Measuring, Marking and Technical Drawing for Jewellery
What is examined
This domain tests accurate measurement, correct use of measuring instruments, marking out, and the ability to read or produce a jewellery drawing with dimensions, tolerances and conventions. Accuracy is assessed numerically, so a small reading error costs marks.
Core rules
- Steel rule: read at eye level to avoid parallax; graduations are usually 0.5 mm or 1 mm. Do not use it as a cutting straightedge.
- Vernier caliper: main scale plus vernier scale. A common least count is 0.02 mm; check zero error before use and apply it to the reading. It measures outside diameter, inside diameter and depth.
- Micrometer: typical least count 0.01 mm; use the ratchet or friction thimble to avoid over-tightening on thin sheet.
- Ring and finger gauges: systems differ - UK letters, US numbers, EU circumference in millimetres. Convert only with a reliable chart, and state the system on the job sheet.
- Marking out: scribe, dividers, marking blue, centre punch and jeweller's saw. Mark on the waste side of the line and preserve the datum edge.
- Technical drawing: orthographic projection, first-angle or third-angle as specified; scale 1:1, 2:1 or 1:2; visible outlines, hidden lines, centre lines, dimension lines with arrowheads, extension lines, section hatching, title block, material and finish notes.
- Tolerance is the permitted variation from nominal size. A drawing that states 17.00 ± 0.05 mm accepts 16.95–17.05 mm; a setting recess may need a tighter tolerance than an outside diameter.
Worked example
A vernier caliper has a least count of 0.02 mm. The main scale reads 12 mm and the 34th vernier line aligns with a main-scale line. Vernier contribution = 34 × 0.02 = 0.68 mm, so the reading is 12.68 mm. If the caliper showed a zero error of +0.03 mm when closed, subtract it: 12.68 − 0.03 = 12.65 mm. State the corrected value, not just the raw scale reading, and record the tolerance on the drawing.
Traps
Ignoring zero error; reading a caliper at an angle; measuring a curved surface with the caliper jaws only at the tips; mixing UK, US and EU ring sizes; dimensioning from two different datums so the dimensions conflict; drawing freehand without scale; forgetting to note tolerance, material or finish; using a worn rule or a caliper with damaged jaws.