Tools, Instruments, and Equipment
Tools, Instruments, and Equipment tests selection and handling: choosing a tool whose range, resolution, and accuracy suit the tolerance, using it correctly, and keeping it in calibration. Questions often show a reading or a misuse and ask what is wrong with it.
Selection rules
- Instrument resolution should be roughly one-tenth of the tolerance being checked; a 0.02 mm caliper cannot police a 0.01 mm tolerance.
- Stay within range, and choose a range that puts the reading in the upper part of the scale.
- Confirm calibration status and check zero before use.
- Match the tool to the feature: micrometer for outside diameters, bore gauge for internal diameters, dial indicator for runout, feeler gauge for gaps, spirit level for level and plumb.
Correct use
- Calipers: measure square to the feature with light jaw pressure; use the depth rod, not the jaw tips, for depth.
- Micrometers: use the ratchet or friction thimble so pressure is consistent; read barrel and thimble; allow for the half-millimetre mark.
- Multimeters: correct function, range, and CAT rating for the circuit; never leave the leads in the current sockets when probing a voltage source.
- Torque wrenches: set to the specified value, apply smoothly to the click, never use as a breaker bar, and recalibrate after a drop or over-range use.
Worked example
A metric micrometer shows the barrel edge past the 7.5 mm mark with the thimble at 22 divisions.
- Barrel: 7 mm plus the half-millimetre mark = 7.50 mm.
- Thimble: 22 × 0.01 mm = 0.22 mm.
- Reading = 7.50 + 0.22 = 7.72 mm. Now check zero: if the instrument reads 0.01 mm with the faces closed, subtract that 0.01 mm zero error. An uncorrected zero error biases an entire batch by the same amount.
Traps that cost marks
- Measuring a moving or hot part.
- Using a caliper as a spanner or a micrometer as a clamp.
- Reading a vernier scale off-axis, producing parallax error.
- Ignoring a damaged or worn measuring face.
- Trusting a tool whose calibration label has expired.