2D CAD Drawings of Physical Machine Elements: Fasteners, Washers & Assembled Joints

---

1. Washers: Standards, Function & Proportions (IS 2016 / IS 3063)

A washer is a thin annular disk placed beneath a nut or bolt head.

Primary Engineering Functions:

1. Load Distribution: Distributes the bolt clamping pressure over a wider contact area on softer clamped materials.
2. Surface Protection: Prevents gouging, scratching, and distortion of the finished joint surface during nut tightening.
3. Locking Against Vibration: Spring washers maintain continuous axial tension under dynamic vibration, preventing accidental unscrewing.

                  PLAIN WASHER                          SPRING (SPLIT) WASHER
           +-------------------------+                       +----+     +----+
          /|       d1 (Hole)         |\                      |    \     /    | Split gap
         + |        +-------+        | +                     +----+ \ / +----+
         | |        |       |        | |  t = 0.15D                  X
         + |        +-------+        | +                     +----+ / \ +----+
          \|                         |/                      |    /     \    |
           +-------------------------+                       +----+     +----+
           |<-------- Do ----------->|

Standard Proportions (for Bolt Diameter $D$):

• Inside Hole Diameter ($d_1$): $d_1 = D + 1\text{ mm}$ (for $D \leq 12\text{ mm}$) or $D + 2\text{ mm}$ (for $D > 12\text{ mm}$).
• Outside Diameter ($D_o$):

• Washer Thickness ($t$):

• Chamfer: $30^\circ\text{ to }45^\circ$ on one outer edge.

---

2. Forms of Machine Screws and Bolts

Machine screws are threaded fasteners used for connecting parts without nuts by screwing directly into tapped holes.

Screw / Bolt Type	Head Geometry	Application & Features
Hexagonal Head Bolt	6-sided prism, $30^\circ$ chamfer	General machinery assembly; tightened with spanner.
Square Head Bolt	4-sided prism	Used in bearings, pipe flanges, and tool-posts where head is captive.
Socket Head Cap Screw (Allen Bolt)	Cylindrical head with internal hexagon socket	High-strength assembly where head is countersunk flush into a counterbore.
Countersunk Screw	Conical head ($90^\circ$ included angle)	Sits completely flush with plate surface (aircraft, machine tables).
Cheese Head Screw	Short cylindrical head with screwdriver slot	Electrical appliances, small mechanisms.
Eye Foundation Bolt	Forged circular eye embedded in concrete	Anchoring heavy dynamic machines (engines, compressors) to concrete foundation beds.

---

3. Assembled Bolted Joint: 2D CAD Layout

When two plates of thickness $t_1$ and $t_2$ are clamped together using a bolt, washer, and nut:

                  Hex Head Bolt
                     +----+
                     |    |
          -----------+----+----------- Plate 1 (t1)
          Clearance  | :  | Hole (d1 = D + 1)
          -----------+----+----------- Plate 2 (t2)
                     |    |
                     +====+ Plain Washer (Do = 2D + 3)
                     |    | Hex Nut (T = D)
                     +----+
                     |    | Projection (2 to 3 thread pitches)
                     +----+

Essential Assembly Rules:

1. Clearance Hole in Plates: The hole drilled through the plates must be slightly larger than the bolt shank ($d_h = D + 1\text{ to }2\text{ mm}$) to prevent binding.
2. Washer Placement: The washer is placed under the rotating nut (not under the bolt head, unless specified).
3. Thread Projection: The threaded bolt shank must project beyond the outer face of the tightened nut by at least $2\text{ to }3$ thread pitches ($2P\text{ to }3P$) to guarantee full nut thread engagement.
4. Hatching across Plates: When sectioning the assembly, Plate 1 is hatched at $+45^\circ$ and Plate 2 is hatched at $-45^\circ$ ($135^\circ$) to clearly distinguish the two components.

---

4. Limits, Fits & Surface Finish Symbols in CAD Drawings

Technical working drawings must specify limits, fits, and manufacturing tolerances (as per IS 919 / ISO 286):

• Hole-Basis System: The basic size is taken as the minimum hole size ($H$-hole, lower deviation $= 0$). The desired fit is achieved by varying the tolerance of the shaft. Preferred worldwide.
• Types of Fits:
  1. Clearance Fit (e.g., $H7/g6$): Minimum hole diameter is strictly greater than maximum shaft diameter $\implies$ free relative rotation or sliding.
  2. Transition Fit (e.g., $H7/k6$): Tolerance zones overlap $\implies$ can result in either slight clearance or slight interference; used for accurate location (locating pins).
  3. Interference Fit (e.g., $H7/p6$): Shaft is strictly larger than the hole $\implies$ requires hydraulic press or thermal shrink fitting; permanent assembly.

---

5. Bill of Materials (BOM) Structure in CAD Assembly Drawings

A technical assembly drawing is incomplete without a standard Bill of Materials (BOM) placed directly above the Title Block:

+----+----------------------+------+---------------+----------+
|ITEM| PART NAME            | QTY  | MATERIAL      | REMARKS  |
+----+----------------------+------+---------------+----------+
| 01 | Body / Pedestal Base |  01  | Cast Iron (CI)| FG 200   |
| 02 | Bearing Cap          |  01  | Cast Iron (CI)| FG 200   |
| 03 | Split Brasses (Steps)|  02  | Gunmetal      | Split    |
| 04 | Square Head Bolt M16 |  02  | Mild Steel    | Grade 4.6|
| 05 | Hex Nut M16          |  02  | Mild Steel    | IS 1363  |
| 06 | Plain Washer 16      |  02  | Steel         | IS 2016  |
+----+----------------------+------+---------------+----------+

---

6. Summary of Key Exam Points

1. Washer Dimensions: $D_o = 2D + 3\text{ mm}$, $t = 0.15D$, hole $= D + 1\text{ mm}$.
2. Spring Washer Function: Prevents loosening of fasteners subjected to mechanical vibration through constant elastic pre-load.
3. Bolt Projection Past Nut: Must extend $2\text{ to }3$ thread pitches ($2P$).
4. Sectioning Solid Fasteners: Bolts, studs, nuts, washers, and keys are never sectioned longitudinally.
5. Hole-Basis Fit: Standardized on the capital letter $H$ for holes (e.g., $H7$), meaning the lower deviation of the hole is exactly zero.