Mechanical Fasteners and Dimensioning Standards

Posted on Mar 10, 2025 in Mechanical Engineering

Dimensioning Standards

Dimensions should be placed in the most descriptive view of the feature being dimensioned. A feature should be dimensioned only once. Each feature should be dimensioned or identified with a note. Dimension features or surfaces should be done to a logical reference point. Center lines or center marks should be used on all circles, holes, and slots. Arrowheads are used to terminate dimension lines. The point of the arrowheads on the leader line must make contact with the feature line.

Staggered Dimension Text

Smaller (shorter) dimensions are located closest to the feature line, followed by larger dimensions. Follow these guidelines:

  • Arrows in, dimension out.
  • Out In.
  • In Out.
  • Out Out.

Dimensioning Holes and Cylinders

  • Holes are dimensioned via diameter (Ø).
  • Arcs or fillets are dimensioned via radius (R).
  • Cylinder: One dimension.
  • Cylinder with a hole: Requires two dimensions.

U.S. and Metric Standards

  • U.S. – ANSI Standard: Uses the decimal inch value. When the decimal inch system is used, a zero is not used to the left of the decimal point for values less than one inch, and trailing zeros *are* used.
  • Metric: Dimensions are expressed in millimeters (mm). No trailing zeros should be used. Millimeter, Gram, Second (MMGS) unit system.

Dimensioning Methods

  • Baseline Dimensioning: Creates dimensions by measuring from a common baseline. Used when the location of features must be controlled from a common reference point or plane.
  • Chain Dimensioning: Creates a series or a chain of dimensions that are placed one after another. Used when tolerances between adjacent features are more important than the overall tolerance of the feature.

Tolerance Accumulation

The distance can be controlled by a single dimension or multiple dimensions. The maximum variation between two features is equal to the sum of the tolerances placed on the controlling dimensions. This is especially relevant in chain dimensioning.

Fasteners

  • A Bolt is designed to be inserted through a hole and secured with a nut.
  • A Screw is designed to be used in a threaded hole – sometimes with a nut.

Commonly Used Fasteners

  • Hex Bolts: A bolt is a fastener having a head on one end and a thread on the other end.
  • Studs: A stud is a rod with threaded ends.
  • Cap Screws: A hexagon cap screw is similar to a bolt except it is used without a nut and generally has a longer thread.
  • Machine Screws: Similar to the slot-head cap screw but smaller.
  • Wood Screws: A tapered shank screw for use exclusively in wood.
  • Sheet Metal Screws: Highly versatile fasteners designed for thin materials.
  • Carriage Bolts: Mostly used in wood with a domed shape top and a square under the head, which is pulled into the wood as the nut is tightened.
  • Socket Screws: Also known as Allen head, are fastened with a hexagon Allen wrench.
  • Set Screws: Used to prevent relative motion between two parts.
  • Nuts: Used to attach machine thread fasteners.
  • Washers: Provide a greater contact surface under the fastener.
  • Keys: Used to prevent relative motion between shafts and wheels, couplings, and similar parts attached to shafts.
  • Rivets: Used to hold sheet metal parts together.

Multiple Threads

If a single helical groove is cut or formed on a cylinder, it is called a single-thread screw. If the helix angle is increased sufficiently for a second thread to be cut between the grooves of the first thread, a double thread will be formed. Double, triple, and even quadruple threads are used whenever a rapid advance is desired, as on valves.

Screw Thread Terminology

  • Major Diameter: The largest diameter (fractional diameter or number) of a screw thread.
  • Minor Diameter: The smallest diameter (fractional diameter or number) of a screw thread.

Example: U.S. Thread Note

.5-13x1 UNC–3B

  • 1/2: Major diameter in inches.
  • 13: Number of threads per inch.
  • 1: Thread depth.
  • UNC: Thread series.
  • 3: Class of thread.
  • B: Internal thread (Female).

Counterbore and Countersink

Sometimes bolt or screw heads need to be flush with the surface. This can be achieved by using either a Counterbore or Countersink hole, depending on the fastener’s head shape.

  • Counterbore: Holes designed to recess a bolt or screw head below the surface of a part.
  • Countersink: Angled holes that are designed to recess screws with angled heads.

Example: Metric Thread Note

M 20 X 1.5–7H

  • M: Symbol for ISO metric threads.
  • 20: Nominal major diameter in millimeters.
  • 1.5: Thread pitch in millimeters.
  • 7: Grade of tolerance.
  • H: Tolerance class.

Methods of Representing Screw Threads

There are three methods of representing screw threads on a drawing:

  1. Detailed Representation: A close approximation of the appearance of an actual screw thread.
  2. Schematic Representation: Uses staggered lines to represent the thread roots and crests.
  3. Simplified Representation: Uses visible and hidden lines to represent the major and minor diameters.