2D CAD Drafting of Machine Fasteners: Screws, Nuts, and Bolts

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1. Introduction to Computer-Aided Drafting (CAD)

Computer-Aided Drafting (CAD) replaces manual drafting instruments with interactive digital graphics software (e.g., AutoCAD, SolidWorks, CATIA, Creo).

Major Engineering Advantages:

• Zero Drawing Errors: Absolute mathematical precision ($16$-digit floating-point accuracy).
• Layer Management: Separation of visible outlines, hidden lines, centerlines, dimensions, and annotations onto independently toggleable layers.
• Parametric Editing: Rapid modification without erasing and redrawing; commands like STRETCH, TRIM, SCALE, and MIRROR.
• Standard Toolboxes & Libraries: Standard reusable hardware libraries (screws, bearings, keys) adhering to BIS/ISO standards.

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2. Standard Proportions of Threaded Fasteners (IS 1363 / ISO 4014)

In engineering drawings, standard fasteners are drawn using empirical proportions based on the nominal diameter ($D$):

                  HEXAGONAL NUT (Across Corners)
                        +---------------+
                       /        |        \  30 deg Chamfer
                      +---------+---------+
                      |         |         |
                      |         |         |  Nut Thickness T = D
                      |         |         |
                      +---------+---------+
                       \        |        /
                        +---------------+
                      |<------- 2D ------>|

Feature / Dimension	Empirical Formula (BIS SP 46)	For $D = 20\text{ mm}$ (Example)
Nominal (Major) Diameter	$D$	$20\text{ mm}$
Core (Minor) Diameter	$d_c \approx 0.84D \text{ to } 0.85D$	$17\text{ mm}$
Width Across Flats ($W$)	$W = 1.5D + 3\text{ mm}$	$33\text{ mm}$
Width Across Corners ($D_1$)	$D_1 = 2D$ (or $W / \cos 30^\circ$)	$40\text{ mm}$
Thickness of Nut ($T$)	$T = D$	$20\text{ mm}$
Height of Bolt Head ($H$)	$H = 0.7D \text{ to } 0.8D$	$16\text{ mm}$
Length of Thread ($b$)	$b = 2D + 6\text{ mm}$ (for $L \leq 125\text{ mm}$)	$46\text{ mm}$
Chamfer Angle	$30^\circ$ with the flat bearing face	$30^\circ$
Chamfer Arc Radii	Central arc $R = 1.2D \text{ to } 1.4D$; Side arcs $r \approx 0.4D$	$R = 28\text{ mm}, r = 8\text{ mm}$

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3. Standard CAD Layer Architecture

A clean CAD drawing requires a structured layer hierarchy:

Layer Name	Line Type	Lineweight	Color	Typical Usage
0_Outlines	Continuous	$0.50\text{ mm}$ (Thick)	White / Black	Visible contours of bolt head, nut, and shank
0_Center	CENTER2	$0.25\text{ mm}$ (Thin)	Red	Central axes of bolt, hole, and pitch circle
0_Hidden	HIDDEN2	$0.25\text{ mm}$ (Thin)	Yellow	Internal threaded holes, hidden clearances
0_Threads	Continuous	$0.25\text{ mm}$ (Thin)	Green	Minor diameter of external thread; major diameter of internal thread
0_Dim	Continuous	$0.25\text{ mm}$ (Thin)	Cyan	Aligned/linear dimensions and leader text

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4. 2D Drafting Workflow in CAD (AutoCAD Command Sequence)

A. Drawing the Top View (Hexagon View):

1. Command: POLYGON
   • Enter number of sides: 6.
   • Specify center point: (0, 0).
   • Option: Inscribed in circle (radius $= D$) or Circumscribed about circle (radius $= W/2$).
2. Nominal Hole Diameter:
   • Command: CIRCLE $\implies$ Center (0, 0), Diameter $= D$ (drawn with continuous thick line).
3. Core Thread Diameter:
   • Command: CIRCLE $\implies$ Center (0, 0), Diameter $= 0.85D$.
   • Command: TRIM $\implies$ Trim away one-quarter of the circle to leave a broken $3/4$ circular arc (standard representation of internal threads as per IS 696).

B. Drawing the Front View (Across Corners):

1. Project vertical lines from the top view corners to define the width across corners ($2D$).
2. Draw the base line and top line offset by nut thickness $T = D$.
3. Through the center, draw vertical line showing central face (width $= D$) and two side faces (each of width $= D/2$).
4. Chamfer Arcs (ARC command):
   • On the central face, construct arc with radius $R = 1.4D$ touching the top edge.
   • On the two side faces, construct arcs with radius $r = 0.4D$.
   • Use CHAMFER or LINE at $30^\circ$ from corners to meet the arc extremities.

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5. Thread Representation Conventions (IS 696 / ISO 6410)

       EXTERNAL THREAD (Bolt)                  INTERNAL THREAD (Nut/Hole)
       +-----------------------+                 +-----------------------+
       |                       |                 |///////////////////////|
       +===+---------------+===+                 +---+---------------+---+  Major Dia (Thin)
       |   |               |   |                 |   |               |   |
  =====+   | (Crest: Thick)|   +=====       =====+   | (Crest: Thick)|   +===== Axis
       |   |               |   |                 |   |               |   |
       +===+---------------+===+                 +---+---------------+---+
       |                       |                 |///////////////////////|
       +-----------------------+                 +-----------------------+
       |<--- b (Thread L) ---->|

• External Thread (Bolt):
  • Major diameter (crest): Continuous Thick line.
  • Minor diameter (root): Continuous Thin line ($0.85D$).
  • End of thread: Continuous thick line across the shank.
• Internal Thread (Tapped Hole in Section):
  • Minor diameter (hole drilled): Continuous Thick line.
  • Major diameter (tapped thread): Continuous Thin line ($1.0D$).
  • In section views, hatching lines continue right through to the minor diameter (thick line).

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6. Summary of Key Exam Points

1. Nut Proportions: Thickness $T = D$; Width across corners $= 2D$; Width across flats $= 1.5D + 3\text{ mm}$.
2. Chamfer Angle: Standard chamfer is strictly $30^\circ$ with the bearing face.
3. End View Thread Convention: A thread root is drawn as a three-quarter broken circle in continuous thin line.
4. CAD Essential Commands:
   • POLYGON (for 6-sided nut head)
   • OFFSET (for thread depth and wall thickness)
   • TRIM (for clearing intersections and chamfer corners)
   • FILLET (for rounding under-head radius $R \approx 0.1D$)
   • LAYER (for assigning appropriate lineweights and colors).