Question & Answer

In a certain chemical blanking operation, a sulfuric acid etchant is used to remove material from a

sheet of magnesium alloy. The sheet is 0.25 mm thick. The screen resist method of masking was

used to permit high production rates to be achieved. As it turns out, the process is producing a large

proportion of scrap. Specified tolerances of ±0.025 mm are not being achieved. The foreman in the

CHM department complains that there must be something wrong with the sulfuric acid. \"Perhaps

the concentration is incorrect,\" he suggests. Analyze the problem and recommend a solution.

In a chemical milling operation on a flat mild steel plate, it is desired to cut an ellipse-shaped pocket

to a depth of 0.4 in. The semiaxes of the ellipse are a = 9.0 in and b = 6.0 in. A solution of

hydrochloric and nitric acids will be used as the etchant. Determine (a) metal removal rate in in3/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.

Chemical milling is used in an aircraft plant to create pockets in wing sections made of an

aluminum alloy. The starting thickness of one workpart of interest is 20 mm. A series of

rectangular-shaped pockets 12 mm deep are to be etched with dimensions 200 mm by 400 mm. The

corners of each rectangle are radiused to 15 mm. The part is an aluminum alloy and the etchant is

NaOH. The penetration rate for this combination is 0.024 mm/min and the etch factor is 1.75.

Determine (a) metal removal rate in mm3/min, (b) time required to etch to the specified depth, and

(c) required dimensions of the opening in the cut and peel maskant to achieve the desired pocket

size on the part.

A wire EDM operation is used to cut out punch-and-die components from 25-mm-thick tool steel

plates. However, in preliminary cuts, the surface finish on the cut edge is poor. What changes in

discharge current and frequency of discharges should be made to improve the finish?

A wire EDM operation is to be performed on a slab of ¾-in-thick aluminum using a brass wire

electrode whose diameter = 0.005 in. It is anticipated that the overcut will be 0.001 in, so that the

kerf width will be 0.007 in. 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 1220°F.

In a wire EDM operation performed on 7-mm-thick C1080 steel using a tungsten wire electrode

whose diameter = 0.125 mm, past experience suggests that the overcut will be 0.02 mm, so that the

kerf width will be 0.165 mm. Using a discharge current = 10 amps, what is the allowable feed rate

that can be used in the operation? Estimate the melting temperature of 0.80% carbon steel from the

phase diagram of Figure 6.4.

A metal removal rate of 0.01 in3/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 2802°F and 2651°F,

respectively.

Suppose the hole in Problem 26.10 were to be cut using EDM rather than ECM. Using a discharge

current = 20 amps (which would be typical for EDM), how long would it take to cut the hole? From

Table 4.1, the melting temperature of iron is 2802°F.

An electric discharge machining operation is being performed on two work materials: tungsten and

zinc. Determine the amount of metal removed in the operation after one hour at a discharge

amperage = 20 amps for each of these metals. Use U.S. Customary units and express the answer in

in3/hr. From Table 4.1, the melting temperatures of tungsten and zinc are 6170°F and 420°F,

respectively.

An electric discharge machining operation is being performed on two work materials: tungsten and

tin. Determine the amount of metal removed in the operation after one hour at a discharge current of

20 amps for each of these metals. Use metric units and express the answers in mm3/hr. From Table

4.1, the melting temperatures of tungsten and tin are 3410°C and 232°C, respectively.

A 3.5 in diameter through hole is to be cut in a block of pure iron (Valence = 2) by electrochemical

machining. The block is 2.0 in thick. To speed the cutting process, the electrode tool will have a

center hole of 3.0 in which will produce a center core that can be removed after the tool breaks

through. The outside diameter of the electrode is undersized to allow for overcut. The overcut is

expected to be 0.005 in on a side. If the efficiency of the ECM operation is 90%, what current will

be required to complete the cutting operation in 20 minutes?

A square hole is to be cut using ECM through a plate of pure copper (valence = 1) that is 20 mm

thick. The hole is 25 mm on each side, but the electrode used to cut the hole is slightly less that 25

mm on its sides to allow for overcut, and its shape includes a hole in its center to permit the flow of

electrolyte and to reduce the area of the cut. This tool design results in a frontal area of 200 mm2.

The applied current = 1000 amps. Using an efficiency of 95%, determine how long it will take to

cut the hole.

In an electrochemical machining operation, the frontal working area of the electrode is 2.5 in². 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 26.1. (a) If the ECM process is 90 percent efficient,

determine the rate of metal removal in in³/hr. (b) If the resistivity of the electrolyte = 6.2 ohm-in,

determine the working gap

The frontal working area of the electrode in an ECM operation is 2000 mm2. The applied current =

1800 amps and the voltage = 12 volts. The material being cut is nickel (valence = 2), whose specific

removal rate is given in Table 26.1. (a) If the process is 90% efficient, determine the rate of metal

removal in mm3/min. (b) If the resistivity of the electrolyte = 140 ohm-mm, determine the working

gap.

A furniture company that makes upholstered chairs and sofas must cut large quantities of fabrics.

