Viscosity of Fluids
5.5 Effect of Manufacturing Processes
The ability to achieve a certain tolerance or surface is a function of the manufactur- ing process. This section describes the general capabilities of various processes in terms of tolerance and surface roughness and surface integrity.
Some manufacturing processes are inherently more accurate than others. Most machining processes are quite accurate, capable of tolerances of 0.05 mm (0.002 in) or better. By contrast, sand castings are generally inaccurate, and tolerances of 10 to 20 times those used for machined parts should be specifi ed. In Table 5.4, we list a variety
5.5
b Compiled from [4], [5], and other sources. For each process category, tolerances vary depending on process parameters. Also, tolerances increase with part size.
TABLE • 5.4 Typical tolerance limits, based on process capability (Section 40.2), for various manufacturing processes.b
Process Typical Tolerance, mm (in) Process
Typical Tolerance, mm (in) Sand casting
Cast iron 1.3 (0.050)
Steel 1.5 (0.060)
Aluminum 0.5 (0.020)
Die casting 0.12 (0.005) Plastic molding:
Polyethylene 0.3 (0.010) Polystyrene 0.15 (0.006) Machining:
Drilling, 6 mm (0.25 in) 0.08/0.03 (0.003/0.001)
Milling 0.08 (0.003)
Turning ±0.05 (±0.002)
Abrasive
Grinding 0.008 (0.0003)
Lapping 0.005 (0.0002)
Honing 0.005 (0.0002)
Nontraditional and thermal
Chemical machining 0.08 (0.003) Electric discharge 0.025 (0.001) Electrochem. grind 0.025 (0.001) Electrochem. machine 0.05 (0.002) Electron beam cutting 0.08 (0.003) Laser beam cutting 0.08 (0.003) Plasma arc cutting ±1.3 (±0.050)
References 115
of manufacturing processes and indicate the typical tolerances for each process. Toler- ances are based on the process capability for the particular manufacturing operation, as defi ned in Section 40.2. The tolerance that should be specifi ed is a function of part size;
larger parts require more generous tolerances. The table lists tolerances for moderately sized parts in each processing category.
The manufacturing process determines surface fi nish and surface integrity. Some processes are capable of producing better surfaces than others. In general, process- ing cost increases with improvement in surface fi nish. This is because additional operations and more time are usually required to obtain increasingly better surfaces.
Processes noted for providing superior fi nishes include honing, lapping, polishing, and superfi nishing (Chapter 24). Table 5.5 indicates the usual surface roughness that can be expected from various manufacturing processes.
[1] American National Standards Institute. Sur- face Texture, ANSI B46.1-1978. American Society of Mechanical Engineers, New York, 1978.
[2] American National Standards Institute. Sur- face Integrity, ANSI B211.1-1986. Society of
Manufacturing Engineers, Dearborn, Michi- gan, 1986.
[3] American National Standards Institute.
Dimensioning and Tolerancing, ANSI Y14.5M-2009. American Society of Mechani- cal Engineers, New York, 2009.
References
TABLE • 5.5 Surface roughness values produced by the various manufacturing processes.a
Process
Typical
Finish Roughness Range b Process
Typical Finish
Roughness Range b Casting:
Die casting Good 1–2 (30–65) Investment Good 1.5–3 (50–100) Sand casting Poor 12–25 (500–1000) Metal forming:
Cold rolling Good 1–3 (25–125) Sheet metal draw Good 1–3 (25–125) Cold extrusion Good 1–4 (30–150) Hot rolling Poor 12–25 (500–1000) Machining:
Boring Good 0.5–6 (15–250)
Drilling Medium 1.5–6 (60–250)
Milling Good 1–6 (30–250)
Reaming Good 1–3 (30–125)
Shaping and planing Medium 1.5–12 (60–500)
Sawing Poor 3–25 (100–1000)
Turning Good 0.5–6 (15–250)
a Compiled from [1], [2], and other sources.
b Roughness range values are given, mm (m-in). Roughness can vary signifi cantly for a given process, depending on process parameters.
Abrasive:
Grinding Very good 0.1–2 (5–75) Honing Very good 0.1–1 (4–30) Lapping Excellent 0.05–0.5 (2–15) Polishing Excellent 0.1–0.5 (5–15) Superfi nish Excellent 0.02–0.3 (1–10) Nontraditional:
Chemical milling Medium 1.5–5 (50–200) Electrochemical Good 0.2–2 (10–100) Electric discharge Medium 1.5–15 (50–500) Electron beam Medium 1.5–15 (50–500) Laser beam Medium 1.5–15 (50–500) Thermal:
Arc welding Poor 5–25 (250–1000) Flame cutting Poor 12–25 (500–1000) Plasma arc cutting Poor 12–25 (500–1000)
[4] Bakerjian, R., and Mitchell, P. Tool and Manu- facturing Engineers Handbook, 4th ed., Vol. VI, Design for Manufacturability. Society of Manu- facturing Engineers, Dearborn, Michigan, 1992.
[5] Brown & Sharpe. Handbook of Metrology.
North Kingston, Rhode Island, 1992.
