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Dimension Application and Limits of Size

4. Dimension Application and Limits of Size. Clearly apply dimensions by complying with the stated general dimensioning guidelines. Apply dimensions to any of the geometric shapes commonly found on mechanical parts.

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Dimension Application and Limits of Size

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  1. 4 Dimension Application and Limits of Size

  2. Clearly apply dimensions by complying with the stated general dimensioning guidelines. • Apply dimensions to any of the geometric shapes commonly found on mechanical parts. • Cite the categories for limits of fit and describe the general condition created by each category. • Calculate and apply limits of size for mating features. • Cite the two rules contained within ASME Y14.5.

  3. Provide examples of the effects that dimensions and tolerances have on manufacturing. • Complete a surface condition specification when provided the allowable variations.

  4. Cylinders • Diameter and length required • Prefer dimensions on noncircular view • Diameter dimension usually between the views • Only one view may be necessary • Diameter symbol required for circular dimension

  5. Cones • Right circular cone • Base diameter and cone height • Base diameter and cone angle • Oblique circular cone • Base diameter, cone height, apex offset

  6. Holes—Specifying Diameter

  7. Holes—Specifying Location • Hole center • Centerlines and extension lines used • Centerlines between holes • Extension lines beyond holes • Dimensions on one side of view

  8. Holes—Specifying Depth • Assumed to go through part, unless depth is specified • When not clear, use THRU • Use depth symbol for blind holes • Depth specified after the diameter

  9. Holes for Machine Screws—Counterbore

  10. Holes for Machine Screws—Countersink

  11. Angles • Angle specification notation • Degrees—30 • Minutes—3015 • Seconds—301528 • Decimal—30.25 • Center of dimension line arc at vertex of angle • Extension lines, if extended, intersect at vertex of angle

  12. Chamfers • Eliminates sharp edge • Facilitates assembly • 45° chamfers may be dimensioned by a note • Dimensioning chamfers other than 45° • Dimension angle and length of one side • Dimension lengths of both sides of chamfer

  13. Taper—Standard Specified

  14. Taper—Taper per Inch Specified

  15. Taper—Included Angle Specified

  16. Features on Curved Surfaces • Location • Angle • Chord • Arc length

  17. Spherical Radii • SR in front of dimension • Dimension to outside surface • Easier to produce • Easier to verify

  18. Irregular Curves

  19. Symmetrical Features • Only one side of symmetry line requires dimensions • Two short lines across each end of center line indicates line of symmetry • Partial or half views allowed

  20. Machining Reliefs

  21. Threads • Minimum information • Nominal size • Threads per inch • Thread form • Thread class • Example • .750-10UNC-2A

  22. Knurls • Machined on cylindrical shapes • Increase friction between parts • Improve grip and appearance • Diametral pitch and shape required • Diameter before knurl required

  23. Sheet Metal Bends • Locate surface intersections or intersecting faces • Specify inside or outside bend radius, but not both • Material type and thickness dictates undimensioned bend radius

  24. Sheet Metal Joggles • Information needed • Location • Height • Length • Bend radius

  25. Standard Sizes and Shapes • Reduces production costs • Dimensions not needed for standard materials • National standards maintain uniformity

  26. Broken Lengths • Long parts have portion removed • Reduces drawing space • Dimension not broken • Dimension value reflects complete part

  27. Limits of Size • General tolerance in notes acceptable • Tolerance on size dimension • Limit dimensions • Bilateral tolerances • Unilateral tolerances • Single limit tolerances

  28. Limit Dimensions • Specifies minimum and maximum • Maximum size above minimum • In a note, minimum size before maximum

  29. Plus and Minus Tolerances • May be bilateral or unilateral • Bilateral tolerance may be equal or unequal

  30. Single Limit Dimensions

  31. Tolerances on Angles • Limit dimensions • Plus-and-minus tolerances • Orientation tolerances • Single limit not practical

  32. Basic Hole System • Preferred system • Limits of size for hole calculated first • Typically use standard tool sizes for hole

  33. Basic Shaft System • May require nonstandard tools for hole • Used when purchasing shaft and mating hole is specified

  34. Calculating Size Limits Using Standard Tables • Function of parts considered • Descriptions of general applications provided • Determine class of fit • Determine tolerance values • Tolerance is proportional to size of feature

  35. Running and Sliding Clearance Fits • Parts move freely • 9 classes of fit • RC1 through RC9

  36. Location Clearance Fits • Locating parts without interference • 11 classes of fit • LC1 through LC11

  37. Location Transition Fits • Locating parts with slight clearance or interference • 6 classes of fit • LT1 through LT6

  38. Location Interference Fits • Locating parts with a press fit (interference) • Not for transmitting loads • 3 classes of fit • LN1 through LN3

  39. Force Fits • Locating parts with a press fit (interference) • Bore pressure constant through range • 5 classes of fit • FN1 through FN5

  40. Limit Calculations Using Tables • Select a class of fit • Select a category of fit from the class • Find appropriate tolerance table • Locate nominal size in table • Calculate tolerances

  41. Rules in ASME Y14.5 • Rule #1: Perfect form at MMC • Exceptions to Rule #1 • Parts subject to free state variations • Stock materials • Rule #2: RFS and RMB assumed

  42. Dimension Interpretations

  43. Machine Capability • Each machine type is uniquely capable • Each machining program for a part is uniquely capable • Smaller tolerances generally equal larger costs • Know your company and supplier’s capabilities • Machining handbooks provide general capabilities

  44. Dimension Revision • Change geometry in CAD • Out-of-scale dimension to be avoided • Reasons for revisions • Design changes • Corrections • Manufacturing-based changes

  45. Surface Texture Dimensions • Determined by function • Surface variations • Roughness • Waviness • Lay • Roughness width • Roughness width cutoff • Roughness cutoff • Sampling length

  46. Surface Texture Symbols

  47. Explain how a single view can be dimensioned to completely define a cylindrical part. • Create one view that shows both the length and diameter of the cylinder and apply both the length and diameter dimensions to the created view.

  48. List the classes of fit. • Running and sliding clearance, location clearance, location transition, location interference, force fits

  49. Calculate the tolerance for a hole that will accept a purchased shaft with the dimension .5.001, and provide a clearance of min .0012, max .0038 (RC6). • Min hole = .501 + .0012 = .5022 • Max hole = .499 + .0038 = .5028 • (Max clearance comes from tol table) • hole tol + min clearance + shaft tol

  50. How can an exception to Rule #1 be specified? • Place the independency symbol adjacent to the excepted dimensions to indicate perfect form at MMC is not required.

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