Question & Answer

A machinability rating is to be determined for a new work material. For the base material (B1112), test data resulted in a Taylor equation with parameters n = 0.29 and C = 490. For the new material, the Taylor parameters were n = 0.23 and C = 430. Units in both cases are: speed in m/min and tool life in min. These results were obtained using cemented carbide tooling. (a) Compute a machinability rating for the new material using cutting speed for a 30-min tool life as the basis of comparison. (b) If the machinability criterion were tool life for a cutting speed of 150 m/min, what is the machinability rating for the new material?

A small company uses a band saw to cut through 2-inch metal bar stock. A material supplier is pushing a new material that is supposed to be more machinable while providing similar mechanical properties. The company does not have access to sophisticated measuring devices, but they do have a stopwatch. They have acquired a sample of the new material and cut both the present material and the new material with the same band saw settings. In the process, they measured how long it took to cut through each material. To cut through the present material, it took an average of 2 minutes, 20 seconds. To cut through the new material, it took an average of 2 minutes, 6 seconds. (a) Develop a machinability rating system based on time to cut through the 2.0-inch bar stock, using the present material as the base material. (b) Using your rating system, determine the machinability rating for the new material.

A machinability rating is to be determined for a new work material using the cutting speed for a 60- min tool life as the basis of comparison. For the base material (B1112 steel), test data resulted in Taylor equation parameter values of n = 0.29 and C = 500, where speed is in m/min and tool life is min. For the new material, the parameter values were n = 0.21 and C = 400. These results were obtained using cemented carbide tooling. (a) Compute a machinability rating for the new material. (b) Suppose the machinability criterion were the cutting speed for a 10-min tool life rather than the present criterion. Compute the machinability rating for this case. (c) What do the results of the two calculations show about the difficulties in machinability measurement?

There are 14 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 score on the quiz is based on the total number of correct answers

24.1 Which of the following criteria are generally recognized to indicate good machinability (four best answers): (a) ease of chip disposal, (b) high cutting temperatures, (c) high power requirements, (d) high value of Ra, (e) long tool life, (f) low cutting forces, and (g) zero shear plane angle?

24.2 Of the various methods for testing machinability, which one of the following is the most important: (a) cutting forces, (b) cutting temperature, (c) horsepower consumed in the operation, (d) surface roughness, (e) tool life, or (f) tool wear?

24.3 A machinability rating greater than 1.0 indicates that the work material is (a) easier to machine than the base metal or (b) more difficult to machine than the base metal, where the base metal has a rating = 1.0?

24.4 In general, which one of the following materials has the highest machinability: (a) aluminum, (b) cast iron, (c) copper, (d) low carbon steel, (e) stainless steel, (f) titanium alloys, or (g) unhardened tool steel?

24.5 Which one of the following operations is generally capable of the closest tolerances: (a) broaching, (b) drilling, (c) end milling, (d) planing, or (e) sawing?

24.6 When cutting a ductile work material, an increase in cutting speed will generally (a) degrade surface finish, which means a higher value of Ra or (b) improve surface finish, which means a lower value of Ra?

24.7 Which one of the following operations is generally capable of the best surface finishes (lowest value of Ra): (a) broaching, (b) drilling, (c) end milling, (d) planing, or (e) turning?

24.8 Which of the following time components in the average production machining cycle is affected by cutting speed (two correct answers): (a) part loading and unloading time, and (b) setup time for th

24.9 Which cutting speed is always lower for a given machining operation: (a) cutting speed for maximum production rate, or (b) cutting speed for minimum cost?

24.10 A high tooling cost and/or tool change time will tend to (a) decrease, (b) have no effect on, or (c) increase the cutting speed for minimum cost?

Which cutting speed is always lower for a given machining operation, cutting speed for minimum cost or cutting speed for maximum production rate? Why?

The unit cost in a machining operation is the sum of four cost terms. The first three terms are: (1) part load/unload cost, (2) cost of time the tool is actually cutting the work, and (3) cost of the time to change the tool. What is the fourth term?

What are the factors on which the selection of feed in a machining operation should be based?

Name some of the steps that can be taken to reduce or eliminate vibrations in machining

What are the parameters that have the greatest influence in determining the ideal surface roughness Ri in a turning operation?

What are the basic factors that affect surface finish in machining?

Why do costs tend to increase when better surface finish is required on a machined part?

Name some of the important mechanical and physical properties that affect the machinability of a work material

What are the criteria by which machinability is commonly assessed in a production machining operation?

Define machinability.

High-speed machining (HSM) is being considered to produce the aluminum part in Problem 22.15. All cutting conditions remain the same except for the cutting speed and the type of insert used in the cutter. Assume the cutting speed will be at the limit given in Table 22.1. Determine (a) the new time to machine the part and (b) the new metal removal rate. (c) Is this part a good candidate for high-speed machining? Explain.

An open side planer is to be used to plane the top surface of a rectangular workpart, 20.0 in by 45.0 in. The cutting speed is 30 ft/min, the feed is 0.015 in/pass, and the depth of cut is 0.250 in. The length of the stroke across the work must be set up so that 10 in are allowed at both the beginning and end of the stroke for approach and overtravel. The return stroke, including an allowance for acceleration and deceleration, takes 60% of the time for the forward stroke. The workpart is made of carbon steel with a tensile strength of 50,000 lb/in2 and a Brinell hardness of 110 HB. How long will it take to complete the job, assuming that the part is oriented in such a way as to minimize the time?