Many of these fabrics are strong and wear-resistant, which properties make them difficult to cut. What nontraditional process(es) would you recommend to the company for this application? Justify

your answer by indicating the characteristics of the process that make it attractive.

Much of the work at the Cut-Anything Company involves cutting and forming of flat sheets of

fiber-glass for the pleasure boat industry. Manual methods based on portable saws are currently

used to perform the cutting operation, but production is slow and scrap rates are high. The foreman

says the company should invest in a plasma arc cutting machine, but the plant manager thinks it

would be too expensive. What do you think? Justify your answer by indicating the characteristics of

the process that make PAC attractive or unattractive in this application.

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 (2.0 in) cube of steel. The overall size of the \"G\"

is 25 by 19 mm (1.0 by 0.75 in), the depth of the hole is 3.8 mm (0.15 in), and its width is 3 mm

(1/8 in).

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

through-hole in the shape of the letter L in a 12.5 mm (0.5 in) thick plate of glass. The size of the

\"L\" is 25 by 15 mm (1.0 by 0.6 in) and the width of the hole is 3 mm (1/8 in).

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 an

engraved aluminum printing plate to be used in an offset printing press to make 275 by 350 mm (11

by 14 in) posters of Lincoln's Gettysburg address.

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 matrix

of 0.1 mm (0.004 in) diameter holes in a plate of 3.2 mm (0.125 in) thick hardened tool steel. The

matrix is rectangular, 75 by 125 mm (3.0 by 5.0 in) with the separation between holes in each

direction = 1.6 mm ( 0.0625 in).

There are 17 correct answers in the following multiple choice questions (some questions have multiple

answers that are correct). To attain a perfect score on the quiz, all correct answers must be given. Each

correct answer is worth 1 point. Each omitted answer or wrong answer reduces the score by 1 point, and

each additional answer beyond the correct number of answers reduces the score by 1 point. Percentage scor the quiz is based on the total number of correct answers.

26.1 Which of the following processes use mechanical energy as the principal energy source (three

correct answers): (a) electrochemical grinding, (b) laser beam machining, (c) conventional milling,

(d) ultrasonic machining, (e) water jet cutting, and (f) wire EDM?

26.2 Ultrasonic machining can be used to machine both metallic and nonmetallic materials: (a) true or

(b) false?

26.3 Applications of electron beam machining are limited to metallic work materials due to the need for

the work to be electrically conductive: (a) true or (b) false?

26.4 Which one of the following is closest to the temperatures used in plasma arc cutting: (a) 2750°C

(5000°F), (b) 5500°C (10,000°F), (c) 8300°C (15,000°F), (d) 11,000°C (20,000°F), or (e) 16,500°C

(30,000°F)

26.5 Chemical milling is used in which of the following applications (two best answers): (a) drilling

holes with high depth-to-diameter ratio, (b) making intricate patterns in thin sheet metal, (c)

removing material to make shallow pockets in metal, (d) removing metal from aircraft wing panels,

and (e) cutting of plastic sheets?

26.6 Etch factor is equal to which of the following in chemical machining (more than one): (a)

anisotropy, (b) CIt, (c) d/u, and (d) u/d; where C = specific removal rate, d = depth of cut, I =

current, t = time, and u = undercut?

26.7 Of the following processes, which one is noted for the highest material removal rates: (a) electric

discharge machining, (b) electrochemical machining, (c) laser beam machining, (d) oxyfuel cutting,

(e) plasma arc cutting, (f) ultrasonic machining, or (g) water jet cutting?

26.8 Which one of the following processes would be appropriate to drill a hole with a square cross

section, 0.25 inch on a side and 1-inch deep in a steel workpiece: (a) abrasive jet machining, (b)

chemical milling, (c) EDM, (d) laser beam machining, (e) oxyfuel cutting, (f) water jet cutting, or

(g) wire EDM?

26.9 Which of the following processes would be appropriate for cutting a narrow slot, less than 0.015

inch wide, in a 3/8-in-thick sheet of fiber-reinforced plastic (two best answers): (a) abrasive jet

machining, (b) chemical milling, (c) EDM, (d) laser beam machining, (e) oxyfuel cutting, (f) water

jet cutting, and (g) wire EDM?

26.10 Which one of the following processes would be appropriate for cutting a hole of 0.003 inch

diameter through a plate of aluminum that is 1/16 in thick: (a) abrasive jet machining, (b) chemical

milling, (c) EDM, (d) laser beam machining, (e) oxyfuel cutting, (f) water jet cutting, and (g) wire

EDM?

26.11 Which of the following processes could be used to cut a large piece of 1/2-inch plate steel into two

sections (two best answers): (a) abrasive jet machining, (b) chemical milling, (c) EDM, (d) laser

beam machining, (e) oxyfuel cutting, (f) water jet cutting, and (g) wire EDM?

Name the four subsystems in a wire EDM process.

Name the four subsystems in a RAM EDM process.

What are two other names for ram type EDMs?

What are the three layers of a part’s surface after undergoing EDM.