Scheda di revisione: Workshop Hand Tools and Measurement

📋 Course Outline

1. Marking Out and Datums
2. Marking Out Tools
3. Marking Out Procedure
4. Testing and Scribing Tools
5. Hacksaws and Files
6. Hammers and Chisels
7. Taps, Dies and Threads
8. Basic Measuring Instruments

📖 1. Marking Out and Datums

🔑 Key Concepts & Definitions

• Marking out : The scratching of lines on a workpiece surface to indicate its outline or the position of holes and slots and to guide machine setup.
• Datum : A reference point, edge or centre line from which all dimensions and measurements are taken.

📝 Essential Points

• Two datums, usually at right angles to each other, are required to position a point on a plane surface.

💡 Memory Hook

Datum first, lines second

📖 2. Marking Out Tools

📝 Essential Points

★ Must-know

• A scriber is used to mark all lines on a metal surface, and its point must remain sharp to produce well-defined lines.

• A divider is used to scribe circles or arcs and to mark a series of lengths such as hole centres, and it is made from tool steel or high-carbon steel.

📌 A centre punch has a 90° point for wide, relatively shallow hole-location marks; a dot punch has a 60° point for witness marks; and a prick punch has a 30° point for light marks positioning dividers.

Further detail

• The three punch types are the centre punch, dot punch and prick punch.

💡 Memory Hook

Scribe, divide, punch

📖 3. Marking Out Procedure

📝 Essential Points

★ Must-know

🔄 Process — To mark out the plate, first find and scribe the centre line and centre-dot the relevant intersection, then use dividers to scribe arcs for hole centres and radii, and finally scribe the tangential profile lines with a precision steel rule.

Further detail

🔄 Process — When scribing the plate profile, dividers are set using the graduations of a precision steel rule and are located on centre dots before each radius is scribed.

• The centre of the small radius is found by measuring from one end a distance equal to the radius plus 1 mm to allow for the extra material left on the end.

💡 Memory Hook

Measure, arc, profile

📖 4. Testing and Scribing Tools

🔑 Key Concepts & Definitions

• Scribing block : A testing tool used to scribe lines parallel to a datum surface, set a job parallel to a datum surface, check job height and parallelism, and set a job concentric with a machine spindle.

📝 Essential Points

📌 A fixed scribing block and a universal scribing block are two forms of scribing block shown for testing and marking operations.

💡 Memory Hook

Check against the datum

📖 5. Hacksaws and Files

🔑 Key Concepts & Definitions

• Hacksaw : Used to cut metal and remove large amounts of waste before a file produces the final size and surface.
• File cuts : A single-cut file has one series of teeth across its face, whereas a double-cut file has two sets of teeth across its face.

📝 Essential Points

★ Must-know

📌 A file must always be used with a handle installed on its tang.

• The standard hacksaw blade is 300 mm long, 13 mm wide and 0.65 mm thick, and is available with 14, 18, 24 or 32 teeth per 25 mm.

Further detail

📌 A hacksaw blade must be selected according to the material hardness and whether the cut is through a thick or thin section, considering its pitch and blade material.

• Flat, equalising or hand, half-round, round or rat-tail, square, triangular, pillar, knife and needle files are used for different finishing, slotting, hole, groove and angle-cutting tasks.

💡 Memory Hook

Cut, then finish

📖 6. Hammers and Chisels

🔑 Key Concepts & Definitions

• Soft hammer : Has a head made from materials such as lead, copper, babbitt, rawhide, wood or plastic and is used to position workpieces without damaging their finish.
• Chisel : Used to chip and cut metal and to remove waste metal from a workpiece.

📝 Essential Points

★ Must-know

📌 A ball-peen hammer has a hemispherical peen for upsetting or riveting metal, a straight-peen hammer spreads or draws metal in line with the handle, and a cross-peen hammer spreads or draws metal at right angles to the handle.

Further detail

📌 Hammer safety requires inspecting the handle, ensuring the head is firmly attached, using a power grip and keeping the other hand away.

📌 Chisel safety requires inspecting the blade and handle, rejecting a mushroomed impact head and pointing the blade away from the body.

💡 Memory Hook

Strike safely

📖 7. Taps, Dies and Threads

🔑 Key Concepts & Definitions

• Tapping : The operation of cutting an internal thread.

📝 Essential Points

★ Must-know

🔄 Process — To tap a hole, drill it to the correct tap-drill size, clamp the workpiece with the hole upright, install the correct taper or plug tap in a wrench, insert it, press down and turn clockwise while keeping the tap parallel to the hole.

📌 The taper or first tap has the longest chamfer lead, the second or intermediate tap has the next longest lead, and the bottoming or plug tap has a very short lead.

📐 Formula — For a metric tap, tap drill size equals major diameter in millimetres minus pitch in millimetres: TDS = major diameter − pitch.

📐 Formula — For an inch tap, tap drill size equals major diameter in inches minus one divided by the number of threads per inch: TDS = major diameter − 1/N.

📌 An adjustable round split die has an adjusting screw that controls thread diameter and fit, whereas a plain round split die is solid and cannot provide different thread fits.

Further detail

📌 During external threading, the die is turned in quarter turns and backed off in quarter turns to remove chips, and cutting oil is always used.

💡 Memory Hook

Drill, tap, back off

📖 8. Basic Measuring Instruments

🔑 Key Concepts & Definitions

• Vernier caliper : Measures external dimensions, internal dimensions and depths using measuring jaws, a depth gauge, a main scale and a vernier scale.

📝 Essential Points

★ Must-know

🔄 Process — To read a micrometer, grip the frame, turn the thimble until the object is gently held, turn the ratchet until it clicks, and add the sleeve-scale reading to the thimble reading multiplied by the least count.

🔄 Process — To read a vernier caliper, place the object between the jaws, close them gently, record the main-scale reading behind the vernier zero, multiply the coinciding vernier division by the least count, and add both readings.

📌 External calipers measure outside dimensions and internal calipers measure inside dimensions; when used with a steel rule, their accuracy is limited to 0.5 mm.

📐 Formula — Micrometer reading equals sleeve-scale reading plus thimble-scale reading multiplied by the least count.

📐 Formula — For the described vernier caliper, the least count is 0.1 cm minus 0.09 cm, which equals 0.01 cm.

Further detail

• A steel ruler is made from stainless steel or tool steel, has inch and metric scales, and is available in metric, fractional and decimal forms.

• The listed micrometer types are blade, point, disk, direct-reading, inside and tubing micrometers.

💡 Memory Hook

Rule, compare, calculate

📊 Synthesis Tables

Punch Types and Uses

Punch	Point angle	Use
Centre punch	90°	Locate holes
Dot punch	60°	Mark witness marks
Prick punch	30°	Position dividers

⚠️ Common Pitfalls & Confusions

1. Marking out is not the machining operation itself; it guides the subsequent setup and machining.
2. A divider scribes arcs and transfers distances; it is not primarily a measuring scale.
3. A scribing block is used for checking and setting relative to a datum, not for cutting metal.
4. A hacksaw removes surplus material, whereas filing provides final sizing and surface finishing.
5. Straight-peen and cross-peen directions are parallel and perpendicular to the handle respectively.
6. Tapping produces an internal thread, whereas dies produce external threads.
7. External and internal calipers are distinguished by the shape and orientation of their legs.