Fitting and Bench Work: Limits, Fits and Tapping
Fitting is the bench trade of making two parts mate within a specified clearance by hand. The exam tests the sequence of fitting operations, the meaning of limits, tolerances and fits, and simple calculations such as tap drill size.
Operations in order
Marking out, chipping, filing (cross filing removes metal, draw filing finishes), scraping to remove high spots (checked with Prussian blue), drilling, countersinking, reaming, tapping, then lapping or honing for a very fine finish.
Limits, fits and tolerances
- Tolerance is the permitted difference between the upper and lower limits of a dimension.
- Clearance fit: the hole is always larger than the shaft, as in a running bearing.
- Interference fit: the shaft is always larger, so it needs pressing or shrinking, as with a bush in a housing.
- Transition fit: may give a small clearance or a small interference. In the hole-basis system the hole is kept constant and the shaft is varied; shaft-basis does the reverse.
Tapping arithmetic
Tap drill diameter = major diameter − pitch. Use taper, then plug, then bottoming tap, lubricate, and reverse half a turn to break the chip.
Worked example
An M12 × 1.75 thread is needed in a mild-steel plate. Tap drill = 12 − 1.75 = 10.25 mm, so drill 10.2 mm and tap M12 × 1.75. If a reamed hole is specified for a dowel pin, drill about 0.2 mm undersize and ream to size, because a reamer removes only a few hundredths of a millimetre and follows the existing hole; it improves size and finish, not position.
Traps
Drilling the tap drill at the major diameter leaves no metal for the thread. Confusing tolerance with allowance. Using a second-cut file where a smooth file is specified for a bearing surface. Believing reaming corrects a hole drilled in the wrong place.