3.3 Hand Tools, Powered Equipment & Operating Safety

3.3 Hand Tools, Powered Equipment & Operating Safety

Approved-Data Control

Values and examples explain principles. Current approved maintenance data, product instructions, organisation procedures, and applicable law control actual limits, materials, intervals, methods, and acceptance.

Aircraft maintenance demands specialized hand and pneumatic tools capable of operating within strict metallurgical tolerances. Using an incorrect screwdriver tip, an improper hacksaw blade pitch, or an undressed chisel can ruin expensive airframe components or inflict severe personal injury. EASA Part-66 Module 7 requires an in-depth understanding of tool geometry, material compatibility, and operational safety.

---

Striking Tools: Hammers & Mallets

Striking tools are categorized by head material and functional application:

• Ball-Peen Hammer: The standard workshop striking tool. Features a flat striking face for driving cold chisels and pin punches, and a hemispherical "peen" on the reverse side. The ball peen is used for expanding rivet shanks, peening metal plates, and forming radiused internal corners. Heads are forged high-carbon steel, heat-treated so the striking face is hard while the eye remains softer to absorb impact without fracturing. Sized by head weight, ranging from 2 oz (light sheet metal work) to 2 lb (heavy structural maintenance).
• Soft-Faced Mallets: Essential when striking finished aircraft alloys, bearing housings, ground shafts, or cadmium-plated hardware. Heads are manufactured from brass, copper, lead, rawhide, rubber, or high-density polyurethane/nylon. They deform slightly upon impact, distributing force over a larger contact area and preventing surface gouging, work hardening, or micro-cracking of structural aluminium.
• Dead-Blow Hammers: Feature a hollow composite or polyurethane head partially filled with steel shot or lead pellets. When swung, the shot lags behind during the initial stroke. Upon impact, the shot shifts forward in a continuous flow, dampening rebound shock, eliminating bounce-back, and transferring 100% of kinetic energy into the workpiece. Crucial for seating precision bearings and assembling close-tolerance interference joints without damaging internal raceways.

---

Screwdrivers & Aerospace Drive Systems

Aerospace fasteners experience high torque loads and require dedicated drive profiles to prevent cam-out (the tendency of the driver tip to slip upward out of the fastener recess under rotational torque):

   PHILLIPS              POSIDRIV               TORQ-SET            TRI-WING
  (Symmetrical)       (45° Ticks & Ribs)    (Offset Cruciform)     (3-Flute Radial)
      |                     |                     |                     |
   ───┼───               ───┼───               ───┼───               ───┼───
      |                     |                   ─┼─                     |
(Tapered corners)    (Parallel flutes)     (Wings offset)       (Tamper-resistant)

---

Cold Chisels: Types, Cutting Angles & Safety

Cold chisels are forged from high-carbon or chrome-vanadium tool steel, hardened and tempered at the cutting edge, with a softer, tougher striking head.

Chisel Classifications

1. Flat Cold Chisel: The most common workshop chisel. Used for chipping flat surfaces, shearing sheet metal along vice jaws, and splitting seized nuts.
2. Cross-Cut (Cape) Chisel: Features a narrow cutting edge with a body that tapers backward to provide clearance. Used for cutting keyways, rectangular slots, and narrow grooves.
3. Diamond-Point Chisel: Tapered to a square section ground diagonally to produce a sharp diamond-shaped cutting point. Used for cutting V-grooves and cleaning out sharp internal 90° corners.
4. Round-Nose Chisel: Features a semi-circular cutting edge. Used for cutting concave oil grooves in bronze sleeve bearings and drawing off-centre drill holes back on track.

+-------------------------------------------------------------+
|              COLD CHISEL CUTTING EDGE GEOMETRY              |
+-------------------------------------------------------------+
| Included Angle: 60° to 70° (for aircraft steels & aluminium)|
| Soft Metals:    30° to 40° (copper, brass, soft materials)  |
| Cutting Edge:   Slightly convex curvature to prevent        |
|                 sharp outer corners from digging in         |
+-------------------------------------------------------------+

Cutting Angles & Edge Geometry

The cutting edge is ground to an included angle of 60° to 70° for general aircraft steels and aluminium alloys. For softer metals (copper, lead, soft brass), the angle may be reduced to 30° to 40°. The cutting edge should be ground with a slight convex curve; this ensures the centre of the edge enters the metal first, preventing the sharp fragile corners from digging in and snapping off.

Striking Head Dressing & Safety

Repeated striking by steel hammers causes the soft steel at the chisel's struck end to mushroom and crack. Mushroomed heads present an extreme safety hazard: subsequent hammer blows will fracture the curled metal burrs, launching razor-sharp steel fragments across the workshop at lethal velocity.

Technicians must dress striking heads regularly on a bench grinder, restoring a clean 45° chamfer around the perimeter. Always wear ballistic safety goggles when striking or grinding chisels.

---

Hacksaws: Frame Types, Blades & TPI Selection

A hacksaw consists of an adjustable or rigid steel frame and a replaceable flexible high-speed steel (HSS) or bi-metal blade. Blades are installed with teeth pointing forward, away from the handle, cutting exclusively on the forward push stroke.

