter = 0.0125 cm. It is anticipated that the overcut will be 0.0025 cm, so that the kerf width will be 0.0175 cm. Using a discharge current = 7 amps, what is the expected allowable feed rate that can be used in the operation? The melting temperature of aluminum is 660°C. Solution: Using Eq. (24.7), R MR 1.23 = 664(7)/660 2 3 = 1.58 mm /sec Frontal area of kerf A = 1.9(0.0175) = 0.033 cm 3 f = 1.58 × (0.1) r × 60/0.033 = 2.872 cm/min 24.19 In a chemical milling operation on a flat mild steel plate, it is desired to cut an ellipse-shaped pocket to a depth of 1 cm. The semiaxes of the ellipse are a = 22.5 cm and b = 15 cm. A solution of hydrochloric and 3 nitric acids will be used as the etchant. Determine (a) metal removal rate in in /hr, (b) time required to etch to depth, and (c) required dimensions of the opening in the cut and peel maskant required to achieve the desired pocket size on the part. Solution: (a) Neglecting the fact that the initial area would be less than the given dimensions of 22.5 cm by 15 cm, and that the material removal rate