Engineering Design Representation. Use of 2D drawing format: Typical Design Annotation. Standardization. Extracted drawings. General dimensions

Transcription

1 Engineering Design Representation Some elements of design representation not easily conveyed through model alone. Many are notational in nature. Examples are: Thread specifications Surface finishes Surface quality Tolerances Use of 2D drawing format: Much of this information is added to extracted 2D drawings. Drawings can be extracted from geometric models. Despite advances, 2D mechanical drawings still most popular format for design documentation. Typical Design Annotation General dimensions Size, location General and local notes Feature specifications (e.g. threads) Surface qualities (finish, treatment) Fits (quality of fits between mating parts) Tolerance (size, form, location) Standardization Annotation for mechanical drawings should conform to accepted standards for: form content placement ASME/ANSI Y14 is the most common also ISO and MIL STANDARD General dimensions Extracted drawings Require all dimensions to completely document design. For drawings extracted from dimensionally -driven systems: Not all dimensions may be included (use of geometric constraints) Dimensions used for constraint may not be those required for manufacture. solid modelers allow drawing data to be extracted software may place some dimensions for user typically driving dimensions for the model in dimensionally-driven systems 1

2 Extracted drawings default dimensions may or may not conform to standards commands supplied to modify/add dimensions and notes software cannot automatically add all notes and tolerances General notes: Apply to the entire component Examples: Reference to repeated feature (size of fillets and rounds) Material specification Overall surface quality General Notes ALL UNSPECIFIED RADII REQ D, CAST ALUMINUM ALL FINISHED SURFACE 64µ REMOVE ALL SHARP EDGES Local notes: Sometimes referred to as callouts, when specification is made to a hole feature. Reference feature through use of a leader line. Specify information for only a finite portion of component or system. Examples of local notes:.50,.62, X 45 CHAM Hole specifications (size dimension) Specified via local note Note written in order of operations performed. (e.g. drill, thread) Written to circular view of hole. (preferred) Leader line points to hole center. Leader arrowhead touches representation of last operation performed. 2

3 Holes as referred to as: Through (passage) Require no specification of hole depth in note. Blind (non-passage) Requires depth specification. Multiple occurrences of hole specification in pattern. Write note to one hole. Include reference to the number of occurrences in note. e.g. 4 x φ.375 Threaded Holes Thread parameters Nominal (major) and root (minor) diameter Pitch (lead, inches/thread) Thread angle Crest and Root Internal vs. External thread Standardization: Threads parameters are standardized Information available in tabular form from many sources: Mechanical Drawing texts Machine Design handbooks Thread specifications Thread form: Classification based upon cross-sectional profile of thread. Standard for English unit threads in U.S. is the Unified (UN) thread form. Thread series: Standard based upon the number of threads/inch for a specific nominal diameter. Standards for English units are: Coarse (C), Fine (F), Extra-Fine (EF) 3

4 Thread specifications Class fit: A specification of how tightly mating external and internal threads will mesh. Based upon a difference in the values of the respective root diameters. For the standard Unified thread form, the classes of fit are: 1 (loose), 2 (standard), 3 (tight). Additional designation is made for external (A) versus internal (B) threads. English unit thread specification examples unf - 2A, LH 1/2-13UNC - 1 Metric thread series M Series: Standard metric thread profile MJ Series: Modified series in which crest and root are specified unef - 2B Metric thread series: Fits Specified by tolerance grade (numeral) and by tolerance position (letter). General purpose fit 6g (external) 6H (internal) Close fit 5g6g (external) 6H (internal) Specification may include pitch dia. fit and minor dia. fit. 4

5 Metric thread specifications MJ6 x 1-4H5H M8 x h6h LH M10 x 1.5-4h5h Note for threaded hole (internal threads) Size (diameter) of pilot hole drilled prior to tapping, and depth (if required) Specification of threads to be cut, and depth (if required) Tap drill diameter: Unique for thread diameter and series. Taken from table of standard data Diameter may be specified as: fraction or decimal (English units) letter (English units) number (English units) decimal values for Metric Form of drill size in note: Value: given as exact decimal equivalent or fraction Letter: give letter and decimal equivalent as reference (in parentheses) Number: give number and decimal equivalent as reference Threaded Hole Callouts Counterbored and countersunk holes: Counterbore: subsequent machining performed to cut hole to an enlarged diameter for a limited depth. Countersink: enlarged angular opening to hole, sometimes referred to as counter drilling. Both used to accommodate fastener head, recessing it flush of below material surface. 5

6 C Bored and C Sk Holes Counterbore specification: Diameter of counterbore, based upon fastener head diameter with standard clearance added. Depth of counterbore, based upon head profile height. Countersink specification: C Bore and C Sink Callouts Angle of countersink and either; 1) depth of countersink 2) diameter of maximum opening (based upon fastener head diameter plus typ. 1/64 or equivalent) Mechanical fastener specification: You should have the following: Ability to write accurate specification of desired fastener. Ability to associate a given specification with the respective fastener. Basic understanding of fastener use. Specification of fastener: Thread specification Head type Fastener type Fastener length Also: material, grade 6

7 Basic head types: Basic Head Types Hexagonal head (HEX HD) Fillister head (FIL HD) Flat head (FLAT HD) Oval head (OVAL HD) Round head (RND HD) Hexagonal socket head (SOC HD) Basic fastener types: Cap screws (CAP SCR) Machine screws (MACH SCR) Set screws (SET SCR) Cap screw/machine screw Used for clamping in conjunction with internally threaded holes. Clamping force applied through bottom face of head and the contact between internal and external heads. CAP SCR Larger sizes (diameters) Threaded end of fastener is chamfered. Minimum thread length is a function fastener nominal diameter. e.g. length of thread = 0.5 *DIA CAP SCR with a nut is referred to as a bolt. MACH SCR Smaller sizes (diameters) Threaded end of fastener not chamfered. Minimum thread length is a function of fastener length: if fastener length > 2, min. thread length = 1.75 if fastener length < 2, min. thread length = fastener length 7

8 SET SCR Function is to restrict or control motion. Headless (except for antiquated square head) Have specified point types. Threaded entire length. SET SCR, Point types: Cone Cup Flat Oval Dog (full or half) Standards for Thread Representation Can be: Detailed Simplified Schematic 8