A shaper is used to reduce the thickness of a 50 mm part to 45 mm. The part is made of cast iron and has a tensile strength of 270 MPa and a Brinell hardness of 165 HB. The starting dimensions of the part are 750 mm x 450 mm x 50 mm. The cutting speed is 0.125 m/sec and the feed is 0.40mm/pass. The shaper ram is hydraulically driven and has a return stroke time that is 50% of the cutting stroke time. An extra 150 mm must be added before and after the part for acceleration and deceleration to take place. Assuming the ram moves parallel to the long dimension of the part, how long will it take to machine?

A part is produced using six conventional machine tools consisting of three milling machines and three drill presses. The machine cycle times on these machines are 4.7 min, 2.3 min, 0.8 min, 0.9 min, 3.4 min, and 0.5 min. The average load/unload time for each of these operations is 1.25 min. The corresponding setup times for the six machines are 1.55 hr, 2.82 hr, 57 min, 45 min, 3.15 hr, and 36 min, respectively. The total material handling time to carry one part between the machines is 20 min (consisting of five moves between six machines). A CNC machining center has been installed, and all six operations will be performed on it to produce the part. The setup time for the machining center for this job is 1.0 hr. In addition, the machine must be programmed for this part (called “part programming”), which takes 3.0 hr. The machine cycle time is the sum of the machine cycle times for the six machines. Load/unload time is 1.25 min. (a) What is the total time to produce one of these parts using the six conventional machines if the total consists of all setups, machine cycle times, load/unload times, and part transfer times between machines? (b) What is the total time to produce one of these parts using the CNC machining center if the total consists of the setup time, programming time, machine cycle time, and load/unload time, and what are the percent savings in total time compared to your answer in (a)? (c) If the same part is produced in a batch of 20 pieces, what is the total time to produce them under the same conditions as in (a) except that the total material handling time to carry the 20 parts in one unit load between the machines is 40 min? (d) If the part is produced in a batch of 20 pieces on the CNC machining center, what is the total time to produce them under the same conditions as in part (b), and what are the percent savings in total time compared to your answer in (c)? (e) In future orders of 20 pieces of the same part, the programming time will not be included in the total time because the part program has already been prepared and saved. In this case, how long does it take to produce the 20 parts using the machining center, and what are the percent savings in total time compared to your answer in (c)?

A three-axis CNC machining center is tended by a worker who loads and unloads parts between machining cycles. The machining cycle takes 5.75 min, and the worker takes 2.80 min using a hoist to unload the part just completed and load and fixture the next part onto the machine worktable. A proposal has been made to install a two-position pallet shuttle at the machine so that the worker and the machine tool can perform their respective tasks simultaneously rather than sequentially. The pallet shuttle would transfer the parts between the machine worktable and the load/unload station in 15 sec. Determine (a) the current cycle time for the operation and (b) the cycle time if the proposal is implemented. What is the percentage increase in hourly production rate that would result from using the pallet shuttle?

The top surface of a rectangular workpart is machined using a peripheral milling operation. The workpart is 735 mm long by 50 mm wide by 95 mm thick. The milling cutter, which is 60 mm in diameter and has five teeth, overhangs the width of the part equally on both sides. Cutting speed = 80 m/min, chip load = 0.30 mm/tooth, and depth of cut = 7.5 mm. (a) Determine the time required to make one pass across the surface, given that the setup and machine settings provide an approach distance of 5 mm before actual cutting begins and an overtravel distance of 25 mm after actual cutting has finished. (b) What is the maximum material removal rate during the cut?

A face milling operation removes 0.32 in depth of cut from the end of a cylinder that has a diameter of 3.90 in. The cutter has a 4-in diameter with 4 teeth, and its feed trajectory is centered over the circular face of the work. The cutting speed is 375 ft/min and the chip load is 0.006 in/tooth. Determine (a) the time to machine, (b) the average metal removal rate (considering the entire machining time), and (c) the maximum metal removal rate.

Solve Problem 22.17 except that the workpiece is 5.0 in wide and the cutter is offset to one side so that the swath cut by the cutter = 1.0 in wide. This is called partial face milling, Figure 22.20(b).

A face milling operation is performed on the top surface of a steel rectangular workpiece 12.0 in long by 2.5 in wide. The milling cutter follows a path that is centered over the workpiece. It has five teeth and a 3.0 in diameter. Cutting speed = 250 ft/min, feed = 0.006 in/tooth, and depth of cut = 0.150 in. Determine (a) the actual cutting time to make one pass across the surface and (b) the maximum metal removal rate during the cut. (c) If an additional approach distance of 0.5 in is provided at the beginning of the pass (before cutting begins), and an overtravel distance is provided at the end of the pass equal to the cutter radius plus 0.5 in, what is the duration of the feed motion

A slab milling operation is performed on the top surface of a steel rectangular workpiece 12.0 in long by 2.5 in wide. The helical milling cutter, which has a 3.0 in diameter and ten teeth, is set up to overhang the width of the part on both sides. Cutting speed is 125 ft/min, feed is 0.006 in/tooth, and depth of cut = 0.300 in. Determine (a) the actual machining time to make one pass across the surface and (b) the maximum metal removal rate during the cut. (c) If an additional approach distance of 0.5 in is provided at the beginning of the pass (before cutting begins), and an overtravel distance is provided at the end of the pass equal to the cutter radius plus 0.5 in, what is the duration of the feed motion

A face milling operation is used to machine 6.0 mm from the top surface of a rectangular piece of aluminum 300 mm long by 125 mm wide in a single pass. The cutter follows a path that is centered over the workpiece. It has four teeth and is 150 mm in diameter. Cutting speed = 2.8 m/s, and chip load = 0.27 mm/tooth. Determine (a) the actual machining time to make the pass across the surface and (b) the maximum metal removal rate during cutting.