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Chapter 10 Tolerancing Printing Instructions: Print handouts Select File, Print Edit the following selections to read: Select the OK button Tolerancing Tolerances are used to control the variation in size that exists on all manufactured parts. The amount that a size is allowed to vary depends on the function of the part & its assembly. – electric drill vs. jet engine The more accuracy required in a part (smaller tolerance) the greater the cost. Tolerances allow for interchangeable parts, which permits the replacement of individual parts in an assy. instead of replacing the whole system if a part goes bad or fails. Tolerance Tolerance is the total amount a dimension may vary. It is the difference between the maximum size and the minimum size. Tolerances can be expressed on a drawing in several ways: – direct limits, or as tolerance values applied directly to a dimension. (Fig. 10.8) – geometric tolerances (Fig. 10.25) – notes referring to specific conditions – A general tolerance note in the titleblock. (Fig. 10.7) Dimensions Limit dimensions: Plus/minus dimensions: – unilateral – bilateral Tolerancing Terms Nominal size: the “name” or general size. Often expressed as a fraction. Basic size: the theoretical size used as a starting point for the application of tolerances. Nominal size in decimal format. Actual size: the measured size of the finished part. Limits: the maximum & minimum sizes shown by the toleranced dimension. Allowance: the min. clearance or max. interference between 2 parts. MMC: the condition of a part in which it contains the most amount of material. EX: biggest shaft or smallest hole. LMC: the condition of a part in which it contains the least amount of material. Tolerance, MMC, LMC? Limit dimensions: Plus/minus dimensions: – unilateral – bilateral Complete Worksheet TOL-1 Remember x.xxxx – I.e. 0.1234 – I.e. 2.1200 Fit Fit: the degree of tightness between mating parts. The three most common types of fit found in industry are: – Clearance fit - there is always a space between the 2 mating parts. (shaft is smaller than the hole) – Interference fit - the 2 mating parts always interfere with one another in assembly. (shaft is bigger than the hole) – Transition fit - sometimes a clearance fit & sometimes an interference fit between the mating parts. Clearance Fit Interference Fit Do on own page 335, Figure 10.1a •Hole Tolerance? •Shaft Tolerance? •Minimum clearance? •Maximum clearance? •Allowance? Complete Number 1 of Worksheet 66 ANSI Standard Fits A group of English usit tolerance relationships called preferred precision fits have been developed. They are specified in ANSI B4.1. The five classes are: – Running & sliding fits (RC) – Clearance locational fits (LC) – Transition locational fits (LT) – Interference locational fits (LN) – Force or shrink fits (FN) These tolerances are specified in Appendix 5 through 9, pages a25-a32. The values in these tables are given in THOUSANDTHS of an inch. Example: 1.2 = 0.0012 Class RC9: Running & Clearance Fit Basic DIA = 2.0000 Hole Shaft Shaft Tolerance = Hole Tolerance = Max. Clearance = Min. Clearance = Class FN2: Medium Drive fit Basic DIA = 0.5000 Hole Shaft Shaft Tolerance = Hole Tolerance = Max Interference = Min Interference = Complete Worksheets Tol-2, Tol-3 and 66 Metric Fit - Terms Basic size: size to which limits of deviation are assigned. (Fig.10.1, p 343) – Must be same for both parts – Basic sizes selected from chart in Table 10.2 Deviation: difference between the actual size and the basic size. (Fig. 10.14) Upper deviation: difference between the max. size limit & the basic size. Lower deviation: difference between the min. size limit & the basic size. Fundamental deviation: the deviation closest to the basic size. (Fig. 10.18) – Capital H indicates Hole – Lowercase letter, I.e. f , indicates shaft Metric Fit – Terms cont’d International Tolerance Grade (IT): a group of tolerances that vary depending upon the basic size, but have the same level of accuracy within a given grade. (Fig 10.18) – There are 18 IT grades. The smaller the grade number, the smaller the tol. zone. – See Appendix 10 for IT grades. Preferred Metric Fits There are 2 systems used to indicate preferred fits in the metric system. – Hole basis: the system of fits where the min. hole size is the basic size. The fundamental deviation for a hole basis system is indicated by a capital “H”. » EX. 50H8 (50 = DIA in mm) (Basic size) (H = using Basic Hole system) (fundamental deviation) (8 = closest running fit) (IT grade) » For mating part 50H8/f7 (Appendix 11) – Hole Limits? – Shaft Limits? – Fit? Preferred Metric Fits The second system used to indicate preferred fits in the metric system. – Shaft basis: the system of fits where the max. shaft size is the basic size. The fundamental deviation for a shaft basis system is indicated by a lowercase letter. » EX. 50h7 (50 = DIA in mm) (Basic size) (h = using Basic Shaft system) (fundamental deviation) (7 = closest running fit) (IT grade) » For mating part 50F8/h7 (Appendix ?) » Limits and Fits? – Hole Limits? – Shaft Limits? – Fit? Try the following problem Basic DIA 41mm Use Hole Basis System Sliding Try the following problem Basic DIA 41mm Use Shaft Basis System Sliding Try the following problem Basic DIA 58mm Use Shaft Basis System Force Fit Look Up What type of fit is a 6 H7/s6? Hole or Shaft Basis System? Complete Worksheets Tol-4 and 67