Slide 002: Tolerances, Fits, and Engineering Drawings¶
Slide Visual¶
Slide Overview¶
This slide covers the concepts of dimensional tolerances, engineering fits, and how to read critical dimensions from machining drawings.
Instruction Notes¶
Understanding Tolerances¶
Every machined dimension has a tolerance—the acceptable range of variation from the nominal (target) dimension. Tolerances are specified because no manufacturing process can produce a perfect dimension.
Tolerance expression formats: - Bilateral: 1.000" ±0.005" (can be 0.995" to 1.005") - Unilateral: 1.000" +0.000/-0.010" (can be 0.990" to 1.000") - Limit dimensions: 0.995" to 1.005" (upper and lower limits stated directly)
General shop tolerances (when no tolerance is stated):
| Decimal Places | Implied Tolerance |
|---|---|
| X.X (1 decimal) | ±0.1" |
| X.XX (2 decimal) | ±0.01" |
| X.XXX (3 decimal) | ±0.005" |
| X.XXXX (4 decimal) | ±0.0005" |
Engineering Fits¶
A "fit" describes the relationship between two mating parts (shaft and hole). The three categories:
Clearance Fit: The shaft is always smaller than the hole. Parts slide freely. Used for bearings, pivots, and parts that must assemble easily. - Example: 1.000" hole, 0.998" shaft = 0.002" clearance
Interference (Press) Fit: The shaft is always larger than the hole. Parts must be pressed or heat-shrunk together. Used for permanent assemblies. - Example: 1.000" hole, 1.001" shaft = 0.001" interference
Transition Fit: Depending on actual dimensions within tolerance, the fit may have slight clearance or slight interference. Used when precise alignment is needed but assembly/disassembly is required.
Reading Machining Drawings¶
Key elements students must identify:
- Title block: Part number, material, finish, tolerances, scale
- Dimensions: Linear dimensions with tolerances
- Geometric tolerances (GD&T): Feature control frames specifying flatness, parallelism, position, etc.
- Surface finish symbols: Ra (roughness average) values indicating required surface smoothness
- Notes: Material specifications, heat treatment, special instructions
- Views: Orthographic projections (front, top, right side) and section views
Surface Finish¶
Surface roughness is measured in Ra (roughness average, in microinches):
| Ra Value | Finish Quality | Typical Process |
|---|---|---|
| 250 μin | Rough | As-sawn, rough turned |
| 125 μin | Medium rough | Rough machined |
| 63 μin | Medium | Standard machined finish |
| 32 μin | Smooth | Fine machined, ground |
| 16 μin | Very smooth | Precision ground, lapped |
| 8 μin | Mirror | Superfinished |
Key Talking Points¶
- Tolerances drive the manufacturing process—tighter tolerances require slower, more careful machining
- Understanding fits is essential for making parts that work together
- Always check the drawing before machining—know the tolerances before you cut
- Surface finish requirements affect tool selection, speed, feed, and finishing operations
Learning Objectives (Concept Check)¶
- Can students interpret bilateral and unilateral tolerances?
- Can students explain the three types of fits?
- Can students identify key elements on a machining drawing?
Last Updated: 2026-03-19