The Fundamental Rule of Pitch Selection

Blade pitch is designated by Teeth Per Inch (TPI). The golden rule of hacksawing mandates that at least 2 to 3 teeth must be in continuous engagement with the workpiece cross-section at all times.

If the material thickness is less than the pitch between two adjacent teeth, the edge will fall into the tooth gullet, causing the tooth to straddle the metal, snag violently, and strip off the blade.

Sawing Technique

Operate the hacksaw at a controlled rhythm of 40 to 50 strokes per minute. Apply downward pressure strictly during the forward stroke. Relieve all cutting pressure on the return stroke; dragging teeth backward under pressure breaks off fine tooth points and dulls the cutting edge prematurely.

---

Engineering Files: Classifications, Care & Safety

Files are classified by their cut (tooth pattern), length, and degree of coarseness:

• Single-Cut: A single series of parallel, unbroken diagonal teeth cut across the face at an angle of 65° to 85° to the centerline. Operates with light pressure to produce a smooth, polished surface. Ideal for draw-filing and deburring thin sheet metal.
• Double-Cut: Two intersecting series of diagonal teeth. The first cut (overcut, 40° to 45°) is overlaid with a second, deeper cut (upcut, 70° to 80°), creating thousands of diamond-shaped cutting points. Designed for rapid metal removal.

Coarseness Hierarchy

Files are graded from coarsest to finest: Rough → Bastard → Second-Cut → Smooth → Dead-Smooth. The spacing of teeth depends on both the grade and the file length (a 12-inch bastard file has coarser teeth than an 8-inch bastard file).

[Coarse / Rapid Stock Removal]                   [Precision Finishing]
   Rough  ──►  Bastard  ──►  Second-Cut  ──►  Smooth  ──►  Dead-Smooth

File Chalking & Pinning Prevention

Soft metals (aluminium, bronze, copper) tend to clog the file teeth, a condition known as pinning. The trapped metal pins score and gouge the workpiece surface. To prevent pinning:

1. Clean teeth regularly using a file card (a stiff wire brush mounted on leather) pulled parallel to the tooth flutes.
2. Apply blackboard chalk rubbed thoroughly across the file face before cutting soft alloys. Chalk acts as a dry lubricant, preventing soft swarf from adhering to the tooth gullets.

Mandatory Workshop Safety Rule

NEVER use a file without a securely fitted, undamaged handle. The pointed, tapered tang is hard and sharp. If a file snags on the forward stroke without a handle, the tang will be driven directly into the technician's palm or wrist, puncturing arteries or nerve bundles.

---

Twist Drills, Powered Equipment & Precision Reamers

Precision hole preparation in airframe structures requires specific drill geometry and careful speed regulation:

+------------------------------------+------------------------------------+
|        118° STANDARD POINT         |          135° SPLIT POINT          |
+------------------------------------+------------------------------------+
| - Point angle: 118°                | - Point angle: 135°                |
| - Lip clearance: 8° to 12°         | - Lip clearance: 8° to 12°         |
| - Standard chisel edge             | - Split chisel (self-centering)    |
| - For: General steels & aluminium  | - For: Titanium, stainless alloys  |
+------------------------------------+------------------------------------+

Twist Drill Point Angles

• 118° Standard Point: The universal point angle for general mild steel, alloy steels, and aluminium alloys. Lip clearance angle is ground to 8° to 12°.
• 135° Split Point: Designed for hard, work-hardening alloys (titanium, stainless steel, Inconel). The flatter angle creates a shorter cutting lip, while the split chisel edge eliminates the blunt chisel center, allowing the drill to self-center without "wandering" and reducing required feed thrust by 40%.
• 90° Point: Used for drilling plastics, Plexiglas/acrylic cabin windows, and composite laminates. Prevents grabbing and breakout cracking on the exit side of the hole.

Microstop Countersink Cages

When preparing holes for flush solid rivets (such as 100° countersunk rivets), hand drilling can easily produce oval or overly deep countersinks that weaken the skin. Technicians use a microstop countersink cage. The tool features a spring-loaded nylon foot that contacts the skin without scratching, and a micrometer-threaded barrel adjustable in 0.001-inch (0.025 mm) increments to ensure perfectly repeatable countersink depth.

Precision Reamers & The Golden Rule

Drilling produces a hole that is slightly out-of-round and rough. Precision aircraft holes for close-tolerance bolts (e.g., NAS or MS bolts) are finished using a precision reamer (hand reamer with a tapered lead or spiral-flute machine reamer). Reaming removes the final 0.10 to 0.25 mm of metal, producing a mirror-smooth surface with dimensional accuracy to within 0.01 mm.

The Golden Rule of Reaming: A reamer must ALWAYS be turned in the clockwise (cutting) direction, both when feeding into the hole and when withdrawing it. Reversing the rotation wedges microscopic metal chips behind the relief faces of the cutting flutes, instantly dulling the cutting edges and scoring deep spiral scratches into the finished bore.