Basic Fitting
What is tested
Basic Fitting covers layout, marking, sawing, filing, drilling, tapping, reaming and bench fitting to a specified fit. The exam looks for correct sequence, tool selection and tolerance control.
Core rules
- Start from a datum face. Mark with a scriber and engineer's blue, centre-punch before drilling, and check diagonals to keep layout square.
- Select hacksaw blade pitch by material thickness: fine teeth for thin tube and sheet, coarser teeth for solid bar. Keep at least two teeth in contact with the work.
- File with the correct cut and clean the file with a file card; do not file a surface that will be used as a datum unless the drawing allows it.
- Drill speed falls as drill diameter rises. Use coolant or cutting oil, clamp the work, and centre-drill first.
- For a metric coarse thread, tap drill size is approximately major diameter minus pitch. Countersink the hole before tapping and use a tap wrench squarely.
Worked example
A worker needs an M10 × 1.5 tapped hole. Tap drill diameter = 10 − 1.5 = 8.5 mm. The reasoning is that the tap needs a core hole about one pitch smaller than the nominal diameter so the thread flanks can cut without the tap binding. After drilling 8.5 mm, countersink, mount the tap squarely, apply cutting oil, rotate a quarter turn forward and half turn back to break chips, and check with the matching bolt.
Traps
Using the nominal diameter as the tap drill, which leaves no thread and often breaks the tap; tapping without lubricant; and not countersinking, which raises a burr and misaligns the tap. Another error is clamping thin sheet without backing, causing distortion. In fitting, removing too much material to make it fit destroys the intended clearance and cannot be corrected.