Marking and Measuring Tools

Steel Rule and Measuring Scales

The steel rule is the most basic linear measuring tool used by a Fitter. It is made of hardened and tempered stainless steel. Metric rules are graduated in mm and 0.5 mm divisions; the smallest readable division is 0.5 mm. Standard lengths available are 150 mm, 300 mm, 600 mm, and 1000 mm. Rules must be held flat on the workpiece to avoid parallax error. Hook rules and narrow rules are special types. Always read the scale at eye level perpendicular to the graduation to get an accurate reading.

Vernier Caliper — Construction and Least Count

The Vernier Caliper measures external diameter, internal diameter, depth, and step distances in one instrument. It has a main scale (fixed jaw) and a vernier scale (sliding jaw). Least Count (LC) = 1 MSD − 1 VSD. For a 25-division vernier: LC = 1 − (24/25) = 0.02 mm. For a 50-division vernier: LC = 1 − (49/50) = 0.02 mm. Reading = Main Scale Reading + (Coinciding VSD × LC). Range is typically 0–150 mm or 0–300 mm. The depth bar at the tail measures slot depth. Always clean jaws before use to avoid zero error.

Marking Tools — Scriber, Divider, Dot Punch, Centre Punch

Scriber: Made of HSS or HCS, tip ground to 15°–20°, used to scribe lines on metal surfaces coated with chalk or Prussian blue. Divider: Both legs are pointed; used to scribe arcs, circles, and to divide lines into equal parts. Dot Punch: Included angle 60°; used to make small indentations to retain scribed lines. Centre Punch: Included angle 90°; used to mark the centre of a hole before drilling. Odd-leg caliper (Jenny caliper) scribes lines parallel to an edge. All punches are made of HCS, hardened and tempered at the tip.

Surface Plate, Angle Plate, V-Block and Try Square

Surface Plate: Provides a true flat reference datum. Made of CI or Granite. Grades — A (inspection), B (toolroom), C (workshop). Size 400×400 mm is common. Angle Plate: Made of CI, has two faces at exactly 90°; used to hold workpieces vertically on a surface plate. V-Block: Made of hardened steel; V-groove angle is 90°; holds cylindrical workpieces for marking. Try Square: Blade and stock at 90°; checks squareness of edges. Combination Set includes square head, protractor head, and centre head on one rule — measures 90°, 45°, and centre of round bars.

01Linear Measuring Tools

• Steel rule — graduations and parallax error
• Vernier caliper — main scale, vernier scale, least count
• Reading a vernier caliper — formula and steps
• Depth micrometer vs. depth bar of vernier

02Marking Tools

• Scriber — material, tip angle, usage with layout dye
• Divider — scribing arcs and bisecting lines
• Odd-leg caliper — scribing parallel lines from an edge
• Trammels — for large radii beyond divider range

03Punches

• Dot punch — 60° angle, preserving scribed lines
• Centre punch — 90° angle, drill starting point
• Bell punch — self-centering on round bars
• Material — HCS, hardened tip, soft head

04Reference and Supporting Tools

• Surface plate — grades A, B, C; CI and Granite types
• Angle plate — 90° faces, clamping workpiece vertically
• V-block — 90° groove, holding cylindrical shafts
• Marking off table vs. surface plate

05Angular and Combination Measuring Tools

• Try square — 90° check, blade lengths 150–300 mm
• Combination set — square, protractor, centre head
• Bevel protractor — least count 5 minutes, measures angles
• Universal bevel protractor — vernier type, LC = 5′