Definition: Feet refers to the degree of tightness or looseness between mating parts.
Interchangeability: The concept that allows for different components to fit together without being made to exact specifications.
Tolerances in Manufacturing
Example: Shaft diameter and hole diameter both targeted at 25 mm.
Tolerance Specification:
Upper and lower limits are specified in drawings (e.g., 25.02 mm upper limit and 25.00 mm lower limit).
Tolerance is calculated as upper limit - lower limit (0.02 mm in this case).
Interchangeability Application: Allows any part within specified tolerances from different lots to fit together, reducing costs and production time.
Types of Fits
Clearance Fit:
Hole size is always larger than shaft size.
Ensures easy assembly without interference.
Interference Fit:
Shaft size is always larger than hole size.
Requires application of force to assemble the parts.
Transition Fit:
Can have either clearance or interference.
Allows for potential fitting issues between components.
Definitions of Fitting
Fitting: Refers to the assembly of parts after adjusting dimensions to secure necessary fits.
Fitting Work: Operations performed to achieve the desired fit using various tools.
Fitting Operations
Deburring: Removal of imperfections (burs) from machined metals using rough files.
Measuring and Marking: Various tools used for accurate measurements (e.g., steel rule, Vernier caliper).
Cutting Operations: Performed using tools like hacksaws, chisels, and files.
Drilling and Reaming: Creating holes and refining them for smooth finishes.
Tapping: Cutting internal threads in holes using a sequence of taps.
Dyeing: Used for cutting external threads.
Measurement Tools
Steel Rule: Basic measuring tool for length.
Vernier Caliper: For precise measurements (internal and external dimensions).
Height Gauge: Measures vertical dimensions from a surface plate.
Combination Set: Used for squaring and marking angles (includes square head, center head, and protractor).
Cutting Tools
Hacksaw: For cutting various materials, adjustable for different thicknesses.
Chisels: For precision cutting; various types (side-cut, round nose) for specific applications.
Finishing Operations
Lapping: Fine finishing done with abrasive slurry on a flat surface.
Honing: Smoothes internal surfaces using rotating stones.
Buffing/Polishing: Uses soft materials to achieve a shiny finish.
Super Finishing: Similar to honing but with finer tolerances and surface finishes.
Summary of Thread Types
Metric Thread: Major diameter and pitch defined (e.g., M8x1.25).
Unified Thread: Similar to metric with threads per inch defined.
Trapezoidal Thread: Used in heavy lifting applications.
Conclusion
Understanding fittings and their operations is crucial in manufacturing and assembly processes. The selection of appropriate tolerances and fits can significantly impact the efficiency and cost-effectiveness of production.