[6] Curtis, M., Handbook of Dimensional Meas- urement, 4th ed. Industrial Press Inc., New York, 2007.
[7] Drozda, T. J., and Wick, C. Tool and Manufac- turing Engineers Handbook, 4th ed., Vol. I, Machining. Society of Manufacturing Engineers, Dearborn, Michigan, 1983.
[8] Farago, F. T. Handbook of Dimensional Meas- urement, 3rd ed. Industrial Press, New York, 1994.
[9] Machining Data Handbook, 3rd ed., Vol. II.
Machinability Data Center, Cincinnati, Ohio, 1980, Ch. 18.
[10] Mummery, L. Surface Texture Analysis—The Handbook. Hommelwerke Gmbh, Germany, 1990.
[11] Oberg, E., Jones, F. D., Horton, H. L., and Ryffel, H. Machinery’s Handbook, 26th ed. Industrial Press, New York, 2000.
[12] Schaffer, G. H. “The Many Faces of Surface Texture,” Special Report 801, American Ma- chinist and Automated Manufacturing, June 1988, pp. 61–68.
[13] Sheffi eld Measurement, a Cross & Trecker Company. Surface Texture and Roundness Measurement Handbook, Dayton, Ohio, 1991.
[14] Spitler, D., Lantrip, J., Nee, J., and Smith, D. A.
Fundamentals of Tool Design, 5th ed. Society of Manufacturing Engineers, Dearborn, Michi- gan, 2003.
[15] L. S. Starrett Company. Tools and Rules. Athol, Massachusetts, 1992.
[16] Wick, C., and Veilleux, R. F. Tool and Manu- facturing Engineers Handbook, 4th ed., Vol.
IV, Quality Control and Assembly, Section 1. Society of Manufacturing Engineers, Dear- born, Michigan, 1987.
[17] Zecchino, M. “Why Average Roughness Is Not Enough.” Advanced Materials & Processes, March 2003, pp. 25–28.
Review Questions
5.1 What is a tolerance?
5.2 What is the difference between a bilateral tolerance and a unilateral tolerance?
5.3 What is accuracy in measurement?
5.4 What is precision in measurement?
5.5 What is meant by the term graduated measur- ing device?
5.6 What are some of the reasons why surfaces are important?
5.7 Defi ne nominal surface.
5.8 Defi ne surface texture.
5.9 How is surface texture distinguished from sur- face integrity?
5.10 Within the scope of surface texture, how is roughness distinguished from waviness?
5.11 Surface roughness is a measurable aspect of sur- face texture; what does surface roughness mean?
5.12 Indicate some limitations of using surface roughness as a measure of surface texture.
5.13 Identify some changes and injuries that can occur at or immediately below the surface of a metal.
5.14 What causes the various types of changes that occur in the altered layer just beneath the surface?
5.15 What are the common methods for assessing surface roughness?
5.16 Name some manufacturing processes that produce very poor surface fi nishes.
5.17 Name some manufacturing processes that pro- duce very good or excellent surface fi nishes.
Problems 117
Problems
5.1(A) (SI units) A GO/NO-GO plug gage will be designed to inspect a 50.00 0.20 mm diameter hole. A wear allowance of 3% of the total toler- ance band is applied to the GO side of the gage.
Determine (a) the nominal sizes of (a) the GO gage and (b) the NO-GO gage.
5.2 (SI units) A GO/NO-GO ring gage is needed to inspect the diameter of a shaft that is 50.00 0.20 mm. A wear allowance of 3% of the en- tire tolerance band is applied to the GO side.
Determine (a) the nominal sizes of (a) the GO gage and (b) the NO-GO gage.
5.3 (USCS units) A GO/NO-GO plug gage is needed to inspect a 1.000 0.020 in diameter hole. A wear allowance is applied to the GO side of the gage. The allowance 2% of the total tolerance band. Determine the nominal sizes of (a) the GO gage and (b) the NO-GO gage.
5.4 (USCS units) A GO/NO-GO snap gage is re- quired to inspect the diameter of a shaft that is 1.000 0.020. A wear allowance of 2% of the entire tolerance band is applied to the GO side.
Determine (a) the nominal sizes of (a) the GO gage and (b) the NO-GO gage.
5.5(A) (USCS units) A sine bar is used to deter- mine the angle of a part feature. The length of the sine bar 8.000 in. The rolls have a diam- eter of 1.000 in. All inspection is performed on a surface plate. In order for the sine bar to match the angle of the part, the following gage blocks must be stacked: 2.0000, 0.5000, 0.2500, and 0.0050. Determine the angle of the part feature.
5.6 (SI units) A 150.00-mm sine bar is used to in- spect a part angle that has a dimension of 35.0 1.0°. The sine bar rolls have a diameter 25.0 mm. A set of gage blocks is available that can form any height from 10.0000 mm to 199.995 mm in increments of 0.005 mm. All inspection is performed on a surface plate.
Determine (a) the height of the gage block stack to inspect the minimum angle, (b) the height of the gage block stack to inspect the maximum angle, and (c) the smallest increment of angle that can be set up at the nominal angle size.
Answers to Problems labeled (A) are listed in the Appendix at the back of the book.