Basic Fitting
Fitting is bench work that produces a part which mates with another within specified limits. The exam tests terminology of limits and fits and the correct sequence of operations.
Fitting operations
Marking off from a datum, centre punching, sawing, filing flat, square, parallel and angular, drilling, counterboring, countersinking, reaming, tapping, scraping and lapping. The rule is rough cut first, finish last, and check the dimension after every operation rather than at the end.
Limits, fits and tolerances
- Basic or nominal size is the size from which limits are derived.
- Tolerance is the upper limit minus the lower limit, that is the total permitted variation, not the plus-or-minus figure.
- Allowance is the intentional difference between the minimum hole and the maximum shaft, and can be negative.
- Clearance fit: the hole is always larger than the shaft (running, sliding fits).
- Interference fit: the shaft is always larger (press and shrink fits).
- Transition fit: may give a small clearance or a small interference (push fit).
- The hole basis system is preferred because holes are produced by fixed-size tools such as drills and reamers.
Worked example
Hole 25.02/25.00 mm, shaft 25.00/24.98 mm. Minimum clearance = 25.00 - 25.00 = 0.00 mm. Maximum clearance = 25.02 - 24.98 = 0.04 mm. Since clearance never becomes negative, the pair gives a clearance fit, with 0.02 mm tolerance on the hole and 0.02 mm on the shaft. Reasoning: always compare the smallest hole with the largest shaft for minimum clearance.
Traps that cost marks
- Calling a plus-or-minus 0.02 mm tolerance 0.02 mm when the total is 0.04 mm.
- Confusing allowance with clearance.
- Assuming a nominal 25 mm shaft is exactly 25 mm when its limits are 24.98/25.00.
- Removing material after the final measurement.