General Hand Working Operations

Soldering is a joining process in which a filler metal (solder) melting below 450 °C is melted into the joint; the parent metals are not melted. The joint forms by wetting and a thin alloy layer, not by fusion.

Soft solder and equipment

• Common soft solder is tin-lead (for example 60% Sn, 40% Pb) which melts at about 190 to 240 °C. Lead-free tin-copper/silver solders are also used.
• Tools: soldering iron (copper bit), blow-lamp or gas torch, flux, file or emery paper.

Steps

1. Clean the surfaces to bright metal with a file or emery cloth to remove oxide, dirt and oil.
2. Apply flux on the clean surfaces.
3. Fit and hold the parts in position with a small gap (0.05 to 0.15 mm) for capillary action.
4. Heat the joint with the clean, tinned iron or torch. Heat the work, not the solder.
5. Touch the solder to the heated joint; it melts and is drawn into the gap by capillary action.
6. Remove the heat and let the joint cool without movement.
7. Clean off flux residue, especially corrosive flux, with water or a solvent.

Functions of flux

• Dissolves and removes oxide film from the metal surface and prevents new oxidation during heating.
• Lowers the surface tension of molten solder so that it spreads and wets the surface.
• Helps heat transfer.

Examples

• Rosin (resin) flux: non-corrosive; for electrical and electronic joints.
• Zinc chloride (killed spirit) or ammonium chloride: active, corrosive; for sheet iron, brass, copper plumbing. Must be washed off.

A tap is a hardened threaded tool with flutes used to cut an internal thread in a drilled hole. It is turned by a tap wrench. A die cuts an external thread on a rod, held in a die stock.

Set of taps

For through holes, taper or plug tap alone is often enough.

Procedure

1. Drill the hole to the tap drill size and chamfer its mouth.
2. Hold the work firmly in a vice with the hole axis vertical.
3. Fit the taper tap in the wrench and start it square to the work by light downward pressure.
4. Turn the wrench 1/2 to 1 turn forward, then 1/4 turn back to break and clear the chips; use cutting oil (not for cast iron).
5. Remove the tap, clear the chips and repeat with the intermediate and plug taps in order.
6. Check the thread with a bolt or a go/no-go gauge.

Tap drill size

Tap drill size = major diameter - pitch (for ISO metric threads of about 75% thread depth).

$$\begin{aligned} d_{tap} &= D - p \\ &= 12 - 1.75 = 10.25\ \text{mm} \end{aligned}$$

The nearest standard drill is 10.2 mm.

Answer: tap drill size = 10.25 mm (use 10.2 mm drill).

Die

A die cuts an external thread on a bar. The bar end is chamfered, the die is started square in the die stock and turned forward with periodic reverse to break chips, with cutting oil.