Technical Knowledge: Fitter Trade
Fitter Trade questions test bench work and precision fitting: measuring instruments, marking, hand tools, and the limits-and-fits system that governs interchangeable manufacture.
Measurement and marking
Vernier caliper: least count = 1 main-scale division - 1 vernier division. With 50 vernier divisions on 49 mm, LC = 0.02 mm. Micrometer: pitch 0.5 mm with 50 thimble divisions gives LC = 0.01 mm. Transfer dimensions from a surface plate using V-block, height gauge and scriber.
Limits, fits and tolerances
- Tolerance = upper limit - lower limit.
- Clearance fit: hole always larger, e.g. H7/g6.
- Interference fit: shaft always larger, e.g. H7/p6.
- Transition fit: either clearance or interference, e.g. H7/k6.
- Hole-basis is preferred because holes are finished with fixed-size reamers.
Worked example
A 25 mm shaft has limits 24.98 and 25.02 mm, so tolerance = 0.04 mm. Its mating hole is 25.00 to 25.03 mm. Minimum clearance = 25.00 - 25.02 = -0.02 mm; maximum clearance = 25.03 - 24.98 = 0.05 mm. Since a negative clearance occurs, the fit is transitional.
Traps
Do not confuse least count with zero error, and always apply zero error with its sign. Read the vernier from the correct jaw. For metric threads, tapping drill size = major diameter - pitch, not the major diameter alone. Reaming removes little material and corrects hole size; drilling does not guarantee accuracy.
Key points
- Vernier LC = 1 MSD - 1 VSD; 50 divisions on 49 mm gives 0.02 mm.
- Micrometer LC = pitch divided by thimble divisions = 0.01 mm.
- Tolerance = upper limit - lower limit.
- H7/g6 is clearance, H7/p6 interference, H7/k6 transition.
- Tapping drill size = major diameter - pitch.